Possible
Mars impact highlights risk to Earth
NewScientist.com
news service
David Shiga
An
asteroid hurtling towards Mars has a 1 in 28 chance of walloping the Red Planet
on 30 January, according to the latest calculations.
The
rock's discovery just a couple of months before a possible impact begs the question
of what would happen if it were instead headed for Earth. With so little warning,
the only option would be to evacuate any inhabited areas it might hit, astronomers
say.
The
asteroid, called 2007 WD5, was discovered on 20 November by a 1.5 metre telescope
near Tucson, Arizona, US, that combs the skies as part of NASA's efforts to detect
asteroids with a chance of hitting Earth.
It
is an estimated 50 metres across, putting it in the same class as the Tunguska
object that exploded over Siberia in 1908, flattening trees in an area extending
many kilometres from the explosion.
The
case of 2007 WD5 shows that Earth is vulnerable to small objects that can evade
detection until it is too late to deflect them, says Don Yeomans, head of NASA's
Near Earth Object Program at the Jet Propulsion Laboratory in Pasadena, California,
US.
The
US congress has asked NASA to find 90% of asteroids down to 140 metres across,
but trying to take this down to 50 metres might prove too costly to be funded,
he says.
No
time
Yeomans
estimates that it would take months to years to build a spacecraft that could
deflect an asteroid and prevent it from hitting Earth. Scientists are still debating
the best method for deflecting asteroids, considering everything from firing lasers
at them to nudging them off course with a "gravity tractor".
But
if an object in a scenario similar to that of 2007 WD5 was discovered just a few
weeks ahead of a potential collision with Earth, there would not be enough time
to mount a deflection mission.
"Then
you just worry about evacuation," Yeomans told New Scientist. "You'd
be able to pinpoint along what sort of ground track the object could hit and then
you'd try to evacuate populated regions within that ground track."
Temporary
increase
Early
calculations gave the asteroid a 1 in 75 chance of striking Mars on 30 January
2008.
Then,
additional observations of the asteroid on 8 November were found in archival images
from the 2.5-metre telescope at the Apache Point Observatory near Cloudcroft,
New Mexico, US. As a result, NASA's Near Earth Object Program reported on 28 December
that the impact probability had increased to 3.9%, or about 1 in 25.
But
new observations taken between 29 December and 2 January using a 2.4-metre telescope
at the Magdalena Ridge Observatory in New Mexico have revised the probability
again, slightly lowering it to 3.6%, or about 1 in 28.
Difficult
target
It
is not unusual for the probability of an impact to temporarily increase with additional
observations, only to decrease again when even more measurements are made. The
probability of an Earth impact in 2036 by a 250-metre-wide asteroid called Apophis
increased from about 1 in 5000 to 1 in 37 in 2004, before dropping to 1 in 45,000
with the latest observations.
Usually,
with more observations, the chance of an impact disappears – which is the
likely scenario for 2007 WD5, according to Yeomans.
The
asteroid is growing dimmer as it recedes from Earth, making it harder to observe.
But scientists are still hoping to obtain new measurements over the weekend and
into the coming week.
In
addition to the Magdalena Ridge telescope, scientists are looking into using a
2.2-metre telescope on Mauna Kea, Hawaii, US, belonging to the University of Hawaii
and the 6.5-metre MMT Observatory on Mount Hopkins near Tucson, Arizona, US.
