Halley’s Comet compared to Earth

Famous short-period comet, returns every 75 to 76 years, next in 2061.

How big is Halley’s Comet compared to Mount Etna?

Halley’s Comet is roughly two fifths as wide as Mount Etna.

Halley’s Comet orbits on average about eighteen times as far from the Sun as Earth does.

Halley’s Comet takes about 75 Earth years to go once round the Sun. That is nearly a whole human lifetime.

One spin: complex, tumbling rotation — no single, simple “day”.

What it does near the Sun: As it nears the Sun, its ice turns straight to gas, creating the glowing coma and long tail that always points away from the Sun.
How it moves: Its period varies from 74 to 79 years. It swings from closer to the Sun than Venus out past Neptune (0.586 AU to 35.08 AU).; How we know: Astronomer Edmond Halley realised in 1705 that comets seen in 1531, 1607 and 1682 all had nearly identical paths — using Newton’s new laws of gravity, he concluded they were one comet and predicted its return in 1758; he died in 1742, and it was seen again on Christmas night 1758.
The machines we sent: The Giotto spacecraft flew past its nucleus in 1986, taking the closest pictures of a comet’s heart anyone had then taken — a week after the Soviet Vega probes took the first.

The numbers

Longest dimension	14.9 km
Distance from the Sun	17.83 AU on average, from 0.586 to 35.08 AU · sunlight takes 2 hours 28 minutes at its average distance
Once round its orbit	about 75 Earth years
Also found there	water, carbon dioxide