This week–

YOU’D think that by now we’d have a firm grip on the trajectory of the solar system’s most worrisome chunk of rock. In fact we have only a hazy understanding of how likely the asteroid Apophis is to strike Earth. What’s more, budget cuts may shut down the telescope that could clarify the situation.

Since Apophis was discovered in 2004, asteroid-watchers have known that it has a slim chance of hitting Earth in 2036. At 270 metres wide, it is too small to rival the object that wiped out the dinosaurs, but it could cause devastating tsunamis were it to hit the ocean. Worrying as this is, we have been able to take comfort in the computed probability of impact, which is just 1 in 45,000.

Now it seems the true risk is unclear, thanks to minute effects that the calculations didn’t take into account. “You really can’t estimate the probability because it’s driven by these unknown physical parameters,” says Jon Giorgini of the Near-Earth Object Program at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Whether Apophis ends up on a collision course with Earth depends on what happens during a close encounter between Apophis and our planet in 2029. During this swingby, the Earth’s gravity could alter the asteroid’s path

enough to put it on a collision course with us when its orbit brings it back in 2036.

One problem, says Giorgini, is that our calculations do not include effects arising from the fact that Earth is not a perfect sphere. This slightly alters its gravitational field and could make a difference to the asteroid’s path when it swings close to Earth.

Yet the most powerful steer could come from the way the sunlit asteroid radiates heat, says Giorgini. Radiation gives rise to a small thrust, and since warmer areas of the asteroid radiate more than cooler ones, there is a net force on the asteroid. This phenomenon – the Yarkovsky effect – means our calculations of Apophis’s path could be out by millions of kilometres, according to Giorgini, who will present his results at the Asteroid, Comets, and Meteors conference in Baltimore, Maryland, on 17 July.

Unfortunately, we can include this effect only if we know how Apophis spins, which has so far been impossible to measure. There will be a chance to do so with the 305-metre Arecibo radio telescope in Puerto Rico when Apophis comes within a few million kilometres of Earth in 2013. The observations made then may evenlengthen the odds of an impact and show that the effects highlighted by Giorgini won’t matter.

Yet the US National Science Foundation is considering cancelling the funding for Arecibo. Ironically, shutting down the telescope might require sending an expensive space mission to orbit the asteroid so it can be tracked. The US House of Representatives has directed NASA to plan such a mission, in a bill now awaiting Senate approval.

Alan Harris of the Space Science Institute in La Canada, California, says watching Apophis will revealmuch about the limitations of our calculations of asteroid motion. He adds that the vast majority of asteroids that could present a risk to Earth have not been seen, and that one of these is much more likely to strike us before 2036. ●

Asteroid threat gets harder to pin down

SOUNDBITES

‹ As it is expressed in the G8 statement, the long-term goal is an empty slogan.›Marthinus van Schalkwyk, South Africa’s environment minister, on the G8 leaders’ pledge to cut carbon dioxide emissions by at least 50 per cent by 2050. Like many critics, he points out that the statement fails to mention whether the cut will be from current emissions or some other figure (Associated Press, 8 July)

‹ They are very susceptible to stress, so if they do have a favourite side to be fed on, it could reduce risk to them.›Are octopuses octidextrous? Claire Littleat the Sea Life Centre in Weymouth, UK, is participating in a Europe-wide study to find out if the creatures favour one or more tentacles (The Daily Telegraph,London, 7 July)

‹ I have decided that while such a cull might work, it might also not work.›UK environment secretary Hillary Benn makes his reasoning clear after announcing the government won’t issue licences to farmers for culling badgers to prevent the spread of bovine tuberculosis (BBC News, 7 July)

‹ Customers are ringing to ask: ‘Do you have any sand this year?’ ›Beach life has been rather diminished at San Cantaldo in southern Italy, says lifeguard Mauro della Valle, because coastal currents are carrying away sand. Local officials are locked in dispute with a nearby beach town, which is refusing to lend any of its own sand (The Daily

Telegraph, London, 8 July)

‹ Use it or lose it.›That’s the message for older men hoping to avoid erectile dysfunction, from a study by Juha Koskimaki and colleagues at the University of Tampere, Finland. They studied 989 men aged 55 to 75 and found those who had regular sex at the start of the five-year study were less likely to develop the condition (Reuters, 7 July)

DAVID SHIGA

12 | NewScientist | 12 July 2008 www.newscientist.com

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“Whether Apophis ends up on a collision course with Earth depends on a close encounter between them in 2029”

–Not a nice way to make a splash–