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The Solar System^[d] is the gravitationally bound system of the Sun and the objects that orbit it.^[11] It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere. Astronomers classify it as a G-type main-sequence star.

The largest objects that orbit the Sun are the eight planets. In order from the Sun, they are four terrestrial planets (Mercury, Venus, Earth and Mars); two gas giants (Jupiter and Saturn); and two ice giants (Uranus and Neptune). All terrestrial planets have solid surfaces. Inversely, all giant planets do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of the Solar System’s mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn.

There is a strong consensus among astronomers^[e] that the Solar System has at least nine dwarf planets: Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna. There are a vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. Some of these bodies are in the asteroid belt (between Mars’s and Jupiter’s orbit) and the Kuiper belt (just outside Neptune’s orbit).^[f] Six planets, seven dwarf planets, and other bodies have orbiting natural satellites, which are commonly called ‘moons’.

The Solar System is constantly flooded by the Sun’s charged particles, the solar wind, forming the heliosphere. Around 75–90 astronomical units from the Sun,^[g] the solar wind is halted, resulting in the heliopause. This is the boundary of the Solar System to interstellar space. The outermost region of the Solar System is the theorized Oort cloud, the source for long-period comets, extending to a radius of 2,000–200,000 AU. The closest star to the Solar System, Proxima Centauri, is 4.25 light-years (269,000 AU) away. Both stars belong to the Milky Way galaxy.

Formation and evolution

Main article: Formation and evolution of the Solar System

Past

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.^[b] This initial cloud was likely several light-years across and probably birthed several stars.^[14] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars.^[15]

As the pre-solar nebula^[15] collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings.^[14] As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU^[14][16] and a hot, dense protostar at the center.^[17][18] The planets formed by accretion from this disc,^[19] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover minor bodies.^[20][21]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the frost line). They eventually formed the rocky planets of Mercury, Venus, Earth, and Mars. Because these refractory materials only comprised a small fraction of the solar nebula, the terrestrial planets could not grow very large.^[20]

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.^[20] Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.^[20]

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion.^[22] As helium accumulates at its core, the Sun is growing brighter;^[23] early in its main-sequence life its brightness was 70% that of what it is today.^[24] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a main-sequence star.^[25] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.^[23]

Following the dissipation of the protoplanetary disk, the Nice model proposes that gravitational encounters between planetisimals and the gas giants caused each to migrate into different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period, the grand tack hypothesis suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the Late Heavy Bombardment of the inner planets.^[26][27]

Present and future

The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.^[28] Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.^[29]

The Sun’s main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun’s pre-remnant life combined.^[30] The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun’s main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (4,220 °F) at its coolest) than it is on the main sequence.^[30]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable (possibly destroying Earth as well).^[31][32] Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf, half the original mass of the Sun but only the size of Earth.^[30] The ejected outer layers may form a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.^[33][34]

General characteristics

Astronomers sometimes divide the Solar System structure into separate regions. The inner Solar System includes Mercury, Venus, Earth, Mars, and the bodies in the asteroid belt. The outer Solar System includes Jupiter, Saturn, Uranus, Neptune, and the bodies in the Kuiper belt.^[35] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.^[36]

Composition

Further information: List of Solar System objects and List of interstellar and circumstellar molecules

The principal component of the Solar System is the Sun, a G-type main-sequence star that contains 99.86% of the system’s known mass and dominates it gravitationally.^[37] The Sun’s four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.^[h]

The Sun is composed of roughly 98% hydrogen and helium,^[41] as are Jupiter and Saturn.^[42][43] A composition gradient exists in the Solar System, created by heat and light pressure from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.^[44] The boundary in the Solar System beyond which those volatile substances could coalesce is known as the frost line, and it lies at roughly five times the Earth’s distance from the Sun.^[5]

Orbits

The planets and other large objects in orbit around the Sun lie near the plane of Earth’s orbit, known as the ecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.^[45][46] Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.^[47]

As a result of the formation of the Solar System, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth’s north pole.^[48] There are exceptions, such as Halley’s Comet.^[49] Most of the larger moons orbit their planets in prograde direction, matching the direction of planetary rotation; Neptune’s moon Triton is the largest to orbit in the opposite, retrograde manner.^[50] Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.^[51]

To a good first approximation, Kepler’s laws of planetary motion describe the orbits of objects around the Sun.^{[52]: 433–437} These laws stipulate that each object travels along an ellipse with the Sun at one focus, which causes the body’s distance from the Sun to vary over the course of its year. A body’s closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion.^[53]: 9-6 With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler’s laws only account for the influence of the Sun’s gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using numerical models,^[53]: 9-6 but the planetary system can change chaotically over billions of years.^[54]

The angular momentum of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components.^[55] Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.^[56][57] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.^[56]

Distances and scales

The radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).^[58] Thus, the Sun occupies 0.00001% (1 part in 10⁷) of the volume of a sphere with a radius the size of Earth’s orbit, whereas Earth’s volume is roughly 1 millionth (10⁻⁶) that of the Sun. Jupiter, the largest planet, is 5.2 AU from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU from the Sun.^[43][59]

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like the Titius–Bode law^[60] and Johannes Kepler’s model based on the Platonic solids,^[61] but ongoing discoveries have invalidated these hypotheses.^[62]

Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.^[63] The largest such scale model, the Sweden Solar System, uses the 110-meter (361-foot) Avicii Arena in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.^[64][65] At that scale, the distance to Proxima Centauri would be roughly 8 times further than the Moon is from Earth.

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth’s diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale.^[66]

Habitability

Main article: Planetary habitability in the Solar System

Comparison of the habitable zones of the Solar System and TRAPPIST-1, an ultracool red dwarf star known to have seven terrestrial planets in stable orbits around the star.

Comparison of the habitable zones for different stellar temperatures, with a sample of known exoplanets plus the Earth, Mars, and Venus. From top to bottom are an F-type main-sequence star, a yellow dwarf (G-type main-sequence star), an orange dwarf (K-type main-sequence star), a typical red dwarf, and an ultra-cool dwarf.

Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun’s magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.^[67]

The zone of habitability of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water.^[68] Habitability might be possible in subsurface oceans of various outer Solar System moons.^[69]

Comparison with extrasolar systems

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.^[70][71] The known Solar System lacks super-Earths, planets between one and ten times as massive as the Earth,^[70] although the hypothetical Planet Nine, if it does exist, could be a super-Earth orbiting in the edge of the Solar System.^[72]

Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no “gap” as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.^[70][73]

The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller orbital eccentricity.^[70] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.^[70][74]

Sun

Main article: Sun

The Sun is the Solar System’s star and by far its most massive component. Its large mass (332,900 Earth masses),^[75] which comprises 99.86% of all the mass in the Solar System,^[76] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.^[77] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.^[78][79]

Because the Sun fuses hydrogen at its core, it is a main-sequence star. More specifically, it is a G2-type main-sequence star, where the type designation refers to its effective temperature. Hotter main-sequence stars are more luminous but shorter lived. The Sun’s temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up about 75% of the fusor stars in the Milky Way.^[80]

The Sun is a population I star, having formed in the spiral arms of the Milky Way galaxy. It has a higher abundance of elements heavier than hydrogen and helium (“metals” in astronomical parlance) than the older population II stars in the galactic bulge and halo.^[81] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun’s development of a planetary system because the planets formed from the accretion of “metals”.^[82]

The region of space dominated by the Solar magnetosphere is the heliosphere, which spans much of the Solar System. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) called the solar wind. This stream spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),^[83] filling the vacuum between the bodies of the Solar System. The result is a thin, dusty atmosphere, called the interplanetary medium, which extends to at least 100 AU.^[84]

Activity on the Sun’s surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.^[85] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth’s magnetic field funnels charged particles into Earth’s upper atmosphere, where its interactions create aurorae seen near the magnetic poles.^[86] The largest stable structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun’s rotating magnetic field on the interplanetary medium.^[87][88]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroids.^[89] Composed mainly of silicates and metals,^[90] the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the frost line, which is a little less than 5 AU from the Sun.^[45]

Inner planets

Main article: Terrestrial planet

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.^[91]

• Mercury (0.31–0.59 AU from the Sun)^{[D 6]} is the smallest planet in the Solar System. Its surface is grayish, with an expansive rupes (cliff) system generated from thrust faults and bright ray systems formed by impact event remnants.^[92] The surface has widely varying temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. In the past, Mercury was volcanically active, producing smooth basaltic plains similar to the Moon.^[93] It is likely that Mercury has a silicate crust and a large iron core.^[94][95] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms.^[96] Mercury has no natural satellites.^[97]
• Venus (0.72–0.73 AU)^{[D 6]} has a reflective, whitish atmosphere that is mainly composed of carbon dioxide. At the surface, the atmospheric pressure is ninety times as dense as on Earth’s sea level.^[98] Venus has a surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.^[99] The planet lacks a protective magnetic field to protect against stripping by the solar wind, which suggests that its atmosphere is sustained by volcanic activity.^[100] Its surface displays extensive evidence of volcanic activity with stagnant lid tectonics.^[101] Venus has no natural satellites.^[97]
• Earth (0.98–1.02 AU)^{[D 6]} is the only place in the universe where life and surface liquid water are known to exist.^[102] Earth’s atmosphere contains 78% nitrogen and 21% oxygen, which is the result of the presence of life.^[103][104] The planet has a complex climate and weather system, with conditions differing drastically between climate regions.^[105] The solid surface of Earth is dominated by green vegetation, deserts and white ice sheets.^{[106][107][108]} Earth’s surface is shaped by plate tectonics that formed the continental masses.^[93] Earth’s planetary magnetosphere shields the surface from radiation, limiting atmospheric stripping and maintaining life habitability.^[109]
  • The Moon is Earth’s only natural satellite.^[110] Its diameter is one-quarter the size of Earth’s.^[111] Its surface is covered in very fine regolith and dominated by impact craters.^[112][113] Large dark patches on the Moon, maria, are formed from past volcanic activity.^[114] The Moon’s atmosphere is extremely thin, consisting of a partial vacuum with particle densities of under 10⁷ per cm⁻³.^[115]
• Mars (1.38–1.67 AU)^{[D 6]} has a radius about half of that of Earth.^[116] Most of the planet is red due to iron oxide in Martian soil,^[117] and the polar regions are covered in white ice caps made of water and carbon dioxide.^[118] Mars has an atmosphere composed mostly of carbon dioxide, with surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.^[119] During the Mars year (687 Earth days), there are large surface temperature swings on the surface between −78.5 °C (−109.3 °F) to 5.7 °C (42.3 °F). The surface is peppered with volcanoes and rift valleys, and has a rich collection of minerals.^[120][121] Mars has a highly differentiated internal structure, and lost its magnetosphere 4 billion years ago.^[122][123]Mars has two tiny moons:^[124]
  • Phobos is Mars’s inner moon. It is a small, irregularly shaped object with a mean radius of 11 km (7 mi). Its surface is very unreflective and dominated by impact craters.^{[D 7][125]} In particular, Phobos’s surface has a very large Stickney impact crater that is roughly 4.5 km (2.8 mi) in radius.^[126]
  • Deimos is Mars’s outer moon. Like Phobos, it is irregularly shaped, with a mean radius of 6 km (4 mi) and its surface reflects little light.^{[D 8][D 9]} However, the surface of Deimos is noticeably smoother than Phobos because the regolith partially covers the impact craters.^[127]

Asteroids

Main article: Asteroid

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice.^[128][129] They range from a few meters to hundreds of kilometers in size. Many asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids.^[130]

• Mercury-crossing asteroids are those with perihelia within the orbit of Mercury. At least 362 are known to date, and include the closest objects to the Sun known in the Solar System.^[131] No vulcanoids, asteroids between the orbit of Mercury and the Sun, have been discovered.^[132][133] As of 2024, one asteroid has been discovered to orbit completely within Venus’s orbit, 594913 ꞌAylóꞌchaxnim.^[134]
• Venus-crossing asteroids are those that cross the orbit of Venus. There are 2,809 as of 2015.^[135]
• Near-Earth asteroids have orbits that approach relatively close to Earth’s orbit,^[136] and some of them are potentially hazardous objects because they might collide with Earth in the future.^[137][138] There are over 37.000 known as of 2024.^[139] A number of solar-orbiting meteoroids were large enough to be tracked in space before striking Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth.^[140]
• Mars-crossing asteroids are those with perhihelia above 1.3 AU which cross the orbit of Mars.^[141] As of 2024, NASA lists 26,182 confirmed Mars-crossing asteroids.^[135]

Asteroid belt

The asteroid belt occupies a torus-shaped region between 2.3 and 3.3 AU from the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System’s formation that failed to coalesce because of the gravitational interference of Jupiter.^[142] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter.^[143] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.^[40] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.^[144]

Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets, a precursor stage before becoming a fully-formed planet (see List of exceptional asteroids):^{[145][146][147]}

• Ceres (2.55–2.98 AU) is the only dwarf planet in the asteroid belt.^[148] It is the largest object in the belt, with a diameter of 940 km (580 mi).^[149] Its surface contains a mixture of carbon,^[150] frozen water and hydrated minerals.^[151] There are signs of past cryovolcanic activity, where volatile material such as water are erupted onto the surface, as seen in surface bright spots.^[152] Ceres has a very thin water vapor atmosphere, but practically speaking it is indistinguishable from a vacuum.^[153]
• Vesta (2.13–3.41 AU) is the second-largest object in the asteroid belt.^[154] Its fragments survive as the Vesta asteroid family^[155] and numerous HED meteorites found on Earth.^[156] Vesta’s surface, dominated by basaltic and metamorphic material, has a denser composition than Ceres’s.^[157] Its surface is marked by two giant craters: Rheasilvia and Veneneia.^[158]
• Pallas (2.15–2.57 AU) is the third-largest object in the asteroid belt.^[154] It has its own Pallas asteroid family.^[155] Not much is known about Pallas because it has never been visited by a spacecraft,^[159] though its surface is predicted to be composed of silicates.^[160]

Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits.^[161] They lie in three linked clusters between Jupiter and the main asteroid belt.

Trojans are bodies located within another body’s gravitationally stable Lagrange points: L₄, 60° ahead in its orbit, or L₅, 60° behind in its orbit.^[162] Every planet except Mercury and Saturn is known to possess at least 1 trojan.^{[163][164][165]} The Jupiter trojan population is roughly equal to that of the asteroid belt.^[166] After Jupiter, Neptune possesses the most confirmed trojans, at 28.^[167]

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane, than planets of the inner Solar System because their lower temperatures allow these compounds to remain solid, without significant sublimation.^[20]

Outer planets

Main article: Giant planet

The four outer planets, called giant planets or Jovian planets, collectively make up 99% of the mass orbiting the Sun.^[h] All four giant planets have multiple moons and a ring system, although only Saturn’s rings are easily observed from Earth.^[91] Jupiter and Saturn are composed mainly of gases with extremely low melting points, such as hydrogen, helium, and neon,^[168] hence their designation as gas giants.^[169] Uranus and Neptune are ice giants,^[170] meaning they are largely composed of ‘ice’ in the astronomical sense (chemical compounds with melting points of up to a few hundred kelvins^[168] such as water, methane, ammonia, hydrogen sulfide, and carbon dioxide.^[171]) Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune’s orbit.^[171][172]

• Jupiter (4.95–5.46 AU)^{[D 6]} is the biggest and most massive planet in the Solar System. On its surface, there are orange-brown and white cloud bands moving via the principles of atmospheric circulation, with giant storms swirling on the surface such as the Great Red Spot and white ‘ovals’. Jupiter possesses a strong enough magnetosphere to redirect ionizing radiation and cause auroras on its poles.^[173] As of 2024, Jupiter has 95 confirmed satellites, which can roughly be sorted into three groups:
  • The Amalthea group, consisting of Metis, Adrastea, Amalthea, and Thebe. They orbit substantially closer to Jupiter than other satellites.^[174] Materials from these natural satellites are the source of Jupiter’s faint ring.^[175]
  • The Galilean moons, consisting of Ganymede, Callisto, Io, and Europa. They are the largest moons of Jupiter and exhibit planetary properties.^[176]
  • Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects.^[177]
• Saturn (9.08–10.12 AU)^{[D 6]} has a distinctive visible ring system orbiting around its equator composed of small ice and rock particles. Like Jupiter, it is mostly made of hydrogen and helium.^[178] At its north and south poles, Saturn has peculiar hexagon-shaped storms larger than the diameter of Earth. Saturn has a magnetosphere capable of producing weak auroras. As of 2024, Saturn has 146 confirmed satellites, grouped into:
  • Ring moonlets and shepherds, which orbit inside or close to Saturn’s rings. A moonlet can only partially clear out dust in its orbit,^[179] while the ring shepherds are able to completely clear out dust, forming visible gaps in the rings.^[180]
  • Inner large satellites Mimas, Enceladus, Tethys, and Dione. These satellites orbit within Saturn’s E ring. They are composed mostly of water ice and are believed to have differentiated internal structures.^[181]
  • Trojan moons Calypso and Telesto (trojans of Tethys), and Helene and Polydeuces (trojans of Dione). These small moons share their orbits with Tethys and Dione, leading or trailing either.^[182][183]
  • Outer large satellites Rhea, Titan, Hyperion, and Iapetus.^[181] Titan is the only satellite in the Solar System to have a substantial atmosphere.^[184]
  • Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects. Phoebe is the largest irregular satellite of Saturn.^[185]
• Uranus (18.3–20.1 AU),^{[D 6]} uniquely among the planets, orbits the Sun on its side with an axial tilt >90°. This gives the planet extreme seasonal variation as each pole points alternately toward and then away from the Sun.^[186] Uranus’ outer layer has a muted cyan color, but underneath these clouds are many mysteries about its climate, such as unusually low internal heat and erratic cloud formation. As of 2024, Uranus has 28 confirmed satellites, divided into three groups:
  • Inner satellites, which orbit inside Uranus’ ring system.^[187] They are very close to each other, which suggests that their orbits are chaotic.^[188]
  • Large satellites, consisting of Titania, Oberon, Umbriel, Ariel, and Miranda.^[189] Most of them have roughly equal amounts of rock and ice, except Miranda, which is made primarily of ice.^[190]
  • Irregular satellites, having more distant and eccentric orbits than the other objects.^[191]
• Neptune (29.9–30.5 AU)^{[D 6]} is the furthest planet known in the Solar System. Its outer atmosphere has a slightly muted cyan color, with occasional storms on the surface that look like dark spots. Like Uranus, many atmospheric phenomena of Neptune are unexplained, such as the thermosphere‘s abnormally high temperature or the strong tilt (47°) of its magnetosphere. As of 2024, Neptune has 16 confirmed satellites, divided into two groups:
  • Regular satellites, which have circular orbits that lie near Neptune’s equator.^[185]
  • Irregular satellites, which as the name implies, have less regular orbits. One of them, Triton, is Neptune’s largest moon. It is geologically active, with erupting geysers of nitrogen gas, and possesses a thin, cloudy nitrogen atmosphere.^[192][184]

Centaurs

Main article: Centaur

The centaurs are icy, comet-like bodies whose semi-major axes are longer than Jupiter’s and shorter than Neptune’s (between 5.5 and 30 AU). These are former Kuiper belt and scattered disc objects (SDOs) that were gravitationally perturbed closer to the Sun by the outer planets, and are expected to become comets or be ejected out of the Solar System.^[39] While most centaurs are inactive and asteroid-like, some exhibit cometary activity, such as the first centaur discovered, 2060 Chiron, which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.^[193] The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi) and is one of the few minor planets possessing a ring system.^[194][195]

Trans-Neptunian region

Beyond the orbit of Neptune lies the area of the “trans-Neptunian region“, with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the “third zone of the Solar System”, enclosing the inner and the outer Solar System.^[196]

Kuiper belt

Main article: Kuiper belt

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.^[197] It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.^[198] There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.^[39] Many Kuiper belt objects have satellites,^[199] and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic.^[200]

The Kuiper belt can be roughly divided into the “classical” belt and the resonant trans-Neptunian objects.^[197] The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU.^[201] Members of the classical Kuiper belt are sometimes called “cubewanos”, after the first of their kind to be discovered, originally designated 1992 QB₁, (and has since been named Albion); they are still in near primordial, low-eccentricity orbits.^[202]

There is strong consensus among astronomers that five members of the Kuiper belt are dwarf planets.^[198][203] Many dwarf planet candidates are being considered, pending further data for verification.^[204]

• Pluto (29.7–49.3 AU) is the largest known object in the Kuiper belt. Pluto has a relatively eccentric orbit, inclined 17 degrees to the ecliptic plane. Pluto has a 2:3 resonance with Neptune, meaning that Pluto orbits twice around the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.^[205]Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra.^[206]
  • Charon, the largest of Pluto’s moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e. they appear to “orbit each other”).
• Orcus (30.3–48.1 AU), is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.^[207] Its eccentricity and inclination are similar to Pluto’s, but its perihelion lies about 120° from that of Pluto. Thus, the phase of Orcus’s orbit is opposite to Pluto’s: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa.^[208] For this reason, it has been called the anti-Pluto.^[209][210] It has one known moon, Vanth.^[211]
• Haumea (34.6–51.6 AU) was discovered in 2005.^[212] It is in a temporary 7:12 orbital resonance with Neptune.^[207] Haumea possesses a ring system, two known moons named Hiʻiaka and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an ellipsoid. It is part of a collisional family of Kuiper belt objects that share similar orbits, which suggests a giant impact on Haumea ejected fragments into space billions of years ago.^[213]
• Makemake (38.1–52.8 AU), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009.^[214] Its orbit is far more inclined than Pluto’s, at 29°.^[215] It has one known moon, S/2015 (136472) 1.^[216]
• Quaoar (41.9–45.5 AU) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea.^[207] It possesses a ring system and one known moon, Weywot.^[217]

Scattered disc

Main article: Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune’s early outward migration. Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs’ orbits can be inclined up to 46.8° from the ecliptic plane.^[218] Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as “scattered Kuiper belt objects”.^[219] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.^[220]

Currently, there is strong consensus among astronomers that two of the bodies in the scattered disc are dwarf planets:

• Eris (38.3–97.5 AU) is the largest known scattered disc object and the most massive known dwarf planet. Eris’s discovery contributed to a debate about the definition of a planet because it is 25% more massive than Pluto^[221] and about the same diameter. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane at an angle of 44°.^[222]
• Gonggong (33.8–101.2 AU) is a dwarf planet in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.^{[D 10]} It has one known moon, Xiangliu.^[223]

Extreme trans-Neptunian objects

Main article: Extreme trans-Neptunian object

Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short.^[224] Generally, ETNOs’ semi-major axes are at least 150–250 AU wide.^[224][225] For example, 541132 Leleākūhonua orbits the Sun once every ~32,000 years, with a distance of 65–2000 AU from the Sun.^{[D 11]}

This population is divided into three subgroups by astronomers. The scattered ETNOs have perihelia around 38–45 AU and an exceptionally high eccentricity of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of gravitational scattering by Neptune and still interact with the giant planets. The detached ETNOs, with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. The sednoids or inner Oort cloud objects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it.^[224]

Currently, there is one ETNO that is classified as a dwarf planet:

• Sedna (76.2–937 AU) was the first extreme trans-Neptunian object to be discovered. It is a large, reddish object, and it takes ~11,400 years for Sedna to complete one orbit. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune’s migration.^[226] The sednoid population is named after Sedna.^[224]

Edge of the heliosphere

The Sun’s stellar-wind bubble, the heliosphere, a region of space dominated by the Sun, has its boundary at the termination shock. Based on the Sun’s peculiar motion relative to the local standard of rest, this boundary is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.^[227] Here the solar wind collides with the interstellar medium^[228] and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath.^[227]

The heliosheath has been theorized to look and behave very much like a comet’s tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind to possibly several thousands of AU.^[229][230] Evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,^[231][232] but the actual shape remains unknown.^[233]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere.^[227] The heliopause is considered the beginning of the interstellar medium.^[84] Beyond the heliopause, at around 230 AU, lies the bow shock: a plasma “wake” left by the Sun as it travels through the Milky Way.^[234] Large objects outside the heliopause remain gravitationally bound to the Sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects.^[84]

Miscellaneous populations

Comets

Main article: Comet

Comets are small Solar System bodies, typically only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.^[235]

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent.^[236] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.^[237] Old comets whose volatiles have mostly been driven out by solar warming are often categorized as asteroids.^[238]

Meteoroids, meteors and dust

Main articles: Meteoroid, Interplanetary dust cloud, and Cosmic dust

Solid objects smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.^[239] By 2017, the IAU designated any solid object having a diameter between ~30 micrometers and 1 meter as meteoroids, and depreciated the micrometeoroid categorization, instead terms smaller particles simply as ‘dust particles’.^[240]

Some meteoroids formed via disintegration of comets and asteroids, while a few formed via impact debris ejected from planetary bodies. Most meteoroids are made of silicates and heavier metals like nickel and iron.^[241] When passing through the Solar System, comets produce a trail of meteoroids; it is hypothesized that this is caused either by vaporization of the comet’s material or by simple breakup of dormant comets. When crossing an atmosphere, these meteoroids will produce bright streaks in the sky due to atmospheric entry, called meteors. If a stream of meteoroids enter the atmosphere on parallel trajectories, the meteors will seemingly ‘radiate’ from a point in the sky, hence the phenomenon’s name: meteor shower.^[242]

The inner Solar System is home to the zodiacal dust cloud, which is visible as the hazy zodiacal light in dark, unpolluted skies. It may be generated by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is materials from planet Mars.^[243] The outer Solar System hosts a cosmic dust cloud. It extends from about 10 AU to about 40 AU, and was probably created by collisions within the Kuiper belt.^[244][245]

Boundary region and uncertainties

See also: Planets beyond Neptune, Planet Nine, and List of Solar System objects by greatest aphelion

Much of the Solar System is still unknown. Regions beyond thousands of AU away are still virtually unmapped and learning about this region of space is difficult. Study in this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.^[246] Many objects may yet be discovered in the Solar System’s uncharted regions.^[247]

The Oort cloud is a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets.^[248][249] No direct observation of the Oort cloud is possible with present imaging technology.^[250] It is theorized to surround the Solar System at roughly 50,000 AU (~0.9 ly) from the Sun and possibly to as far as 100,000 AU (~1.8 ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.^[248][249]

As of the 2020s, a few astronomers have hypothesized that Planet Nine (a planet beyond Neptune) might exist, based on statistical variance in the orbit of extreme trans-Neptunian objects.^[251] Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.^{[252][253][254]} However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.^[255] An alternative hypothesis has a close flyby of another star disrupting the outer Solar System.^[256]

The Sun’s gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.^[257] Most of the mass is orbiting in the region between 3,000 and 100,000 AU.^[258] The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun.^[259] The Sun’s Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.^[260] It was calculated by G. A. Chebotarev to be 230,000 AU.^[7]

Celestial neighborhood

Main articles: List of nearest stars, List of nearest exoplanets, and List of nearby stellar associations and moving groups

Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble’s G-Cloud.^[262] Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to the Sun, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.^[263]

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud’s edge.^[264] Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble.^[264] The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.^[265]

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length.^[266] All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.^[267]

Groups of stars form together in star clusters, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is the Ursa Major moving group, which is around 80 light-years away within the Local Bubble. The nearest star cluster is Hyades, which lies at the edge of the Local Bubble. The closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.^[268]

Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz’s Star, which approached to ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud.^[269] There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System.^[270]

Galactic position

See also: Location of Earth, Galactic year, and Orbit of the Sun

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars.^[271] The Sun is part of one of the Milky Way’s outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.^[272][273] It is a member of the thin disk population of stars orbiting close to the galactic plane.^[274]

Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.^[271] This revolution is known as the Solar System’s galactic year.^[275] The solar apex, the direction of the Sun’s path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.^[276] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.^[c]

The Sun follows a nearly circular orbit around the Galactic Center (where the supermassive black hole Sagittarius A* resides) at a distance of 26,660 light-years,^[278] orbiting at roughly the same speed as that of the spiral arms.^[279] If it orbited close to the center, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life.^[279]

The Solar System’s location in the Milky Way is a factor in the evolutionary history of life on Earth. Spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.^[279] However, according to the controversial Shiva hypothesis, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth.^[280][281]

Discovery and exploration

Main article: Discovery and exploration of the Solar System

Humanity’s knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle Ages–Renaissance, astronomers from Europe to India believed Earth to be stationary at the center of the universe^[282] and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system.^[283][284]

Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables, which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler’s elliptical orbits.^[285][286]

In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it.^[287] Christiaan Huygens followed on from these observations by discovering Saturn’s moon Titan and the shape of the rings of Saturn.^[288] In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun.^[289] Halley’s friend Isaac Newton, in his magisterial Principia Mathematica of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity apply on Earth and in the skies.^[52]: 142

The term “Solar System” entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets.^[290] In 1705, Halley realized that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun,^[291] though Seneca had theorized this about comets in the 1st century.^[292] Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.^[293]

Uranus, having occasionally been observed since 1690 and possibly from antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.^[294] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth’s motion around the Sun, providing the first direct, experimental proof of heliocentrism.^[295] Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.^[296] Mercury’s orbital anomaly observations led to searches for Vulcan, a planet interior of Mercury, but these attempts were quashed with Albert Einstein‘s theory of general relativity in 1915.^[297]

In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space since the 1960s.^[298] By 1989, all eight planets have been visited by space probes.^[299] Probes have returned samples from comets^[300] and asteroids,^[301] as well as flown through the Sun’s corona^[302] and visited two dwarf planets (Pluto and Ceres).^[303][304] To save on fuel, some space missions make use of gravity assist maneuvers, such as the two Voyager probes accelerating when flying by planets in the outer Solar System^[305] and the Parker Solar Probe decelerating closer towards the Sun after its flyby of Venus.^[306]

Humans have landed on the Moon during the Apollo program in the 1960s and 1970s^[307] and will return to the Moon in the 2020s with the Artemis program.^[308] Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006, hence the demotion of Pluto to a dwarf planet,^[309] and further interest in trans-Neptunian objects.^[310]

Chemical warfare (CW) involves using the toxic properties of chemical substances as weapons.^[1][2] This type of warfare is distinct from nuclear warfare, biological warfare and radiological warfare, which together make up CBRN, the military acronym for chemical, biological, radiological, and nuclear (warfare or weapons), all of which are considered “weapons of mass destruction” (WMDs), a term that contrasts with conventional weapons.^[3]

The use of chemical weapons in international armed conflicts is prohibited under international humanitarian law by the 1925 Geneva Protocol and the Hague Conventions of 1899 and 1907.^[4][5] The 1993 Chemical Weapons Convention prohibits signatories from acquiring, stockpiling, developing, and using chemical weapons in all circumstances except for very limited purposes (research, medical, pharmaceutical or protective).^[6][7]

Definition

[edit]

Chemical warfare is different from the use of conventional weapons or nuclear weapons because the destructive effects of chemical weapons are not primarily due to any explosive force. The offensive use of living organisms (such as anthrax) is considered biological warfare rather than chemical warfare;^[8] however, the use of nonliving toxic products produced by living organisms (e.g. toxins such as botulinum toxin, ricin, and saxitoxin) is considered chemical warfare under the provisions of the Chemical Weapons Convention (CWC). Under this convention, any toxic chemical, regardless of its origin, is considered a chemical weapon unless it is used for purposes that are not prohibited (an important legal definition known as the General Purpose Criterion).^[9][10]

About 70 different chemicals have been used or were stockpiled as chemical warfare agents during the 20th century. The entire class, known as Lethal Unitary Chemical Agents and Munitions, has been scheduled for elimination by the CWC.^[11]

Under the convention, chemicals that are toxic enough to be used as chemical weapons, or that may be used to manufacture such chemicals, are divided into three groups according to their purpose and treatment:

• Schedule 1 – Have few, if any, legitimate uses. These may only be produced or used for research, medical, pharmaceutical or protective purposes (i.e. testing of chemical weapons sensors and protective clothing). Examples include nerve agents, ricin, lewisite and mustard gas. Any production over 100 grams (3.5 oz) must be reported to the Organisation for the Prohibition of Chemical Weapons (OPCW) and a country can have a stockpile of no more than one tonne of these chemicals.^{[citation needed]}
• Schedule 2 – Have no large-scale industrial uses, but may have legitimate small-scale uses. Examples include dimethyl methylphosphonate, a precursor to sarin also used as a flame retardant, and thiodiglycol, a precursor chemical used in the manufacture of mustard gas but also widely used as a solvent in inks.
• Schedule 3 – Have legitimate large-scale industrial uses. Examples include phosgene and chloropicrin. Both have been used as chemical weapons but phosgene is an important precursor in the manufacture of plastics, and chloropicrin is used as a fumigant. The OPCW must be notified of, and may inspect, any plant producing more than 30 tons per year.

Chemical weapons are divided into three categories:^[12]

• Category 1 – based on Schedule 1 substances
• Category 2 – based on non-Schedule 1 substances
• Category 3 – devices and equipment designed to use chemical weapons, without the substances themselves

History

[edit]

Main article: History of chemical warfare

Simple chemical weapons were used sporadically throughout antiquity and into the Industrial Age.^[13] It was not until the 19th century that the modern conception of chemical warfare emerged, as various scientists and nations proposed the use of asphyxiating or poisonous gasses.

Multiple international treaties were passed banning chemical weapons based upon the alarm of nations and scientists. This however did not prevent the extensive use of chemical weapons in World War I. Chlorine gas, among others chemicals, was used by both the Allied and Central powers to try to break the stalemate of trench warfare. Though largely ineffective over the long run, it decidedly changed the nature of the war. In many cases the gasses used did not kill, but instead horribly maimed, injured, or disfigured. Some 1.3 million gas casualties were recorded, which may have included up to 260,000 civilian casualties.^[14][15][16]

The interwar years saw the occasional use of chemical weapons, mainly to put down rebellions.^[17]

World War II

[edit]

Although significant effort went into the development and stockpiling of chemical weapons in World War II, they saw little battlefield use in the European Theatre. Nazi Germany dedicated much research to the development of potent nerve agents,^[18] but used them very sparingly, likely due to fears that the Allies would retaliate with their own chemical weapons.^{[citation needed]} These fears were not unfounded: the Allies made comprehensive plans for defensive and retaliatory use of chemical weapons, and stockpiled large quantities.^[19][20] Japanese forces, as part of the Axis, used chemical weapons more widely in the Pacific Theatre against Asian Allied enemies, as they also feared that using it on Western powers would result in retaliation.^{[citation needed]} Chemical weapons were frequently used against both the Kuomintang and People’s Liberation Army troops in China.^[21]

However, Nazi Germany extensively used poison gas against civilians, particularly Jews, in the Holocaust. Vast quantities of Zyklon B and carbon monoxide gas were used in the systematic extermination of some three million victims. This remains the deadliest use of poison gas in history.^{[22][23][24][25]}

Post-War

[edit]

During the Iran–Iraq War, some 100,000 Iranian troops were casualties of Iraqi chemical weapons.^[26][27][28] Iraq also used mustard gas and nerve agents against the kurdish population killing more than 5000 people and injuring many in the 1988 Halabja chemical attack.^[29]

The Cuban intervention in Angola saw limited use of organophosphates.^[30]

Terrorist groups have also used chemical weapons, notably in the Tokyo subway sarin attack and the Matsumoto incident.^[31][32]

21st Century

[edit]

The Ba’athist regime in Syria has used sarin, chlorine, and mustard gas in numerous deadly chemical attacks against civilian populations in the Syrian civil war.^[33][34][35]

During the Russian invasion of Ukraine, Russia has been reported to deploy CS gas through K-51 grenades dropped by unmanned drones.^[36] On 13 December 2024, the Ukrainian military stated that over 4,800 incidents involving chemical weapons against Ukrainian forces have been record since the war began, which resulted in over 2,000 Ukrainian soldiers having been hospitalized, and 3 deaths. The use of gas was often hidden by heavy Russian “intense artillery, rocket, and bomb attacks”, forcing Ukrainian soldiers out of their positions. They saw less use of chemical gas in cold weather, as it reduced the effectiveness of the K-51 gas grenades.^[37] A recent US aid package to Ukraine included “nuclear, chemical and radiological protective equipment”.^[38]

Technology

[edit]

See also: Chemical weapon

Year	Agents	Dissemination	Protection	Detection
1914	Chlorine Chloropicrin Phosgene Sulfur mustard	Wind dispersal	Gas masks, urine-soaked gauze	Smell
1918	Lewisite	Chemical shells	Gas mask Rosin oil clothing	Smell of geraniums
1920s		Projectiles with central bursters	CC-2 clothing
1930s	G-series nerve agents	Aircraft bombs		Blister agent detectors Color change paper
1940s		Missile warheads Spray tanks	Protective ointment (mustard) Collective protection Gas mask w/ whetlerite
1950s
1960s	V-series nerve agents	Aerodynamic	Gas mask w/ water supply	Nerve gas alarm
1970s
1980s		Binary munitions	Improved gas masks (protection, fit, comfort)	Laser detection
1990s	Novichok nerve agents

Although crude chemical warfare has been employed in many parts of the world for thousands of years,^[39] “modern” chemical warfare began during World War I – see Chemical weapons in World War I.

Initially, only well-known commercially available chemicals and their variants were used. These included chlorine and phosgene gas. The methods used to disperse these agents during battle were relatively unrefined and inefficient. Even so, casualties could be heavy, due to the mainly static troop positions which were characteristic features of trench warfare.

Germany, the first side to employ chemical warfare on the battlefield,^[40] simply opened canisters of chlorine upwind of the opposing side and let the prevailing winds do the dissemination. Soon after, the French modified artillery munitions to contain phosgene – a much more effective method that became the principal means of delivery.^[41]

Since the development of modern chemical warfare in World War I, nations have pursued research and development on chemical weapons that falls into four major categories: new and more deadly agents; more efficient methods of delivering agents to the target (dissemination); more reliable means of defense against chemical weapons; and more sensitive and accurate means of detecting chemical agents.

Chemical warfare agents

[edit]

See also: List of chemical warfare agents

The chemical used in warfare is called a chemical warfare agent (CWA). About 70 different chemicals have been used or stockpiled as chemical warfare agents during the 20th and 21st centuries. These agents may be in liquid, gas or solid form. Liquid agents that evaporate quickly are said to be volatile or have a high vapor pressure. Many chemical agents are volatile organic compounds so they can be dispersed over a large region quickly.^{[citation needed][42]}

The earliest target of chemical warfare agent research was not toxicity, but development of agents that can affect a target through the skin and clothing, rendering protective gas masks useless. In July 1917, the Germans employed sulfur mustard. Mustard agents easily penetrate leather and fabric to inflict painful burns on the skin.

Chemical warfare agents are divided into lethal and incapacitating categories. A substance is classified as incapacitating if less than 1/100 of the lethal dose causes incapacitation, e.g., through nausea or visual problems. The distinction between lethal and incapacitating substances is not fixed, but relies on a statistical average called the LD₅₀.

Persistency

[edit]

Chemical warfare agents can be classified according to their persistency, a measure of the length of time that a chemical agent remains effective after dissemination. Chemical agents are classified as persistent or nonpersistent.

Agents classified as nonpersistent lose effectiveness after only a few minutes or hours or even only a few seconds. Purely gaseous agents such as chlorine are nonpersistent, as are highly volatile agents such as sarin. Tactically, nonpersistent agents are very useful against targets that are to be taken over and controlled very quickly.

Apart from the agent used, the delivery mode is very important. To achieve a nonpersistent deployment, the agent is dispersed into very small droplets comparable with the mist produced by an aerosol can. In this form not only the gaseous part of the agent (around 50%) but also the fine aerosol can be inhaled or absorbed through pores in the skin.

Modern doctrine requires very high concentrations almost instantly in order to be effective (one breath should contain a lethal dose of the agent). To achieve this, the primary weapons used would be rocket artillery or bombs and large ballistic missiles with cluster warheads. The contamination in the target area is only low or not existent and after four hours sarin or similar agents are not detectable anymore.

By contrast, persistent agents tend to remain in the environment for as long as several weeks, complicating decontamination. Defense against persistent agents requires shielding for extended periods of time. Nonvolatile liquid agents, such as blister agents and the oily VX nerve agent, do not easily evaporate into a gas, and therefore present primarily a contact hazard.

The droplet size used for persistent delivery goes up to 1 mm increasing the falling speed and therefore about 80% of the deployed agent reaches the ground, resulting in heavy contamination. Deployment of persistent agents is intended to constrain enemy operations by denying access to contaminated areas.

Possible targets include enemy flank positions (averting possible counterattacks), artillery regiments, command posts or supply lines. Because it is not necessary to deliver large quantities of the agent in a short period of time, a wide variety of weapons systems can be used.

A special form of persistent agents are thickened agents. These comprise a common agent mixed with thickeners to provide gelatinous, sticky agents. Primary targets for this kind of use include airfields, due to the increased persistency and difficulty of decontaminating affected areas.

Classes

[edit]

Chemical weapons are agents that come in four categories: choking, blister, blood and nerve.^[43] The agents are organized into several categories according to the manner in which they affect the human body. The names and number of categories varies slightly from source to source, but in general, types of chemical warfare agents are as follows:

Class of agent	Agent Names	Mode of Action	Signs and Symptoms	Rate of action	Persistency
Nerve	Cyclosarin (GF)Sarin (GB)Soman (GD)Tabun (GA)VXVRSome insecticidesNovichok agents	Inactivates enzyme acetylcholinesterase, preventing the breakdown of the neurotransmitter acetylcholine in the victim’s synapses and causing both muscarinic and nicotinic effects	Miosis (pinpoint pupils)Blurred/dim visionHeadacheNausea, vomiting, diarrheaCopious secretions/sweatingMuscle twitching/fasciculationsDyspneaSeizuresLoss of consciousness	Vapors: seconds to minutes;Skin: 2 to 18 hours	VX is persistent and a contact hazard; other agents are non-persistent and present mostly inhalation hazards.
Asphyxiant/Blood	Most ArsinesCyanogen chlorideHydrogen cyanide	Arsine: Causes intravascular hemolysis that may lead to renal failure.Cyanogen chloride/hydrogen cyanide: Cyanide directly prevents cells from using oxygen. The cells then use anaerobic respiration, creating excess lactic acid and metabolic acidosis.	Possible cherry-red skinPossible cyanosisConfusionNauseaPatients may gasp for airSeizures prior to deathMetabolic acidosis	Immediate onset	Non-persistent and an inhalation hazard.
Vesicant/Blister	Sulfur mustard (HD, H)Nitrogen mustard (HN-1, HN-2, HN-3)Lewisite (L)Phosgene oxime (CX)	Agents are acid-forming compounds that damages skin and respiratory system, resulting burns and respiratory problems.	Severe skin, eye and mucosal pain and irritationSkin erythema with large fluid blisters that heal slowly and may become infectedTearing, conjunctivitis, corneal damageMild respiratory distress to marked airway damage	Mustards: Vapors: 4 to 6 hours, eyes and lungs affected more rapidly; Skin: 2 to 48 hoursLewisite: Immediate	Persistent and a contact hazard.
Choking/Pulmonary	ChlorineHydrogen chlorideNitrogen oxidesPhosgene	Similar mechanism to blister agents in that the compounds are acids or acid-forming, but action is more pronounced in respiratory system, flooding it and resulting in suffocation; survivors often suffer chronic breathing problems.	Airway irritationEye and skin irritationDyspnea, coughSore throatChest tightnessWheezingBronchospasm	Immediate to 3 hours	Non-persistent and an inhalation hazard.
Lachrymatory agent	Tear gasPepper spray	Causes severe stinging of the eyes and temporary blindness.	Powerful eye irritation	Immediate	Non-persistent and an inhalation hazard.
Incapacitating	Agent 15 (BZ)	Causes atropine-like inhibition of acetylcholine in subject. Causes peripheral nervous system effects that are the opposite of those seen in nerve agent poisoning.	May appear as mass drug intoxication with erratic behaviors, shared realistic and distinct hallucinations, disrobing and confusionHyperthermiaAtaxia (lack of coordination)Mydriasis (dilated pupils)Dry mouth and skin	Inhaled: 30 minutes to 20 hours;Skin: Up to 36 hours after skin exposure to BZ. Duration is typically 72 to 96 hours.	Extremely persistent in soil and water and on most surfaces; contact hazard.
Cytotoxic proteins	Non-living biological proteins, such as: RicinAbrin	Inhibit protein synthesis	Latent period of 4-8 hours, followed by flu-like signs and symptomsProgress within 18-24 hours to:Inhalation: nausea, cough, dyspnea, pulmonary edemaIngestion: Gastrointestinal hemorrhage with emesis and bloody diarrhea; eventual liver and kidney failure.	4–24 hours; see symptoms. Exposure by inhalation or injection causes more pronounced signs and symptoms than exposure by ingestion	Slight; agents degrade quickly in environment

There are other chemicals used militarily that are not scheduled by the CWC, and thus are not controlled under the CWC treaties. These include:

• Defoliants and herbicides that destroy vegetation, but are not immediately toxic or poisonous to human beings. Their use is classified as herbicidal warfare. Some batches of Agent Orange, for instance, used by the British during the Malayan Emergency and the United States during the Vietnam War, contained dioxins as manufacturing impurities. Dioxins, rather than Agent Orange itself, have long-term cancer effects and for causing genetic damage leading to serious birth defects.
• Incendiary or explosive chemicals (such as napalm, extensively used by the United States during the Korean War and the Vietnam War, or dynamite) because their destructive effects are primarily due to fire or explosive force, and not direct chemical action. Their use is classified as conventional warfare.
• Viruses, bacteria, or other organisms. Their use is classified as biological warfare. Toxins produced by living organisms are considered chemical weapons, although the boundary is blurry. Toxins are covered by the Biological Weapons Convention.

Designations

[edit]

Further information: chemical weapon designation

Most chemical weapons are assigned a one- to three-letter “NATO weapon designation” in addition to, or in place of, a common name. Binary munitions, in which precursors for chemical warfare agents are automatically mixed in shell to produce the agent just prior to its use, are indicated by a “-2” following the agent’s designation (for example, GB-2 and VX-2).

Some examples are given below:

Blood agents:	Vesicants:
Cyanogen chloride: CKHydrogen cyanide: AC	Lewisite: LSulfur mustard: H, HD, HS, HT
Pulmonary agents:	Incapacitating agents:
Phosgene: CG	Quinuclidinyl benzilate: BZ
Lachrymatory agents:	Nerve agents:
Pepper spray: OCTear gas: CN, CS, CR	Sarin: GBVE, VG, VM, VX

Delivery

[edit]

The most important factor in the effectiveness of chemical weapons is the efficiency of its delivery, or dissemination, to a target. The most common techniques include munitions (such as bombs, projectiles, warheads) that allow dissemination at a distance and spray tanks which disseminate from low-flying aircraft. Developments in the techniques of filling and storage of munitions have also been important.

Although there have been many advances in chemical weapon delivery since World War I, it is still difficult to achieve effective dispersion. The dissemination is highly dependent on atmospheric conditions because many chemical agents act in gaseous form. Thus, weather observations and forecasting are essential to optimize weapon delivery and reduce the risk of injuring friendly forces.^{[citation needed]}

Dispersion

[edit]

Dispersion is placing the chemical agent upon or adjacent to a target immediately before dissemination, so that the material is most efficiently used. Dispersion is the simplest technique of delivering an agent to its target. The most common techniques are munitions, bombs, projectiles, spray tanks and warheads.

World War I saw the earliest implementation of this technique. The actual first chemical ammunition was the French 26 mm cartouche suffocante rifle grenade, fired from a flare carbine. It contained 35 g (1.2 oz) of the tear-producer ethyl bromoacetate, and was used in autumn 1914 – with little effect on the Germans.

The German military contrarily tried to increase the effect of 10.5 cm (4.1 in) shrapnel shells by adding an irritant – dianisidine chlorosulfonate. Its use against the British at Neuve Chapelle in October 1914 went unnoticed by them. Hans Tappen, a chemist in the Heavy Artillery Department of the War Ministry, suggested to his brother, the Chief of the Operations Branch at German General Headquarters, the use of the tear-gases benzyl bromide or xylyl bromide.

Shells were tested successfully at the Wahn artillery range near Cologne on January 9, 1915, and an order was placed for 15 cm (5.9 in) howitzer shells, designated ‘T-shells’ after Tappen. A shortage of shells limited the first use against the Russians at the Battle of Bolimów on January 31, 1915; the liquid failed to vaporize in the cold weather, and again the experiment went unnoticed by the Allies.

The first effective use were when the German forces at the Second Battle of Ypres simply opened cylinders of chlorine and allowed the wind to carry the gas across enemy lines. While simple, this technique had numerous disadvantages. Moving large numbers of heavy gas cylinders to the front-line positions from where the gas would be released was a lengthy and difficult logistical task.

Stockpiles of cylinders had to be stored at the front line, posing a great risk if hit by artillery shells. Gas delivery depended greatly on wind speed and direction. If the wind was fickle, as at the Battle of Loos, the gas could blow back, causing friendly casualties.

Gas clouds gave plenty of warning, allowing the enemy time to protect themselves, though many soldiers found the sight of a creeping gas cloud unnerving. This made the gas doubly effective, as, in addition to damaging the enemy physically, it also had a psychological effect on the intended victims.

Another disadvantage was that gas clouds had limited penetration, capable only of affecting the front-line trenches before dissipating. Although it produced limited results in World War I, this technique shows how simple chemical weapon dissemination can be.

Shortly after this “open canister” dissemination, French forces developed a technique for delivery of phosgene in a non-explosive artillery shell. This technique overcame many of the risks of dealing with gas in cylinders. First, gas shells were independent of the wind and increased the effective range of gas, making any target within reach of guns vulnerable. Second, gas shells could be delivered without warning, especially the clear, nearly odorless phosgene—there are numerous accounts of gas shells, landing with a “plop” rather than exploding, being initially dismissed as dud high explosive or shrapnel shells, giving the gas time to work before the soldiers were alerted and took precautions.

The major drawback of artillery delivery was the difficulty of achieving a killing concentration. Each shell had a small gas payload and an area would have to be subjected to saturation bombardment to produce a cloud to match cylinder delivery. A British solution to the problem was the Livens Projector. This was effectively a large-bore mortar, dug into the ground that used the gas cylinders themselves as projectiles – firing a 14 kg (31 lb) cylinder up to 1,500 m (5,000 ft). This combined the gas volume of cylinders with the range of artillery.

Over the years, there were some refinements in this technique. In the 1950s and early 1960s, chemical artillery rockets and cluster bombs contained a multitude of submunitions, so that a large number of small clouds of the chemical agent would form directly on the target.

Thermal dissemination

[edit]

Thermal dissemination is the use of explosives or pyrotechnics to deliver chemical agents. This technique, developed in the 1920s, was a major improvement over earlier dispersal techniques, in that it allowed significant quantities of an agent to be disseminated over a considerable distance. Thermal dissemination remains the principal method of disseminating chemical agents today.

Most thermal dissemination devices consist of a bomb or projectile shell that contains a chemical agent and a central “burster” charge; when the burster detonates, the agent is expelled laterally.

Thermal dissemination devices, though common, are not particularly efficient. First, a percentage of the agent is lost by incineration in the initial blast and by being forced onto the ground. Second, the sizes of the particles vary greatly because explosive dissemination produces a mixture of liquid droplets of variable and difficult to control sizes.

The efficacy of thermal detonation is greatly limited by the flammability of some agents. For flammable aerosols, the cloud is sometimes totally or partially ignited by the disseminating explosion in a phenomenon called flashing. Explosively disseminated VX will ignite roughly one third of the time. Despite a great deal of study, flashing is still not fully understood, and a solution to the problem would be a major technological advance.

Despite the limitations of central bursters, most nations use this method in the early stages of chemical weapon development, in part because standard munitions can be adapted to carry the agents.

Aerodynamic dissemination

[edit]

Aerodynamic dissemination is the non-explosive delivery of a chemical agent from an aircraft, allowing aerodynamic stress to disseminate the agent. This technique is the most recent major development in chemical agent dissemination, originating in the mid-1960s.

This technique eliminates many of the limitations of thermal dissemination by eliminating the flashing effect and theoretically allowing precise control of particle size. In actuality, the altitude of dissemination, wind direction and velocity, and the direction and velocity of the aircraft greatly influence particle size. There are other drawbacks as well; ideal deployment requires precise knowledge of aerodynamics and fluid dynamics, and because the agent must usually be dispersed within the boundary layer (less than 60–90 m or 200–300 ft above the ground), it puts pilots at risk.

Significant research is still being applied toward this technique. For example, by modifying the properties of the liquid, its breakup when subjected to aerodynamic stress can be controlled and an idealized particle distribution achieved, even at supersonic speed. Additionally, advances in fluid dynamics, computer modeling, and weather forecasting allow an ideal direction, speed, and altitude to be calculated, such that warfare agent of a predetermined particle size can predictably and reliably hit a target.

Protection against chemical warfare

[edit]

Ideal protection begins with nonproliferation treaties such as the CWC, and detecting, very early, the signatures of someone building a chemical weapons capability. These include a wide range of intelligence disciplines, such as economic analysis of exports of dual-use chemicals and equipment, human intelligence (HUMINT) such as diplomatic, refugee, and agent reports; photography from satellites, aircraft and drones (IMINT); examination of captured equipment (TECHINT); communications intercepts (COMINT); and detection of chemical manufacturing and chemical agents themselves (MASINT).

If all the preventive measures fail and there is a clear and present danger, then there is a need for detection of chemical attacks,^[44] collective protection,^[45][46][47] and decontamination. Since industrial accidents can cause dangerous chemical releases (e.g., the Bhopal disaster), these activities are things that civilian, as well as military, organizations must be prepared to carry out. In civilian situations in developed countries, these are duties of HAZMAT organizations, which most commonly are part of fire departments.

Detection has been referred to above, as a technical MASINT discipline; specific military procedures, which are usually the model for civilian procedures, depend on the equipment, expertise, and personnel available. When chemical agents are detected, an alarm needs to sound, with specific warnings over emergency broadcasts and the like. There may be a warning to expect an attack.

If, for example, the captain of a US Navy ship believes there is a serious threat of chemical, biological, or radiological attack, the crew may be ordered to set Circle William, which means closing all openings to outside air, running breathing air through filters, and possibly starting a system that continually washes down the exterior surfaces. Civilian authorities dealing with an attack or a toxic chemical accident will invoke the Incident Command System, or local equivalent, to coordinate defensive measures.^[47]

Individual protection starts with a gas mask and, depending on the nature of the threat, through various levels of protective clothing up to a complete chemical-resistant suit with a self-contained air supply. The US military defines various levels of MOPP (mission-oriented protective posture) from mask to full chemical resistant suits; Hazmat suits are the civilian equivalent, but go farther to include a fully independent air supply, rather than the filters of a gas mask.

Collective protection allows continued functioning of groups of people in buildings or shelters, the latter which may be fixed, mobile, or improvised. With ordinary buildings, this may be as basic as plastic sheeting and tape, although if the protection needs to be continued for any appreciable length of time, there will need to be an air supply, typically an enhanced gas mask.^[46][47]

Decontamination

[edit]

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Decontamination varies with the particular chemical agent used. Some nonpersistent agents, including most pulmonary agents (chlorine, phosgene, and so on), blood gases, and nonpersistent nerve gases (e.g., GB), will dissipate from open areas, although powerful exhaust fans may be needed to clear out buildings where they have accumulated.

In some cases, it might be necessary to neutralize them chemically, as with ammonia as a neutralizer for hydrogen cyanide or chlorine. Riot control agents such as CS will dissipate in an open area, but things contaminated with CS powder need to be aired out, washed by people wearing protective gear, or safely discarded.

Mass decontamination is a less common requirement for people than equipment, since people may be immediately affected and treatment is the action required. It is a requirement when people have been contaminated with persistent agents. Treatment and decontamination may need to be simultaneous, with the medical personnel protecting themselves so they can function.^[48]

There may need to be immediate intervention to prevent death, such as injection of atropine for nerve agents. Decontamination is especially important for people contaminated with persistent agents; many of the fatalities after the explosion of a WWII US ammunition ship carrying sulfur mustard, in the harbor of Bari, Italy, after a German bombing on December 2, 1943, came when rescue workers, not knowing of the contamination, bundled cold, wet seamen in tight-fitting blankets.

For decontaminating equipment and buildings exposed to persistent agents, such as blister agents, VX or other agents made persistent by mixing with a thickener, special equipment and materials might be needed. Some type of neutralizing agent will be needed; e.g. in the form of a spraying device with neutralizing agents such as Chlorine, Fichlor, strong alkaline solutions or enzymes. In other cases, a specific chemical decontaminant will be required.^[47]

Sociopolitical climate

[edit]

There are many instances of the use of chemical weapons in battles documented in Greek and Roman historical texts; the earliest example was the deliberate poisoning of Kirrha’s water supply with hellebore in the First Sacred War, Greece, about 590 BC.^[49]

One of the earliest reactions to the use of chemical agents was from Rome. Struggling to defend themselves from the Roman legions, Germanic tribes poisoned the wells of their enemies, with Roman jurists having been recorded as declaring “armis bella non venenis geri”, meaning “war is fought with weapons, not with poisons.” Yet the Romans themselves resorted to poisoning wells of besieged cities in Anatolia in the 2nd century BC.^[50]

Before 1915 the use of poisonous chemicals in battle was typically the result of local initiative, and not the result of an active government chemical weapons program. There are many reports of the isolated use of chemical agents in individual battles or sieges, but there was no true tradition of their use outside of incendiaries and smoke. Despite this tendency, there have been several attempts to initiate large-scale implementation of poison gas in several wars, but with the notable exception of World War I, the responsible authorities generally rejected the proposals for ethical reasons or fears of retaliation.

For example, in 1854 Lyon Playfair (later 1st Baron Playfair, GCB, PC, FRS (1818–1898), a British chemist, proposed using a cacodyl cyanide-filled artillery shell against enemy ships during the Crimean War. The British Ordnance Department rejected the proposal as “as bad a mode of warfare as poisoning the wells of the enemy.”

Efforts to eradicate chemical weapons

[edit]

See also: List of chemical arms control agreements

Nation	CW Possession[citation needed]	Signed CWC	Ratified CWC
Albania	Eliminated, 2007	January 14, 1993[51]	May 11, 1994[51]
China	Probable	January 13, 1993	April 4, 1997
Egypt	Probable	No	No
India	Eliminated, 2009	January 14, 1993	September 3, 1996
Iran	Possible	January 13, 1993	November 3, 1997
Iraq	Eliminated, 2018	January 13, 2009	February 12, 2009
Israel	Probable	January 13, 1993[52]	No
Japan	Probable	January 13, 1993	September 15, 1995
Libya	Eliminated, 2014	No	January 6, 2004 (acceded)
Myanmar (Burma)	Possible	January 14, 1993[52]	July 8, 2015[53]
North Korea	Known	No	No
Pakistan	Probable	January 13, 1993	November 27, 1997
Russia	Eliminated, 2017	January 13, 1993	November 5, 1997
Serbia and Montenegro	Probable	No	April 20, 2000 (acceded)
Sudan	Possible	No	May 24, 1999 (acceded)
Syria	Known	No	September 14, 2013 (acceded)
Taiwan	Possible	n/a	n/a
United States	Eliminated, 2023[54]	January 13, 1993	April 25, 1997
Vietnam	Possible	January 13, 1993	September 30, 1998

• August 27, 1874: The Brussels Declaration Concerning the Laws and Customs of War is signed, specifically forbidding the “employment of poison or poisoned weapons”, although the treaty was not adopted by any nation whatsoever and it never went into effect.
• September 4, 1900: The First Hague Convention, which includes a declaration banning the “use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases,” enters into force.
• January 26, 1910: The Second Hague Convention enters into force, prohibiting the use of “poison or poisoned weapons” in warfare.
• February 6, 1922: After World War I, the Washington Arms Conference Treaty prohibited the use of asphyxiating, poisonous or other gases. It was signed by the United States, Britain, Japan, France, and Italy, but France objected to other provisions in the treaty and it never went into effect.
• February 8, 1928: The Geneva Protocol enters into force, prohibiting the use of “asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices” and “bacteriological methods of warfare”.

Chemical weapon proliferation

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Main article: Chemical weapon proliferation

Despite numerous efforts to reduce or eliminate them, some nations continue to research and/or stockpile chemical warfare agents.

In 1997, future US Vice President Dick Cheney opposed the signing ratification of a treaty banning the use of chemical weapons, a recently unearthed letter shows. In a letter dated April 8, 1997, then Halliburton-CEO Cheney told Sen. Jesse Helms, the chairman of the Senate Foreign Relations Committee, that it would be a mistake for America to join the convention. “Those nations most likely to comply with the Chemical Weapons Convention are not likely to ever constitute a military threat to the United States. The governments we should be concerned about are likely to cheat on the CWC, even if they do participate,” reads the letter,^[55] published by the Federation of American Scientists.

The CWC was ratified by the Senate that same month. In the following years, Albania, Libya, Russia, the United States, and India declared over 71,000 metric tons of chemical weapon stockpiles, and destroyed a third of them. Under the terms of the agreement, the United States and Russia agreed to eliminate the rest of their supplies of chemical weapons by 2012, but ended up taking far longer to do so as shown in the previous and following section of this article.

Chemical weapons destruction

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India

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In June 1997, India declared that it had a stockpile of 1044 tons of sulphur mustard in its possession. India’s declaration of its stockpile came after its entry into the Chemical Weapons Convention, that created the Organisation for the Prohibition of Chemical Weapons, and on January 14, 1993, India became one of the original signatories to the Chemical Weapons Convention. By 2005, from among six nations that had declared their possession of chemical weapons, India was the only country to meet its deadline for chemical weapons destruction and for inspection of its facilities by the Organisation for the Prohibition of Chemical Weapons.^[56][57] By 2006, India had destroyed more than 75 percent of its chemical weapons and material stockpile and was granted an extension to complete a 100 percent destruction of its stocks by April 2009. On May 14, 2009, India informed the United Nations that it has completely destroyed its stockpile of chemical weapons.^[58]

Iraq

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See also: Iraqi chemical warfare

The Director-General of the Organisation for the Prohibition of Chemical Weapons, Ambassador Rogelio Pfirter, welcomed Iraq’s decision to join the OPCW as a significant step to strengthening global and regional efforts to prevent the spread and use of chemical weapons. The OPCW announced “The government of Iraq has deposited its instrument of accession to the Chemical Weapons Convention with the Secretary General of the United Nations and within 30 days, on 12 February 2009, will become the 186th State Party to the Convention”. Iraq has also declared stockpiles of chemical weapons, and because of their recent accession is the only State Party exempted from the destruction time-line.^[59]

Japan

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During the Second Sino-Japanese War (1937–1945) Japan stored chemical weapons on the territory of mainland China. The weapon stock mostly containing sulfur mustard-lewisite mixture.^[60] The weapons are classified as abandoned chemical weapons under the Chemical Weapons Convention, and from September 2010 Japan has started their destruction in Nanjing using mobile destruction facilities in order to do so.^[61]

Russia

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Russia signed into the Chemical Weapons Convention on January 13, 1993, and ratified it on November 5, 1995. Declaring an arsenal of 39,967 tons of chemical weapons in 1997, by far the largest arsenal, consisting of blister agents: Lewisite, Sulfur mustard, Lewisite-mustard mix, and nerve agents: Sarin, Soman, and VX. Russia met its treaty obligations by destroying 1 percent of its chemical agents by the 2002 deadline set out by the Chemical Weapons Convention, but requested an extension on the deadlines of 2004 and 2007 due to technical, financial, and environmental challenges of chemical disposal. Since, Russia has received help from other countries such as Canada which donated C$100,000, plus a further C$100,000 already donated, to the Russian Chemical Weapons Destruction Program. This money will be used to complete work at Shchuch’ye and support the construction of a chemical weapons destruction facility at Kizner (Russia), where the destruction of nearly 5,700 tons of nerve agent, stored in approximately 2 million artillery shells and munitions, will be undertaken. Canadian funds are also being used for the operation of a Green Cross Public Outreach Office, to keep the civilian population informed on the progress made in chemical weapons destruction activities.^[62]

As of July 2011, Russia has destroyed 48 percent (18,241 tons) of its stockpile at destruction facilities located in Gorny (Saratov Oblast) and Kambarka (Udmurt Republic) – where operations have finished – and Schuch’ye (Kurgan Oblast), Maradykovsky (Kirov Oblast), Leonidovka (Penza Oblast) whilst installations are under construction in Pochep (Bryansk Oblast) and Kizner (Udmurt Republic).^[63] As August 2013, 76 percent (30,500 tons) were destroyed,^[64] and Russia leaves the Cooperative Threat Reduction (CTR) Program, which partially funded chemical weapons destruction.^[65]

In September 2017, OPCW announced that Russia had destroyed its entire chemical weapons stockpile.^[66]

United States

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See also: United States and weapons of mass destruction § Chemical weapons

On November 25, 1969, President Richard Nixon unilaterally renounced the offensive use of biological and toxic weapons, but the U.S. continued to maintain an offensive chemical weapons program.^[67]

From May 1964 to the early 1970s the U.S. participated in Operation CHASE, a United States Department of Defense program that aimed to dispose of chemical weapons by sinking ships laden with the weapons in the deep Atlantic. After the Marine Protection, Research, and Sanctuaries Act of 1972, Operation Chase was scrapped and safer disposal methods for chemical weapons were researched, with the U.S. destroying several thousand tons of sulfur mustard by incineration at the Rocky Mountain Arsenal, and nearly 4,200 tons of nerve agent by chemical neutralisation at Tooele Army Depot.^[68]

The U.S. began stockpile reductions in the 1980s with the removal of outdated munitions and destroying its entire stock of 3-Quinuclidinyl benzilate (BZ or Agent 15) at the beginning of 1988. In June 1990 the Johnston Atoll Chemical Agent Disposal System began destruction of chemical agents stored on the Johnston Atoll in the Pacific, seven years before the Chemical Weapons Treaty came into effect. In 1986 President Ronald Reagan made an agreement with German Chancellor Helmut Kohl to remove the U.S. stockpile of chemical weapons from Germany. In 1990, as part of Operation Steel Box, two ships were loaded with over 100,000 shells containing Sarin and VX were taken from the U.S. Army weapons storage depots such as Miesau and then-classified FSTS (Forward Storage / Transportation Sites) and transported from Bremerhaven, Germany to Johnston Atoll in the Pacific, a 46-day nonstop journey.^[69]

In the 1980s, Congress, at the urging of the Reagan administration, Congress provided funding for the manufacture of binary chemical weapons (sarin artillery shells) from 1987 until 1990, but this was halted after the U.S. and the Soviet Union entered into a bilateral agreement in June 1990.^[67] In the 1990 agreement, the U.S. and Soviet Union agreed to begin destroying their chemical weapons stockpiles before 1993 and to reduce them to no more than 5,000 agent tons each by the end of 2002. The agreement also provided for exchanges of data and inspections of sites to verify destruction.^[70] Following the collapse of the Soviet Union, the U.S.’s Nunn–Lugar Cooperative Threat Reduction program helped eliminate some of the chemical, biological and nuclear stockpiles of the former Soviet Union.^[70]

The United Nations Conference on Disarmament in Geneva in 1980 led to the development of the Chemical Weapons Convention (CWC), a multilateral treaty that prohibited the development, production, stockpiling, and use of chemical weapons, and required the elimination of existing stockpiles.^[71] The treaty expressly prohibited state parties from making reservations (unilateral caveats).^[71] During the Reagan administration and the George H. W. Bush administration, the U.S. participated in the negotiations toward the CWC.^[71] The CWC was concluded on September 3, 1992, and opened for signature on January 13, 1993. The U.S. became one of 87 original state parties to the CWC.^[71] President Bill Clinton submitted it to the U.S. Senate for ratification on November 23, 1993. Ratification was blocked in the Senate for years, largely as a result of opposition from Senator Jesse Helms, the chairman of the Senate Foreign Relations Committee.^[71] On April 24, 1997, the Senate gave its consent to ratification of the CWC by a 74–26 vote (satisfying the required two-thirds majority). The U.S. deposited its instrument of ratification at the United Nations on April 25, 1997, a few days before the CWC entered into force. The U.S. ratification allowed the U.S. to participate in the Organisation for the Prohibition of Chemical Weapons, the organization based in The Hague that oversees implementation of the CWC.^[71]

Upon U.S. ratification of the CWC, the U.S. declared a total of 29,918 tons of chemical weapons, and committed to destroying all of the U.S.’s chemical weapons and bulk agent.^[72] The U.S. was one of eight states to declare a stockpile of chemical weapons and to commit to their safe elimination.^[73] The U.S. committed in the CWC to destroy its entire chemical arsenal within 10 years of the entry into force (i.e., by April 29, 2007),^[72] However, at a 2012 conference,^[74] the parties to the CWC parties agreed to extend the U.S. deadline to 2023.^[72][74] By 2012, stockpiles had been eliminated at seven of the U.S.’s nine chemical weapons depots and 89.75% of the 1997 stockpile was destroyed.^[75] The depots were the Aberdeen Chemical Agent Disposal Facility, Anniston Chemical Disposal Facility, Johnston Atoll, Newport Chemical Agent Disposal Facility, Pine Bluff Chemical Disposal Facility, Tooele Chemical Disposal Facility, Umatilla Chemical Disposal Facility,^[74] and Deseret Chemical Depot.^[75] The U.S. closed each site after the completion of stockpile destruction.^[74] In 2019, the U.S. began to eliminate its chemical-weapon stockpile at the last of the nine U.S. chemical weapons storage facilities: the Blue Grass Army Depot in Kentucky.^[72] By May 2021, the U.S. destroyed all of its Category 2 and Category 3 chemical weapons and 96.52% of its Category 1 chemical weapons.^[73] The U.S. is scheduled to complete the elimination of all its chemical weapons by the September 2023 deadline.^[72] In July 2023 OPCW confirmed the last chemical munition of the U.S., and that the last chemical weapon from the stockpiles declared by all States Parties to the Chemical Weapons Convention was verified as destroyed.^[76]

The U.S. has maintained a “calculated ambiguity” policy that warns potential adversaries that a chemical or biological attack against the U.S. or its allies will prompt a “overwhelming and devastating” response. The policy deliberately leaves open the question of whether the U.S. would respond to a chemical attempt with nuclear retaliation.^[77] Commentators have noted that this policy gives policymakers more flexibility, at the possible cost of decreased strategic unpreparedness.^[77]

Anti-agriculture

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Herbicidal warfare

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See also: Herbicidal warfare

Although herbicidal warfare use chemical substances, its main purpose is to disrupt agricultural food production and/or to destroy plants which provide cover or concealment to the enemy.

The use of herbicides by the U.S. military during the Vietnam War has left tangible, long-term impacts upon the Vietnamese people and U.S. veterans of the war.^[78][79] The government of Vietnam says that around 24% of the forests of Southern Vietnam were defoliated and up to four million people in Vietnam were exposed to Agent Orange. They state that as many as three million people have developed illness because of Agent Orange while the Red Cross of Vietnam estimates that up to one million people were disabled or have health problems associated with Agent Orange. The United States government has described these figures as unreliable.^[80][81][82] During the war, the U.S. fought the North Vietnamese and their allies in Laos and Cambodia, dropping large quantities of Agent Orange in each of those countries. According on one estimate, the U.S. dropped 475,500 US gallons (1,800,000 L) of Agent Orange in Laos and 40,900 US gallons (155,000 L) in Cambodia.^[83][84][85] Because Laos and Cambodia were officially neutral during the Vietnam War, the U.S. attempted to keep secret its military involvement in these countries. The U.S. has stated that Agent Orange was not widely used and therefore hasn’t offered assistance to affected Cambodians or Laotians, and limits benefits American veterans and CIA personnel who were stationed there.^[84][86]

Anti-livestock

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During the Mau Mau Uprising in 1952, the poisonous latex of the African milk bush was used to kill cattle.^[87]