10 Interesting Facts about Halley’s Comet (1P/Halley)

10 Interesting Facts about Halley’s Comet

Halley’s Comet, also designated 1P/Halley as it was the first comet to be recognized as periodic, is the only comet that is known to be visible without optical aid during each apparition, which occurs once every 74 to 79 years. Although Halley’s Comet will next appear in mid-2061, technology has now improved to the point where astronomers can actually observe it at any point in its orbit. Below are ten more facts about Comet Halley that you may not have known:

Halley was the first comet to be recognized as periodic

Up until the Renaissance, it was commonly believed that comets were mere disturbances in Earth’s atmosphere. However, while Tycho Brahe used parallax measurements to demonstrate that comets were located beyond the Moon, it was Edmond Halley who showed in his treatise called ‘Synopsis of the Astronomy of Comets’ that the comets that appeared and were recorded in 1531, 1607, 1680 and 1682 were in fact the same comet, but seen at different points in its orbit. In 1910, the comet’s pass was particularly impressive as it flew within 13.9 million miles (22.4 million kms) of the Earth, or around 1/15 of the distance separating the Earth and Sun

Halley was first recorded in 240 BC

Chinese observers recorded the appearance of a comet in the ‘Records of the Grand Historian’, which recording is now believed to be the first authenticated sighting of Comet Halley. This recording describes the apparition as having “appeared in the east, and moved north”. One prior sighting, that of 164 BC, is recorded on two Babylonian clay tablets, although it is yet to be confirmed whether this record is of Halley or not.

Halley associated with two meteor showers

Since Halley passes close by Earth twice in a single orbit, we pass through two debris trails, the first of which is the debris that we see as the Orionids in late October each year. While Comet Halley is recognized as the origin of the Orionids meteor shower, it is not yet certain whether the comet is also the origin of the Eta Aquarid meteor shower, or whether the comet’s close passage merely perturbs the debris that we see as the Eta Aquariids shower in May.

Halley creates its own atmosphere

When the comet approaches the Sun, sublimating ices that include water, carbon monoxide, and carbon dioxide ice create an “atmosphere” that can be as much as 100,000 km across- which is pretty impressive, given that the comet’ nucleus is only about 15 km long, about 8 km wide, and about 8 km thick. Much of this atmosphere is blown away by the solar wind to create a tail that can be as long as 10 million km.

Halley has lost most of its mass

Although Halley is thought to have a lifetime of about 10 million years and has a current mass of about 2.2 hundred trillion kg, and a density of roughly 0.6 g per cm3, recent studies based on the amount of material that is lost through sublimation at each close approach to the Sun, suggest that the comet had lost at least 80% -90% of its original mass over its last 2,000-3,000 orbits. Based on this, it is believed by many investigators that the comet will lose enough mass over the next few tens of millenia to either evaporate completely, split into two pieces, or be expelled from the solar system a few hundred thousand years from now.

Halley was not the Star of Bethlehem

While many astronomers and theologians through the ages have identified the appearance of Halley’s Comet in the year 12 BC as the Star of Bethlehem, the fact is that there are many recorded instances of planetary conjunctions and comets that had occurred/appeared between the year 12 BC and the conventionally accepted date of the birth of Jesus Christ. One possible candidate is the comet that appeared between March 9th and April 5th in the year 5 BC, which appearance was noted by Chinese observers.

Halley’s Comet really brought bad luck

King Harold II and Halley's CometAlthough we may scoff at the idea that comets are harbingers of disaster today, the fact is that Halley’s Comet really brought war, famine, and ill-fortune to many. For instance, the 1456 apparition of the comet coincided with the invasion of the Hungarian Kingdom by the Ottoman Empire, which invasion ended with the Siege of Belgrade in July of that year.

A previous apparition of the comet in 1066 accompanied the overthrow of the King Harold II, by William the Conqueror at the Battle of Hastings. In the Anglo-Saxon Chronicle by Eilmer of Malmesbury, the apparition of the comet in the year 989 AD apparition of the comet is described in 1066 thus:

“You’ve come, have you? … You’ve come; you source of tears to many mothers, you evil. I hate you! It is long since I saw you; but as I see you now, you are much more terrible, for I see you brandishing the downfall of my country. I hate you!”

Halley’s perihelion is delayed at each orbit

While it is thought that the comets’ orbit has been stable for between 16,000, and 200,000 years, analysis of its orbit has shown that its perihelion (which is when the comet is closest to the Sun) is being delayed by about four days on each occasion. It is generally accepted that gravitational influences play a role in this, but the main driver of the delay seems to be the huge effect sublimation has on the comet’s orbit. In effect, the jet blasts that are created when ice sublimates are literally blowing the nucleus off its course. This effect has caused the comet’s orbital period to vary by as much as five years since the sighting of 240 BC. Interestingly, the aphelion (furthest) and perihelion (closest)
that Halley’s Comet gets from the Sun is 35 AU (Astronomical Units) and 0.57 AU respectively, meaning that it can get as close to the Sun as Venus or a far away as Pluto.

Halley is a snowy dirt ball

Halley's Comet Although comets are for the most part dirty snowballs, as Fred Whipple predicted in 1950, Halley’s composition is that of a pile of rubble that is loosely bound by gravity. Observations made by the Vega 1 space craft showed that most of the comet’s nucleus was covered in a pitch black layer of dust, with only pockets of various ices that sublimate when it approaches the Sun. This is in stark contrast with most other comets that consist primarily of various ices with only pockets of rubble frozen into the matrix.

Halley has the highest velocity relative to Earth in the solar system

Since the comet orbits the Sun in a direction opposite to that of the planets, it is moving faster relative to Earth’s movement around the Sun than any other body in the solar system. During its passage in 1910, the comet was measured to move at 70.56 km/sec, relative to Earth, which translates into a speed of 254,016 km/h (157,838 mph).

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