Space

Celestial visitor

Print edition : December 13, 2013

Comet ISON is seen in this five-minute exposure taken at NASA's Marshall Space Flight Centre on November 8 at 10.40 GMT. At the time this picture was taken, Comet ISON was 156 million km from the earth. Photo: Aaron Kingery/NASA/Reuters

The bust of Edmond Halley in the Museum of the Royal Greenwich Observatory, London. Photo: Klaus-Dieter Keller

The newly discovered Comet ISON has excited the scientific community in India, which is going all out to ensure that as many people as possible are able to view it.

NO interested reader of Bengali literature would have missed the children’s nonsense writer Sukumar Roy’s play Jhalapala (meaning blaring nonsense), written after Halley’s Comet’s 1910 visit. In it, all catastrophes and imponderables (such as the hot weather, a cat dying of heat stroke, storms, rain, earthquake, plague, famine, the Allahabad exhibition, beriberi and even worms on the betel leaves) are attributed to the comet. But times have changed. Comet ISON, first spotted by two members of the Russian astronomy group called the International Scientific Optical Network, on September 21, 2012, has evoked tremendous interest the world over. Indian enthusiasts from different backgrounds formed the platform “Eyes on ISON” in April 2013 (www.eyesoncometison.in) to organise mass viewing of the comet from October 2013, when it begins to get bright, to February 2014, when it will fade away. They are now training 2,000 master trainers, who will subsequently train many others in every State. Their national workshops, conducted in Bangalore, Guwahati and Bhopal, have evoked great interest, and when they presented their plan at the International Astronomical Union’s conference in Warsaw (Communicating Astronomy with the Public, 2013), it was greeted with deep astonishment for its sheer scale of operation. But India is a country of over a hundred crore people: while the sky is a universal laboratory, to “universalise the universe”, such large numbers have to be aimed for since this programme also dovetails with the demand for “universalisation of quality education for all”.

The discovery of Comet ISON was unexpected. “The earth hath bubbles, as the water has,” said Shakespeare in Macbeth. Comets are not bubbles but they appear and vanish, “as breath into the wind”. Comet ISON will pass by the sun on November 28, 2013, a “mere” 1,100,000 (1.1 million) kilometres above the sun’s surface, becoming brighter after that. Its mass is centred in a tiny nucleus, not more than 10 km in diameter—too tiny to be seen from the great distance of 300 million km away. As it comes close to the sun, it will profusely vaporise because of the heat. The evaporating matter will gush out of the main body of the object, surrounding the nucleus with a fuzzy gaseous and dusty cloak. This will form the coma, which could be a few thousands to a million kilometres in diameter. The rest of the vapour will eject as a jet, which will form the “tail of the comet”, perhaps up to a hundred thousand kilometres in length.

The coma and the tail will be large enough to be seen, and the sunlight scattered by the cometary dust and gas will make them visible. The solar light beam will push the evaporating gas away from the sun because of what is called radiation pressure. The tail will, thus, point away from the sun. Comet ISON will be visible from November 2013 to February 2014. But everything will depend upon what happens to it as it approaches the sun. Will the sun wipe it off by rapid evaporation? Or will the comet simply split into fragments owing to tidal forces arising out of the sun’s gravitation? These are the imponderables. Comet ISON, tiny though it may be, weighs 10,000 trillion kilograms, that is, a billionth of the weight of the earth. At the perihelion (closest approach to the sun), it will lose 60 kg of water and 51,000 kg of dust a minute. The intriguing point is that it has so much less water and so much more dust than many other comets. Comets also have carbon monoxide, carbon dioxide, methane (marsh gas), hydrocyanic acid, etc. Although the comet will pass by the earth at a minimum distance of 150 million km, these gases from it will be too depleted by the time they reach the earth to cause any harmful effects here.

How is all this known, one may ask. Spectroscopic methods (essentially the analysis of colours and their relative brightness), available since the 19th century, can tell one about the chemical composition of objects, their temperature and their pressure. The fate of Comet ISON depends largely on the orbit it follows, that is, how close it gets to the sun. Its path can be predicted with great precision using Newton’s laws of motion and Newton’s theory of gravitation. The story that follows describes how Isaac Newton’s ideas about the mechanics of objects moving under gravitation were verified by studying Halley’s Comet.

The comet of 1682 and Edmond Halley

Most comets, for example, Comet ISON, are named after their discoverers but Halley’s Comet is an exception. Edmond Halley was born in 1656, and in 1673, he entered The Queen’s College, Oxford. Gregarious, fond of company, Halley frequented the coffee houses and taverns, where “Halley now talks, swears and drinks brandy like a sea-captain”.

In his time, Europe and Britain in particular were witnessing great social change. After the discovery of America and the sea routes to India and the East Indies, sea trade had multiplied. These trading interests gave support to astronomy as stars were the guides for the sea-faring traders. The English East India Company was set up in 1600, under a royal decree. It offered Halley a job. In 1677, he was sent to the island of St Helena in the southern hemisphere (where Napoleon was finally banished in 1815 and where he lived until his death in 1821) to create accurate star charts for navigation. Halley returned to London after about a year with a catalogue of 300 stars. He discussed the data with fellow scientists like Robert Hooke. Trying to connect his data with Kepler’s laws of planetary motion, they got a clue about the inverse square law of gravitation. In 1684, to firmly establish this connection, Halley went to Cambridge to meet Newton. Newton astonished Halley by telling him that he had already solved the problem a few years ago but had not preserved the papers and was not eager to redo the proofs. But thanks to Halley’s patient persuasion, an unwilling Newton finally published the proof in 1687 in Principia. To get this published, Halley spent his own money. Newton’s law of gravitation states that the gravitational force between two objects is inversely proportional to the square of the distance between them. Newton also said that the law was universal, that is, it operates in heaven and earth, as Galileo had first said about the nature of physical laws. Kepler’s laws followed from Newton’s law of gravitation.

Halley got interested in the comets of 1680, 1681 and 1682 during his visit to France. About a century before, Tycho Brahe observed the position of the moon and the 1577 comet (not Halley’s Comet) from two different points, several hundred kilometres apart. He found that as the observer’s location moved, the moon’s shift was more than that of the comet. This established that comets were farther away than the moon. But Tycho’s illustrious one-time assistant, Johannes Kepler, had made an error in judging the paths of comets. Kepler, who had discovered that planets move in elliptical orbits around the sun, found the paths of comets to be straight. This was because he had observed very distant comets, so the curvature in their paths could not be clearly seen.

Halley found that the 1682 comet did not move along a straight path. It roughly followed the paths of the 1607 and 1531 comets, yet did not match their paths exactly. There was still another problem. While planetary orbits were seen to be remarkably periodic, the intervals between the visits of the 1456, 1378, 1301 comets were 76-77 years, on an average, but were not identical.

However, by 1705, Halley found that the answer was in Newton’s law of gravitation: comets being material objects must feel the sun’s gravitational pull the same way that planets do. They must thus move in periodic elliptical orbits and visit the solar system periodically. But the visits are slightly irregular because comets are much lighter than planets and their orbits can be altered significantly by the gravitational influences of heavier planets such as Jupiter and Saturn.

The periods are thus not the same but are 76 years on an average. Thus, Halley declared in1705 that the comets of 1531, 1607 and 1682 were the one and the same and that it would appear again in 1758: “All the elements [of these comets] agree except that there is an inequality in the times of revolution; but this is not so great that it cannot be attributed to physical causes. For example, the motion of Saturn is so disturbed by other planets, and especially by Jupiter, that its periodic time is uncertain to several days.”

The same cause, he argued, could cause disturbances in the periods of comets: “The identity of these comets is confirmed by the fact that in 1456 a comet was seen, which passed in retrograde direction between the earth and the sun, in nearly the same manner; and although it was not observed astronomically, yet from its period and its path I infer that it was the same comet as that of the years 1531, 1607, 1682. I may, therefore, with some confidence predict its return in the year 1758. If this prediction is fulfilled, there is no reason to doubt that other comets will return.”

Because of their peculiar appearance, like a “sword hanging over the city”, comets were held in awe and fear. In Shakespeare’s Julius Caesar, one reads: “When beggars die, there are no comets seen; the heavens blaze forth the death of princes.” And in Henry VI, one is told that comets are capable of “importing change of time and states”.

When Halley wrote his seminal conclusions, they did not create a stir. Interest returned in 1757, by which time Halley was dead (his last act was to drink a glass of wine, sitting in his observatory, a few minutes before he passed away at the age of 86). Charles Messier in France first saw Halley’s Comet in November 1758 but waited for further confirmation. On the Christmas night of 1758, a Saxon farmer, Johann Palitsz, saw it and announced the confirmation of Halley’s prediction: Newton stood vindicated. More vindications would follow, like William Herschel’s discovery (based on observations from 1779 onwards) that every member of a stellar binary moves around its companion, as predicted by Kepler’s laws. So, gravitation is universal. New standards were thus being set for the exactness of the exact sciences!

And Halley was right. Now more than 100 comets are known with a period less than 200 years and about 50 comets with a period less than 10 years. The shortest period, 1,209 days, is seen in Comet Encke. Thus, it has returned about 15 times in the past 50 years. This gives one enough opportunity to test Kepler’s laws and Newton’s theory of gravitation over and over again. The orbits of artificial satellites are calculated in terms of Newton’s laws. There is no evidence to question Newton’s laws unless one travels at speeds very close to that of light or one is in the vicinity of very massive objects. But there are many things that have been discovered since the time of Newton and Halley, particularly about the composition and evolution of comets. Astronomy is now not merely celestial mechanics but involves the entire gamut of physics.

Although Halley’s prediction did not get the attention it deserved in his time, the comet’s later visits were widely covered by the press. The 1986 visit was shown in the visual media and was accompanied by mass viewing. Very few people have seen two of its visits. But those who did told the press in 1986 about their experience in 1910. One such person was Salim Ali (1896-1987), the renowned ornithologist. My colleague Vasundhara Raju, a cometary physicist herself, recalled her late father-in-law’s thrill on learning about Halley’s Comet’s 1986 visit since he had also seen it in 1910.

A person whose life had a great overlap with Halley’s Comet was Mark Twain. “I came with the Halley’s Comet of 1835. It is coming again next year and I expect to go out with it,” Mark Twain said in 1909. Irreverent of authority, he continued: “It will be the greatest disappointment of my life if I don’t go out with Halley’s Comet. The Almighty has said, no doubt, ‘Now, here are two unaccountable freaks; they came in together, they must go out together.’” Mark Twain did not have to live with that disappointment. He passed away just a day after Halley’s Comet reached its perihelion during its 1910 visit. A great coincidence indeed!

By all indications, Comet ISON will survive, and lakhs and lakhs of people in India will be able to watch it together, breaking economic, social, ethnic, cultural and gender barriers between them. With the involvement of all the 35 groups of the All India People’s Science Network, all the State Councils for Science and Technology, Vigyan Prasar (the science communication centre of the Government of India), the National Council for Science & Technology Communication, science museums, amateur astronomy groups, and academics, this campaign promises to be one of the largest public science popularisation initiatives attempted so far in India.

S. Chatterjee is an Associate Professor at the Indian Institute of Astrophysics, Bangalore.

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