9.17: Halley’s Comet
Perhaps the best-known comet, Halley’s Comet, has been a regular visitor through the solar system for thousands of years. Halley is pronounced like Valley … hæli. The comet has been observed and recorded since at least 240 BC. Records of Halley’s passages were made by the Chinese, Babylonians, and medieval Europeans. Recall, Edmund Halley was the first astronomer to determine that Halley’s comet was the same comet that passed by Earth every 75-76 years. The 1910 Earth passage of Halley’s Comet brought much scientific revelation, and much fear. Cyanogen gas was discovered by astronomers during the 1910 passage, leading to mass panic among the public. People were selling comet pills, comet umbrellas, and gas masks.
Halley’s Comet was the first comet to be visited by spacecraft, in 1986; Vega 1 (USSR) and Giotto (European Space Agency). And, interestingly, it turns out that in the last 2,000 years the least-visible or poorest passage of Halley’s Comet to Earth was in 1986. The next close passage of Halley’s Comet will be July 2061; so mark your calendars!