An asteroid the size of two football fields could wipe out a
large city or set off a series of tsunamis across the world. The threat of such
an Earth-smashing
asteroid has lead scientists to dream up several methods of defending the
planet against such a catastrophe.
Solutions have ranged from pushing the asteroid with a
spacecraft to mounting a thruster on its surface. But pushing it would require
too much fuel and could break up the asteroid. Also, asteroids rotate, which
could complicate the firing of a surface thruster.
Now, two NASA astronauts have presented a plan for an
"asteroid tractor"--an unmanned, 20-ton spacecraft that uses the invisible bond
of gravity to gently pull an asteroid into a new, non-threatening orbit.
"You can think of it like a big elastic band between the two
pulling them together," said Edward Lu, who presents the concept for the
spacecraft with fellow astronaut Stanley Love in the Nov. 10 issue of the
journal Nature.
The tractor would hover above the surface of the asteroid,
without touching it, and use gravity as a towline. If the spacecraft maintains
a consistent distance between it and the asteroid, and always tows in the same
direction, this method won't disturb the asteroid's rotation or composition.
Despite the urge to give the asteroid a hardy tug, the key
to moving an asteroid with gravity is to be gentle. An asteroid is likely to be
loosely packed material, so tugging on it too hard could break it into
unmanageable pieces. Or, the force from the spacecraft's thrusters could break
up the asteroid or stir up unwanted dust if fired too vigorously.
To make sure the thrusters couldn't break up the asteroid--or
hinder the net towing force by pushing the asteroid away--Lu and Love angled the
thrusters slightly away from the body of the spacecraft.
"The jets fire off to the side, not quite as efficient as
firing straight down. It's like rowing forward by pushing off to the sides--it
keeps you moving forward, but very slowly," Lu told SPACE.com. "It takes
less than a pound of force--about what you need to hold up a cup of coffee."
In 2029,
the 1,000-foot (320 meters) asteroid 99942 Apophis (2004 MN4) will whiz by Earth at a distance of
about 18,600 miles (30,000 kilometers). That's about as close as many
geosynchronous satellites. It will swing by the Earth again in either 2035 or
2036, and scientists predict it has a small chance of hitting the planet on
this pass.
"The kind of spacecraft we've talked about could move an
asteroid 650 feet (200 meters) across provided we have decades of advanced
warning," Lu said. "That's not out of line with what you'd expect - we can
predict the orbit of an asteroid decades in advance."
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