COMET HALLEY SUMMARY

Jet Propulsion Laboratory

Office of Public Information

In discussing Comet Halley, we should place the term "facts" in quotes, because much of our "knowledge" of Halley is based upon qualitative observations made in 1909-1911 as well as comparison with other comets observed more recently.

Our knowledge of all comets is limited, even for those seen in recent years. This is because the nucleus, which is the real comet and the source of the visible coma and tails, is too small to be observed except as a point of light. Only our knowledge of cometary motions is on reasonably firm ground. That is one fundamental reason for the intensive International Halley Watch campaign now under way.

Then the "facts" presented here represent, in many cases, the current best guesses.

COMET ORIGINS

Comets are believed to have condensed from the solar nebula at the same time that the Sun and planets formed about 4.5 billion years ago.

Most comets are thought to reside in the furthermost reaches of the solar system, 30,000 to 70,000 astronomical units from the Sun, although recent evidence suggests that the region from 1,000 to 30,000 astronomical units may also be heavily populated. (One astronomical unit is defined as the average distance of Earth from the Sun and is equal to about 150 million kilometers, or 93 million miles.)

It is believed that comets remain far from the Sun unless they are disturbed by the gravitational perturbations of passing stars and forced into closer orbits. The distant aggregation from which comets are gravitationally "pulled" is known as the Oort Cloud, after the Dutch astronomer Jan H. Oort (1900- ), who first theorized about it.

Because of their small size and great distance from the Sun, comets are believed to be the most pristine, unaltered samples of the early solar system. Planets, moons, even some asteroids, by comparison, undergo various changes as a result of melting, volcanism and other processes. This is one reason why astronomers are so interested in comets, whose unique status may hold many clues to a better understanding of the origin and evolution of the solar system.

Long-period comets have orbits from 200 years to many millions of years. The aphelion (most distant point from the Sun) of a long-period comet may be more than 50,000 astronomical units away.

Short-period comets have orbits of less than 200 years, and most reach aphelion near the orbit of Jupiter. Of the approximately 100 known short-period comets, Halley is the brightest and most famous. (Actually, Halley belongs to a group astronomers are beginning to call "intermediatel" period: not short enough to be regularly influenced by Jupiter but still with periods less than 200 or so years.)

Our knowledge of comets comes from observations of their activity as they approach the Sun. As a comet nears the Sun, the nucleus‹which according to the "dirty snowball" theory proposed in the 1950s by the American astronomer Fred Whipple is a solid mass of frozen water and gases mixed with rocky material‹is heated enough to shed molecules containing such elements as carbon, hydrogen and nitrogen.

The results of this heating are the visible portions of a comet: the coma, or glowing cloud, that surrounds the nucleus, and the long, beautiful tails that can stretch millions of kilometers through space.

Edmond Halley's Comet

Until the time of English astronomer Edmond Halley (1656-1742), comets were assumed to make only single passes through the solar system.

But in 1705, Halley used Isaac Newton's theories of gravitation and planetary motions to compute the orbits of several comets. Halley noted that the computed orbits for bright comets seen in 1531, 1607 and 1682 were similar, and he suggested that the three comets observed were actually one comet making return trips. Halley correctly predicted the next return of the comet‹which occurred in 1758-1759, 16 years after his death‹and the comet now bears his name.

The comet's appearance has since been traced back as long ago as 240 B.C. in ancient Chinese documents. Comet Halley's recovery on October 16, 1982, marked its 30th recorded appearance. Until recently, the 164 B.C. appearance was considered "lost," since no records of the comet from that time could be found. In 1984, however, British scholars announced their discovery, contained in ancient Babylonian clay tablets, of several lines of cuneiform characters referring to the 164 B.C. apparition.

More specific details about Comet Halley are presented in the remainder of this section. Distances are given throughout in three units of measurement: astronomical units (AU), kilometers (km) and miles. Masses are given in kilograms (kg) and pounds (lb).

Motion and Brightness

Comet Halley moves backward (opposite to Earth's motion) around the Sun in a plane tilted 18 degrees to that of the Earth's orbit. Halley's backward, or retrograde, motion is unusual among short-period comets, as is its aphelion at a distance beyond the orbit of Neptune.

Halley's period is, on average, 76 years. This corresponds to an orbital circumference around the Sun of about 12.2 billion km (7.6 billion miles). The period varies from appearance to appearance because of the gravitational effects of the planets. Measured from one perihelion passage to the next, Halley's period has been as short as 74.42 years (1835-1910) and as long as 79.25 years (451-530).

The comet's closest approach to Earth occurred in 837, at a distance of 0.033 AU (4.94 million km; 3.07 million miles). At that time, April 10, 837, Halley reached a total apparent brightness of about magnitude -3.5,* nearly that of Venus at greatest brilliance. The light of Halley was spread over an extended area, however, so its surface brightness was less than that of Venus.

* Astronomers measure an object's brightness with a scale in magnitudes. The scale is "backward," with larger numbers meaning a fainter object.

(Estimates of Halley's brightness during past appearances are based in each case upon the comet's closest passage of Earth and the assumption that the comet behaved as it did in 1909-1911, when it reached a total apparent brightness of about magnitude -1 and passed within about 0.15 AU [ 23 million km: 14 million miles] of Earth.)

During its current appearance, Halley's nearest approach to Earth will occur on the outbound leg of the trip, at a distance of 0.42 AU (63 million km: 39 million miles). April 11, 1986, the comet will reach a total apparent brightness of about magnitude 2.0, just slightly brighter than the north star Polaris, but again spread over a much larger area than a point-like star. On that date, Halley will be far south (-47 degrees declination) and best seen in the Southern Hemisphere, actually passing through the zenith for observers at 47 degrees south latitude. The comet will be very low in the sky but still visible as far north as the middle United States.

Inbound to the Sun, Halley makes a close approach to Earth on November 27, 1985, at a distance of 0.62 AU (93 million km: 58 million miles).

At aphelion in 1948, Halley was 35.25 AU (5.27 billion km: 3.28 billion miles) from the Sun, well beyond the distance of Neptune. The comet was moving only 0.91 kilometers per second (2,000 mph).

At perihelion on February 9, 1986, Halley will be only 0.5871 AU (87.8 million km: 54.6 million miles) from the Sun, well inside the orbit of Venus. Halley will be moving 54.55 kilometers per second (122,000 mph).

On its way toward the Sun, Halley passes:

‹The mean distance of Jupiter from the Sun on Jan. 10, 1985.

‹The distance of Mars on Nov. 28, 1985.

‹ The distance of Earth on Jan. 1, 1986.

‹ The distance of Venus on Jan. 21, 1986.

Departing, Halley again passes:

‹The mean distance of Venus on Feb. 28, 1986.

‹The distance of Earth on March 20, 1986.

‹The distance of Mars on April 24, 1986.

‹The distance of Jupiter on March 12, 1987.

Nucleus

Among short-period comets, Halley is judged to be large. Its heart, the nucleus, is believed to be an irregularly shaped sphere, roughly 6 km (4 miles) in diameter, rotating approximately every 2.2 days. The axis of the nucleus is tipped perhaps 45 degrees to the orbit plane. The estimated mass is 100 million million kg (100 billion tons).

On March 10, 1986, during the peak of spacecraft activity, the nucleus is expected to produce gas at a rate of molecules per second; of this, some 80 percent is expected to be water. Seen another way, this production is equal to 3,000 kg per second (6,600 lb/sec).

Estimated dust production on that date: general, 600 kg/sec (1,300 lb/sec), and in jets, 900 kg/sec (2,000 lb/sec), for a total of 1,500 kg/sec (3,300 lb/sec).

Coma

The dust emitted by Halley's nucleus will extend roughly 100,000 km (60,000 miles) in the sunward direction and as much as twice that distance perpendicular to the sunward direction in mid-March 1986. Various gases will extend far beyond these distances, but the visible gas coma will probably be no larger than the dust coma. The visible gas is largely composed of C2 (molecular carbon) with small contributions from many molecular fragments containing such elements as carbon, hydrogen and nitrogen (C3, CH and CN, for example). Various visible coma structures will be the product of dust jets.

Tails

All comets have two tails: an ion tail and a dust tail.

The first is formed when solar heating causes neutral molecules to be shed from the comet nucleus. These molecules are given an electric charge by solar ultraviolet radiation and X-radiation. When they encounter the solar wind and its magnetic field, the now-charged molecules are carried outward to form a tail that may stretch millions of kilometers through space. This is known as the ion tail, "ion" being the general term for atoms and molecules that carry an electric charge.

The gases streaming from the nucleus also carry off a fine dust. This dust, blown away from the Sun by the pressure of the solar wind, forms a second tail that is seen by reflected sunlight. This is known as the dust tail.

When the two tails are superimposed on each other as seen from Earth, the comet appears to have only one tail. This happened in 1910 for Halley. For smaller, less active comets, one or both tails may be too faint to be seen or even photographed, and the comet will appear simply as a fuzzy blob of light. All comets appear this way when far from the Sun. Tails usually begin to become visible only within about 1-1/2 AU of the Sun.

In 1986, the ion and dust tails of Halley should be fairly well separated. In mid-March, the ion tail should extend about 15 degrees‹ representing a maximum length of about 80 million km (50 million miles)‹in the direction opposite the Sun. At that time, the dust tail will appear to be spread out like a fan, rather than edgewise as in 1910. It will appear to the north of the ion tail as a sort of heart-shaped blob, 2 degrees in length, with its point at the coma. (For scale, the Moon is about half a degree in diameter.)

The tails will be visible only in clear, Moon-less skies far from any city lights and air pollution, looking like faint, misplaced pieces of the Milky Way.

Lifetime

With each orbit around the Sun, a comet the size of Halley loses an estimated 1 to 3 meters (3 to 10 feet) of material from the surface of its nucleus. Thus, as a comet ages, it eventually dims in appearance and may lose all the ices in its nucleus. The tails disappear at that stage, and the comet finally evolves into a dark mass of rocky material or perhaps dissipates into dust.

Scientists calculate that an average periodic comet lives to complete about 1,000 trips around the Sun. Halley has been in its present orbit for at least 16,000 years, but it has shown no obvious signs of aging in its recorded appearances.

References

Belton, Michael J.S., 1979, "The Impact of the Halley/Tempel 2 Mission on Cometary Physics" in COMENTARY MISSIONS, PROCEEDINGS OF A WORKSHOP ON COMENTARY MISSIONS, W. I. Axford, Hugo Fechtig, Jergen Rahe, eds., Bamberg, West Germany: Astron. Institute der Universitat Erlangen Nurnberg, p. 221.

Calder, Nigel, 1980, THE COMET IS COMING, New York: The Viking Press, p. 44.

Cravens, Gwyneth, 1983, "Toasting Halley's Comet" in "Our Footloose Correspondents," THE NEW YORKER, 59 (June 27, 1983), p. 76-79.

Hughes, David W., 1983, "How to Say Halley" in "News and Views," NATURE, 304 (July 14, 1983), p. 119-120.