Space Probes Begin Examination of Comet Halley

Date March 8, 1986

When Comet Halley made its fourth predicted return after its orbit was first calculated by Edmond Halley following his observations of the comet in 1682, five space probes—launched by the European Space Agency, the Soviet Union, and Japan—were dispatched to examine it at close range.

Locale Elliptical orbit around the Sun

Key Figures

• Edmond Halley (1656-1742), English astronomer who first calculated the orbit of the comet named for him
• Giotto (c. 1266-1337), Florentine painter who produced a key image of the comet
• Joseph M. Laufer (fl. late twentieth century), American amateur astronomer who participated enthusiastically in the International Halley Watch

Summary of Event

From the calculations Edmond Halley made in association with his observations of the comet of 1682, he inferred that it was the same object that had previously been observed in 1531 and 1607, and thus predicted its return in 1758. The comet’s return on Christmas Day of that year was a triumphant vindication of the authority of astronomical observation, and the subsequent returns of Comet Halley have all been eagerly anticipated.

In the time following the 1758 sighting, the technology available to support astronomical observations made spectacular progress; the comet was photographed for the first time in 1910, and by the time it returned in 1986—when its closest approach to Earth was 39 million kilometers (about 24 million miles)—it was possible to send space probes to intercept it. Such was the symbolic importance of the comet that no less than five such probes were prepared and launched, as scientists attempted to make the closest study yet of a cometary nucleus.

The European Space Agency named its probe Giotto, for the Florentine Renaissance painter, because it had been realized after Halley’s death that Giotto had used Halley’s comet in 1301 as a model for the Star of Bethlehem in his painting The Adoration of the Magi, adding an extra element to the comet’s iconic mystique (even though the original Star of Bethlehem could not have been Halley’s comet). Although the National Aeronautics and Space Administration (NASA) in the United States redirected a probe to intercept the comet, the principal American effort was concentrated on collaboration with the Giotto probe, launched on July 2, 1985, which was intended to photograph the comet’s nucleus. The buildup to the encounter and its aftermath were elaborately chronicled by the International Halley Watch, a group whose members compiled a huge archive, including twenty-one issues of a publication titled Halley’s Comet Watch Newsletter produced by Joseph M. Laufer. The newsletter offered a gradually unfolding commentary on the event’s progress and significance that preserved a useful insight into the popular excitement generated by the comet’s return.

The two Japanese probes, Sakigake (Pioneer) and Suisei (Comet), launched, respectively, on January 7 and August 18, 1985, carried instruments whose purpose was to study the comet’s coma—the “atmosphere” surrounding its nucleus—and its interaction with the charged particles of the solar wind. The two Russian Vega probes, launched on December 15 and 21, 1984, had a dual mission, the first part of which was to make a close passage of the planet Venus, and were not as well equipped for the rendezvous with the comet as the other three. Vega got closer to the comet than did Giotto, but the images it returned were inferior to those captured by the European probe.

The two Japanese probes reached the comet ahead of Giotto, Suisei making its closest passage on March 8, 1986, at a distance of 151,000 kilometers (almost 94,000 miles), but observers on Earth were paying far more attention to Giotto, waiting expectantly for the images relayed by its camera. Giotto passed within 605 kilometers (376 miles) of the comet on the night of March 13, 1986. Unfortunately, the craft was hit by a particle of dust and its camera ceased to function fourteen seconds before its closest approach, so the best image of the nucleus was obtained from 1,675 kilometers (1,041 miles).

Giotto’s images revealed that the nucleus of the comet was darker than had been expected, on the basis of the theory that cometary nuclei were composed of various ices. It appeared to have a black crust pockmarked with various bumps and craters, which occasionally cracked to release jets of evaporating gases from a core whose main constituent was water ice. The nucleus proved to be an ovoid measuring about 15 kilometers (a little more than 9 miles) on the long axis and 8 kilometers (about 5 miles) on the shorter ones, with a total mass somewhere between 50 and 100 billion metric tons. The nucleus was rotating on its long axis, with a period of approximately 55 hours.

Giotto’s mass spectrometer produced good analyses of the ejected gases and dust, demonstrating that the relative abundance of the lighter elements in the comet was similar to that observed in the Sun. The data sent back by Sakigake and Suisei confirmed that the comet’s coma extended for millions of kilometers and that the neutral particles ejected by the nucleus were excited by the solar wind and converted to ions, which formed the comet’s tail.

The cameraless but intact Giotto was rerouted after its encounter with Comet Halley to a second rendezvous with Comet P/Grigg-Skjellerup, which it passed in July, 1992, returning further valuable information about cometary composition even in the absence of visual images. This postscript served to underline the heroic nature of the enterprise, symbolizing the vast technological strides that had been made since the comet’s previous visit in 1910. Sakigake and Suisei were similarly rerouted toward Comet Giacobini-Zimmer, with which they were intended to rendezvous in the late 1990’s, but neither probe was able to complete its journey, further highlighting Giotto’s remarkable achievement.

Significance

The data that the five probes sent back from Comet Halley were important primarily because they confirmed that comets are, as previously theorized, “dirty ice balls.” The probes also provided important information concerning the mechanisms by which comets produce their comas and tails. What Giotto added to the theoretical picture was a new awareness of the structure of the cometary nucleus, particularly of the nature and activity of its surrounding crust. That new knowledge was further augmented some years after the passage of the space probes, in February, 1991, when observers using a telescope at the observatory at La Silla in Chile saw that the comet had unexpectedly flared up, presumably because of a major outburst of gases from its crust.

Giotto’s measurements of Comet Halley’s composition confirmed that the Oort cloud, from which comets come, must have condensed from the primordial solar nebula at about the same time as the Sun, and the Japanese probes added an abundance of hard data to existing theses regarding cometary comas and tails. The multiple rendezvous was a spectacular triumph of international cooperation in technological expertise, and the data collected provided a fine demonstration of the accuracy with which astronomical theorists had calculated the nature and behavior of comets.

The probes’ greatest significance, however, was the manner in which they captured the popular imagination. Although the comet was disappointing in terms of Earthly observations, failing to provide a spectacular display of the sort associated with younger comets that cross Earth’s orbit at more favorable points, Giotto allowed Comet Halley to recover and retain its iconic significance as a symbol of the scope and exactitude of astronomical science, refreshing the sense of wonder associated with that science.

Bibliography

Grewing, M., F. Praderie, and R. Reinhard, eds. The Exploration of Halley’s Comet. New York: Springer-Verlag, 1989. Massive volume collects papers presented at a 1986 scientific symposium on the data collected by the probes.

Hughes, D. W. “The History of Halley’s Comet.” Philosophical Transactions of the Royal Society of London (Series A) 323 (September 30, 1987): 349-367. Presents a succinct summary of the comet’s history, including a brief evaluation of data gathered during its 1986 return.

Laufer, Joseph M. Halley’s Comet Watch Newsletter 1 (May/June, 1982)-5 (November, 1986). These twenty-one issues provide a wide-ranging account of the author’s personal involvement with the International Halley Watch, including his research, relevant travels, and responses to the unfolding events.

Schaaf, Fred. Comet of the Century: From Halley to Hale-Bopp. New York: Copernicus, 1997. Places the 1986 return of Halley’s comet in a broader historical context, comparing it with other major contemporary observations of such phenomena.

Sekanina, Zdenek. The Comet Halley Archive Summary Volume. Pasadena: Jet Propulsion Laboratory and California Institute of Technology, 1991. Presents a retrospective summary account of the work of the International Halley Watch.