April 10, 837 – Halley’s Comet makes its closest approach to Earth at a distance equal to 0.0342 AU (5.1 million kilometres/3.2 million miles).
Halley’s Comet is a short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is clearly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.
Halley’s returns to the inner Solar System have been observed and recorded by astronomers since at least 240 BC. Clear records of the comet’s appearances were made by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object at the time. The comet’s periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named.
During its 1986 apparition, Halley’s Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple’s “dirty snowball” model, which correctly predicted that Halley would be composed of a mixture of volatile ices – such as water, carbon dioxide, and ammonia – and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, now it is understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.
In 1986, Great Britain issued this 31-pence stamp portraying Halley’s Comet and the phrase “Maybe Twice in Lifetime.” The use of the word ‘maybe’ on the 31p British stamp issued in 1986 to commemorate the comet’s appearance that year reflects the increased chance of one’s opportunity to see the comet more than once during their life. The artist Ralph Steadman drew the four stamps issued by Royal Mail for this event and received a BBC Design Award for the set.