Earth Passed Through Halley's Comet's Tail

When Halley’s Comet swung past Earth in 1910, it put on a show no living human had witnessed before. The comet reached its closest approach on the night of May 18-19, 1910, passing within roughly 22 million kilometres of Earth, and for about six hours our planet drifted through the faint outer fringe of its enormous tail.

The science behind the spectacle had stirred dread. Earlier that year, astronomers using spectroscopy announced traces of cyanogen, a poisonous gas, in the comet’s tail. Sensational newspapers seized on the finding, and some warned the gas might smother all life on Earth.

Con artists hawked “anti-comet pills” and gas masks while frightened families sealed their windows shut.

In reality the tail was so diffuse it was nearly a vacuum, and the trace gas posed no danger whatsoever. Earth passed through with no measurable effect on the atmosphere. The 1910 visit was also historic for astronomers: it was the first apparition ever recorded in photographs and the first studied with spectroscopic instruments.