[meteorite-list] Risk of Asteroid Smashing into Earth Reduced (99942 Apophis)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon May 22 12:27:40 2006
Message-ID: <200605221618.JAA21890_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9203-risk-of-asteroid-smashing-into-earth-reduced.html

Risk of asteroid smashing into Earth reduced
Kelly Young
New Scientist
22 May 2006

The danger to Earth from an asteroid called Apophis, which once looked
relatively likely to hit the Earth, appears to be waning.

The odds of an Earth impact by Apophis in 2036 have now been reduced
from 1 in 5500 to 1 in 24,000, following new radar measurements taken by
the Arecibo Observatory in Puerto Rico.

On 6 May 2006, with the asteroid 42 million kilometres away, astronomers
with NASA's Jet Propulsion Laboratory measured the speed of the asteroid
with radar. They found its speed was 6 millimetres per second different
than what they had expected.

"It's just a small correction in the orbit that propagates forward in
time enough to reduce the probability of impact," says Jon Giorgini,
senior analyst with the Solar System Dynamics Group at JPL in Pasadena,
California, US.

Before this observation, it was thought that in 2029 Apophis would
approach our planet to within about 5.86 Earth radii. With these new
measurements, the estimated distance has been pushed back to 5.93 Earth
radii.
          
Through the keyhole

This seemingly small difference is crucial since if the asteroid's path
carries it through a certain "keyhole" - in this case, a specific region
of space near Earth just 600 metres across - Apophis's orbit could be
perturbed enough to put it on a collision course with Earth in 2036.

The asteroid, which is about 320 metres in diameter, ranks a 1 on the
Torino scale. Zero means an object has no chance of hitting Earth and a
10 means certain impact. At one point, Apophis rated a 4.

Because the asteroid will travel too close to the Sun for radar
measurements, this will probably be the last chance to get good radar
data of Apophis for several years. But in 2013, the asteroid should be
in a position where astronomers can track it with radar again.

"I think we'll be able to put this to rest one way or the other,"
Giorgini told New Scientist.

Giggle factor

However, astronomers may be able to get optical data on Apophis before
then, says Dave Tholen, an astronomer with the University of Hawaii, US.

In January, Tholen and his team plan to observe Apophis to determine its
rotation rate. This is important because if the asteroid is heated
unevenly by sunlight, the radiation can impart a small force on the
asteroid. That force may be miniscule, but when it accumulates over 20
years, it can make the difference in whether Apophis passes through the
keyhole or not, he says.

Even though the odds of this particular asteroid hitting Earth have been
reduced dramatically, its early threat alerted the public to the
potential of asteroid impacts. "Compared to 10 years ago, I'd say the
giggle factor has been reduced considerably," Tholen says.
Received on Mon 22 May 2006 12:18:40 PM PDT