Giotto Sees Halley, Halley Sees Giotto

By Tanya Kucak

II Computing Volume 1 Number 1
October / November 1985

In 1310, Giotto saw Halley’s Comet. Next year, Halley’s Comet will see Giotto.

The first Giotto is the Florentine painter Giotto di Bondone (1266?-1337), who saw the comet from Padua, Italy. Fie depicted the comet as the Star of Bethlehem in his famous fresco Adoration of the Magi, which is in the Scrovegni chapel in Padua.

The second Giotto is a space probe launched by the European Space Agency (ESA) to photograph and analyze the comet. One of the most important goals of the Giotto mission is to determine the exact chemical composition of the comet. Astronomers believe comets consist of primordial debris — dust and frozen gases — left over from the formation of the solar system more than 4.5 billion years ago. As such, comets may preserve the material from which the solar system formed.

Sir Edmond Halley, in studying the comet that now bears his name, was the first person to determine that its periodic sightings were returns of the same object. He theorized that comets orbit the Sun in elongated ellipses. As a comet approaches the Sun, its frozen gases vaporize, thus liberating gas as well as dust particles and pushing the comet’s lengthening tail away from the Sun.

Near the Sun, the intense solar ultraviolet light causes the comet’s tail to glow. Moreover, the comet grows in size as its density decreases, thus lessening the effect of the Suns gravitational pull and enabling the cornet to swing back toward Neptune. The comet’s nucleus grows denser as its frozen gases condense once again as it moves from the Sun. The attenuated dust-and-gas tail eventually spreads along the entire orbit of the comet, and when the Earth passes through this dust trail, a meteor shower results. Halley’s Comet is responsible for two meteor showers: the Eta Aquarid of early May and the Orionid of late October.

Indeed, scientists expect that cometary dust, traveling with velocities over 50 times faster than a speeding bullet, will destroy Giotto shortly after its closest approach— within 500 km of the 5to 10-km-diameter nucleus of Halley’s Comet — on March 13, 1986. The craft may last a few hours to a few days, during which time it will transmit information from ten scientific instruments, including a multicolor camera (image resolution of 50 meters anticipated), a photopolarimeter, dust detectors, and several plasma experiments.

Fifteen subcontractor companies from ten European nations developed Giotto. The $52 million craft was built at British Aerospace Dynamics Bristol factory; tested at the Centre Spatiale de Toulouse in France; sent to ESA’s Space Technology Center in Noordwijk, Holland, for final adjustments; and launched in July from the Guiana Space Center in Kourou, French Guiana.

At the time of launch, scientists could estimate the position of the comet’s core to within 30,000 km. Data from two Soviet space probes, also en route to Halley’s Comet, enable Giotto scientists to make midcourse corrections. The two Soviet crafts, Vega 1 and Vega 2, launched December 1984, will pass within 10,000 km and 3,000 km (respectively) of the comet’s core in March 1986. Since the U.S. chose not to launch a probe to investigate the comet, the Vega craft carry American experiments as well, making the Halley’s Comet missions a model of international cooperation.

Altogether, five space probes will rendezvous with Halley’s Comet. Besides the European and Soviet missions, Japan launched a test vehicle, MST5, in early 1985 and its scientific probe, Planet A, in August 1985. Planet A carries only two instruments, an ultraviolet camera and a solar wind analyzer. //

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