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A Mission to Discover the Secrets of a Comet: Close Encounter

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In March 1986 a small fleet of spacecraft intercepted Halley’s comet as it plunged through the inner solar system, 100 million miles from earth. The boldest of the robot explorers was a probe called Giotto. It flew close to the comet’s heart and received a battering that knocked out nearly half of the instruments it carried. But mankind’s first close encounter with a comet was triumphantly successful.

Halley’s comet requires 75 years to travel its elongated orbit. But for most of the time it is in the outer reaches of the solar system – a cold, dark object invisible to earthbound astronomers. But when Halley approaches the inner planets, the warmth of the sun drives off a cloud of gas and dust thousands of miles wide. The pressure of sunlight and the solar wind – a stream of electrically charged particles constantly flowing out from the sun – sweep the gas and dust out a great tail, millions of miles long.

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Before the Halley missions, the precise nature of comets was a mystery. Astronomers knew that comets were made up of material left over after the formation of the planets and their satellites, but the exact composition of comets remained obscure. Several explanations had been proposed. Most favored was the “dirty snowball” theory suggested by the American astronomer Fred Whipple in the 1950’s. Whipple believed that the nucleus of a comet, hidden within the gas and dust of its head, consisted of a compact mass of ice, dust, and rock.

A Violent Core

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Two Japanese probes, Sakigake (“Forerunner”) and Suisei (“Comet”), and two Russian ones, Vega 1 and 2, were the first to investigate Halley. The information they provided about its motion was used to guide Giotto on its near-collision course. Passing 5,000 miles from the comet, the Soviets’ two Vega craft were the first to spot the nucleus.

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Observing Halley – Although not as bright as on previous visits, Halley’s comet was visible from earth when it made its recent appearance in the summer of 1985 – 86. Taken through the Schmidt telescope in Great Britain, even this high speed photograph make the starts appear to be streaks of the light. The tail streams out behind the core of the comet.

Their television cameras revealed a potato-shaped object nine moles long and five miles wide that rotated end over end every 53 hours.

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On the side facing the sun, bright jets of gas mingled with dust that rose from the surface as ice was heated by sunlight and turned into vapor. Fortunately this violent activity subsided before Giotto met the comet. Otherwise the probe would have encountered an even fiercer dust storm.

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The Giotto satellite, built by a British company and launched by a French Ariane rocket, and carried experiments designed by British, French, and German scientists. It encountered the comet at a speed of 40 miles per second. A large two-layer dust shield surmounting the probe doubled as a dust detector, recording the impact of the minute grains of dust that bombarded the spacecraft.

Hazard in Space

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As Giotto plunged toward the comet’s nucleus on March 13, its camera sent a stream of pictures back to earth. But just 12 seconds before the closest approach, impact from the dust particle set the probe wobbling. Its pencil-thin radio beam, which had been accurately pointed at the earth, swung wildly.

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For 30 tense minutes, as the probe passed the nucleus at a distance of only 400 miles, scientists and engineers at mission control in Darmstadt, West Germany, thought that Giotto had been destroyed. Then the probe’s automatic stabilizing equipment finally locked the beam back onto earth, and the stream of data resumed.

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Of the 14 instruments on board, six had been knocked out, including the camera. But the remaining equipment continued to function and provide data that will keep scientists busy for years.

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Some of the most important questions about comets have already been answered. Whipple’s dirty snowball theory has been confirmed: the icy part of the comet does consist of water mixed with frozen gases.

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Most intriguing of all, the surface of the nucleus is coated with a layer of sooty material containing carbon. Probably no more than half an inch thick, it makes the nucleus one of the darkest bodies in the solar system. Since such materials can be produced by living matter, scientists conjecture that bacteria may live on comets and spread diseases to earth.

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The working lives of the comet probes are not yet over. Vega 1 and 2 are continuing their journey to take a close-up look at the asteroid Adonis, another little-changed relic of the early days of the solar system. And in 1992, after six years of orbiting the sun, Giotto was directed toward a comet called Grigg-Skjellerup. The goal is to discover whether Halley’s comet is unique or just like the thousands of comets that swarm the sun.