Cosmic Collisions

Meteorites and comets are more dangerous 
than the Hiroshima bomb, reports Biplab Das 

About 50,000 years ago, a meteorite weighing 20 million tones crashed into Earth's surface, carving out a crater. It is the third largest impact crater on the planet, and gave rise to the Lonar lake in Maharashtra. The energy released by the impact was nearly 500 times that of the Hiroshima bomb. "That is why it is necessary to keep close observation on comets and meteorites, so that possible collisions can be predicted well in advance," said Prof. Jayant Vishnu Narlikar, in his lecture on 'Challenges and Benefits of Studying Astronomy', at the Indian Science Congress Association (ISCA) on March 21. 

On the occasion, Prof. Narlikar, founder director and Homi Bhabha Professor of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), was presented the Jawaharlal Nehru Birth Centenary Award, 2001-2002. The award is given by ISCA in fundamental sciences. 

Prof. Narlikar described the progress of astronomy from the beginning of civilisation to the latest advances. "Long before the invention of the telescope, with the naked eye, ancient people gazed at the night sky dotted with stars and from their observations, they developed a calendar system of their own. That was the humble beginning of astronomy," said he. 

The calendar was used to anticipate the changes of seasons and flooding of rivers. It also helped sea voyagers in navigation. 

All ancient civilisation had a geo-centric concept of the Universe. They also thought that the planets moved at random without any external influence. Aristotle and other Greek philosophers began an extensive and systematic study of celestial bodies. Nicolaus Copernicus overturned their ideas with a new model which put the sun at the center of solar system. But his theory that planets move around the sun in circular orbits was wrong. Tycho Brahe corrected that mistake by proposing elliptical orbits. 

Unfortunately, Brahe lacked the proper mathematical tool to prove his point. It was accomplished by his assistant, Johannes Kepler. "With his profound knowledge of mathematics Kepler gave three empirical laws of planetary motions, which proved the prediction of elliptical orbits," said Prof. Narlikar. But he did not attempt to explain what caused the movement of planets. "This missing piece of the puzzle was provided by Sir Isaac Newton, who uncovered the secret of planetary movements by discovering the laws of gravitation," he said. 

Not only was the geo-centric Universe banished for good, gravity turned out to be invaluable for physics too. As the murky picture of the Universe became clearer, scientists tried to discover the source of the sun's vast energy. "In the late 19th century, William Kelvin and Baron von Helmholtz theorised that it was gravitational collapse which kept the sun hot," said Prof. Narlikar. "Going by this hypothesis, the estimated age of sun and earth was 24 million years." 

But by the turn of the 20th century, it had been proved wrong. British astrophysicist Arthur Eddington not only unravelled the mystery of the sun's energy he revised its age too. "According to Eddington, deep inside the sun, high temperatures triggered a unique reaction called nuclear fusion which converted hydrogen into helium, generating the sun's huge energy," said Prof. Narlikar. 

Eddington was derided by atomic physicists who ruled out the possibility of such high temperatures inside the sun. Unyielding, he advised them in his book Internal Constitution of Stars, to "go and seek out a hotter place." The revised age of the sun according to Eddington is more than 4.5 billion years. 

Besides the sun, much is being learnt about our neighbouring planets too. While doing so, astronomers have found stony stragglers called asteroids that orbit the sun between Mars and Jupiter. "Asteroids like meteorites and comets have been accused of being a threat to the earth's very existence," said Prof. Narlikar. "Meteorites or asteroids which come within the earth's gravitational pull slam into it, with devastating results. An asteroid, 10 km across, can hit the earth with the impact of 1 million Hiroshima bombs, wiping out most existing life forms," warned Prof. Narlikar. 

Can we avert such catastrophe? Prof. Narlikar suggested a solution, which he had provided in a sci-fi story 25 years ago. In it, an amateur astronomer in Calcutta discovers that a comet is headed for the earth. He proposes that a spacecraft loaded with nuclear bombs be detonated near the comet, generating a shock wave which would divert the comet's path. "NASA scientists are now thinking of similar strategies to avoid meteorite collisions," said Prof. Narlikar. 

In the last 40 years, technological advances have helped to discover nearly 100 extra-solar planets, none of which supports life. But a study has revealed that a comet's tail deposits particles resembling microbial colonies in the earth's atmosphere as it brushes past, and astronomers like the late Fred Hoyle have thought that life on earth started from microbes brought by meteors which found a hospitable atmosphere here. Which is why Prof. Narlikar feels that when man does find alien life, it will be microbial rather than sentient.