Mining Asteroids

ILLUSTRATION HOME RUN PICTURES

Artist's impression of a possible future manned mission to mine an asteroid. Such sights may become a reality later this century.

One day this century an unmanned space probe will touch down on a dormant comet. The probe will drill through the comet's gravel-like shell to reach the ice beneath. Next, a tube will descend into the drilled hole, and, using heat from solar mirrors, will slowly melt the ice, pumping the melt into a giant balloon-like tank. As the tank fills, the water in it will freeze once again. Some of the water will be diverted into the probe, where it will be heated later, again by means of solar energy. The resulting steam will be used to supply the thrust needed for the probe's return to Earth orbit [see diagram, "Mining Water From a Stone"].

Arriving there months later, the probe's icy cargo might be used to steam-power a follow-up mission or to supply drinking water to orbital outposts like the Alpha Space Station. Or it might be used to form a frozen ring to shield those outposts from harmful radiation.

True, a steam-belching rocket ferrying a balloon full of ice through space isn't as exciting as a manned expedition to Mars. Nonetheless, a fledgling group of researchers believes a mission similar to the one described here is not only possible using present-day technology, but could make money for its organizers.

ILLUSTRATION: STEVE STANKIEWICZ

Mining Water From a Stone: In one proposed profile for a mining expedition, a spacecraft lands on an asteroid or dormant comet and drills below the surface into an ice pocket. The ice is melted and pumped into a large bladder, where it refreezes. On the return journey to a space station or storage facility in Earth orbit, some of the ice can be boiled using heat from a solar collector and the resulting steam harnessed to propel the spaceship.

Indeed, a veritable El Dorado awaits in the so-called near Earth objects (NEOs) within the solar system. The term NEO refers to both dormant comets (comets that no longer produce distinctive tails) and asteroids that travel about the sun, often in highly elliptical orbits. Unlike the space rubble that lies in the Asteroid Belt between Mars and Jupiter, the NEOs' orbits occasionally bring them quite close to Earth, some even to the point of impact [see diagram].

The Celestial Neighborhood: Most asteroids inhabit the Asteroid Belt, but some are in orbits that bring them close to Earth. For example, the Amor asteroid group's closest approach to the sun (perihelion) is outside our orbit, while the Apollo and Aten groups have perihelions that are closer to the sun than Earth orbit.

Astronomers have catalogued many types of NEOs. The dormant-comet variety contains mostly water mixed in with bits of sand and loose rock. Other NEOs, called C types, are a rough mixture of volatiles (made up of clays, hydrated salts, and water), plus silicates along with metals like iron, nickel, and platinum.

John Lewis, who co-directs the Space Engineering Research Center at the University of Arizona at Tucson, studied one C-type asteroid, a 2-km-wide NEO called Amun. He concluded that the monetary value of Amun's platinum group metals (pgms)—platinum, iridium, osmium, palladium, and so on—is more than US $6 trillion. Amun's iron and nickel might be worth something on the order of $8 trillion. Add another $6 trillion for Amun's cobalt deposits, and the asteroid's value totals a spectacular $20 trillion!

To get at these valuable resources, Amun's metallic ores would need to be separated out from the asteroid's silicates and volatiles. But another kind of asteroid, the M-type, is almost pure metal, mostly iron. Some M-types, like the unassumingly named 1986 DA, are mountain-sized blends of iron, nickel, and cobalt—in other words, naturally occurring stainless steel. In all, roughly 2000 NEOs about the size of 1986 DA are known to exist, with as many as 50 more being discovered each year.

Your Space Velocity May Vary: Going places in the inner solar system is not dependent just on distance, but also on the velocity required to overcome Earth's gravity, measured in terms of a quantity known as delta-V (ΔV). Most of the total ΔV is needed just to reach low Earth orbit.

Gravity's rainbow

NEOs came to the public's attention last February, when NASA's Near Earth Asteroid Rendezvous probe made a controlled landing on the asteroid Eros. But researchers have pondered for decades ways that asteroids might be profitably mined. Their interest has everything to do with gravity. Because of the negligible gravity of NEOs, sending a probe to reach one takes less energy than a visit to any other celestial body, including the moon.

In space, a mass continues in motion forever unless it collides with something. In navigating among orbiting bodies in space, the primary measure of how hard it is to get from point A to point B becomes not distance but a quantity called delta-V (ΔV). Escaping one planet's gravity, adjusting orbit so as to synchronize the time of arrival with the destination's own path through space, and finally slowing enough to land gently or enter orbit upon arrival—all require considerable changes in velocity, or ΔV.

The amount of energy required to travel between destination points (and hence how much fuel must be carried) increases with the total ΔV and the mass of the spacecraft. The DV needed to ascend to low Earth orbit (LEO) is a crippling 9 km/s or so—most of the mass of a rocket has to be fuel and engines, not payload. But as science fiction writer Robert Heinlein noted, after you reach Earth orbit, you're halfway to anywhere. That's because a rocket, sitting on a launch pad, is considered to have zero velocity. (Strictly speaking, the earth's rotation makes a difference: a rocket launched at the equator in the direction of that rotation has an initial velocity advantage over one launched at a higher latitude in the same direction.)

But once the rocket is in LEO, the additional increases in velocity needed to reach many destinations in the solar system are smaller—and hence the energy required is less, too [see graph]. To go another 340 000 km from LEO and land gently on the moon, for instance, requires an additional velocity change of a little over 6 km/s. But a voyage from Earth orbit to an NEO would require only 5 km/s, possibly even less than 4.3 km/s, depending on the asteroid's size and trajectory in relation to Earth.

Even greater reductions in ΔV could be achieved on return trips from NEOs. Lifting off the lunar surface and traveling to Earth orbit would take a ΔV of 3 km/s, assuming Earth's atmosphere is used for aerobraking. In contrast, a trip from an asteroid would require just 1 km/s or less, because many NEOs possess negligible gravity, so hardly any energy is required to lift objects off their surfaces. This ΔV difference between NEOs and the moon for the return trip is important, since it is during that portion of the trip that the probe would carry its bulky cargo of ores.

Any reduction in required velocity, of course, translates directly into rocket fuel savings. In addition, many asteroids are thought to be richer in metals and volatiles than the lunar surface. All told, mining NEOs would take less energy and time and so yield a higher return than would be the case with the moon.

Big science

With these facts in mind, elaborate plans have been drawn up for NEO missions. In a typical plan, a ship departs Earth for an asteroid when the two bodies' orbits are such that the lowest ΔV is required. Mining operations might last many months, and meanwhile, Earth and the NEO would be moving farther and farther apart. When the two orbits coincided once again, the mined materials could be shipped back to Earth.

As with most, if not all, speculative space ventures, debate has raged over whether these missions should be manned or robotic [see "Modes of Mining in Orbit"]. But early blueprints of NEO mining missions put human crews squarely at the helm.

One of the first detailed plans for an asteroid mission emerged in 1977, as part of a NASA study on space colonization. The plan, co-authored by space-futurist Brian O'Leary, came soon after the huge expenditures of the Apollo program. Accordingly, it employed a philosophy of striking with overwhelming force.

O'Leary's task force was charged with devising ways to retrieve raw materials from an NEO. The group's solution was to send a large crew of astronaut-miners to a C-type asteroid. Over the course of their three-year mission, volatiles would be baked out of the rock, using a 600 °C solar furnace. The volatiles, which would include water and potential fuel-producing substances such as nitrogen, carbon, sulfur, and phosphorus, would supply the fuel needed to separate out the asteroid's metals and other materials, which would be catapulted back to LEO for further processing.

The study determined that to retrieve half the mass of a million-metric-ton asteroid, some 10 000 metric tons of materials would need to be lifted into LEO at an assumed cost of $240/kg (1977 dollars). The total cost of the mission was put at $31 billion, including R&D costs. To ship the same quantity of mined materials from Earth's surface would cost a prohibitive $663 billion.

Islands of ice

In 1993, perhaps to accommodate diminished expectations in space, Lewis, of the University of Arizona, published a similar study that examined the cost of mining fuels and other materials on the moon, versus mining them on NEOs. For starters, Lewis reasoned that the lunar mission could use as fuel for its return trip hydrogen from the ice supposed to exist at the lunar poles. The resulting payback after 10 lunar missions would be 8:1. That is, eight times more fuel could be transported than consumed. His calculations included the amount of fuel needed to send the vessel to the moon so that fuel recovery operations could begin.

When Lewis looked at NEOs as a source of water and fuel, the potential payback improved considerably. He estimated that an NEO mission could return three times as much fuel or water as it consumed in making the voyage—and that was just for the first trip and after factoring in the fuel cost of getting the probe to the asteroid to be refueled for the return voyage.

If the ship could be reused five times, the payback ratio would rise to 15:1. "Each trip makes considerable masses of propellant available for other uses in near Earth space," Lewis wrote.

But the actual distances must be taken into account; if many round trips from the moon to LEO can be made in the same time as one round trip from an NEO to Earth orbit, lunar mining will still prove to be the most economical. This constraint indicates a need for an approach to NEO mining with continuous operations and multiple transport ships, increasing start-up costs.

Many researchers agree with the choice of water as the likely first target of an NEO mining mission. "Water is peculiarly easy to handle and peculiarly useful," said Mark Sonter, a mining engineer based in Sydney, Australia, who has written several papers on profitable ways to mine NEOs. "You're almost guaranteed 10-30 percent recoverable water from water-bearing asteroids," he said. "Once you've extracted it at the asteroid, you can use it directly as propellant in a steam rocket or you can split it into hydrogen and oxygen and use it in a classic chemical rocket, and using some of it, you can return the rest to Earth orbit, where it can be used as propellant, as life support, or as radiation shielding."

A market for water already exists in LEO, believes Kevin Reed, a research scientist for the aerospace-defense firm BAE Systems, Farnborough, UK. Like Sonter and many another NEO mining enthusiast, Reed earns a living at an unrelated day job, but spends much of his free time devising ways to profit from asteroid mining. The market for water, he said, is the Alpha Space Station, which currently gets its water as a byproduct of visiting space shuttles' fuel cells. As the station grows, so will its need for consumables. "You could sell [Alpha] water and oxygen," he says. And in the future, "If the Russians got the capability of supplying water and oxygen to a Mir 2, they could take up as many [space tourists like] Denis Tito as they want."

All that glitters

But why bother ferrying water around instead of mining metals and returning them to Earth's surface—especially since trillions of dollars worth of high-valued metals are ripe for the taking?

The fact is that transporting materials back to Earth changes the economics of the equation. For starters, the logistical problems greatly increase. "One of the things that we're discovering is just how fragile atmospheric physics is," said Richard Gertsch, an assistant professor in the mining and materials engineering department at Michigan Technological University, Houghton. Thus, any miscalculation could turn an ore-bearing shuttle into a hailstorm of molten metal. Disasters aside, developing a craft that transports materials back to Earth as efficiently as ships and rail cars now transport ores from terrestrial mines is a tall order.

An even bigger problem from an economic standpoint is that asteroidal supplies of iron may easily exceed demand, depressing prices. A huge influx of space metals—or even the expectation that they might come onto the market—would result in a price collapse. Also, any venture aimed at returning these materials from space has to compete with the highly efficient terrestrial mining techniques already in use.

Still, given that asteroids contain platinum metals worth trillions, why isn't it possible to profitably return these to earth? "The whole idea of space resources is that the resources are huge. How you use them has been the real problem," Gertsch laments. "Everyone goes back to high-value metals, the pgms."

Vexed by the problem, Gertsch co-wrote a study with his wife Leslie, another assistant professor at Michigan Technological University with a special interest in asteroid mining. They envisioned a project equivalent in scale to the Anglo-French Channel Tunnel. It would cost at least $5 billion and take up to 12 years to finish. The study assumed that the asteroid mined would be made up of 150 parts per million of pgms, a concentration thought to occur in about one in 10 platinum-bearing asteroids.

Finding a suitable asteroid and mounting a mission would consume up to four years of the project, the Gertsches reasoned. On arrival, miners would need to sift through 500 million metric tons of material in order to extract enough platinum—some 68 thousand metric tons, at an assumed price of about $13 per gram—to generate a return of 100 percent on the project.

However, even a 100 percent return rate would not attract the needed billions in risk capital, given the 12-year timetable and the high probability of failure, the Gertsches concluded.

ILLUSTRATION HOME RUN PICTURES

A Private Enterprise: SpaceDev plans to launch this Near Earth Asteroid Prospector, shown in an artist's rendering, in three to five years. In this first commercial deep space mission, a scientific payload will assess the value of the asteroid's minerals.

Space is the place

Another proponent of expeditions to the asteroids is Jim Benson, chief executive officer of a publicly traded company called SpaceDev, in Poway, Calif. At a meeting he attended with a prominent venture capitalist, "I was told they wouldn't consider plans in which they would only make 100 times their money," Benson said. "Unless they were going to make 1000 times their money, they're not even interested."

For this reason, researchers have tried to promote the idea of mining materials in space for use in orbit. "I don't think these resources need to be brought back," Benson said. Since it costs $10 000/kg to lift anything into space, any material in orbit already has a putative value of $10 000, he explained. His company hopes to raise the $12 million needed to set a probe down on an asteroid, assay its resources, then legally claim it [see artist's rendering]. Actual mining missions would come later.

Last November a group called the Space Resources Roundtable met to discuss similar plans to bootstrap NEO mining operations. The group's members are drawn from the space, mining, and financial communities and have been meeting under the auspices of the Colorado School of Mines, in Golden, since 1999. The roundtable bases its ideas on the thinking of Lewis and others that NEO mining can be launched cheaply and multiple missions can be undertaken. The end result should be an increasingly valuable bank of water or other materials in LEO.

Another strategy is offered by Brad Blair, a former mining company engineer and now a doctoral candidate at the Colorado School of Mines. He proposes turning the second stages of commercial launch rockets into transports for space miners and their equipment to NEOs. Once depleted, these stages are normally allowed to burn up in the atmosphere, but Blair claims they could be modified to run on steam.

Beyond this horizon

How long before any of this begins to happen? Many cite 20 years as a realistic figure, especially if companies from the private sector lead the way. "The problem is not the technology [but] companies' perceptions of what the risks would be and their perceptions of how it would be received by the investment community," said mining engineer Sonter.

But that may change as investors continue to search for the Next Big Thing. "It's going to become increasingly obvious to people with money that this is going to be the new Internet," said SpaceDev's Benson. His company has allied with a Canadian nonprofit group called the Northern Center for Advanced Technology Inc. in Sudbury, Ont., Canada. The group, in part, represents mining interests from that region, and hopes to develop new markets for local technological know-how as mines there gradually become depleted. Ironically, those mines contain ores from an asteroid impact some 100 million years ago.

Stephen Cass, Editor