Rendezvous with Halley's comet; a major international venture to study visitor from space

UNESCO Courier, March, 1986 by Howard Brabyn

LIKE courtiers dancing a stately quadrille, eight instrument-packed space probes (one launched by the European Space Agency, two by the USSR, two by Japan, and three by the USA) are weaving intricate patterns in space in the biggest international astronomic co-operative venture ever mounted--the investigation of Halley's comet.

Two space shuttle missions are also being in part devoted to observation of the comet, the trajectory of which will be continuously monitored by the world's most powerful land-based telescopes. Altogether some 900 professional astronomers from forty-seven countries are taking part in the investigation.

Why all this interest in what veteran US astronomer Fred Whipple has described as "a large, dirty snowball"?

The answer is that astronomers believe that comets consist of the remnants of the building blocks left over from the birth of the solar system some 4,500 million years ago. The history of the solar system is recorded, so it is thought, within their frozen hearts.

Today, for the first time ever, thanks to the genius of two seventeenth-century English scientists, Isaac Newton and Edmond Halley, and the achievements of modern space technology, we have in our hands an entrance ticket to one of these refrigerated, flying museums. For Halley's comet is the only vigorously active comet whose orbit brings it relatively close to the earth (this time round, its thirtieth recorded apparition, it is coming t within 62 million kilometres of our planet) and has been determined precisely enough to allow of detailed scientific planning for investigation well in advance; and for the very first time space technology is sufficiently advanced to enable us to go out and meet a comet beyond the blanket of the earth's atmosphere instead of waiting for it to come to us, to penetrate its coma (the fuzzy gaseous cloud, up to 100,000 kilometres across, that shields the nucleus from our sight), to have a close up view of its nucleus and to analyse the components of its tail, in short, to understand the elements from which our solar system was constructed.

In 1950, the Dutch astronomer Jan Hendrik Oort postulated that comets originate from a vast cloud of hundreds of millions of small bodies (now known as the Oort Cloud) that orbits the solar system at a distance of one light-year. The nucleus, or core, of a comet is thought to consist of about 25 per cent dust and lumps of rock or metallic matter and 75 per cent of ice, in which are mixed compounds containing ammonia, methane and carbon dioxide radicals. The 700 or so comets that have been scientifically recorded so far have had nuclei ranging in size from 0.5 to 70 kilometres in diameter.

From time to time a passing star sends a gravitational ripple through the Oort Cloud sending a flurry of these "dirty snowballs" hurtling off into space. Some of them, the "short-period" comets, like Halley's comet, are projected towards the solar system and, influenced by its gravitational pull, become members of the inner solar system, completing their orbits around the sun in less than 200 years. The "long-period" comets can take millions of years to complete their orbital cycles. An average of five new comets are discovered every year.

The outer surface of a comet nucleus is thought to consist mainly of dust. As a comet approaches to within three astronomical units of the sun (one astronomical unit equals the distance between the earth and te sun, roughly 150 million kilometres) this outer crust heats up and the sub-surface ice begins to sublimate (a process by which a solid is converted directly into vapour without going through the liquid stage). The resulting gas streams out from the comet carrying dust particles with it to form the coma.

As it gets closer to the sun a comet develops two tails, a yellowish curved tail composed of dust particles released during the process of sublimation, and a blueish stream of gas called the plasma tail which is formed when the gases released from the comet's nucleus become charged by solar radiation.

In a few months' time, when all the data have been gathered in, we shall have made a tremendous leap forward in our knowledge of the nature of comets and of the origins of the solar system. We should not, however, forget that this will be due not only to the brilliance of modern astronomers but also to a quite unprecedented effort of international co-operation.

Launched on 15 December 1984, the Soviet space probe Vega 1 flew through Halley's coma on 6 March 1986, at a distance of about 9,600 kilometres from the nucleus, and photographed and analysed the gases around it. Sister satellite Vega 2 encountered Halley's comet on 9 March 1986 and analysed its coma.

The Japanese probe Sakigake (pioneer), launched on 7 January 1985, rendezvoused with Halley's comet on 9 March 1986 and measured the speed and temperature of the solar wind blowing against the comet, whilst on 8 March 1986 its companion satellite Suisei (comet) flew past Halley's comet at a distance of 144,000 kilometres and studied te enormous cloud of hydrogen gas surrounding the comet.