1682

Halley’s Comet

Edmond Halley (1656–1742)

Comets are occasional, often dramatic interlopers among the otherwise clockwork regularity of the solar system. Early Chinese astronomers recorded them as “broom stars” with long tails. Isaac Newton suspected that at least some of them orbit the Sun, though he didn’t pursue proving the idea.

But the idea was soon picked up by Newton’s friend and colleague Edmond Halley, an English astronomer, geophysicist, and mathematician. Halley observed a bright comet that appeared in 1682; later, using historical records and Newton’s Laws to calculate its orbit, he proposed that it was the same comet that had been seen in 1531 and 1607. These spacings of approximately 76 years enabled him to predict that the comet would return again in 1758. It did, and while Halley did not live to see it return, astronomers named it Halley’s comet (now just “Halley”) in his honor.

More modern telescopic and photographic studies of the comet were made during its 1835 and 1910 passes, and the 1986 Soviet Vega and European Giotto probes flew close by Halley’s central nucleus, revealing it to be surprisingly small (approximately 9 × 5 × 5 miles [15 × 8 × 8 kilometers]), peanut-shaped, rugged, porous (with a density of approximately 0.6 grams per cubic centimeter), and dark as coal. Bright jets of ices composed of water, carbon monoxide, and carbon dioxide sublime from the surface and interior, releasing dust and organic molecules and creating a long tail.

The nucleus of Halley’s comet, photographed by the European Space Agency Giotto spacecraft on March 13, 1986, from inside the coma of gas and dust surrounding the comet.

Halley was the first discovered periodic comet. Nearly 500 other “short-period” (less than 200 years) comets are now known; most come from the Kuiper Belt, beyond Neptune, but some, including Halley, may have started as long-period comets from the distant Oort Cloud. Astronomers have found that more than 20 bright comets recorded between 240 BCE and 1682 CE were all actually previous apparitions of Halley’s comet. Its next pass will be in the summer of 2061.

Halley’s comet, in a negative photo from Mauna Kea Observatory on March 5, 1986, just after its closest approach to the Sun during its latest 76-year journey.