Published online 9 March 2007 | Nature | doi:10.1038/news070305-15


Astronomers hash out defense against asteroids

A billion dollars needed to spot potential killer impacts.

Crash landing: how do we stop this from happening?Crash landing: how do we stop this from happening?Don Davis/NASA

Astronomers trying to save the world from Earth-threatening asteroids have this week composed a white paper outlining the threat and what needs to be done about it.

Although it isn't their first white paper on the subject — that was released in 2004 — it is the first mandated by Congress. This, scientists hope, may mean that their conclusions will be taken more seriously by decision-makers in Washington.

In 2005, Congress passed a bill authorizing NASA to search for asteroids as small as 140 metres that could possibly strike the Earth. The bill, however, provided no money for the search. Scientists at the Planetary Defense Conference, held 5-8 March in Washington DC to compose the new white paper, were quick to point this out.

On Monday, Simon 'Pete' Worden, director of NASA's Ames Research Center, said that the cost of finding at least 90% of the 20,000 estimated potential Earth-killers by 2020 would cost about $1 billion. US government employees, including NASA scientists, don't usually make public requests for more cash, but Worden was clear: "We know what to do, we just don't have the money."

The impact of a moderately large asteroid, say 10 kilometres across, would have catastrophic consequences: tsunamis, earthquakes, blast waves, climate change and perhaps more than a billion deaths, the conference heard. The likelihood of an asteroid making a direct hit any time soon is rather unlikely, noted Clark Chapman of the Southwest Research Institute in San Antonio, Texas: a 15-kilometre-wide asteroid is thought to strike the Earth once every 25 million years.

But even a small asteroid, on the order of tens of metres across, exploding several kilometres above Earth's surface would have an impact similar to that of Tunguska - where an asteroid burst levelled about 2,000 square kilometres of forest in 1908. Such an event is thought to happen once every 100-250 years.

Blown away

If Worden and his colleagues had their way, they would catalogue and track all objects of 140 metres or larger. This would require a dedicated space-based telescope and more time on the two ground-based radar facilities currently used to spot incoming asteroids, scientists suggested at the meeting. This would hopefully provide enough time to deflect a big, bad rock.

Scientist at the conference were keen to focus on how to do this. One idea would be to use a robotic spacecraft as a 'tugboat' that would physically attach itself to the asteroid and push it into a new orbit. The problem is that the surface of an asteroid would probably be rough and unconsolidated. An alternative would be to have a spacecraft hover above the asteroid surface, using gravity as a towline (see 'Gravity tractors beat bombs').

Other proposals call for nuclear detonations to push the asteroid off target, anchoring a tether onto a rock so that a spacecraft can sling it athwart, or attaching solar sails to blow it off course.

Spread the warning

In the meantime, astronomers acknowledge that they still have a lot to learn about the nature of asteroids in order to better assess our vulnerability. Only this week researchers confirmed one aspect of asteroid mechanics that they had predicted but never seen: the motley appearance of an asteroid means that sunlight reflects more strongly off some parts than others, which can make its spin speed up and possibly alter its trajectory. In Nature and Science this week, astronomers describe seeing this in action for the first time1,2.


Astronomers hope that the white paper, which should be completed and released soon, will be read by people who haven't previously thought about the possibility of Earth-threatening asteroids. William Ailor of the Aerospace Corporation and general chair of the conference, says it will be posted on the conference web page when finalised. But he hopes it won't stop there.

"I know Congress is getting interested in this and I think that's very encouraging," Ailor says.

Visit our hashoutdefensea.html">newsblog to read and post comments about this story.  

  • References

    1. Kaasalainen M., et al. Nature, advance online publication (2007) doi:10.1038/nature05614.
    2. Taylor P., et al. Science, express (2007) doi:10.1126/science.1139040.