Halley's Comet

Halley’s comet is arguably the most famous of all comets. It is a periodic comet that returns to Earth every 75 years. The last time it was here was in 1986, and it is projected to return in 2061. It is named after the Engish astronomer Edmond Halley, who examined the reports of a comet approaching the Earth in 1531, 1607 and 1682. He later concluded that these comets were the same returning over and over again.

Origin and Orbit of Halley’s Comet

Halley’s comet is believed to have originated from the Kuiper Belt. Some of the blocks of rock and ice – which are essentially the leftover matter from the formation of the Solar System some 4.6 billion years ago – are pulled deeper into the Solar System and become active
comets. Halley’s comet is classified under a periodic or a short-period comet, with the orbit lasting for 20 years or less. This contrasts with long-period comets whose orbit lasts for thousands of years and that which originates from the Oort cloud. The orbit of Halley’s comet around the Sun is highly elliptical and retrograde. This means that it orbits around the Sun in the opposite direction to the planets. Due to its retrograde orbit, its velocity is highest relative to the Earth.

Structure and Composition

As Halley approaches the Sun, it expels jets of sublimating gases from its surface, which knocks it very slightly off its orbital path. This process causes the comet to form a bright tail of ionized gas (ion tail), and a faint one made up of dust particles. The ion tail is also known as a coma (a small atmosphere) which spans up to 100,000 km across and consists of volatiles such as water, methane, ammonia and carbon dioxide.

As Halley approaches the Sun, it expels jets of sublimating gases from its surface that knock it off very slightly from its orbital path. This process results in the comet to form a bright tail of ionized gas and a faint one made up of dust particles. The ion tail is known as a coma that spans up to 100,000 km across and consists of volatiles such as methane, water, ammonia, and carbon dioxide.

Despite the vast coma, Halley’s comet is relatively small with a nucleus of 15 kilometres long, 8 kilometres wide and roughly 8 kilometres thick. Its mass is estimated to be 242.5 billion tons and its average density to be 0.6 g/cm3. It was observed that the gases ejected were made of 80% water, 3-4 % carbon dioxide, 17 % carbon monoxide with traces of hydrocarbons.

The last time Halley’s comet was seen was in 1986, and it is expected to be seen next in 2061. As expected, some are choosing to prepare for the worst, indicating the end of life. Others are contemplating if they will live long enough to witness it.

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