By Jonathan Fildes
Science and technology reporter, BBC News, San Francisco
A draft UN treaty to determine what would have to be done if a giant asteroid was on a collision course with Earth is to be drawn up this year.
Nasa is tracking many asteroids
The document would set out global policies including who should be in charge of plans to deflect any object.
It is the brainchild of the Association of Space Explorers, a professional body for astronauts and cosmonauts.
At the moment, Nasa is monitoring 127 near-Earth objects (NEO) that have a possibility of hitting the Earth.
The association has asked a group of scientists, lawyers, diplomats and insurance experts to draw up the recommendations.
The group will have its first meeting in Strasbourg in May this year. It is hoped the final document will be presented to the UN in 2009.
"We believe there needs to be a decision process spelled out and adopted by the United Nations," said Dr Russell Schweickart, one of the Apollo 9 astronauts and founder of the Association of Space Explorers.
The threat of an asteroid hitting the Earth is being taken more and more seriously as more and more NEOs are found.
In the US, Congress has charged Nasa with the task of starting a more detailed search for life-threatening space rocks.
"Congress has said that Nasa's efforts to date are not sufficient to the threat," said the US space agency's Dr Steven Chesley.
"They have changed Nasa's targets so that the cataloguing and tracking of asteroids is part of its mandate."
Congress has asked the agency to mount a much more aggressive survey.
At the moment, Nasa tracks all objects greater than 700m (2,300ft) in diameter. The agency's new goal is to track all objects greater than 70m (230ft) in diameter.
To do this, the agency needs to use a new suite of telescopes.
Alternatives include building a new Nasa-owned system or investing in other proposed telescopes such as the Large Synoptic Survey Telescope (LSST) or the Panoramic Survey Telescope & Rapid Response System (Pan-Starrs).
Pan-Starrs is a wide-field telescope being developed at the University of Hawaii, whilst the LSST is a proposed ground-based instrument being developed by the not-for-profit LSST corporation based in the US.
Nasa estimates that there are about 20,000 potentially threatening asteroids yet to be discovered.
"Out of those thousands, there will be without question many that look like they might hit the Earth with a high enough probability that the public and everyone else will be concerned," said Dr Schweickart.
"This has gone from being an esoteric statistical argument to talking about real events," added Dr David Morrison, an astronomer at the Nasa's Ames Research Center.
The UN draft treaty would establish who should be in charge in the event of an asteroid heading towards Earth, who would pay for relief efforts and the policies that should be adopted.
In addition, it would set out possible plans to deflect the object.
Ideas could include hitting the asteroid with a spacecraft or rocket to deflect its orbit.
Other less destructive proposals include a "gravity tug" that would simply hover over the asteroid and use gravity as a "towline" to change its path.
But any decision to deflect an NEO could come with its own set of conundrums for the UN, as changing its path may simply alter its final target.
"It's important to understand when you start to deflect an asteroid that certain countries are going to have accept an increase in risk to their populations in order to take the risk to zero for everyone," said Dr Schweickart.
It is difficult decisions like this that can only be addressed by the UN, the Association of Space Explorers believes.
And it is under no illusion that the process can be sorted out quickly.
"You have to act when things look like they are going to happen - if you wait until you know for certain, it's too late," said Dr Schweickart.
Experts who will draw up the treaty include Lord Rees, the English Astronomer Royal and head of the Royal Society; the ex-director of science at the European Space Agency, Roger Bonnet; and former UK government advisor Sir Crispin Tickell.
The proposals were outlined at the American Association for the Advancement of Science (AAAS) annual meeting in San Francisco, US.