Efforts are under way to agree a UN-based treaty on the threat posed by asteroids orbiting near the earth. The goal is to reach international agreement on how humanity would respond should an asteroid or comet threaten life on the planet.
A series of four international meetings are planned with the first taking place in May in Strasbourg, according to former astronaut Russell Schweickart of the Association of Space Explorers, the host organisation for the meetings.
The very real danger of an asteroid collision was brought home just over two years ago with the discovery of asteroid Apophis in December 2004. For a time scientists believed there was a genuine possibility that it might strike the earth, but this risk has diminished, with just a one in 45,000 chance that it will hit earth in 2036.
Locating any threatening "near earth objects" (NEOs) and deciding what to do about them provided an intriguing session at the American Association for the Advancement of Science meeting in San Francisco.
"Yes there really is a danger from impacts by asteroids," said David Morrison of Nasa's astrobiology institute. "This grew from being just an esoteric statistical argument to a real possibility."
Years of searching the skies for NEOs of more than one kilometre in diameter, an impactor that could destroy a large fraction of life on earth, has identified about 90 per cent of the large bodies that pose a threat, Dr Steven Chesley of the Jet Propulsion Laboratory in Pasadena told the meeting.
The new goal is to refine the search to discover 90 per cent of all NEOs larger than 140 metres in diameter by 2020.
Mr Morrison expects the number of bodies discovered by this survey to increase 50 to 100-fold. "It doesn't represent an increased danger but an increased awareness of the danger," he said.
It will however, force us to make decisions about how to respond should a NEO on an impact trajectory be discovered, stated Dr Edward Lu of Nasa's Johnson Space Centre in Houston. "Apophis forced us to really think about what we would do with an asteroid threat."
He believes world consensus should follow the "cosmic do no harm principle", which argues against using impacting spacecraft or explosive devices in an attempt to deflect an asteroid. "You can end up with a situation where you are worse off," he said. "The danger of hitting it with something is the risk of breaking off a chunk," with a large asteroid forming several smaller but still deadly impactors.
Yet we are not powerless to protect ourselves, Dr Lu argues. "It is possible to save the earth from something like an Apophis." He suggests the use of a "gravitational tractor", a spacecraft weighing about a tonne and sent out to position itself adjacent to the approaching asteroid. The craft would use engines to keep it away from the asteroid while exerting a gravitational pull that would gradually tow the impactor off its earth-bound trajectory.
It would be a slow process taking many years, but eventually the asteroid could be moved onto a path that would never cross that of the earth. "It would work on about half of the asteroids known including Apophis," Dr Lu said.
Meanwhile, Mr Schweickart is hopeful the UN will sponsor an international treaty on NEOs.
