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Chip Design

Melting trapped ice could turn a profit for private companies, with metal processing not far behind
Mining Asteroids

By Mark Ingebretsen, Contributing Editor

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].

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.

aster01.jpgIndeed, 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].

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

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.


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 (DV). 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 DV.

The amount of energy required to travel between destination points (and hence how much fuel must be carried) increases with the total DV 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 DV could be achieved on return trips from NEOs. Lifting off the lunar surface and traveling to Earth orbit would take a DV 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 DV 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 DV 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.



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