History of Halley's Comet

It was August in 1682 when the faint gleam first appeared in the night sky. From one night to the next, it grew slightly brighter and changed position among the background stars. The pioneers of astronomy peered at it through that new-fangled invention, the telescope, and watched it begin to sprout a tail. Among the majority of people, however, the instinctive reaction was fear. “A comet,” they whispered. “A comet has appeared! And plague, famine, and war will surely follow.”

In an age when people believed the sky could foretell the future, comets were regarded as evil omens. A comet appeared without warning, hung in the sky for a few weeks—just long enough for panic to spread—and then mysteriously vanished. In contrast, the motions of the stars and planets were orderly and predictable. No wonder comets were regarded as the forerunners of evil events! Considering how common plagues, amines, and wars were in those days, it is not surprising that one or the other often followed a comet’s appearance, even though the two events were purely coincidence.

Fact of Fiction

One observer of the 1682 comet chose not to regard it with superstition. A young scientist at Cambridge University, Edmund Halley (rhymes with “valley”), suspected that comets moved in orbits that could be calculated and predicted. At the time, not even the planets' motions could be predicted precisely, although Halley's friend, Isaac Newton, had made a breakthrough in that direction. When Newton's ideas were finally published in 1689, Halley had the information he needed to try to calculate comets' orbits.

Newton's Law of Gravity showed that the orbits of the planets are ellipses. Ellipses have two “centers” called foci. If the foci are very close to each other, as they are in planet orbits, the ellipse resembles a circle.

Halley wondered if the orbits of comets might be ellipses whose foci were far apart. Such ellipses are very long and skinny and don't resemble circles at all.

Calculating orbits for several comets, Halley discovered that they were indeed elliptical and that the comet of 1682 should return every 75 years. Checking through the records for 1607, 75 years before, Halley found that there had been a comet with a similar orbit. For the first time, there was evidence that a comet might return more than once! Going farther back, Halley discovered that yet another comet in 1531 had a similar orbit. Between 1531 and 1607 there was an interval of 76 years, not 75. Could it still be the same comet, and if it was, what had caused the change?

Carefully, Halley checked the positions of the planets over the years in question. He found that the comet had passed near Jupiter on one of its orbits and that the gravitational pull of the giant planet had probably been enough to alter its path a bit—76 years was the usual interval. Confidently, Halley published his findings and predicted the comet's return around 1758. When it finally appeared in 1759 (slightly late because of the gravitational tugs of the giant planets), it was hailed as a confirmation of Newton's Law of Gravity and a triumph of science. In tribute to the man who calculated its path, it was named Halley's Comet.

The Return

1986 marked the fourth predicted return in the history of Halley's Comet (since 1758, it has also been observed in 1835 and 1910). But the comet has been seen many other times in the past before anyone realized it was the same object appearing again and again. The first recorded sighting of Halley's Comet was in 240 B.C., when Chinese astronomers noticed a “hairy star” in the sky. No records have been found of its next predicted return in 164 B.C., but since its appearance in 87 B.C. it has been observed at every passage.

By coincidence, the history of Halley's Comet lined up with several important periods in history and was taken by many as an evil omen, though, of course, it had no real effect on events. For instance, its appearance in 66 A.D. was thought to be a forerunner of the destruction of Jerusalem by the Romans four years later. In 1066, Halley's Comet was visible a few months before the defeat of King Harold of England by William the Conqueror. And in 1456, when Europeans feared the Turks who had conquered Constantinople three years earlier, the comet's appearance led to special prayers for protection against its evil effects.

Even as late as 1910, enough superstition lingered that some people bought “comet pills” to protect themselves from predicted ill effects when the Earth passed through the tail of Halley's Comet. The pills did nothing, of course, but then neither did the comet. By that time, enough people were scientifically educated that many could simply enjoy the comet as a remarkable spectacle without believing it to be an evil omen.

An Intriguing Disappointment

The view in 1986, however, was not quite as thrilling as the world had thought it would be. Halley's Comet spent most of its time on the opposite side of the Sun from Earth, and as a consequence, it was faint, probably fourth magnitude or dimmer. (That's about as bright as the star cluster M-44 in Cancer.) It became first visible to the unaided eye in January 1986, low in the west after sunset. The comet rapidly approached the Sun's glare and was visible all of February, passing closest to the Sun on February 9. In early March, it appeared in the east just before sunrise and remained visible through the first days of April. Early in April, it brightened to third magnitude, since it got closer to the Earth then. At its closest point, Halley's Comet will still be quite far from us, about 40 million miles away, and it will be too far south to be seen from all but the southernmost part of the United States. In late April 1986, the comet returned to the evening sky, probably fourth magnitude or fainter.

Assembling the Halley Armada

Because of this poor view we had from Earth, and also because of developed space technology, many scientists believed we could get our best view of Halley’s Comet by sending a probe to it. A probe may tell us exactly what the comet is made of, which may in turn give hints about the origin of our Solar System. Halley's Comet is the only bright, active comet whose orbit is predictable enough to be investigated by a spacecraft. If it is not explored at this passage, it may be many years before we have another chance for a close-up view of such an outstanding comet.

Know as the Halley Armada, five spacecrafts were sent toward Halley’s Comet: two by Japan, two by the Soviet Union and France, and one by the European Space Agency (ESA). Giotto, a European robotic spacecraft mission from the ESA, flew by the comet and became the first to make up close observations. On March 13, 1986, the mission succeeded and approached Halley’s nucleus from a distance of 596 kilometers. The main Japanese Probe, Planet-A, passed at a great distance from the comet’s nucleus, about 60,000 miles. It studied the huge coma and took ultraviolet pictures. MS-T5, a test probe that was launched about eight months before Planet-A, performed test maneuvers and gathered data on the solar wind. This information was useful in interpreting the Planet-A data, and the ESA probe provided the best images of the nucleus, photographing it from the closest range, about 400 miles. The French and Soviet probes, Vega 1 and Vega 2, helped provide further scientific analysis on the comet, showing it had formed 4.5 billion years ago from volatiles (mainly ice) that had condensed onto interstellar dust particles.

Recommended Reading

While Halley's Comet is one of the best-known comets that becomes visible to Earth, it is not the only, and perhaps not the most interesting. To learn about other comets, or to learn more about Halley's Comet, read COMETS!: Visitors from Deep Space by David J. Eicher.

Dadvid J. Eicher takes a journey through the past, present, and future of comets in COMETS!: Visitors from Deep Space. This book explores the cutting-edge science surrounding what comets are, how they behave, how they are related to one another, and much more.