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Recently Discovered Asteroid Could Hit Mars in January
Steve Chesley and Paul Chodas
NASA/JPL Near-Earth Object Program Office
December 21, 2007
A recently discovered asteroid which passed close to the Earth in
November, is now
headed towards a very close passage by Mars in late January, and
there is
a small chance that it could hit that planet. The probability of a
collision is
only 1 chance in 75, but scientists are excited about the possibility. If
it happens,
the impact would occur on January 30, 2008 at around 10:55 UT (2:55 a.m.
PST).
The current position of asteroid 2007 WD5, with its orbit shown in blue.
The asteroid's orbit stretches from just outside the Earth's orbit at its
closest point to the Sun, to the outer reaches of the asteroid belt at its
farthest.
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Uncertainty Region for 2007 WD5 at encounter with Mars, shown as white dots.
The thin white line is the orbit of Mars. The blue line traces the motion of
the center of the uncertainty region, which is the most likely position of
the asteroid.
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In the likely event that the asteroid misses Mars, it could come back to
the vicinity
of the Earth years or decades later, but our routine hazard monitoring
shows that
there is no threat of an impact with the Earth.
Animation showing the motion of the uncertainty region of 2007 WD5 as it
approaches Mars.The thin white line is the orbit of Mars. The blue line
traces the motion of the center of the uncertainty region, which is the most
likely position of the asteroid.
Large Version (600x600)
Small Version (300x300)
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Designated 2007 WD5, the asteroid was discovered on Nov. 20, 2007 by the
NASA-funded
Catalina Sky Survey using a 1.5m telescope on Mt. Lemmon, near Tucson. The
object
had already passed within 7.5 million km (5 million miles) of the Earth on
Nov. 1,
before it was discovered. Based on its magnitude, we estimate the asteroid
to be about
50 meters (160 feet) across. As the accompanying diagram shows, it has
already reached
the halfway point between Earth and Mars. When it closes in on Mars, it
will approach
from the day side, and would then be very difficult to observe from any of the
spacecraft on or around Mars. Our current best estimate predicts the
asteroid will miss
Mars by 50,000 km, but the miss distance is highly uncertain because the
asteroid's path
is not known with sufficient accuracy. The uncertainty region during the
Mars encounter
currently extends over a million kilometers (700,000 miles) along a very
slender
ellipsoid only 1200 km (700 miles) wide, but the ellipsoid does intersect
Mars. The
zone of potential impact on the surface of Mars is approximately 800 km
wide, and sweeps
across the Martian equator from southwest to northeast, crossing the
equator at roughly
30 deg W longitude. The MER Opportunity rover is close to the southern edge
of this
possible impact zone but clearly outside it.
The asteroid is becoming increasingly difficult to observe, since it is
receding from
the Earth and the waxing Moon is approaching the same part of the sky. But
it should
become observable again early in January. These new measurements will lead
to a
significant improvement in the orbit accuracy, and we will then be able to
refine the
probability that the asteroid might collide with Mars.
If the asteroid is indeed on a collision course, it would hit Mars with a
velocity of
about 13.5 km/s (8.4 miles per second), and would produce an explosion
equivalent to
about 3 MT of TNT. We can only speculate as to the effects of such an
impact, but it
would be reasonable to expect a crater nearly a kilometer across and a
significant
amount of dust lifted into the atmosphere.
An impact would not be unprecedented: 21 fragments of Comet Shoemaker-Levy 9
impacted Jupiter in July, 1994. Those impacts were predicted with near
certainty
almost a year before the impact. But, with a 1-in-75 chance, this
asteroid's possible
impact with Mars is far from certain.
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