Mining Near-Earth Asteroids Makes Little Sense

So why does the idea persist?

The continued interest in mining near-Earth asteroids is partly based on myths that afflicted the first generation of investor-backed space resource companies, and partly by architectures that seem to make sense on paper until you remember the Moon exists.

In terms of mythos, infographics like this were floated in the era of Planetary Resources and Deep Space Industries:

There’s a lot of issues here, like why are the elements being compared to the crust of Earth? We don’t mine the bulk crust, we mine extremely concentrated ores where elements are orders of magnitude more concentrated. No single asteroid contains both high volatile and high metal contents; these have been mixed and matched here from different meteorites classes. Though errant thinking like this, an idea has been built up of asteroids as a vast treasure trove of nearly unlimited resources, and it seems intuitive we should start by going after those closest to us: the near-Earth asteroids (NEAs). Below, I offer five arguments why mining NEAs makes little sense.

1. There’s nothing found on near-Earth asteroids you can’t get on the Moon. 

All the elements of the periodic table are found on every planetary body, it’s just a question of concentration. If we compare asteroid material with lunar material, there’s really nothing special or unique about asteroids, and they’re deficient in several areas.

Water

NEAs: Locked in hydrated clays with max ~10% water, needs to be heated to >400 C

Moon: Found as ice at up to 100% concentration, only needs modest heating

Supervolatiles as sources of carbon, nitrogen, etc.

NEAs: Not thermally stable

Moon: Thermally stable at the poles and confirmed by LCROSS

Metals

NEAs: Abundant on only the rarest types of asteroids, the M-types

Moon: Can be produced from bulk regolith

Nuclear fuels (U, Th)

NEAs: Not concentrated on any type of asteroid

Moon: Could be concentrated if “KREEP ores” exist at local scales, regular KREEP materials are too low in concentration

The only area where asteroids eke out any advantage is in base metals found in an already chemically reduced form, but there are sources of these on the Moon. As well, an industrial technology stack of solar power and electrochemistry will make it easy to transform lunar regolith into metallic products.

2. It takes forever to get to and from NEAs, and little of that time is spent mining. 

Once an accessible NEA is located, it can take several years for the orbits to line up for a favorable launch, hundreds of days to get out to the asteroid and rendezvous, then hundreds of days to get back. Almost none of this time is spent mining. The ostensible benefit of NEAs is that a subpopulation of them have a delta-v requirement lower than reaching the lunar surface. But is it really worth saving half a km/s to have a mining operation with an uptime in the single digit percent range? How many mines on Earth would be profitable if they operated for five hours a year? By contrast, the Moon is 3 days away, and at most it takes about a month for orbits to line up for any landing site, even at the poles. 

3. Mining small asteroids is an operational nightmare. 

Nobody has come up with what I would consider a viable, well thought out plan to actually manipulate a small asteroid in order to mine it. These asteroids do not have solid surfaces with gravity like that depicted in Armageddon:

Almost every small NEA is an irregular shaped rubble pile–or basically a space sandcastle of loose dust and boulders–held weakly together by cohesion and microgravity, and spinning rapidly. If you so much as gently tap on the surface, material explodes and goes flying in every direction. We know this because we’ve done it:

You cannot “land” on a small NEA per se. You cannot anchor to it. Any mechanical system that touches the surface will blow material everywhere. If you put a structure around the asteroid, it will completely fill up with debris. This is an extremely hard problem that has received little attention. By contrast, the Moon is a solid surface with useful gravity, and with Starship in the mix you can bring down many hundreds of tons of heavy equipment to set up a full-fledged mining operation. Terrestrial excavation techniques need a bit of modification, but not much. 

4. There is very little mass in the accessible NEAs.

The total mass of the NEAs is estimated at 10^16 kg, and the subpopulation that are actually feasible to get to would bring this number down by orders of magnitude. By contrast, the mass of the main belt asteroids is five orders of magnitude higher at ~10^21 kg. So, while asteroids do make sense as a space resource target in the longer term, their main advantage is accessible volume and this is only realized in the main belt, not the NEAs. One could argue there’s more than enough mass in the NEAs for near-term needs, but again this falls down when the Moon is considered: the total NEA mass is equivalent to just an 18 cm deep layer of regolith on the Moon. There’s more than enough material for near-term needs on the Moon too, and it’s far closer and easier to operate on.

5. The Moon is a better stepping stone to mining main belt asteroids than NEAs are.

One could claim that if we want to eventually learn how to mine asteroids in the main belt, the NEAs are a nearer term, accessible first step toward that path. But most of the mass in the belt is taken up by the largest asteroids like Ceres and Vesta. Mining operations on these bodies will be much more reminiscent of the Moon than they will small rubble piles: decent gravity, long-term sustained operations with heavy landed mass, a solid surface, etc. In this case, the Moon is a more relevant first step to develop the technologies and best practices before taking them further out in the solar system.