Halley's Comet facts for kids

Halley's Comet on 8 March 1986

Halley's Comet (Comet Halley) is a comet which comes round every 75 or 76 years. When it is near, it can be seen with the naked eye. It will return in 2061.

When the comet came close to the Earth in 1986, it was visited by several space-probes. The probe Giotto from the European Space Agency managed the closest approach to the comet.

The number of years that the comet finishes its full cycle can vary depending on the effect of another planet's gravitational pull.

In 1986, Halley's Comet was the first to be observed in detail by spacecraft. It gave the first data on the structure of a comet nucleus and how the coma (nebulous envelope around the core or nucleus) and the tail formed. These observations supported Fred Whipple's "dirty snowball" model. This correctly predicted that Halley would be composed of a mixture of volatile ices – such as water, carbon dioxide, ammonia, – and dust. The missions also adjusted these ideas. For example, it is now known that the surface of Halley is mostly dusty, non-volatile materials, and that only a small portion of it is icy.

Orbit and origin

The orbital path of Halley, against the orbits of the planets (animation)

Orionid meteor originating from Halley's Comet streaking the sky below the Milky Way and to the right of Venus

Halley's orbital period has varied between 74 and 79 years since 240 BC. The perihelion, the point in the comet's orbit when it is nearest the Sun, is 0.59 au (88 million km). This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 au (5.2 billion km) (roughly the distance of Pluto). 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.

Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less. This contrasts it with long-period comets, whose orbits last for thousands of years.

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.

Structure and composition

The nucleus of Halley's Comet, imaged by the Giotto probe in 1986. The dark coloration of the nucleus can be observed, as well as the jets of dust and gas erupting from its surface.

The Giotto and Vega missions gave planetary scientists their first view of Halley's surface and structure. Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as water, carbon monoxide, carbon dioxide and other ices) begin to sublimate from the surface of its nucleus. This causes the comet to develop a coma, or atmosphere, up to 100,000 kilometres (62,000 mi) across. Evaporation of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as fluorescence, whereas dust particles scatter the solar light. Both processes are responsible for making the coma visible. As a fraction of the gas molecules in the coma are ionized by the solar ultraviolet radiation, pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long tail, which may extend more than 100 million kilometres into space. Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.

Despite the vast size of its coma, Halley's nucleus is relatively small: barely 15 kilometres (9.3 mi) long, 8 kilometres (5.0 mi) wide and perhaps 8 kilometres (5.0 mi) thick. Its shape vaguely resembles that of a peanut shell. Its mass is relatively low (roughly 2.2 × 10¹⁴ kg) and its average density is about 0.6 grams per cubic centimetre (0.35 oz/cu in), indicating that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile.

History

Before 1066

Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22 and 28 September 164 BC, Babylon, Iraq. British Museum

Halley may have been recorded as early as 467 BC, but this is uncertain. A comet was recorded in ancient Greece between 468 and 466 BC; its timing, location, duration, and associated meteor shower all suggest it was Halley.

Report of Halley's Comet by Chinese astronomers in 240 BC (Shiji)

The first certain appearance of Halley's Comet in the historical record is a description from 240 BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north. The only surviving record of the 164 BC apparition is found on two fragmentary Babylonian tablets, now owned by the British Museum.

The apparition of 87 BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month. This appearance may be recalled in the representation of Tigranes the Great, an Armenian king who is depicted on coins with a crown that features, according to Vahe Gurzadyan and R. Vardanyan, "a star with a curved tail [that] may represent the passage of Halley's Comet in 87 BC."

Possible report of Halley's Comet in the Talmud (MS Paris 1337).

In 837, Halley's Comet may have passed as close as 0.03 au (3.2 million miles; 5.1 million kilometres) from Earth, by far its closest approach. Its tail may have stretched 60 degrees across the sky. It was recorded by astronomers in China, Japan, Germany, the Byzantine Empire, and the Middle East; Emperor Louis the Pious observed this appearance and devoted himself to prayer and penance.

1066

Halley's Comet in 1066 depicted in the Bayeux Tapestry

Halley's Comet seen from London on 6 May 1066 as simulated by Stellarium. The Moon, Mars, Jupiter, and Saturn are also visible.

In 1066, the comet was seen in England and thought to be an omen: later that year Harold II of England died at the Battle of Hastings and William the Conqueror claimed the throne. The comet is represented on the Bayeux Tapestry and described in the tituli as a star. Surviving accounts from the period describe it as appearing to be four times the size of Venus, and shining with a light equal to a quarter of that of the Moon. Halley came within 0.10 au of Earth at that time.

Chaco Native Americans in New Mexico may have recorded the 1066 apparition in their petroglyphs.

1145–1378

The Adoration of the Magi (circa 1305) by Giotto, who purportedly modelled the star of Bethlehem on Halley, which had been sighted 4 years before that painting.

The 1145 apparition was recorded by the monk Eadwine. The 1986 apparition exhibited a fan tail similar to Eadwine's drawing. Some claim that Genghis Khan was inspired to turn his conquests toward Europe by the 1222 apparition. The 1301 apparition may have been seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-colored comet in the Nativity section of his Arena Chapel cycle, completed in 1305.

1456

In 1456, the year of Halley's next apparition, the Ottoman Empire invaded the Kingdom of Hungary, culminating in the siege of Belgrade in July of that year. In a papal bull, Pope Callixtus III ordered special prayers be said for the city's protection. In 1470, the humanist scholar Bartolomeo Platina wrote in his Lives of the Popes that,

A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestilence, dearth and some great calamity. Calixtus, to avert the wrath of God, ordered supplications that if evils were impending for the human race He would turn all upon the Turks, the enemies of the Christian name. He likewise ordered, to move God by continual entreaty, that notice should be given by the bells to call the faithful at midday to aid by their prayers those engaged in battle with the Turk.

1456 comet in Zodiac

After witnessing a bright light in the sky which most historians have identified as Halley's Comet, Zara Yaqob, Emperor of Ethiopia from 1434 to 1468, founded the city of Debre Berhan (tr. City of Light) and made it his capital for the remainder of his reign.

1531

In the Sikh scriptures of the Guru Granth Sahib, the founder of the faith Guru Nanak makes reference to "a long star that has risen" at Ang 1110, and it is believed by some Sikh scholars to be a reference to Halley's appearance in 1531.

1531–1759

Illustrations of prior comet appearances in
the January 1910 Popular Science Monthly magazine

1682	1759	1835

Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict it would return. One key breakthrough occurred when Halley talked with Newton about his ideas of the laws of motion. Newton also helped Halley get Flamsteed's data on the 1682 apparition. By studying data on the 1531, 1607, and 1682 comets, he came to the conclusion these were the same comet, and presented his findings in 1696.

One difficulty was accounting for variations in the comet's orbital period, which was over a year longer between 1531 and 1607 than it was between 1607 and 1682. Newton had theorized that such delays were caused by the gravity of other comets, but Halley found that Jupiter and Saturn would cause the appropriate delays. In the decades that followed, more refined mathematics would be worked on, notable by Paris Observatory; the work on Halley also provided a boost to Newton and Kepler's rules for celestial motions.

1910

A photograph of Halley's Comet taken during its 1910 approach

Halley in April 1910, from Harvard's Southern Hemisphere Station, taken with an 8-inch Bache Doublet

Infographic from the January 1910 issue of Popular Science Monthly magazine, showing how Halley's tail points away from the Sun as it passes through the inner Solar System

The 1910 approach, which came into naked-eye view around 10 April and came to perihelion on 20 April, was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained. Furthermore, the comet made a relatively close approach of 0.15 au, making it a spectacular sight.

The comet added to the unrest in China on the eve of the Xinhai Revolution that would end the last dynasty in 1911.

1986

Halley's Comet in 1986

1986 USSR miniature sheet, featuring Edmond Halley, Comet Halley, Vega 1, Vega 2, Giotto, Suisei (Planet-A)

Daily motion across sky during the 1986 passage of Halley's Comet

Animation of 1P/Halley orbit - 1986 apparition
      1P/Halley ·       Earth ·       Sun

The 1986 apparition of Halley's Comet was the least favourable on record. In February 1986, the comet and the Earth were on opposite sides of the Sun, creating the worst possible viewing circumstances for Earth observers during the previous 2,000 years. Additionally, increased light pollution from urbanization caused many people to fail in attempts to see the comet. With the help of binoculars, observation from areas outside cities was more successful. Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April 1986, with best opportunities occurring when the comet could be sighted close to the horizon at dawn and dusk, if not obscured by clouds.

In 1986, scientists had the opportunity to study the comet closely and several probes were launched to do so. The Soviet Vega 1 probe began returning images of Halley on 4 March 1986, captured the first-ever image of its nucleus, and made its flyby on 6 March. It was followed by the Vega 2 probe, making its flyby on 9 March. On 14 March, the Giotto space probe, launched by the European Space Agency, made the closest pass of the comet's nucleus. There also were two Japanese probes, Suisei and Sakigake. Unofficially, the numerous probes became known as the Halley Armada.

Based on data retrieved by the largest ultraviolet space telescope of the time, Astron, during its Halley's Comet observations in December 1985, a group of Soviet scientists developed a model of the comet's coma. The comet also was observed from space by the International Cometary Explorer (ICE). Originally the International Sun-Earth Explorer 3, the spacecraft departed the Sun-Earth L₁ Lagrangian point in order to intercept comets 21P/Giacobini-Zinner and Halley. ICE flew about 40.2 million km (25 million mi) from Halley's Comet on 28 March 1986.

Two U.S. Space Shuttle missions—STS-51-L and STS-61-E—had been scheduled to observe Halley's Comet from low Earth orbit. The STS-51-L mission carried the Shuttle-Pointed Tool for Astronomy (SPARTAN-203) satellite, also called the Halley's Comet Experiment Deployable (HCED). The mission ended in disaster when the Space Shuttle Challenger exploded in flight, killing all seven astronauts onboard. Scheduled for March 1986, STS-61-E was a Columbia mission carrying the ASTRO-1 platform to study the comet, but the mission was canceled following the Challenger disaster and ASTRO-1 would not fly until late 1990 on STS-35.

After 1986

Halley's Comet observed in 2003 at 28 au from the Sun

On 12 February 1991, at a distance of 14.4 au (2.15×10⁹ km) from the Sun, Halley displayed an outburst that lasted for several months, releasing a cloud of dust 300,000 km (190,000 mi) across. The outburst likely started in December 1990, and then the comet brightened from magnitude 24.3 to magnitude 18.9. Halley was most recently observed in 2003 by three of the Very Large Telescopes at Paranal, Chile, when Halley's magnitude was 28.2. The telescopes observed Halley, at the faintest and farthest any comet has ever been imaged, in order to verify a method for finding very faint trans-Neptunian objects. Astronomers are now able to observe the comet at any point in its orbit.

On 9 December 2023, Halley's Comet will reach the farthest and slowest point in its orbit from the Sun when it will be traveling at 0.91 km/s (2,000 mph) with respect to the Sun.

2061

Animation of 1P/Halley orbit - 2061 apparition
      Sun ·       Venus ·       Earth ·       Jupiter ·       1P/Halley

The next perihelion of Halley's Comet is 28 July 2061, when it will be better positioned for observation than during the 1985–1986 apparition, as it will be on the same side of the Sun as Earth. The closest approach to Earth will be one day after perihelion. It is expected to have an apparent magnitude of −0.3, compared with only +2.1 for the 1986 apparition. On 9 September 2060, Halley will pass within 0.98 au (147,000,000 km) of Jupiter, and then on 20 August 2061 will pass within 0.0543 au (8,120,000 km) of Venus.

2134

Halley will come to perihelion on 27 March 2134. Then on 7 May 2134, Halley will pass within 0.092 au (13,800,000 km) of Earth. Its apparent magnitude is expected to be −2.0.

Interesting facts about Halley's Comet

• Comet Halley is commonly pronounced rhyming with valley, or rhyming with daily.
• The comet is named after Edmond Halley (1656–1742), an English astronomer, who in 1705 predicted the comet's return.
• Halley's Comet was the first comet to be recognized as periodic. (Periodic means that it comes by Earth regularly.)
• It is the only naked-eye comet that can appear twice in a human lifetime.
• Halley has probably been in its current orbit for 16,000–200,000 years.
• Halley is the most active of all the periodic comets.
• Halley's appearance in 12 BC, only a few years distant from the conventionally assigned date of the birth of Jesus Christ, has led some theologians and astronomers to suggest that it might explain the biblical story of the Star of Bethlehem.
• The comet was used in an advertising campaign of Le Bon Marché, a well-known department store in Paris.
• American satirist and writer Mark Twain was born on 30 November 1835, exactly two weeks after the comet's perihelion. In his autobiography, published in 1909, he said,

  I came in with Halley's comet in 1835. It is coming again next year, and I expect to go out with it.'

  Twain died on 21 April 1910, the day following the comet's subsequent perihelion.

Related pages

• List of comets

See also

In Spanish: Cometa Halley para niños