Space

The chances of the asteroid Apophis hitting Earth in 2036 are lower than we thought. But those worried about deep impacts should add a new entry to their calendar: 2068.

When Apophis was first spotted in 2004, the 250-metre-wide rock was briefly estimated to have a 2.7 per cent chance of hitting Earth in 2029. Further observations quickly showed that it will miss Earth that year – but should it pass through a 600-metre-wide "keyhole" in space, it will return to hit Earth in 2036.

For the past several years, the probability of such a collision has been considered to be 1 in 45,000. But new calculations suggest the chance of an impact in 2036 is far lower – about 1 in 250,000.

The revised estimate is in part a result of additional observations of the asteroid as well as refinements on the position of Apophis in older data.

New positions

Astronomers use stars to gauge the position of objects in the solar system. But David Tholen of the University of Hawaii at Manoa and colleagues found the star catalogue that was used to estimate the position of Apophis had inaccurate star positions. The team used a newer catalogue to recalculate the asteroid's position in old images.

The revised data was used to simulate Apophis's orbit and showed that a 2036 impact was much less likely than had been thought. But the new projections also revealed that the asteroid has a 1-in-400,000 chance of smashing into Earth in 2068. The findings are to be presented on Thursday at a meeting of the American Astronomical Society's Division for Planetary Sciences in Fajardo, Puerto Rico.

Despite the demotion in threat, Apophis keeps its ranking as the object that is the fourth most likely to collide with Earth – out of those we know about. However, we are less certain about the orbits of the top three than we are about that of Apophis, says Donald Yeomans, manager of the near-Earth object programme office at NASA's Jet Propulsion Laboratory in Pasadena, California.

"It's still a poster child for near-Earth objects, because it comes so close to Earth. It's going to be a naked-eye object when it flies past in 2029," Yeomans told New Scientist.

On radar

Apophis will come fairly close in 2013, which will allow astronomers to use radar to get better estimates of how quickly the asteroid is moving along Earth's line of sight. "Then we'll have the orbit nailed, and I'll guess the very slight 2036 impact probability could go away altogether," Yeomans says.

The radar observations could also help pin down how fast the asteroid is spinning, which influences how re-radiated heat from the sun can steer it. This effect is now the biggest source of uncertainty in Apophis's orbit.

Improving the errors in the star catalogue will also help improve estimates of the orbits of other objects as well, Yeomans says. "That's important not only for Apophis but also for the other half-million comets and asteroids that have been discovered to date."

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.