By Tim Lambert
Comets are balls of ice and organic material ‘leftover’ from when the Solar System formed. They originate on the outer limits of the Solar System, past the orbit of Neptune. However, most comets have highly elliptical orbits, so for part of their orbits, they approach the Sun more closely. As they approach the Sun on their long orbits, some of their mass turns to gas, which is visible on Earth as a comet’s tail.
People have observed comets for thousands of years, but for most of history, they did not know what they were. On 30 March 239 BC, Chinese astronomers first recorded seeing Halley’s Comet.
In Europe, in the Middle Ages, comets were seen as bad omens. It was widely believed that they portended the death of a king or some other disaster. In 1066, a comet appeared (Halley’s comet), and many people thought it was a bad omen. The comet is shown on the Bayeux Tapestry.
In 1577, Tycho Brahe observed a comet, and he proved it was further away than the Moon. The Greek philosopher Aristotle thought comets were atmospheric phenomena, but Tycho showed that this was not the case.
The Great Comet of 1680 was discovered by astronomer Gottfried Kirch. It was a very bright comet, and it greatly helped scientific understanding of comets. Isaac Newton calculated its orbit.
In 1705, Halley published A Synopsis of the Astronomy of Comets. In it, he showed that comets sighted in 1531, 1607, and 1682 must all be the same comet returning at regular intervals. He predicted this comet would return in 1758. After his death, the comet returned in 1758 as he had predicted. It was named Halley’s Comet after him.
The astronomer Caroline Herschel (1750-1848) discovered several new comets. In 1847, Maria Mitchell became the first American woman to discover a comet.
In 1985, Japan launched its first interplanetary spacecraft. It was called Sakigake, and it was sent to explore a comet, P/Halley.
In 2004, the Stardust space probe collected dust from a comet’s tail and later returned it to Earth. It was the first time material from a comet had been collected.
In 2014, Philae became the first spacecraft to land on a comet.
