Asteroid belt

The asteroid belt is a torus-shaped region between the orbits of Mars and Jupiter, 2.1 to 3.3 astronomical units from the Sun,^[10] populated by millions of rocky bodies, leftover building blocks from the solar system's formation that Jupiter's gravity never allowed to assemble into a planet. The belt matters as an archive and a source: its bodies preserve 4.6-billion-year-old material, and its collisions supply most meteorites and most near-Earth asteroids.

Despite its name and its portrayal in film, the asteroid belt is overwhelmingly empty space: all of its material together amounts to only about 3 percent of the mass of the Moon,^[9] and sizable asteroids are separated by roughly a million kilometers on average. It is less an obstacle than a working destination: spacecraft have orbited its two largest members, sampled its escaped fragments, and demonstrated that an asteroid's path can be deliberately changed.

What the belt actually is#

The main belt spans roughly 2.1 to 3.3 astronomical units from the Sun and holds an estimated 1.1 to 1.9 million bodies larger than one kilometer, plus millions of smaller ones.^[10] The catalogue is closing on that estimate: the Minor Planet Center listed about 1.43 million main-belt asteroids in August 2026, out of roughly 1.55 million known minor planets of every class.^[11] The four largest members are Ceres, Vesta, Pallas, and Hygiea; Ceres, about 940 km across, is the only one classed as a dwarf planet. The belt is not the debris of a destroyed planet; there was never enough material there for one, largely because Jupiter's shifting gravity stirred the region and ejected most of its original mass. Resonances with Jupiter still sweep asteroids out of zones called Kirkwood gaps, feeding some onto planet-crossing orbits. The widest gaps sit at the 3:1, 5:2, 7:3, and 2:1 resonances, where an asteroid's orbital period would be a simple fraction of Jupiter's and repeated tugs at the same point in its orbit pull it out of place.^[12] Every outbound spacecraft has crossed the belt without incident, beginning with Pioneer 10 in 1972 and including the Voyager probes and New Horizons.

Ceres and Vesta: the Dawn mission#

NASA's ion-propelled Dawn spacecraft, launched in 2007, became the first mission to orbit two extraterrestrial destinations, the asteroid belt's two largest bodies, Vesta and Ceres. At Vesta (July 2011 to September 2012) it surveyed a 525 km body with a giant south-pole impact basin, confirming Vesta as the parent of a common meteorite family. Vesta was read for decades as a protoplanet with a dense iron core, but a reanalysis of Dawn's radio tracking and imaging published in April 2025 found its polar moment of inertia close to that of a uniform sphere. The interior is far more homogeneous than expected, with a very small core or none at all, which suggests either that Vesta never fully differentiated or that it is debris from a larger body broken apart by an impact.^[13] At Ceres, which it orbited from March 2015 until its fuel ran out in October 2018, Dawn found an ice-rich dwarf planet 940 km across. The bright spots in the 80 km crater Occator turned out to be sodium carbonate and ammonium chloride, salts left behind by brine that seeped upward after the crater-forming impact fractured the crust above a deep reservoir about 20 million years ago, evidence that Ceres is a relic ocean world.^[1] Dawn also mapped Ahuna Mons, a solitary mountain interpreted as a young cryovolcano.

How much of Ceres is ice has been revised sharply upward. Modelling how the craters Dawn photographed have deformed over billions of years, a 2024 study put the upper crust at close to 90 percent ice by volume, tailing off to none about 117 km down, against a previous working figure of less than 30 percent. On that reading Ceres is a frozen former ocean, muddy rather than clean, closer in kind to Europa than to the dry rock it was long taken for.^[14]

Spacecraft visits#

Close-up images of main-belt asteroids have come mostly from spacecraft passing through on the way somewhere else. Galileo's encounter with 951 Gaspra on October 29, 1991 was the first close look at any asteroid, and the same spacecraft found the first known moon of an asteroid, Dactyl, at 243 Ida two years later.^[15]

Mathilde, which NEAR Shoemaker passed in 1997 on its way to the near-Earth asteroid Eros, turned out to be heavily cratered and much less dense than expected, an early sign that large asteroids can be substantially empty inside.^[16] Rosetta's two flybys on its way to comet 67P returned the best asteroid images of their day: the diamond-shaped Steins at 800 km in 2008 and, in 2010, Lutetia, the largest asteroid seen up close before Dawn reached Vesta.^[17][18]

Lucy is the busiest current visitor, and its two main-belt encounters were rehearsals for a Jupiter Trojan tour that begins in 2027.^[21] Both returned more than practice data. At Dinkinesh in 2023 the satellite Selam turned out to be two touching lobes, the first contact binary found orbiting an asteroid, apparently assembled from material shed when a quarter of Dinkinesh slumped along a trough under its own rotational stress.^[19] Donaldjohanson, a fragment of a larger carbon-rich body shattered about 150 million years ago, is itself a contact binary roughly 8 km long and 3.5 km wide, with an oddly shaped neck between the lobes.^[20] Results published in June 2026 added a rotation that wobbles: it turns end over end once every 10.5 days while swinging around its long axis every 26.5 days. Its spectra show iron-rich clays, which form only in liquid water, but not the magnesium clays that indicate longer soaking at Bennu and Ryugu, so the water exposure appears to have been brief.^[22]

Asteroid types#

Astronomers group asteroids by reflectance spectra. C-type (carbonaceous) asteroids make up more than three quarters of the known population, dominate the outer belt, and contain water-bearing clays and organic carbon; the sampled asteroids Ryugu and Bennu belong to this family. S-type (silicaceous) asteroids of rocky, metal-bearing composition account for roughly a fifth and dominate the inner belt; Itokawa and Eros are examples. The remaining few percent covers everything else, including the M-type (metallic) bodies that may be fragments of shattered planetesimal cores, with 16 Psyche the largest.^[23] The arrangement is not random: the dark, water-rich material sits farther from the Sun and the drier, stonier material closer in. Many small asteroids of every class are rubble piles rather than solid rock: when OSIRIS-REx tagged Bennu in 2020, its arm sank half a meter into surface that behaved like a ball pit.

Near-Earth objects and surveys#

Asteroids nudged out of the belt onto orbits coming within 1.3 astronomical units of the Sun are classed as near-Earth objects (NEOs). The Minor Planet Center's tally passed 42,000 near-Earth asteroids during 2026, of which 870 are larger than a kilometer and 2,542 are flagged as potentially hazardous.^[11] Ground-based surveys such as Catalina Sky Survey, Pan-STARRS, and ATLAS find most of them, joined in 2025 by the Vera C. Rubin Observatory, which turned up 2,104 previously unknown asteroids in about ten hours of test observations before its ten-year survey had formally begun.^[25] Coverage is still incomplete: Congress directed NASA in 2005 to catalogue 90 percent of near-Earth objects larger than 140 meters, the size that could devastate a region, by the end of 2020, and fewer than half of the estimated 25,000 such objects have been found.^[2][24] NASA's answer is NEO Surveyor, an infrared space telescope now in assembly for launch no earlier than September 2027, designed to find two thirds of those objects within five years.^[3]

2024 YR4 showed what that pipeline is for. The roughly 60-meter asteroid, found by the ATLAS survey in Chile in December 2024, briefly carried an Earth impact probability of about 3 percent for December 22, 2032 before more observations eliminated it, and the residual uncertainty then shifted to the Moon, where the odds peaked near 4 percent. James Webb Space Telescope observations in February 2026 ruled out the lunar impact as well, putting the asteroid about 21,000 km from the Moon's surface at closest approach.^[26]

Planetary defense: DART and Apophis#

Planetary defense moved from theory to practice on September 26, 2022, when NASA's 570 kg DART spacecraft struck Dimorphos, the 160-meter moon of asteroid Didymos, at 6.1 km/s. NASA's first measurement put the shortening of Dimorphos's orbit around Didymos at 32 minutes, against a 73-second threshold for success, and credited the recoil from debris thrown off the surface, which acted like exhaust from a rocket, with multiplying the momentum transfer.^[4] Analysis of the full observing campaign refined the change to 33.0 minutes, give or take one.^[27] Impact simulations matched to the ejecta indicate that Dimorphos is a rubble pile so weak, with a cohesive strength under a few pascals, that the collision deformed and resurfaced the entire moonlet rather than digging a clean crater.^[28]

The European Space Agency's Hera spacecraft, launched October 7, 2024, is on course to inspect the result. Three engine burns and a correction spread over four weeks in February and March 2026 changed its velocity by 367 m/s and consumed 123 kg of propellant, the largest manoeuvre of the mission, tilting its orbit to match the asteroids'; a second series starting in October 2026 brakes it into a rendezvous.^[29] Arrival is set for November 2026, a month earlier than first planned, after which Hera and two CubeSats, Milani and Juventas, will measure Dimorphos's mass and interior and turn the experiment into a calibrated deflection technique.^[5][29]

Attention now turns to 99942 Apophis, an S-type asteroid roughly 340 meters across that will pass within about 32,000 km of Earth's surface on April 13, 2029, closer than geostationary satellites and visible to the naked eye across parts of Europe, Africa, and western Asia. Tracking has ruled out any impact for at least a century, making the flyby a pure science opportunity: Earth's tides may measurably quake and resurface the asteroid.^[6] NASA's OSIRIS-APEX, the redirected OSIRIS-REx spacecraft, reaches Apophis in June 2029 for an 18-month campaign that includes firing its thrusters at the surface to blow away the top layer and expose the material underneath.^[6][30] ESA's Ramses mission, approved at the agency's November 2025 ministerial council, is moving quickly: an 81.2 million euro build contract went to OHB Italia on February 10, 2026, and cooperation agreements with JAXA followed on May 7, adding Japanese solar arrays, a thermal infrared imager, and a launch on an H3 rocket.^[7][31] Launch is planned for spring 2028, with arrival at Apophis ahead of the flyby so that Ramses can watch the close approach as it happens.^[7]

Sample returns, Psyche, and mining#

Sample return has become asteroid science's workhorse technique, though every target so far has been a near-Earth asteroid rather than a body still in the belt, since fetching a fragment that has already drifted inward is far cheaper than a round trip past Mars. Japan's Hayabusa2 delivered 5.4 grams of Ryugu in December 2020; analyses found amino acids and uracil, a component of RNA. The spacecraft kept flying, and on July 5, 2026 it passed about 400 meters above the surface of Torifune, formerly 2001 CC21, a near-Earth asteroid roughly 840 meters along its long axis; JAXA records it as the closest flyby of any solar system body flown to date, off the aim point by 120 meters.^[37] Its cameras, near-infrared spectrometer, thermal imager, and laser ranging instrument all worked through the approach, returning images of an elongated, two-lobed body.^[33] Its final target, 1998 KY26, follows in 2031. NASA's OSIRIS-REx returned 121.6 grams of Bennu in September 2023, rich in carbon compounds and evaporite minerals from an ancient brine.

The CNSA's Tianwen-2 launched on May 29, 2025 and reached Kamo'oalewa on July 6, 2026, holding station about 20 km out after roughly 400 days and a billion kilometers of cruise. Kamo'oalewa is not a belt object: it is a quasi-satellite that circles the Sun in step with Earth, and the first spacecraft images make it about 20 meters across, smaller than ground-based estimates had implied. Spectra taken from Earth raised the possibility that it is a chip of lunar rock, which is one of the questions a returned sample would settle. The plan is to survey and sample until April 2027, release the capsule during an Earth flyby that November, and continue on to comet 311P.^[32]

The mining question centers on metal. NASA's Psyche spacecraft, launched on a Falcon Heavy in October 2023, is en route to 16 Psyche, a metal-rich asteroid about 280 km at its widest that may be an exposed planetesimal core. The cruise has not been uneventful. A pressure drop in the primary xenon line shut down all four electric thrusters in early April 2025; engineers traced the fault to a valve, switched to the identical backup line in late May, and had full-time thrusting back on June 16.^[34] The probe then completed a Mars gravity assist on May 15, 2026, passing about 4,600 km above the surface and exercising its imager, magnetometer, and gamma-ray and neutron spectrometer against a real target, with sustained thrusting due to resume in the autumn.^[8] Psyche's gravity is scheduled to capture the spacecraft in late July 2029, with the prime mission, about two years of orbital survey, beginning in August.^[35] It is a science mission, but its inventory of a metal world will inform any future extraction case. Commercial efforts have so far outrun the technology: Planetary Resources and Deep Space Industries folded in the late 2010s before flying prospectors, and startup AstroForge lost contact with its first deep-space probe, Odin, days after its February 2025 launch. The company says its next spacecraft will attempt the first commercial rendezvous with a body outside a planetary gravity well, with launch targeted for the last quarter of 2026.^[36] For now, samples measured in grams are the only asteroid resources ever returned to Earth.

References#

1. Mystery Solved: Bright Areas on Ceres Come From Salty Water Below - NASA.
2. NEO Discovery Statistics - NASA JPL Center for Near-Earth Object Studies.
3. NASA's Next-Gen Near-Earth Asteroid Space Telescope Takes Shape - NASA Science.
4. NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space - NASA, October 11, 2022.
5. ESA's Hera targets early arrival at Didymos asteroids - European Space Agency.
6. OSIRIS-APEX - NASA Science.
7. ESA awards contracts for Ramses mission to Apophis - European Space Agency, February 2026.
8. NASA's Psyche Mission Aces Mars Flyby, Targets Metal-Rich Asteroid - NASA Jet Propulsion Laboratory.
9. Masses of the main asteroid belt and the Kuiper belt from the motions of planets and spacecraft - Pitjeva and Pitjev, Astronomy Letters 44 (2018), pages 554-566; arXiv preprint of the published paper.
10. Asteroids: Facts - NASA Science.
11. Minor Planet Center summary - IAU Minor Planet Center, running totals accessed August 2026.
12. Kirkwood gaps - Encyclopaedia Britannica.
13. NASA Studies Reveal Hidden Secrets About Interiors of Moon, Vesta - NASA Jet Propulsion Laboratory, on the Nature Astronomy paper of April 23, 2025.
14. Asteroid Ceres is a former ocean world that slowly formed into a giant, murky icy orb - ScienceDaily, on Pamerleau and Sori, Nature Astronomy (2024), DOI 10.1038/s41550-024-02350-4.
15. Galileo - NASA Science.
16. NEAR Shoemaker - NASA Science.
17. Rosetta status report No. 104: asteroid Steins flyby - European Space Agency.
18. Rosetta reveals mysterious Lutetia - European Space Agency.
19. NASA Lucy Images Reveal Asteroid Dinkinesh to be Surprisingly Complex - NASA.
20. Asteroid Donaldjohanson - NASA Science.
21. NASA's Lucy Spacecraft Completes Asteroid Donaldjohanson Flyby - NASA Science, April 20, 2025.
22. NASA's Lucy Reveals Wobbling, Peanut-Shaped Asteroid - NASA Science, June 2026, on the study published in Science.
23. Types of asteroids - European Space Agency.
24. New Report Assesses Status of Detecting Near-Earth Asteroids - NASA.
25. Ever-changing universe revealed in first imagery from NSF-DOE Vera C. Rubin Observatory - U.S. National Science Foundation, June 23, 2025.
26. New NASA Asteroid Observations Eliminate Chance of 2032 Lunar Impact - NASA Science.
27. Orbital Period Change of Dimorphos Due to the DART Kinetic Impact - Thomas et al., accepted in Nature (2023); arXiv preprint of the published paper.
28. Physical properties of asteroid Dimorphos as derived from the DART impact - Raducan et al., Nature Astronomy 8 (February 25, 2024), record at OSTI.
29. Hera on course for asteroid rendezvous - European Space Agency, March 17, 2026.
30. OSIRIS-APEX, NASA's asteroid Apophis chaser - The Planetary Society.
31. ESA and JAXA team up on planetary defence, Ramses mission to asteroid Apophis - European Space Agency, May 7, 2026.
32. Tianwen-2 arrives at asteroid Kamo'oalewa, first image revealed - SpaceNews.
33. Hayabusa2 captures images of asteroid Torifune - JAXA, July 6, 2026.
34. NASA's Psyche Spacecraft Resumes Full-Time Propulsion - NASA Science, June 20, 2025.
35. Psyche - NASA Science.
36. AstroForge - AstroForge, company statement of its next mission.
37. Asteroid Explorer Hayabusa2 sets a new record for closest approach during a Solar System body flyby mission - JAXA, July 30, 2026.