Asteroids and comets

Small bodies are the least altered material in the solar system, which makes them scientifically valuable, and they are the only natural objects that pose a predictable existential hazard, which makes them strategically relevant. They are also, mechanically, unlike any other destination.

Populations

Spectral classification divides asteroids into three broad groups. C-type, carbonaceous, are roughly 75% of the population, dark, and rich in hydrated minerals and organics. S-type, silicaceous, are about 17%, brighter and stony. M-type are metallic, thought to be exposed cores of differentiated bodies that were shattered by collisions.

The distinction between asteroids and comets is less clean than the vocabulary suggests. Some objects classified as asteroids show intermittent activity, and some comets become dormant once their surface volatiles are exhausted. The useful physical distinction is volatile content, not orbital family.

Navigating in microgravity

Operating at a small body is not a scaled-down version of operating at a planet. It is a different regime.

Bennu is representative. With a mean radius of about 245 m and a mass of 7.3 × 10¹⁰ kg:

Several consequences follow, and each one broke an assumption from planetary missions.

Solar radiation pressure is comparable to gravity. At these gravitational scales, the force from sunlight on the spacecraft is not a perturbation to be modelled but a dominant term. Trajectory design must treat it as a primary force, and the same is true of the body’s own irregular gravity field.

The gravity field is not a point mass. A body a few hundred metres across and irregularly shaped has a gravity field with no useful spherical approximation at close range, and it must be mapped from tracking data on arrival before close operations can be planned.

Touchdown is not landing. OSIRIS-REx contacted Bennu, collected a sample and backed away within about sixteen seconds, never coming to rest. Anchoring was judged riskier than touch-and-go. Philae, which attempted to anchor on comet 67P, had its harpoons fail and bounced for two hours before settling in shadow where it could not recharge.

Surface strength is unpredictable. OSIRIS-REx sank far deeper into Bennu’s surface than expected. The regolith behaved as a loosely bound fluid rather than a solid, and the spacecraft would have continued sinking without its backaway burn. That was the mission’s most surprising result and a serious lesson for future landers.

Missions flown

Dawn is worth singling out because it demonstrated something no chemical mission could do. Entering orbit around Vesta, spending a year, then leaving and entering orbit around Ceres required about 11.5 km/s of delta-v, more than any spacecraft before it, delivered by ion propulsion from roughly 425 kg of xenon. See Electric propulsion.

Planetary defence

The hazard is real, quantifiable and unusually tractable, which distinguishes it from most catastrophic risks.

The 140 m threshold is the current survey target, and the gap between 40% and completeness is the argument for the NEO Surveyor infrared space telescope. Ground-based surveys are biased against dark objects and against those approaching from the sunward direction; an infrared space telescope detects objects by their thermal emission rather than reflected light, which largely removes the albedo bias.

DART tested deflection directly. In September 2022 it impacted Dimorphos, the moonlet of the binary asteroid Didymos, and changed its orbital period around the primary by about 33 minutes against a success threshold of 73 seconds.

The result exceeded expectations by a wide margin for an instructive reason. The momentum transferred was substantially more than the spacecraft’s own momentum, because ejecta thrown off the surface carried additional momentum in the opposite direction. This momentum enhancement factor, roughly 3.6, means the technique is several times more effective than a naive calculation suggests, but it also depends on surface properties that vary between objects and are not known in advance for an untested target.

Kinetic impact works only with warning. Deflection requires changing the asteroid’s velocity by a small amount long before the encounter, so that the change accumulates into a large miss distance. Decades of warning make deflection easy; months make it impossible. That is what makes cataloguing the population, rather than developing deflection hardware, the highest-value activity.

The resource case

Asteroid mining recurs in investment cycles and deserves an honest assessment.

The physical case is sound. C-type asteroids contain water in hydrated minerals, which yields propellant. M-type bodies contain metals including platinum group elements at concentrations far above terrestrial ores. Delta-v to some near-Earth asteroids is lower than to the lunar surface, because there is no gravity well to descend into.

The economic case is not established, and the obstacles are specific.

Returning metals to Earth destroys their value

Platinum group markets are small enough that any substantial supply collapses the price that motivated the venture.

The plausible market is in space

Propellant and structural material sold in orbit avoids the launch cost of bringing them up. But that market barely exists yet, so the demand must be created before the supply can be sold.

Round trips are slow

Synodic geometry means years between departure opportunities, so capital is committed for a long time before any return.

Nobody has processed anything

The largest sample ever returned is 121.6 g. The gap between that and industrial extraction is enormous and has never been bridged even in demonstration.

Legal position is unsettled

The Outer Space Treaty forbids national appropriation. Several states have passed laws recognising resource ownership, but the international position is contested.

The reasonable conclusion is that asteroid resources are a plausible long-term proposition contingent on a substantial in-space economy existing first, and not a near-term business. The companies founded on the near-term thesis in the 2010s did not survive it.

References

• Michel, P., DeMeo, F. E. and Bottke, W. F. (eds.) Asteroids IV, University of Arizona Press, 2015.
• Lauretta, D. S. et al. “Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu”, Science, 377:285, 2022.
• Daly, R. T. et al. “Successful kinetic impact into an asteroid for planetary defence”, Nature, 616:443, 2023.
• Cheng, A. F. et al. “Momentum transfer from the DART mission kinetic impact on asteroid Dimorphos”, Nature, 616:448, 2023.
• Watanabe, S. et al. “Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu”, Science, 364:268, 2019.
• National Academies, Finding Hazardous Asteroids Using Infrared and Visible Wavelength Telescopes, 2019.