Comet Halley | Universe Today

Written by Jerry Coffey

Halley's Comet or Comet Halley is the best known of the periodic comets. The reason it is so famous is that it can be seen by the unaided eye every 75-76 years. It is also the only periodic comet that a person can possible see with the unaided eye twice in their life time. The comet has been observed since 240 B.C., was not recognized as a periodic comet until its orbit was calculated by Edmund Halley in the eighteenth century. Comet Halley last appeared in the inner solar system in 1986, and will next appear in mid-2061.

The orbit of the comet Halley is a rough ellipse with the perihelion, at 0.6 astronomical units (AU). The aphelion, is 35 AU, which is about how far Pluto is from the Sun. The comet has many irregular things about it. It is something of an irregularity for a short-period comet because they normally have an average orbit of 6.5 years and most of them have orbital periods shorter than 20 years.Halley's is more than thrice that. Comets with orbits similar to Halley’s Comet – with orbital periods longer than 20 years and inclinations from 0 to 90 degrees – are called Halley-type comets. Scientists believe that Halley-type comets used to be long-period comets until the gravity of the gas giants threw them into the inner solar system. Unusual for an object in the solar system, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or clockwise from above the Sun's north pole. Its orbit is highly inclined (18°) to the ecliptic, with much of it lying below the orbits of the planets. Due to Halley's highly eccentric orbit, it has one of the highest velocities relative to the Earth in the Solar System. The 1910 passage had an approximate velocity of 70.56 km per second.

The Giotto space mission was the first to flyby comet Halley. It gave planetary scientists their first view of Halley comet's surface and structure. At times the comet's coma may extend 100 million km into space, Halley's nucleus is relatively small, barely 15 km long, 8 km wide and around 8 km thick. In fact, JPL lists its average diameter as only 11 km and roughly peanut-shaped. Its mass is extremely low; roughly 2.2×10¹⁴kg. Its average density is about 0.6 g/cm³, indicating that it is very loosely constructed. Its albedo is about 4%, meaning that only 4% of the sunlight hitting it is reflected. Making it about as reflective as tar.

There are many good articles about comet Halley on the internet. Try this one for some good information. Here on Universe Today we have several great articles about the comet. These two are the best: here and here. Comet Halley spends most of its time in the icy outer solar system. Astronomy Cast offers a good episode about those cold hinterlands.

Filed under: Astronomy