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Asteroids and comets visited by spacecraft as of 2019 (except Ceres and Vesta), to scale

Asteroids were first discovered in the early 19th century through telescopic observations, which later progressed to exploration by visiting spacecraft. Spacecraft have performed multiple flybys, orbits, and landings on these small bodies, including sample-return missions that collected material directly from their surfaces. Study of these objects is aided by the relatively close proximity of near-Earth asteroids to the Earth, but their small sizes and irregular shapes make detailed mapping and characterization challenging without close-up inspection.

Technical requirements

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The design of an asteroid mission begins with target selection and orbital analysis. Near-Earth asteroids are often preferred because of their relative accessibility, particularly those with a Minimum Orbit Intersection Distance (MOID) of less than 0.05 astronomical units (7,500,000 km; 4,600,000 mi).[1] Mission planners develop flight trajectories by solving orbital transfer problems and balancing launch energy requirements with the total velocity change needed for interception, rendezvous, and, where applicable, sample return. Asteroid size is also an important consideration, as objects smaller than approximately 200 metres (660 ft) in diameter often rotate rapidly or exhibit irregular rotational motion, increasing operational complexity during close-proximity activities.[2]

Deep Space 1 being lifted from its work platform, showing the deployable solar arrays required to power its solar electric propulsion system for deep-space transit to asteroid 9969 Braille.

Spacecraft operations near asteroids present challenges that differ from those encountered around planets. Small asteroids possess weak and irregular gravitational fields, making orbital motion difficult to predict and maintain.[3] In these environments, non-gravitational forces such as solar radiation pressure can significantly influence spacecraft trajectories.[4] As a result, missions typically begin with detailed observations to determine the asteroid's shape, rotation, volume, and surface characteristics before low-altitude operations are attempted.[3][5]

Communication delays between Earth and deep-space spacecraft require a high degree of onboard autonomy. During rendezvous and approach phases, spacecraft use automated navigation systems to combine ranging measurements with optical observations.[4] State estimation algorithms process navigation data to improve trajectory accuracy and account for local gravitational variations.[3] Autonomous imaging systems, including natural feature tracking techniques, compare real-time images with preloaded surface maps to support precise navigation and touch-and-go maneuvers. These operations require accurate attitude control and stable spacecraft pointing.[4][5]

Asteroid missions also place significant demands on propulsion and power systems. Solar electric propulsion technologies, including ion engines and Hall-effect thrusters, are commonly used because they provide high specific impulse and efficient long-duration thrust. These systems require substantial electrical power, which is typically supplied by large deployable solar arrays. The arrays must remain lightweight while generating sufficient power as solar intensity decreases with increasing distance from the Sun.[1]

Past missions

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The robotic exploration of asteroids began with flyby missions conducted during deep-space journeys and later expanded to include orbiters, landers, and sample-return missions. In July 1972 and April 1973, NASA launched Pioneer 10 and Pioneer 11, the first spacecraft to traverse the main asteroid belt.[6] Their successful passages demonstrated that the belt did not present a major hazard to spacecraft navigation.[7] The first close flyby of a targeted asteroid occurred on 29 October 1991, when the Galileo spacecraft passed within 1,600 kilometers (990 miles) of asteroid 951 Gaspra while en route to Jupiter. Galileo later flew past asteroid 243 Ida on 28 August 1993, leading to the discovery of Dactyl, the first confirmed asteroid moon.

Asteroid exploration advanced from flybys to long-term orbital missions with the NEAR Shoemaker spacecraft. Launched in 1996, it conducted a flyby of asteroid 253 Mathilde in 1997 before entering orbit around 433 Eros on 14 February 2000, becoming the first spacecraft to orbit an asteroid. After completing a year of observations, it performed a controlled landing on Eros on 12 February 2001 and transmitted data from the surface. JAXA's Hayabusa mission arrived at asteroid 25143 Itokawa in 2005 and successfully collected surface material despite experiencing technical difficulties. The spacecraft returned the first asteroid samples to Earth in June 2010.

Subsequent missions expanded scientific investigation of asteroids. NASA's Dawn spacecraft, launched in 2007, became the first mission to orbit two extraterrestrial bodies, entering orbit around 4 Vesta in 2011 and later around the dwarf planet Ceres in 2015. JAXA's Hayabusa2 reached asteroid 162173 Ryugu in 2018, deployed small rovers, collected surface and subsurface material, and returned samples to Earth in December 2020. NASA's OSIRIS-REx mission orbited asteroid 101955 Bennu, collected a sample in October 2020, and returned it to Earth in September 2023.

Asteroids have also been used to test planetary defense technologies. On 26 September 2022, NASA's DART spacecraft intentionally impacted Dimorphos, the moon of asteroid Didymos, to alter its orbital period. The impact and resulting debris plume were observed by the Italian cubesat LICIACube, which had been released before the collision.

Overview of missions

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Pioneer 10

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On 15 July 1972, Pioneer 10 became the first spacecraft to enter the asteroid belt en route to Jupiter.[6] Mission planners anticipated a safe passage through the region, and the spacecraft's trajectory remained at least 8.8 million kilometers (5.5 million miles) from any known asteroid. One of its closest approaches occurred on 2 December 1972, when it passed near asteroid 307 Nike.[7]

Galileo

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Image of Gaspra. Colors are exaggerated

On 29 October 1991, the Galileo spacecraft became the first spacecraft to conduct a close flyby of an asteroid en route to Jupiter,[8] when it passed within 1,604 kilometers (997 miles) of the S-type asteroid 951 Gaspra.[9] Images and measurements showed that Gaspra is an irregularly shaped, heavily cratered body measuring approximately 19 by 12 by 11 kilometers (11.8 by 7.5 by 6.8 mi). Observations suggested that the asteroid may have originated from the fragmentation of a larger parent body,[10] while measurements of its surrounding environment raised the possibility of a magnetic field.[8]

Image of Ida and Dactyl

On 28 August 1993, Galileo flew within 2,410 kilometers (1,500 miles) of the S-type asteroid 243 Ida. Analysis of the images revealed Dactyl, a small natural satellite orbiting Ida, making it the first asteroid moon to be discovered.[11] Subsequent studies indicated that both Ida and Dactyl may have formed from the breakup of a larger parent body.[10][12]

NEAR Shoemaker

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On 27 June 1997, NEAR Shoemaker flew past asteroid 253 Mathilde at a distance of approximately 1,200 kilometers (750 miles). During the encounter, the spacecraft returned more than 500 images, covering about 60 percent of the asteroid's surface.[13] The flyby also provided gravitational measurements that enabled scientists to determine Mathilde's size and mass.[14]

Image of Eros from NEAR Shoemaker

On 20 December 1998, NEAR experienced a major anomaly when its planned rendezvous burn with asteroid 433 Eros was aborted. The spacecraft entered safe mode and began tumbling, resulting in the loss of approximately 29 kilograms (64 pounds) of propellant and preventing its scheduled orbital insertion.[15] Instead, NEAR conducted a flyby of Eros on 23 December 1998, passing within 3,827 kilometers (2,378 miles) while collecting images and scientific data. Throughout 1999, a series of trajectory-correction maneuvers gradually reduced the spacecraft's relative velocity and prepared it for a later rendezvous with the asteroid.[16]

Eros from approximately 250 meters altitude (area in image is roughly 12 meters across). This image was taken during NEAR's descent to the surface of the asteroid.[17]

NEAR successfully entered orbit around Eros on 14 February 2000, becoming the first spacecraft to orbit an asteroid. The insertion followed a 13-month heliocentric trajectory closely matching Eros's orbit. Prior to orbital insertion, rendezvous maneuvers reduced the spacecraft's relative velocity, while searches conducted in January and February 2000 found no satellites around the asteroid. NEAR initially entered an elliptical orbit, which was progressively lowered to a 35 kilometers (22 miles) circular polar orbit by July 2000. The mission later transitioned to a 100 kilometers (62 miles) circular orbit and performed a close flyby on 26 October 2000, passing within 5.3 kilometers (3.3 miles) of the surface.[16]

On 12 February 2001, NEAR completed a controlled descent and became the first spacecraft to achieve a soft landing on an asteroid.[18] After landing on Eros, the spacecraft continued transmitting scientific data, including measurements of the asteroid's composition and surface properties,[19] until communications ended on 28 February 2001.[20]

Cassini

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On 23 January 2000, the Cassini spacecraft conducted a flyby of asteroid 2685 Masursky.[21] The spacecraft captured images of the asteroid five to seven hours before the encounter from a distance of about 1,600,000 kilometers (990,000 miles). Based on these observations, Masursky was estimated to have a diameter of approximately 15 to 20 km (9.3 to 12.4 mi). Due to the large flyby distance (approximately 4 lunar distances), the images showed the asteroid only as a small point of light rather than a resolved object.[22]

Deep Space 1

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On 29 July 1999, NASA's Deep Space 1 spacecraft conducted a flyby of asteroid 9969 Braille while traveling toward comet 19P/Borrelly. The spacecraft passed within 26 kilometers (16 miles) of the asteroid,[23] although imaging and spectral observations were collected from an approximate distance of 14,000 kilometers (8,700 miles) due to problems with the spacecraft's tracking system.

During the encounter, DS1's ultraviolet spectrometer was no longer operational, but the spacecraft returned two medium-resolution CCD images and three infrared spectra. The flyby provided valuable scientific data, as Braille was the smallest asteroid to have been observed at such a close range at the time. The encounter was conducted as part of the mission's broader objective of testing advanced spacecraft technologies.

Stardust

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On 2 November 2002, the Stardust spacecraft flew past asteroid 5535 Annefrank at a distance of 3,079 km (1,913 mi). The encounter was conducted primarily as an engineering test of spacecraft systems and mission operations in preparation for Stardust's flyby of comet 81P/Wild in 2003.[24]

Hayabusa

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Rosetta

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Dawn

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Chang'e 2

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Hayabusa2

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DART and LICIACube

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Current missions

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Asteroid exploration in the 2020s has focused on detailed mapping, long-duration missions, and continued operations around near-Earth objects. Following the return of asteroid samples to Earth in 2023, the OSIRIS-REx spacecraft was assigned an extended mission and renamed OSIRIS-APEX. The spacecraft is scheduled to rendezvous with asteroid 99942 Apophis in 2029, shortly after the asteroid's close approach to Earth.

Several active missions are conducting investigations of multiple asteroid targets. NASA's Lucy spacecraft, launched in 2021, completed flybys of asteroid 152830 Dinkinesh in 2023 and asteroid 52246 Donaldjohanson in 2025. The mission is continuing toward Jupiter's trojan asteroids, where it will conduct the first dedicated survey of multiple objects in that population. NASA's Psyche mission, launched in 2023, is en route to the metal-rich asteroid 16 Psyche and is expected to enter orbit in 2029. The mission aims to improve understanding of the composition and evolution of planetary interiors.

International asteroid exploration has also expanded. ESA's Hera spacecraft, launched in 2024, is traveling to the Didymos–Dimorphos system and is expected to arrive in 2026. Its objectives include studying the effects of the DART impact and measuring the resulting changes to Dimorphos. China's Tianwen-2 mission, launched in 2025, is targeting the near-Earth asteroid 469219 Kamoʻoalewa. The mission is designed to collect samples and return them to Earth before continuing toward the active asteroid or main-belt comet 311P/PanSTARRS for further study.

Overview of missions

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New Horizons

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OSIRIS-REx and OSIRIS-APEX

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Lucy

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Psyche

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Hera

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Tianwen-2

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Future and proposed missions

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DESTINY+

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Ramses

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MBR Explorer

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Cancelled and failed missions

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Human exploration

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Main article: Colonization of the asteroid belt

Human exploration and colonization of asteroids have been proposed as possible approaches for expanding human activity beyond Earth.[25] These concepts are motivated by goals such as increasing the long-term resilience of human civilization and utilizing asteroid resources. Asteroids contain materials including metals and water that could support space-based infrastructure and future settlements. Their low gravity could also reduce the energy required to transport materials compared with larger planetary bodies.[26][27]

Artist's depiction of a crewed Orion mission to an asteroid

Asteroid colonization presents several technical and biological challenges. Long travel durations, extreme temperature variations, radiation exposure, and low-gravity environments create difficulties for human habitation. Extended exposure to microgravity can lead to health effects such as bone density loss and muscle deterioration, requiring potential solutions including artificial gravity systems.[28] Asteroids also lack natural radiation shielding, although underground habitats could provide protection.[29][25]

The asteroid belt contains numerous potential exploration targets, including Ceres, the largest object in the region.[30] Ceres contains water resources and has greater gravity than most asteroids, which may make it a more suitable location for certain settlement concepts.[31] Other proposals include using asteroids primarily for resource extraction or establishing support bases near Mars or its moons to enable further exploration.[32] However, human missions and permanent settlements in the asteroid belt remain beyond current technological capabilities and would require advances in transportation, life-support systems, and space habitation.

Mining

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Main article: Asteroid mining

Asteroid mining is the proposed extraction of resources from asteroids and other minor planets.[33] Interest in the concept is based on the potential availability of water, metals, and other materials that could support space exploration or have economic value.[34][35] Different asteroid classes are believed to contain varying amounts of water, iron, nickel, and precious metals, while near-Earth asteroids are often regarded as the most accessible targets because of their relatively low travel-energy requirements.[36][37][38]

Artist's concept from the 1970s of asteroid mining

The development of asteroid mining faces significant technical and economic obstacles. These include the high cost of space missions, the challenge of identifying suitable resource-rich targets, and the difficulty of extracting and processing materials in microgravity environments.[33] Sample-return missions such as Hayabusa, Hayabusa2, and OSIRIS-REx have demonstrated the ability to collect and transport asteroid material, although only limited quantities have been returned to Earth.[39]

Since the 2010s, several government agencies and private companies have investigated asteroid-mining technologies and business models. However, no commercial asteroid-mining operations have been carried out.[40] As of the 2020s, advances in spacecraft technology and reductions in launch costs have renewed interest in the field, but asteroid mining remains a proposed future activity rather than an established industry.[41]

References

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  1. ^ a b Brophy, John R.; Friedman, Louis; Culick, Fred (March 2012). "Asteroid retrieval feasibility". 2012 IEEE Aerospace Conference. pp. 1–16. doi:10.1109/AERO.2012.6187031.
  2. ^ Beshore, Edward; Lauretta, Dante; Boynton, William; Shinohara, Christopher; Sutter, Brian; Everett, David; Gal-Edd, Jonathan; Mink, Ronald; Moreau, Michael; Dworkin, Jason (March 2015). "The OSIRIS-REx asteroid sample return mission". 2015 IEEE Aerospace Conference. pp. 1–14. doi:10.1109/AERO.2015.7118989.
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