IEEE Spectrum May, 2015 - 35

A NEAR-EARTH ASTEROID CENSUS

Asteroids, thousands

(Estimated
total is
unknown)

■ Known asteroids
■ Estimated total

Source: NASA Wide-field
Infrared Survey Explorer
(WISE)

bottom: nasa

the most dramatic and effective option seemed
to be a nuclear bomb: Explode one near an asteroid and you could generate enough pressure to
force the asteroid off course without breaking it
into many Earth-threatening pieces.
But in 2002, astronauts Ed Lu and Rust y
Schweickart, planetary scientist Clark Chapman,
and astrophysicist Piet Hut joined forces to explore
other ways to def lect or destroy a threatening
asteroid-should one be discovered. By 2010, the
B612 Foundation (named for the asteroidal home
of the title character in Antoine de Saint-Exupéry's
The Little Prince) and the rest of the space community had settled on a few ways that humankind
could feasibly deflect all but the largest asteroids.
One option, devised by Lu and fellow astronaut
Stan Love, is called the gravity tractor. It would
take advantage of the small gravitational attraction between a hovering spacecraft and the threatening asteroid. Given enough time, the tug of the
spacecraft could nudge the asteroid's orbit so that
it would pass slightly ahead of or behind Earth.
Another possibility is to send a robotic spacecraft
on a high-velocity collision course, one that would
transfer the craft's considerable momentum to
the asteroid. These methods are attractive alternatives to the prospect of sending nuclear weapons into space. There's only one hitch: Because
of the small changes in velocity these deflection
approaches produce, any such mission would have
to be launched years or even decades before the
calculated date of impact.
Regardless of method, we can't deflect an asteroid if we don't know it's coming. Finding potential threats has been the prime focus of planetary
defense research for the last 20 years or so, and it
continues to be the area that requires the most effort.

<100

100-300

300-500
Meters

500-1,000

> 1,000

tHE cElEstial swarm: the orbits of potentially hazardous asteroids-

near-Earth asteroids at least 140 meters in size and in orbits that pass quite
close to Earth's-are shown here. more than 1,400 objects of this class had
been discovered at the time this graphic was released in 2013.

tHE organizEd astEroid Hunt began with the very biggest
objects in 1998, when the U.S. Congress directed NASA to initiate the
Spaceguard Survey, with the aim of finding 90 percent of NEOs larger
than 1 km within 10 years. To reveal these objects, NASA mounted
a ground-based campaign of telescopic observation from multiple
sites. By 2011, the agency met its goal, finding more than 900 of the
estimated 1,000 or so NEOs larger than 1 km. Happily, none of them
seem poised to strike Earth anytime soon.
The remaining NEOs of this size are on long, elliptical orbits that
will only occasionally pass by Earth and so may take decades longer
to find. Extending the search to smaller objects that could also be
quite dangerous was the next logical step. Even before the Spaceguard
Survey was complete, Congress did just that, calling for NASA to find
90 percent of NEOs that are at least 140 meters in diameter by 2020.
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May 2015

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Table of Contents for the Digital Edition of IEEE Spectrum May, 2015