Possible Mars impact highlights risk to Earth
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.
http://space.newscientist.com/article/dn13141-possible-mars-impact-highlights-risk-to-earth.html
