trapped ice could turn a profit for private companies, with
metal processing not far behind
Ingebretsen, Contributing Editor
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
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
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.
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].
impression of a possible future manned mission to mine an asteroid.
Such sights may become a reality later this century.
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.
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 onis 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
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 cobaltin
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.
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.
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 arrivalall require considerable changes in
velocity, or DV.
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 somost
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.)
the rocket is in LEO, the additional increases in velocity
needed to reach many destinations in the solar system are
smallerand 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.
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.
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.
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.
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.
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.
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.
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