IEEE Spectrum April, 2012 - 60

of solar radiation to maneuver. This method
solar sail, which would use the tiny pressure
has the advantage of working equally well on
that sunlight exerts on it to maneuver through
all classes of objects-metallic, stony, or rich
space. A craft that hit the Earth-threatening
in ice. It would take a very long time, however,
rock with a relative velocity of about 60 km/s
to do the job.
would impart a kinetic energy of 1.8 billion
joules per kilogram. If the aim was perfect
For the 2007 NASA Marshall study
(no small feat at such a relative velocity), the
I began working on yet another scheme,
collision could significantly alter the orbit of
called the solar collector. H. Jay Melosh
an asteroid-one that's sturdy enough to take
of the University of Arizona and Ivan V.
the impact without falling apart. Of course, the
Venus, ho: in late 2010,
Nemchinov and Yu. I. Zetzer, both then
thing could fragment, which might just make
Japan's iKArOS became
affiliated with the Russian Academy of
things worse.
the first spacecraft to use
Sciences, first proposed this approach
But if we had several decades to plan the
a solar sail for interplanetary
travel. Photo: Jaxa sPace agency
in 1994. For this strategy, the spacecraft
intervention, we could apply force more genwould deploy a large parabolic ref lector
tly and with better control. We could wrap
that would always face the sun. Although
a solar sail around the offending object, like
the reflector would resemble a typical solar sail, its purpose
aluminum foil around a potato, changing the degree to which
would be solely to concentrate sunlight onto a smaller flat
it reflects light and thus the effective pressure that sunlight
mirror, known as a thruster sail. The thruster sail would
exerts on it. In this fashion, the sail could gradually alter the
direct concentrated sunlight onto the offending asteroid.
object's orbit around the sun, converting an impending Earth
If the object contained volatile material, the intense beam
impact into a near miss. This wrapping method ought to work
would heat things up enough to vaporize part of the surface.
for any kind of asteroid or comet.
The gas shooting into space as a result would, over time,
Apollo 9 astronaut Rusty Schweickart and Bong Wie of Iowa
impart enough momentum to nudge the body's solar trajecState University have proposed yet another universally applicatory away from a projected impact with Earth. It wouldn't
ble solar-sail technique, called the gravity tractor. Here a solar
take much of a push, because with asteroids, unlike horsesail would maintain position near the threatening body for
shoes, a near miss doesn't count.
decades, exerting a small but significant gravitational attraction Whitepaperad_1-4:Layout
1 9/12/08 1:17 PM Page 1
The version of this approach that I worked on for the
on that object, which over time would alter its course. The sail
NASA Marshall study in 2007 assumed that gas would shoot
would move into position and remain there using the pressure

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Table of Contents for the Digital Edition of IEEE Spectrum April, 2012