Search not destroy... the public is being asked to help find asteroids that could collide with Earth. Photograph: Dieter Spannknebel/GettyThe White House and Nasa will ask the public for help finding asteroids that potentially could slam into the Earth with catastrophic consequences.
Citing planetary defence, the government has decided that the search for killer rocks in space should be the latest in a series of "Grand Challenges", in which the US government sets an ambitious goal, helps create public-private partnerships and sometimes offers prize money for innovative ideas.
"This is really a call to action to find all asteroid threats to human populations and know what to do about them," Nasa Deputy Administrator Lori Garver said last week. She said the asteroid hunt would help prove that "we're smarter than the dinosaurs".
There is a second, overlapping agenda at work: the Nasa human spaceflight programme needs to find a target rock for what is now being called the Asteroid Redirect Mission (formerly the Asteroid Retrieval Mission), or ARM.
The proposed mission, which is early in the planning stages, would send astronauts to visit an asteroid that had been redirected into a high lunar orbit. But first a robotic spacecraft would have to rendezvous with the asteroid and capture it. And even before that, scientists would have to find the right asteroid.
The target rock has to be moving at a leisurely pace relative to the Earth, and ideally would come close to the Earth-moon system sometime in the early 2020s. Nasa has a shortlist of possible targets, but all need further scrutiny to see if they have the size, shape, spin rate and composition that the asteroid mission would require.
Two recent feasibility studies used as their reference a rock discovered in 2009, but Nasa scientists aren't sure that it will meet the mission requirements. For one thing, it might turn out to be too small. They plan to study it this fall with the Spitzer Space Telescope.
Nasa scientists are eager to speed up the rate of discovery of small asteroids, and thus expand the pool of candidate rocks for the ARM mission.
The Earth coexists with a swarm of asteroids of varying sizes. Thanks to a number of asteroid searches in the past 15 years, some funded by Nasa, about 95% of the near-Earth objects (NEOs) larger than 1km in diameter have already been detected, and their trajectories calculated. None poses a significant threat of striking the Earth in the foreseeable future.
The science is clear: catastrophic impacts, such as the one implicated in the extinction of the dinosaurs about 65m years ago, are very rare, and no one needs to panic about killer rocks.
But as one goes down the size scale, these objects become more numerous and harder to detect. In 2005 congress charged Nasa with finding all the asteroids greater than 140 metres in diameter. Asteroids that size are generally regarded as large enough to take out a city.
According to Nasa, there are also probably about 25,000 near-Earth asteroids that are 100 metres or larger. Only 25% of those have been detected, many through Nasa's Near Earth Object Programme. The administration is asking Congress to double the budget for asteroid detection, to 40m, Garver said.
The Grand Challenge would elicit help from academics, international partners and backyard astronomers. The search for NEOs took on greater urgency on 15 February, when, on the very day that a previously detected asteroid was about to make a close pass of the Earth, an unknown 15-metre-diameter rock came out of the glare of the sun and fireballed through the atmosphere above the Russian city of Chelyabinsk.
The asteroid's disintegration caused a shock wave that shattered windows and caused hundreds of injuries and major property damage. It was the first recorded instance of an asteroid causing human casualties. (In 1908 an asteroid exploded over Siberia and flattened trees in a vast, unpopulated area.)
"Even though these smaller asteroids don't pose a threat to human civilisation, they can still cause major damage and casualties on a regional level," said Tom Kalil, deputy director for technology and innovation at the White House Office of Science and Technology Policy. The administration's Grand Challenges include efforts to understand the human brain and cure brain disorders, make solar energy cost-competitive by the decade's end and make electric cars as affordable as petrol-powered vehicles.
