New asteroid top of Earth's hit list
A space rock capable of sub-continent scale devastation has about a one in 1000 risk of colliding with Earth early next century, the highest of any known asteroid, watchers say.
The rock, 2004 VD17, is about 500 metres long and has a mass of nearly a billion tonnes.
If it were to hit Earth, it would deliver 10,000 megatonnes of energy, equivalent to all the world's nuclear weapons.
VD17 was spotted in late November 2004 and was swiftly identified as a rock that potentially crosses Earth's orbit. At the time watchers gave it one in 3000 risk of collision on 4 May 2102.
Further observations and calculations have prompted the risk on that day to be upgraded to "a bit less than 1 in 1000", says NASA near-Earth object (NEO) expert Dr David Morrison.
"The risk of an impact within the next century [is] higher than that of any other known asteroid," he says, stressing however that the likelihood of a hit is small.
"Fortunately, it is nearly a century before the close pass from VD17. This should provide ample time to refine the orbit and, most probably, determine that the asteroid will miss the Earth."
VD17 was previously categorised as a grade green, which means merits careful watching, on the Torino scale of NEO hazards. But it has been upgraded to grade yellow, meaning meriting attention.
There are two more grades beyond this, orange or close encounter, and red or collision is certain, involving objects capable of inflicting regional or global devastation.
The asteroid's closest proximity to Earth on the 2102 flyby was not given by Morrison or the hazard list maintained by NASA and the US Jet Propulsion Laboratory.
VD17's place at the top of the list was briefly snatched in December 2004 by a rock called 99942 Apophis.
Further observations, however, downgraded Apophis' risk to a one in 5000 chance of collision, making it a grade green risk.
Apophis, measuring 300 metres across and with a mass of less than 100 million tonnes, will fly by at a distance of 36,350 kilometres from the Earth's surface on 13 April 2029.
This is slightly higher than the altitude of geosynchronous satellites, according to the website of the International Astronomical Union's Minor Planet Center.