Deep Impact

(Spotted by the Friendly Stranger)

New asteroid at top of Earth-threat list

* 11:52 01 March 2006
* NewScientist.com news service
* Kimm Groshong

Observations by astronomers tracking near-Earth asteroids have raised
a new object to the top of the Earth-threat list.

The asteroid could strike the Earth in 2102. However, Don Yeomans,
manager of NASA's Near Earth Object Program at the Jet Propulsion
Laboratory in Pasadena, California, US, told New Scientist: "The most
likely situation, by far, is that additional observations will bring
it back down to a zero."

He adds: "We're more likely to be hit between now and then by an
object that we don't know about."

On 23 February, new observations allowed researchers to more
accurately calculate the orbit of the asteroid, named 2004 VD17, which
was originally detected by the Massachusetts Institute of Technology's
LINEAR project. Since the improvement did not rule out a potential
collision with the Earth on 4 May 2102, they increased the asteroid's
rating to level 2 on the Torino Scale, a relatively rare event.

Degrees of danger

The Torino Scale, adopted in 1999, is akin to a Richter scale for
asteroid impacts. The vast majority of the 4000 or so near-Earth
objects (NEOs) detected so far have been assigned to level zero on the
Torino scale, meaning they have "no likely consequences".

Level 1, colour-coded green, suggests a possible impact that "merits
careful monitoring". Beyond that, the risk continues to rise along the
scale – levels 2, 3 and 4 are yellow; 5, 6 and 7 are orange; while 8,
9 and 10 earn red.

The highest level ever reached by an asteroid was level 4 by Apophis
(2004 MN4) in December 2004, but subsequent calculations downgraded
that concern to a level 1. So VD17 currently claims the top spot on
NASA's online list of potential asteroid impacts.

Despite the rarity of the yellow designation, Yeomans says "Torino 2
is not very alarming." He notes that the scale does not take account
of how soon an impact may occur, unlike its rival, the Palermo Scale.

Based on current observations, he says the asteroid has a 1 in 1600
chance of striking the Earth in 2102 and a 1 in 500,000 chance of
hitting two years later. But further observations will soon refine the
orbit calculation for VD17 – and hopefully ease minds.

NEO hunter, Andrea Milani Comparetti of the University of Pisa, Italy,
says VD17 "is a serious problem, but not for our generation". He also
notes that VD17 is dim and distant and is not projected to pass close
by the Earth before 2102. "You will need fairly powerful telescopes to
see it before it arrives," he told New Scientist.
Smaller threats

Since 1998, NASA has had a US Congressional mandate to locate 90% of
all NEOs of 1 kilometre or larger by 2008. Yeomans says that 830 out
of a predicted 1100 have been found so far, along with thousands of
smaller objects.

In the NASA Authorization Act of 2005, Congress directed the space
agency to study and report back on the best way to cost-effectively
locate 90% of all asteroids down to a diameter of just 140 metres.
Yeomans says there are likely to be about 100,000 such NEOs.

Yeomans and Bill Bottke of the Southwest Research Institute in
Boulder, Colorado, were both members of a team that reported in 2003
that a survey to locate such small asteroids would be cost-effective,
considering the damage an impact could cause. Bottke says the group
found that to find 90% of the remaining hazard would cost roughly $300
million.

2004 VD17 is estimated to have a diameter of about 580 metres. An
asteroid of that size would produce an impact crater about 10
kilometres wide and an earthquake of magnitude 7.4 if it struck land.