Asteroid brushing past earth

2004-09-29 13:30

Paris - On Wednesday, earth got its closest known shave this century from a major asteroid, a monster big enough to extinguish billions of lives were it ever to collide with our home.

But, in contrast to the warnings of a handful of doomsayers, scientists say the peril from this rock is beyond negligible.

In fact, they say this particular risk is zero and will remain so for several centuries, thanks to an increasingly successful effort to spot space bruisers and calculate their future orbits around the sun.

The asteroid in question is 4179 Toutatis, a behemoth some 4.6km long by 2.4km across.

Spinning like a dumbbell, Toutatis will be at its closest to earth at 13:37 GMT on Wednesday, when it will be 1 549 719 kilometres away, according to the Near-Earth Object (NEO) Programme run by Nasa's Jet Propulsion Laboratory (JPL).

That may seem comfortingly far. But in galactic terms it is narrower than a whisker: Just four times the gap between earth and the moon.

Panicky rumours

Six months ago, panicky rumours spread on the internet that there was little point to booking next year's summer holidays - that Nasa had got it wrong or lied, and we were all heading for The Big One.

Websites run by Christian zealots and individuals in contact with aliens predicted the Second Coming of Jesus or a secret US nuclear missile strike to neutralise the asteroid.

True, Toutatis is so big and will be so close "it should be visible in the night sky in the Southern hemisphere, if you point a pair of binoculars in the region of Alpha Centauri", says Benny Peiser of Liverpool John Moores University.

Were Toutatis to collide with earth, the energy released would be equivalent to tens of thousands of hydrogen bombs, kicking up dust clouds which would shield out the sunlight, plunging the planet and its inhabitants into a lethal "impact winter".

Earth's atmosphere protects us from NEOs up to a diameter of 40 metres, an impact energy of about three megatons.

Beyond that size, the news is bad. NEOs between 40 metres and one kilometre across can inflict local damage equivalent to thousands of nuclear bombs, as evidenced by the massive explosion in Tanguska, Siberia in 1908.

The NEO that whacked into Mexico 65 million years ago, ending the long reign of the dinosaurs, is estimated to have been between five and 15 kilometres across, packing a 30-to 100-million-megatonne punch.

There's good news

The good news, though, is that big advances are being made to spot the biggest threats.

Spaceguard Survey, a US programme, is already two-thirds towards its goal of identifying by the end of 2008, 90% of the estimated 1 000-1 100 asteroid NEOs that are more than one kilometre across.

That does not include, however, comets that take centuries to loop around the sun and whose paths thus remain uncharted. Yet only a tiny number of these are likely to be any potential threat.

"Any NEO that is going to hit the earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard," says Nasa expert David Morrison.

"In almost all cases, we will either have a long lead time," he points out, "or none at all".

Asteroids are commonly thought of as chunks of rock left over from the formation of the solar system 4.5 billion years ago. The vast majority safely trundle around the sun in the asteroid belt between Mars and Jupiter.

But the gravitational tug of giant planets such as Jupiter, as well as collisions with other asteroids, can nudge rocks out of the belt. Those whose orbits may regularly bring them close to earth are classified as NEOs.

Discovered in 1989, Toutatis is probably one of the most studied asteroids of all because its most recent circuits have brought it so close to earth.

Toutatis owes its name to a trio of French astronomers, who baptised it after a Celtic god well-known in France for the comic book hero Asterix.

Protected by Toutatis, Asterix and his friends fear nothing except the idea of the sky falling on their heads. On Wednesday, too, they should have nothing to worry about.

- AFP