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.
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.
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.