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Halley's Comet

Halleys Comet

Halley's Comet, 1910. Wikipedia

Halley's comet is probably the most well known of all the comets. It approaches the sun every 75-76 years and and its visits may have been recorded as early as 467 BC.

Our desktop app shows the position of 1P/Halley (Halley's Comet) from 1600 AD to 2200 AD. The data is from the NASA's JPL website. Outside of those dates the position cannot be calculated with much certainty. The Halley's Comet shown on our comets page uses a certain amount of averaging to get an approximate standard orbit for the comet. This page is more precise and shows how the orbit has changed on each successive pass of the Sun.

Where is Halley's Comet right now?

In desktop version mode, you will see the current location of Halley's Comet. You can then wind time forwards and backwards to accurately see the position of Halley's Comet from any time between 1600 AD to 2200 AD. In Mobile version mode, a video shows the position of Halley's comet from 1900 to 2090 with a close up, slowed down section for the 2061 visit.

The NASA data for comets only covers the years 1600 AD - 2200 AD. This is because it is difficult to predict accurately the path of comets over any great time. This is due to the fact that the comet is ejecting matter on every flyby of the sun which means its mass is constantly changing and the ejection of the gasses can act as tiny rocket motors gently pushing the comet off course.

When will Halley's Comet Return?

Halley's comet will continue to journey away from the Sun until it reaches it's aphelion (furthest point from the Sun at 35.1 Au) around November/December 2023 (different sources give different dates). It will then begin it's long 37.6 year fall back towards the Sun.

It is predicted that Halley's Comet will next reach perihelion (closest point to the Sun) on 28 July 2061. The Earth will be in a better viewing position than it was in 1985-1986 since it will be on the same side of the Sun as the comet. The comet is expected to be about as bright as the brightest stars (apparent magnitude -0.3).

In 2134, Halley's Comet will pass very close to the earth (0.09 AU = 13 million km) and be much brighter than the brightest star (apparent magnitude -2).

Halley's Comet Orbit

Halley's comet has a long thin orbit that stretches out past the orbit of Neptune at its furthest point and comes in closer than the orbit of Venus at its nearest point. Its orbit is in a plane that is an angle of 18 degrees to the plane of the ecliptic and it spends about 98% all of its life on the southern side of the ecliptic. Only when the comet approaches the Sun does it loop over onto the northern side of the ecliptic.

You can see its distance from the ecliptic by enabling the 3D view - using the 2D/3D button.

Unusually, Halley's Comet orbits the sun in the opposite direction to most other bodies. This means its inclination to the plane of the ecliptic is officially 162 degrees to take into account its retrograde motion.

It is thought that Halley has been in its current orbit for around 60,000 to 200,000 years. However it is very difficult to calculate any comets previous orbits to much accuracy due to the fact that the orbit is always altered as they pass close by the Sun. This is because the heat of the Sun causes the comet to warm up which means that some of its ice turns into water vapour (through sublimation) and is ejected into space which results in gas and dust particles being released which form the comets tails. However the loss of mass and the velocity at which it's released alter the comets orbit in a manner which could not/cannot be predicted. This means a comet needs to be observed to determine its new trajectory after each encounter with the Sun.

Halley's orbit can vary by quite a bit due to many factors. One is that it looses mass and the ejection of that mass acts like rocket motors, pushing the comet off course. The other effects are gravitational speed-ups and slow downs that can occur as the comet passes by the gas giants Jupiter, Saturn, Uranus and Neptune.


Halley's Comet Nucleas

Halley's Comet Nucleus taken by the spacecraft Giotto in 1986. Source: ESA.

Halley's Comet has a small nucleus which is peanut shaped and about 15km long by 8km wide and deep. It weighs around 2.2x1014 kg and rotates once very 2.2 days. It has a low density and is thought to be a "rubble pile" in that it is a loose conglomeration of various small particles. The analogy that comets are a dirty snowball is reasonably correct for Halley except that its surface is extremely dark - about as black as coal. This means that a more correct analogy might be that Halley is an snowy dirt-ball rather than a dirty snowball!

When the Giotto spacecraft visited Halley on its last visit in 1986, it found that the gases ejected from the nucleus were 80% water vapour, 10% carbon monoxide and 2.5% methane and ammonia with traces of hydrocarbons, iron and sodium. The dust ejected from the comet was comprised mostly of particles the size of cigarette smoke although the spacecraft Giotto was damaged and knocked out of alignment by a particle of between 0.1 gram to 1 gram. Two kinds of dust were seen: one with carbon, hydrogen, nitrogen and oxygen; the other with calcium, iron, magnesium, silicon and sodium.

Giotto also determined that the comet was ejecting matter at 16 tonnes per second which caused the nucleus to wobble over a long time period.

Giotto Encounter

Above is a good video describing comets and the fleet of spacecraft that went to meet Halley's comet in 1986.

If you've got 90 minutes to spare then why not watch the BBC's excellent coverage of the Giotto Encounter below:

This video has a great deal of information on the spacecraft, the comet, and also captures the excitement of this risky encounter.