[meteorite-list] Taking Out A Killer Asteroid - With A Tame One

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Apr 28 12:07:46 2006
Message-ID: <200604272008.NAA13601_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9063-taking-out-a-killer-asteroid--with-a-tame-one.html

Taking out a killer asteroid - with a tame one
Maggie McKee
New Scientist
26 April 2006

It sounds like a Hollywood blockbuster. A potentially deadly asteroid is
heading for Earth, and scientists mount a mission to intercept it -
using another asteroid. But that is exactly what two French researchers
propose in a plan to capture and "park" a small asteroid near the Earth
for just such emergencies.

But a second group of researchers says shooting a spacecraft into the
asteroid would be simpler and more effective. Other experts warn that
both plans risk having fragments of the initial asteroid strike the
planet, but say they highlight the need for governments to devise
strategies to ward off such impacts.

Relatively small asteroids, about 100 metres wide, are thought to hit
the Earth every few hundred years. But their effects could be deadly.
One such impact unleashed the force of 1000 atomic bombs when it struck
an unpopulated area of Siberia in 1908.

"Asteroid impacts are very rare but they can potentially kill many, many
people," says Dan Durda, a planetary scientist at the Southwest Research
Institute in Boulder, Colorado, US. "And they are 100% preventable."

Various plans have been put forward to deflect incoming asteroids. For
example, lasers or giant space mirrors could evaporate ices on their
surfaces, creating jets that propel them away from Earth. And
half-painting an asteroid could make it radiate heat differently on each
side, slowly nudging the object off course.

But many of these plans require several years of advance warning in
order to push the asteroids into safe orbits. If an asteroid or comet is
found barrelling towards the planet with a year or less to impact,
"that's a case where perhaps our only option is to attempt a big kinetic
kill", says Durda.

Parked space rock

Now, Didier Massonnet and Beno?t Meyssignac of France's National Centre
for Space Studies have come up with a new projectile to fire at the
asteroid in such a "kill". They advocate capturing a small, 40-metre
asteroid and "parking" it a stable Lagrange point 1.5 million kilometres
from Earth, where the gravity of the Earth and the Sun balance.

If a larger asteroid were then found to be on a collision course with
Earth, the small rock could be moved into its path within eight months,
says the team. This "David's stone" would be too puny to cause any
damage to Earth if things went awry, says the team. "Such an asteroid
capture would be one of the most remarkable achievements of mankind,"
they write in Acta Astronautica.

But other experts say the plan is not realistic. It relies on using a
small hopping robot to excavate rock at tens of metres per second from
the little asteroid in order to provide the force to capture it and send
it towards the larger rock. The capture would take a year of digging and
would require the robot to remove 66% of the small rock's mass.

"To have a mechanical device work all on its own - without a person to
kick it - in an essentially unknown surface environment full of dust and
debris, is a very difficult thing to do technically," Durda told New
Scientist.

Gerhard Hahn and Ekkehard K?hrt of the German Aerospace Centre in Berlin
agree. "It sounds rather like science-fiction," they told New Scientist
in an email.
          
Hit and run

But Dario Izzo, an aerospace engineer at the European Space Agency's
Advanced Concepts Team in The Netherlands, says the capture is
technically feasible. "We can do it, but it would be really expensive,"
he told New Scientist.

Izzo is now working on a strategy based on ESA's plans for its Don
Quijote mission. That mission is designed to put one spacecraft in orbit
around an asteroid to watch as another is sent crashing into it. Don
Quijote will be a technology demonstration mission, but Izzo's team has
been working on ways to use just an impactor spacecraft to deflect a
dangerous asteroid.

As a test case, the team used the orbital parameters of Apophis, a
400-metre-wide asteroid that will pass by Earth in 2029. During that
pass, it may change course enough to hit Earth when it returns again in
2036 - a possibility that now has a one in 5000 chance of happening.

The team developed formulae to find out how much Apophis could be
deflected by a 700 kilogram (1540 pound) spacecraft. "We found there are
loads of trajectories, of launch windows, that would allow us to obtain
a deflection," Izzo says. If a spacecraft were to launch by 2026, it
could hit Apophis and change its speed by 0.01 millimetres per second -
a tiny change, but enough to prevent it from colliding with Earth a
decade later, he says.

Gravity tractor

But NASA astronaut and physicist Ed Lu says any plan that involves
striking an asteroid risks breaking it into fragments. Some fragments
would have their orbital periods unchanged, and "any such debris will
strike the Earth," he told New Scientist.

"For any realistic scenario where the Earth is threatened by an
asteroid, would we really choose a method that cannot guarantee that it
will not make the problem worse?" he says. Lu has previously proposed a
"gravity tractor" <http://www.newscientistspace.com/article/dn8291.html>
plan to fly a spacecraft near an asteroid and use its own gravity to
pull it off course.

But Durda, who is also president of the B612 Foundation, which aims to
build a spacecraft to test the gravity tractor idea, says it is
important to consider many different plans for protecting the Earth from
impacts. "We want to have a lot of options at our beck and call," he
says. "Depending on the lead time and properties of an asteroid, a
technique that might work very well in one circumstance might not work
at all in another."

Currently, the US government has not assigned any agency responsibility
for dealing with any impact threat that may be identified. But NASA is
trying to identify what those threats are.

It is expected to finish a survey of potentially dangerous asteroids
larger than 1 km across by 2008. But Durda says it has not yet been
funded to begin a congressionally mandated programme to find all such
space rocks larger than 140 m
<http://www.newscientistspace.com/article/dn8510.html> across.

Journal reference: Acta Astronautica (DOI:
10.1016/j.actaastro.2006.02.030, DOI: 10.1016/j.actaastro.2006.02.002)
Received on Thu 27 Apr 2006 04:08:07 PM PDT


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