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10 October 2007

Serious Or Joking?

The James Bond film, Die Another Day, had a fictitious North Korean terrorist scheming to set the Earth ablaze with harnessed solar energy. Now, some scientists want to use the idea to burn out asteroids rushing headlong towards us. Surajit Dasgupta differentiates science from fiction

Last week, a large section of the popular Indian media was abuzz with the possibility of an asteroid, Apophis, hitting the Earth after William Ailor, director of the Centre for Orbital and Re-entry Studies, Aerospace Corporation, predicted that the collision could occur in 2036 if not in 2029.

Elsewhere, Boris Shustov, director of the Institute of Astronomy, Russia, said on October 1 that the impact of the asteroid, of size equal to three football fields, would cause far more devastation than what the asteroid that hit Siberia in 1908 did. The Tunguska astral event affected 2150 sq km and blasted eight crore odd trees. The force of the impact was about 1,000 times more powerful than that by the atomic bomb the US dropped on Hiroshima, Japan, at the end of World War II and measured 5.0 on the Richter scale.

The scare scenario is significant as Apophis's latest predicted track would bring it about 28,730 km close to Earth in 2029.

What is curious, however, is why the media suddenly found the news so interesting when such predictions are made on a regular basis by different space observatories spread all over the world. Several years ago, NASA scientists predicted that a large asteroid 99942 (Apophis) would smash into Earth in 2029. After more research with readings from Cornell-run Arecibo Observatory in Puerto Rico -- they had claimed their calculation was "extremely precise"! -- people were assured that Apophis would miss Earth by 169,000 km. Now, is the renewed interest owing to our reduced distance (and hence greater chance) from a catastrophe?

Even if so, the Indian media is a full one month late. From Times to The Economist, most reputed papers in the West had reported the prediction -- and soon rubbished it too -- in September. The history of human discovery of asteroids is much older. And some or the other scientist has been scaring people of impending collapses of these massive rocks on us almost ever since.

On January 1, 1801, an object, initially thought to be a comet, was sighted by Giuseppe Piazzi. But after ascertaining its orbit better, it appeared more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno). By the end of the 19th century, there were several hundred.

Several hundred thousand asteroids have been discovered and given provisional designations so far. Their diameters range from 1 km to 200 km. Will any -- even the smallest of them are big enough to destroy a city -- fall on us?

Well, the chances of an asteroid colliding with Earth are very small. Hermes did come as close as 777,000 km. But the closest distance of an asteroid's trajectory with the Earth is being perpetually reduced by the observatories almost every alternate year.

And make no mistake about it, if an asteroid has the potential to hit us, the Hollywood-style solution, as was shown in the film Armageddon -- of deep core drillers approaching it from behind, landing on it, drilling to 800 feet, and planting a nuclear bomb in the shaft and detonating it remotely after evacuation -- is likely to be more harmful than helpful.

Instead, they thought in 2004, a micro-satellite could nudge it into a safer orbit. The latest in the series of solutions, which sounds like make-believe, is to thwart the approaching asteroid using, hold your breath, flying mirrors!

A chain of mirrors harnessing the power of the Sun might 'burn' out the huge rocks hurtling towards us. While one may be inclined to dismiss this 'solution' as an inspiration from the James Bond flick Die Another Day, some researchers from the University of Glasgow are dead serious about the project.

About 5,000 mirrors placed at vantage points with respect to each other to augment the reflected solar power could be used to focus sunlight on to the asteroid, melting the rock and altering its orbital path away from the Earth.

The ingenious scheme was devised after a team at the university compared nine methods of deflecting near-Earth objects. The results were unveiled at the Jodrell Bank observatory in Cheshire while observing the 50th anniversary of the launch of the Soviet satellite Sputnik I.

The research team compared the mirror technique with eight other methods, including nuclear explosion and -- believe it or not -- fixing a propulsion system to the asteroid! The propeller solution obviously had to be rejected, but the spare time or inclination of some scientists to joke cannot be denied to them. As such, scientific rigour takes much fun out of life. And the nuclear impact theory wouldn't hold, as it would be dangerous to turn the near-Earth space radioactive.

A mass-driver system where material is excavated and catapulted away from the asteroid, would also take too much time. A kinetic impactor which would knock the asteroid out of its orbit, was thought to require the launch of too big a space craft as was the option of using a large craft's own gravitational pull to drag the asteroid away from the earth.

The orbiting mirrors would be used to focus sunlight on an area of the asteroid between 0.5 and 1.5 metres wide, heating the rock to around 2,100 °C, hot enough to melt the surface of the asteroid and create a thrust which would nudge it off course.

The orbit of an asteroid of diameter 150 m could be sufficiently modified by a network of 100 mirrors in a few days. For an asteroid the size of the one believed to have wiped out Earth's dinosaurs, a 5,000-strong fleet of mirror-craft would need to focus a beam of sunlight on the surface for three or more years.

This could well be true. With only ten spacecraft flying in formation, each with a 20 m mirror, a asteroid, the size of which exploded at Tunguska, could be deflected to a safe orbit in about six months. This technology is genuinely feasible and there is no risk from fragments, unlike in the case of impactors. For a 10-satellite formation, a launch mass for each individual spacecraft will be around 500kg. This is a smaller and lighter satellite constellation than, say, the Galileo positioning system, so is well within our launch capabilities.

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Surajit Dasgupta treats no individual, organisation or institution as a holy cow.