Some time in the next decade, a US president will probably be presented with this dilemma: is it worth spending US$1 billion to deflect a space rock that may never hit Earth?

A NASA panel is wrestling with this question, which is growing more pertinent as scientists' ability to find asteroids that pose a potential risk, termed near-Earth objects (NEOs), outstrips their capacity to track them accurately. The Ad-Hoc Task Force on Planetary Defense, set up to suggest ways for the agency to protect Earth against a deadly impact, is expected to release its report next month. But public deliberations and interviews with its members have revealed their thinking.

The dilemma stems from a 2005 congressional mandate directing NASA to log 90% of the estimated 20,000 NEOs larger than 140 metres in diameter by 2020. NASA seems unlikely to meet the goal, but the agency is stepping up its detection and tracking of smaller objects.

Missions to deflect all potential threats will be prohibitively expensive. ,

That will create a new problem: if the pace of NEO detections (see graph) grows but precision tracking of orbits lags behind, observers will start to find more rocks — perhaps a few per year — that seem, at first, to have a significant chance of hitting Earth, say panel members. "I don't think that issue has been understood outside the NEO community," says Lindley Johnson, NEO programme officer at NASA and a member of the panel. Launching missions to track or deflect all potential asteroid threats will be prohibitively expensive, but even a small probability of regional or global devastation may not be politically palatable.


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One solution from the panel is to increase the amount that the United States invests in NEO detection and tracking from the current $5.5 million a year. The panel may also recommend the launch of a survey telescope into a solar orbit similar to that of Venus. It would orbit faster than Earth and, looking outwards, would see asteroids in Earth-crossing orbits more often than would ground-based instruments (see diagram). This could improve follow-up observations, narrow estimated trajectories and remove as many asteroids as possible from the threat list. It could also spot and track asteroids on the sunward side of Earth, removing a worrisome blind spot in ground-based surveys. "It is a wonderful rapid technique to track bodies down to 140 metres and smaller," says Tom Jones, a former astronaut and panel co-chair.

Ball Aerospace and Technologies Corporation, a manufacturer of spacecraft based in Boulder, Colorado, has proposed building such a remote scope at a cost of $600 million. But Irwin Shapiro, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who chaired the 2010 Committee to Review Near-Earth-Object Surveys and Hazard Mitigation Strategies for the US National Research Council, says that ground-based observatories such as the planned Large Synoptic Survey Telescope (LSST) on Cerro Pachón in Chile are better value for money than space telescopes, because they last longer and are less expensive. He says the LSST is also more likely to command funding, as it is the top priority recommended by the Astronomy and Astrophysics Decadal Survey, released in August by the National Academies. Putting a space telescope in a Venus-like orbit "would in effect start from scratch", he says.

Owing to a 2008 law passed by Congress, the White House Office of Science and Technology Policy has until 15 October to decide which agency will be responsible for protecting the planet from an asteroid strike. Members of the task force say NASA expects to be given part or all of that responsibility. To meet it, the panel discussed the creation of a Planetary Protection Coordination Office (PPCO) within NASA, with an annual budget of $250 million–$300 million. It would detect and track asteroids — and develop a capability to deflect them. "You want to use a proven capability when you're talking about an actual threat," says Rusty Schweickart, a former astronaut and the other panel co-chair.

The PPCO would also challenge other countries to fund defence against asteroids, perhaps through the United Nations. Canada already plans to launch the NEO Survey Satellite in 2011, and Germany's AsteroidFinder is slated for launch in 2012, but neither is expected to come close to the NEO-logging goal by 2020.

Shapiro stresses that it is unclear whether Congress will give further funds to planetary protection, noting that if it doesn't, there is a risk of the money being taken away from space science. Yet without better detection and tracking there will inevitably be uncertainty about asteroid positions in the future — and even greater expense if the uncertainty leads to unnecessary efforts to thwart an apparent pressing threat.

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