Devangshu Datta: A close encounter with Toutatis

TECHNO BEAT

The honour of naming a celestial object is usually offered to the discoverer. When French astronomer C Pollas found asteroid no: 4179 on January 4, 1989, he named it "Toutatis".

As Asterix fans know, Toutatis was a Gaulish god worshipped by ancient warriors whose only fear was that the sky might fall on their heads. The name may turn out to be more appropriate than we would wish: among all known asteroids, Toutatis has the highest chance of hitting the Earth.

By the time you read this, it will have passed within 1.5 million km of Earth "" only about four times as far out as the moon. Through late September, it's visible (on telescopes) as a fast-moving object in the southern hemisphere; northern observers will pick it up in early October.

This is the closest a large celestial object is slated to come to Earth during the 21st century. Toutatis' wobbly orbit may bring it even closer in 2562. An impact would have terrible consequences "" it may set off mass extinctions across the globe.

Even a close brush could be catastrophic. Toutatis is large enough to strip off the atmosphere. The rock measures about 4.6 by 2.5 km. It is weirdly shaped and looks like a dumbbell with two unevenly-sized "bells" joined by a "neck".

That shape causes a complex rotation cycle that leads to a wobbly four-year orbit around the Sun. Instead of fixed poles, Toutatis' axis of rotation wanders through two separate cycles of 128 hours and 171 hours.

Despite observation since 1992, it's been impossible to predict its position to better than "plus-minus 10 km" at a given instant. Over centuries, those errors may add up.

Asteroids are large, often irregularly-shaped rocks, scattered across a belt between the orbits of Mars and Jupiter. Some theories suggest that a single planet broke up millions of years ago leaving these rocks as debris.

Some asteroids are large while others are tiny. Many have eccentric orbits, crossing the Earth's. Toutatis tops the "potentially hazardous asteroid" list due to size and proximity.

Asteroid and meteor impacts are blamed for the extinction of the dinosaurs. Many scientists also believe that the moon may have been created by an asteroid hit that knocked a chunk of matter off the Earth.

The close encounter will help us study Toutatis and learn more about asteroid formation. Radar observations will refine the spin rate and orbit and tell us about the features of this object. Ideally, a spacecraft rendezvous could be planned.

Toutatis has been a test case for radar observation. Scientists have learnt a lot by bouncing waves off it since 1992. Radar-imaging uses Doppler signature (offering "sideways" views) as well as measuring the time-delay of echoes to generate precise values of range, speed, shape and so on.

One key factor is the knowledge and control over the radar-signal. Radar gives an idea of the density as well as surface features. Toutatis seems equally dense all through, which is unusual, and it has a pitted, beaten-up surface, which is normal.

Prior to radar astronomy, asteroid orbit predictions were often wrong by 100,000 km or more. The two best radar facilities are the Arecibo Observatory in Puerto Rico and Nasa's Goldstone Radar System in California.

Arecibo has twice the range and "sees" three times as much sky as Goldstone. But Goldstone has greater steerability. An upgrade of Arecibo will soon give it access to main-belt asteroids.

The rationale for the searches is compelling. Every Earth-crossing asteroid of, say, 100 metres plus, needs to be mapped "" and there are estimated to be more than 100,000 rocks of this size.

If we discover one of these objects is due for collision or even near-miss, measures must be taken to prevent disaster (an appropriate term "" "disaster" means "bad star").

The best options for averting collision involve intercepting and blowing up or diverting the object by hitting it with fusion bombs. That provides perhaps the best pacifist rationale for maintaining ballistic missiles and nuclear bombs.