Models for planet formation begin with small dust grains coalescing in the protoplanetary disk, before subsequently growing into metre and kilometre-sized planetesimals1,2. These objects had significant compositional diversity; smaller bodies retained a primitive composition, whereas larger planetesimals (≳20–50 km3) experienced widespread melting and differentiation. Planetesimals which formed outside the water snow-line (1.8–2.7 AU4) accreted water ice which subsequently melted (due to radioactive decay5 or impacts6), hydrating minerals7. Others formed further out, where ices such as CO and ammonia were also available. Some of these planetesimals are thought to have contributed significantly to Earth’s volatile budget8,9.

Asteroids are left over, often shattered, planetesimals and their collisional fragments that never grew into larger planetary embryos (and eventual planets), offering us an opportunity to study those initial stages of planetary evolution. Combining space-based missions and laboratory analysis of returned samples offers unparalleled opportunities for future endeavours in asteroid science.

Past missions: Hayabusa, Hayabusa2 and OSIRIS-REx

Whilst it is clear asteroids are key to understanding how Solar Systems form, there are a myriad of methods for studying them. Meteorites are a significant resource of rocky or metallic samples that have been ejected from asteroids (and some planetary bodies) and have travelled to Earth. These objects offer opportunities to study asteroids in laboratories using high-resolution techniques; however, they lack geological context, are not representative of the asteroid population and are often terrestrially weathered and contaminated10,11. Remote sensing is another powerful technique, making use of instrumentation and telescopes on Earth or in orbit, allowing us to observe asteroids in-situ and in environments uncontaminated by the terrestrial atmosphere, e.g., refs. 12,13,14. Telescopes, however, are limited in what they can observe by their distance from the asteroid targets. Spacecraft missions are a key method for studying asteroids. “Flyby” missions, where a spacecraft has serendipitously observed an asteroid whilst on the path to a different target, can only provide imagery and basic compositional information15,16. Dedicated asteroid missions, observing a body from close orbit, provide greater information about their formation and evolution. Examples of these include NEAR-Shoemaker, which visited asteroid 433 Eros17, and Dawn, which visited asteroid 4 Vesta and dwarf planet 1 Ceres18. However, observations and measurements are limited to what instruments can be loaded onto a spacecraft.

Advanced analysis is only possible in Earth-based laboratories, underpinning the necessity for returning samples from objects of interest. In 2003, JAXA launched Hayabusa to near-Earth asteroid 25143 Itokawa: the first asteroid sample return mission. Hayabusa returned ∼90 mg of material, and those samples confirmed a relationship between S-type asteroids and ordinary chondrites. S-type asteroids are siliceous bodies dominated by olivine and pyroxene and are the most common type of asteroid in the Solar System, while ordinary chondrites are the most numerous meteorites on Earth, composed of silicate minerals, metal and millimetre-sized silicate spherules (termed chondrules) which formed in the early Solar System. Remote observations of S-type surfaces had previously suggested they were unrelated, leading to the “the ordinary chondrite paradox”19. Itokawa samples solved this conundrum by showing evidence for significant space-weathering, including solar irradiation and micro-meteorite impacts19. These processes cause remote observations of asteroids to be altered: for S-type bodies, their surfaces become darker and the slope of the spectral flux received is changed20. This effect was what led to the paradox, but samples of Itokawa allowed the effect of space weathering to be quantified and essentially removed from remote observations of S-type asteroids.

JAXA launched Hayabusa2 in 201421, and NASA launched Origins, Spectral Interpretation, Resource Identification and Security–Regolith Explorer (OSIRIS-REx) in 201622, both sample return missions visiting near-Earth asteroids. Both spacecraft rendezvoused with their target asteroids in 2018: Hayabusa2 with ∼900 m asteroid 162173 Ryugu and OSIRIS-REx with ∼500 m asteroid 101955 Bennu. Each spacecraft identified the presence of phyllosilicates: minerals formed through the interaction of rock with a fluid23,24. On Bennu, high albedo carbonate veins were also observed, indicating extensive water flow25. The presence of these phases suggests that Bennu and Ryugu formed in the outer Solar System alongside volatile ices, and that these primitive asteroids were potentially representative of the material which delivered volatiles to the early Earth.

Ryugu and Bennu have similar oblate spheroidal “spinning top” shapes with an equatorial ridge due to their “rubble-pile” nature: formed via gravitational reaccumulation after a larger asteroid parent body was catastrophically disrupted26,27,28. Due to their higher shock absorption potential, rubble piles can survive for billions of years, an order of magnitude longer than intact, monolithic asteroids29. This has implications both for what stage of Solar System evolution they represent, and also for planetary defence: Ryugu and Bennu would be much harder to disrupt than an intact asteroid29.

The first sampling of Ryugu was of surface material, but the second occurred after a projectile created an artificial crater30. Hayabusa2 returned around 5.4 g of material in December 202031. Hayabusa2 also became the first mission to successfully deploy probes on an asteroid surface, including the Mobile Asteroid Surface Scout (MASCOT), which successfully operated for 16 h32. During the sampling operation at Bennu, the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampler head penetrated 0.5 m into the regolith, collecting a bulk sample and also trapping fine surface dust. OSIRIS-REx returned 121.6 g of Bennu to Earth in September 202333.

Sample analysis and current objectives

On Earth, analysis of the Ryugu and Bennu samples is ongoing34,35. Measurements have shown the oxygen isotopic compositions of Ryugu and Bennu are similar to CI carbonaceous chondrites: primitive meteorites which formed in the outer Solar System and closely match the composition of the surface of the sun33,36. Both samples have a hydrated mineralogy produced through extensive aqueous alteration and contain abundant organic molecules37,38. Fluid inclusions in a Ryugu sulphide crystal contain halogens, nitrogen, sulphur, CO2 and dissolved organic compounds: a direct sample of fluid which formed in the outer Solar System39. In Bennu, salt minerals such as Na-bearing phosphates and carbonates have been found; incredibly fragile minerals which have not been observed in meteorites, indicating the presence of brines on Bennu, environments where complex prebiotic organic molecules might have been able to form40. These discoveries would not have been possible without sample return, as salts would have rapidly dissolved and lost in meteorites which spent any time exposed to the terrestrial atmosphere, showcasing its necessity for the future of asteroid analysis.

Both Hayabusa2 and OSIRIS-REx have had their (space-based) missions extended. OSIRIS-REx has become OSIRIS-APEX (OSIRIS-Apophis Explorer) and will rendezvous with 320 m asteroid 99942 Apophis orbiting it from 2029 until 203141. Hayabusa2 will arrive at asteroid 1998 KY26 in 2031, a 30 m asteroid with a rotational period of 10 min42. Findings should provide new information about two further types of asteroids: Apophis is a potentially hazardous object, so its study should help with planetary defence methods and 1998 KY26’s fast rotation suggests it may represent a different stage of planetary evolution from other asteroids visited43.

Outlook: future missions, advanced analytical techniques

Studying asteroids addresses possibly the broadest number of unknowns in planetary science, including questions on astrobiology, Solar System dynamics, terrestrial planet formation and evolution and planetary defence. In the next sections, I will identify and discuss some of these unknowns and how the study of asteroids can contribute.

Astrobiology

For life as we know it to develop, a liquid solvent (water), stable environment, energy source and chemical building blocks (C, H, N, O, P, and S) are required44. How those chemical building blocks combine into organic molecules, and subsequently into more complex prebiotic compounds, is a fundamental question when considering the development of life not only on Earth, but potentially elsewhere. Organic molecules have been identified in returned asteroid samples: Ryugu samples have a diverse array of 20,000 organic species37; and in Bennu, 14 of the 20 standard protein amino acids used in terrestrial biology have been found38. Understanding whether more complex biologically relevant compounds can form from these organic materials on asteroids will have significant astrobiological implications. These materials are likely widespread throughout our Solar System and may indicate the presence of more complex compounds in potentially habitable extraterrestrial environments than previously understood.

We currently have an idea of the abundance and variety of the organics in Ryugu and Bennu, but the next step will be understanding how these organics formed and evolved—was it through parent body aqueous alteration, or did it occur earlier in the Solar Nebula? Understanding the mineral-organic relationship will be key: probing at the micro-scale whether organics are concentrated in specific minerals should tackle whether catalytic effects play a role in organic development in extraterrestrial environments45. Better understanding of the timings of aqueous alteration episodes is also required, and how the presence (or lack) of alteration cycles interacts with organic material. Analogue experiments may play a role: by replicating the briny environment thought to have been present on the Bennu parent body, we could theoretically design an environment with a variety of initial compositions in order to test formation pathways. Fully establishing the role asteroids play in the transport of organic material requires further study into returned asteroid samples.

Solar System formation and dynamics

The current distribution of material in our Solar System reflects a complex process including orbital migration and planetesimal growth. Asteroids offer an opportunity to probe each of these processes.

Bennu and Ryugu likely formed in the outer Solar System and were perturbed into near-Earth space via orbital migration of the gas giant planets8. Isotopic analyses on returned samples may confirm where they formed46, and further interpretation of their space weathering history (e.g., the presence of specific space-weathering products can indicate the dwell time of an asteroid at a specific orbit47,48). In combination with orbital models, this may improve our understanding of Solar System dynamics.

NASA’s Lucy mission will perform flybys of six asteroids in Jupiter’s orbit in 2027 and 2033. These asteroids likely formed in the outer Solar System, where volatile ices were prevalent49, potentially preserving material from different formation zones to Ryugu and Bennu. Determining the composition of these asteroids will help us understand the distribution of diverse hydrated asteroids within the Solar System. JAXA’s DESTINY+ will perform a flyby of near-Earth asteroid 3200 Phaethon in 2030. This is an unusual body with comet-like activity that may have migrated from the main asteroid belt relatively recently50,51,52. Understanding Phaethon’s composition and dynamical history will aid our understanding of the movement of material from the asteroid belt to near-Earth space, which has additional implications for planetary defence and terrestrial planet evolution.

Also of note, there are increasing global fireball networks on Earth identifying meteorite falls, and determining their orbits prior to their Earth entry, e.g., ref. 53. This can indicate meteorite source regions, and based on analysis of properties such as cosmic-ray exposure age (which can inform on time since a meteoroid has been ejected from an asteroid), can inform on their asteroid dynamics.

The other aspect of Solar System evolution, which we arguably know even less about, is how planets form and grow. For example, how large does a body needs to be for differentiation to occur? There are a number of large asteroids that we cannot confirm are differentiated (e.g., 2 Pallas54) with our limited surface observations. We are unable to directly observe or sample even our own planet’s deep interior. NASA’s Psyche mission aims to address this and is enroute to the 220 km metallic asteroid 16 Psyche, which it will orbit for nearly two years from 2029. The Psyche asteroid is thought to be a metal-rich remanent from the core of a differentiated planetesimal55, and the mission aims to determine its origin by taking measurements of its magnetic field, density and composition. This will be the first time we have been able to look inside the interior of a differentiated body, giving us unprecedented information on the process of planetary growth.

Terrestrial planet evolution

Planets were built from planetesimals, some of which survive in our Solar System today as asteroids. These objects can aid in building models of terrestrial planet evolution, including what their initial compositions were and how the Earth and Mars came to have moons. It is the latter question which current missions are focussing on: the CNSA Tianwen-2 mission launched in 2025 to the asteroid 469219 Kamo’oalewa, a quasi-satellite of Earth and will return samples in 202756. This small object (∼50 m) may be debris from impacts on the Earth’s Moon57,58. If a source crater can be identified, we will have additional geological context for the samples returned. Additionally, as Kamo’oalewa is further away from the Earth than the Moon, comparing Kamo’oalewa samples to lunar samples should help investigate and verify the composition and evolution of Earth’s magnetosphere56.

Also planned is the Martian Moons eXplorer (MMX), a JAXA mission which will launch in 2026 and return samples from the Martian moon Phobos59. Whilst not obviously an asteroid mission, there are two prevailing theories into Phobos’ formation—either it is a captured asteroid, or it formed in-situ via co-accretion or a giant impact60,61,62. Remote observations of Phobos look similar to asteroids found in Jupiter’s orbit63, and if confirmed as an asteroid, it will tell us about early Solar System material transport, and how that interacted with the terrestrial planets, particularly Mars. Regardless of whether Phobos is an asteroid or not, it is expected to contain ∼250 ppm material that has been ejected from Mars, making it likely this mission will additionally be the first to return samples of Mars. Mars sample return will be the first opportunity to study Martian material with geological context, and with controlled exposure to the terrestrial environment in Earth-based laboratories.

The next generation of asteroid science should focus on establishing the initial material of the terrestrial planets - what are the building blocks of Earth? There are no surviving rock records from Earth’s earliest history64, however, asteroids record the composition of accreting material at this early stage. Enstatite-bearing meteorites and asteroids may represent the material Earth is built from (based on similarities in isotopic compositions)65, however Earth appears to be more enriched in s-process (slow neutron capture) components66, so the starting composition of Earth is still poorly constrained67. Further study into the variety of inner Solar System asteroids is required, and returned samples from enstatite-bearing asteroids are needed to directly compare their composition to enstatite-bearing meteorites and Earth materials. Combining this information with the developments in understanding of organic chemistry from analysis of Ryugu and Bennu samples may allow us to begin to get a precise picture of what the ancient Earth looked like, both in terms of its geology and its early prebiotic chemistry.

Planetary defence

For asteroid scientists, the probability of a significant asteroid impact is a frequently asked question from the general public. In fact, asteroid mitigation strategies which would prevent an object from impacting Earth have been in development for a number of decades and include either disruption of the body or deflection of its orbit59. Both techniques consider a form of explosives or kinetic impact on the asteroid, although (with enough warning) orbit deflection strategies could also consider manipulation of the Yarkovsky effect (anisotropic emission of thermal radiation) or solar sails, lasers and other developing technologies68,69,70,71.

To investigate the effect of an impact onto an asteroid, in 2022, the NASA spacecraft DART crashed into Dimorphos, the small satellite of asteroid 65803 Didymos, in order to see if the satellite’s orbit around its asteroid could be changed. The impact altered the orbital period by −33 ± 1.0 min, proving the kinetic impact method is an effective method of orbit deflection72. Building on this initial mission, ESA’s Hera mission will rendezvous with Didymos and Dimorphos in late 2026, aiming to validate the change in orbit and evaluate the crater produced by DART. Hera will fully characterise the composition and internal structure of Dimorphos, allowing for accurate estimation of the transfer of momentum during the DART impact, which should aid in designing more precise methods for altering a potentially hazardous asteroid’s orbit73.

The necessity of sample return?

Most missions mentioned here are not sample return missions. Observing asteroids in-situ with spacecraft is valuable, however studying samples in the laboratory is the only way to fully probe the formation and evolution of an asteroid. Sample return missions are a technological and financial challenge, and so scientific output should be maximised where possible.

One key consideration is the preservation of material for future analysis. The Apollo Next Generation Sample Analysis (ANGSA) initiative showcases this practice. Since 1973, some returned material from the moon has been stored in pristine environments to “wait” for developments in analytical techniques, curation and sample handling74. This has proved worthwhile with the invention of tools such as micro X-ray computed tomography (XCT), which has been widely adopted to produce high-resolution, non-destructive 3-dimensional imagery75. In the case of Ryugu and Bennu material, sample analysis plans have a portion of the returned material earmarked for long-term storage and future analysis26,76. Despite the advent of XCT, analytical methods still tend to be destructive, including the loss of material when creating polished sections for microscopy77,78 and the loss of spatial context when powdering samples. This conundrum is well understood, and the evolution of next-generation techniques is focussing on non-destructive methodologies to preserve material79.

The question then becomes whether effort and funding should focus on development of Earth-based laboratories, methodologies and curation, rather than investing in future sample return missions themselves. Both are needed.

Our current asteroid sample inventory is limited to three collections where the asteroid is unambiguously known (Itokawa, Ryugu and Bennu), and meteorites that lack geological context due to unconfirmed parent body asteroids. Additionally, our meteorite collection is biased towards large samples travelling at sufficient low velocity to survive passage through the Earth’s atmosphere and, critically, that are discoverable once on the ground80. The meteorites that make it into our collection are therefore not representative of the true asteroid diversity. In contrast, returned asteroid samples have geological context, but these three S- and C-type asteroids cannot fully represent the ∼1000 asteroids >1 km diameter in near-Earth space81, and likely >1 million asteroids in the main asteroid belt82. Additionally, all of the asteroid samples have shown the effect of space weathering, which can hide true composition. There are abundant asteroids of different spectral types in our Solar System, but analysis from Hayabusa, Hayabusa2 and OSIRIS-REx have shown we will only confirm the true composition of an asteroid by performing sample return. In order to fully understand the role asteroids played in Solar System formation, we need samples from other known asteroids (Table 1), and to address key science questions we need to maximise scientific output by developing non-destructive techniques for studying the samples in Earth-based laboratories.

Table 1 An asteroid target “wish-list” for future missions, and the unknowns they could address
Full size table

As we move into the next decades of planetary science, a continued emphasis on asteroids and sample return missions in particular will fundamentally improve our understanding of the formation and evolution of the Solar System.