Analysis: Should Space Miners Target the Moon, or Asteroids?

Planetary Resources made a media splash recently by announcing a plan to mine asteroids for metals and water. But those same materials are available on the moon, too, and other would-be extraterrestrial prospectors are eyeing lunar missions. So which target is more promising? PM contributor Rand Simberg looks into it here.

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Last week, a new company called Planetary Resources announced that it was starting a private venture to mine asteroids. The mission, backed by billionaires such as Google's Larry Page, plans to send robots to procure heavy metals and water from space rocks. But for years now, other would-be space miners have had another, closer target in mind: the moon. For instance, the Shackleton Energy Company was formed to make rocket fuel from it. Its founders also plan to mine other volatiles—such as ammonia and methane—that were also discovered, which would provide nitrogen and carbon for fertilizer and other uses.

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Although these concepts sound like far-off pipedreams, the ingredients that will make such off-Earth mining possible—including low-cost launch to orbit, electric spacecraft, and advanced sensors and computers for remote operations—are coming. So does it make more sense to try to mine an asteroid, as Planetary Resources wants, or our own natural satellite?


Planetary Resources says it has two main mining targets for asteroids: heavy platinum-group metals and water. Let's start with the metals.

When the Earth formed, it was rich in heavy metals, but they sunk to the core as the planet cooled. As a result, most or all of the heavy metals mined on Earth are mined at impact sites—they came from space aboard asteroids.

Asteroids are the primary source for accessible heavy metals in the solar system because they are the debris of a planet that never formed, and so they are made up of the same material that sunk to the cores of the other rocky planets when they were formed billions of years ago. But the moon is probably rich in these metals, too. Would-be lunar prospectors expect to find metal-rich asteroid material all over the moon just beneath the regolith, because every one of the countless lunar craters is the result of an asteroid strike.


It's easy to understand Planetary Resources' goal to mine asteroids for platinum-group metals, which fetch high prices here on Earth (for instance, platinum itself is currently about $1600 per ounce). But the company has also made clear that it hopes to find water on asteroids and harvest it. This one's a little harder to understand intuitively: Since Earth is awash in water, why send robots or astronauts into space to get it from asteroids?

Water is almost free here, but getting it to space incurs a huge cost per pound (thousands of dollars with current rockets, though if the SpaceX Falcon Heavy flies soon it could reduce that cost to about $1000 per pound). And water can do more than provide the drinking water and oxygen needed to keep humans on space missions alive. Broken down into its constituents of oxygen and hydrogen, Planetary Resources founder Peter Diamandis says, water could provide the fuel to take humans or robots not only back and forth to the moon, but also to other bodies in the solar system, whether NASA or private individuals or corporations fund those missions. Tapping water beyond Earth could enable the large-scale development and settlement of space.

So: Asteroids or the moon? Diamandis says that the water from just one typical smallish asteroid could fill the external tanks of every space shuttle mission flown. And lunar ore is not as rich in water as a carbonaceous chondrite asteroid is, because much of it and other so-called volatiles (such as methane and ammonia) tend to boil off into space from the heat generated when asteroids strike the lunar surface. You'd need to process a lot more lunar regolith to get a gallon than you would material on a water-rich asteroid.


The moon has one major advantage over asteroids, though: It's much closer. The moon is just three days away, while asteroid missions would take months. In addition, the time delay for light (and radio) to the moon is just a couple seconds each way, allowing remote operation of robotic rovers. Asteroids can be several light-minutes away, which would demand much more autonomy for rovers, given the unacceptable transmission delays. You could solve this problem by sending humans, but that would increase the project costs—and put their lives at risk.

Asteroid partisans, like those at Planetary Resources, argue that the transportation energy requirements for asteroids can be much lower than those for going to and from the lunar surface. They say that spacecraft could travel to and from asteroids with propellant-sipping ion drives, whereas moon materials would need to use relatively inefficient chemical propulsion—ion drives don't have the thrust levels necessary to climb into orbit, even from the moon, but that's not a problem with asteroids. Yes, it takes months to travel to the asteroids, Planetary Resources says, but once a pipeline of spacecraft starts making steady circuits, the delay wouldn't matter.


Beyond the technology and economics, the legal issues of mining the moon versus asteroids are different, as well. There are established property rights for bringing samples back from the moon and trading them, based on the precedent of Soviet lunar sample returns, so it is currently legally possible to own and sell what you take away from the moon. Those same rules might apply to an asteroid, too, if a company chipped off a piece of it and brought it back to Earth.

But what if the company, say, moved an entire asteroid closer to Earth to make it easier to access? This seems to be the plan, at least potentially, for Planetary Resources. Under some interpretations of the 1967 Outer Space Treaty, such an act might make the asteroid no longer legally a "celestial object" since its orbit was artificially changed, and the entire thing might then become the asteroid wrangler's property. However, this also would make the asteroid's proud new owner liable if it smacked into anything—such as the Earth. And under that treaty and the later 1972 Liability Convention, the U.S. government would be responsible for such actions by its nationals, which means asteroid miners might face a heap of legal issues before they could ever tap a space rock for platinum or water.

So What?

Planetary Resources is not the first company formed to mine water in space. It's just the splashiest, and its announcement—plus the credentials of its backers—may dilute the "giggle factor" that has traditionally been one of the barriers to investors for such ventures. And on the same day as Planetary Resources' announcement last week, Moon Express, a company with similar lunar ambitions that is funded by Silicon Valley technologists (no doubt spurred by the announcement of its new competition), announced the addition of new members to its scientific advisory board. As its founder, Naveen Jain, points out, you don't have to go to the asteroids to mine asteroids, because over the eons, they've been coming to us in the Earth–moon system.

For now, no one knows the answer to which business approach is the best, but the bets are being placed, and the water rush to the new frontier of space seems to be on. If the government can create a market by using propellant depots for its own deep-space exploration needs, that market will figure it out.

Correction: The above discussion about the relative availability of water on the moon refers to the lunar surface in general. There are locations in craters at the poles, permanently shielded from the sun, that contain abundant amounts of free water ice that may be easier to extract than the water from nearby asteroids.

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