SkyEye

When beggars die there are no comets seen;
The heavens themselves blaze forth the death of princes.
— William Shakespeare, "Julius Caesar", 1599

What is a Comet?

Comets have been famously described as 'dirty snowballs' and this is probably not so far from the truth. These are ancient objects, left over from the formation of the solar system several billion years ago. Most of them live in the farthest regions of the solar system. The analysis of long-period comets led the Dutch astronomer Jan Oort to hypothesize the existance of a cloud of icy objects existing far outside the solar system but well within the Sun's gravitational influence. Collisions between comets or gravitational nudges by nearby stars disturb these objects, sometimes sending one towards the Sun. The Oort cloud remains a theory, having not yet been observed.

Shortly after Oort's work in 1950, Dutch-American astronomer Gerard Kuiper came up with the idea of another belt of comets outside the orbit of Neptune which could account for the shorter-period comets. Now known as the Kuiper Belt, it too remained a theory for over 40 years until 1992 when astronomers Jewitt and Luu photographed 1992 QB1, the first Kuiper Belt Object.

Except for the shortest-period objects, comets spend most of their time in the icy wastes of the outer solar system. The nucleus of the comet is thought to be a conglomeration of various ices and dirt with a dark crust. As the comet approaches the Sun, the crust absorbs heat and some of the ice begins to turn to gas. Soon the pressure builds up, the crust is broken and a shell of gas and dust called a coma develops. The size of the coma varies, depending on the size of the nucleus and the distance from the Sun. The coma of C/1811 F1 'Great Comet of 1811' was the same diameter as the Sun!

There are two types of cometary tail. The dust tail is comprised of dust particles pushed out of the coma by solar radiation pressure. The long white curving tail so often photographed from Earth is the dust tail. The ion tail or plasma tail is formed when photons from the Sun ionise the gas in the coma. These ions follow the magnetic fields carried by the solar wind and so this tail usually appears pointing directly away from the Sun. Often appearing blue in colour and fainter than the dust tail, the ion tail can look ropey or knotted as the ion bunch together in the solar magnetic field.

Comet Names

The International Astronomical Union controls the naming of comets. Most but not all comets are named for their discoverers. However, every comet receives an alphanumerical designation which conveys information regarding its discovery. Comets are named according to the year in which they were discovered, followed by a letter indicating the half-month and a number showing the order of discovery. (Note that the letter I is omitted and the letter Z is unused in the half month scheme.)

In addition, a prefix is used to describe what kind of comet it is:

P/

A periodic comet with a revolution period of less than 200 years or confirmed observations at more than one perihelion passage.

C/

A nonperiodic comet or a periodic comet whose revolution period is too great or has only been observed at one perihelion passage.

D/

A periodic comet which has disappeared or no longer exists.

X/

A comet for which no meaningful orbit can be computed.

Periodic comets are also numbered in order of discovery.

Examples

• 1P/Halley was the first comet shown to be periodic.
• C/1995 O1 Hale-Bopp was the first comet discovered in the second half of July (O1) in 1995. It had not been previously observed and so gets a C prefix. The surnames of the discoverers are Hale and Bopp.
• D/1993 F2 Shoemaker-Levy 9 was the second comet discovered in the second half of March (F2) in 1993, the ninth one by Carolyn and Gene Shoemaker and David Levy. The comet collided with Jupiter in 1994 and was destroyed, hence the D prefix.
• First spotted in early February 1106, the orbit of X/1106 C1 'Great Comet of 1106' has never been accurately determined, hence the X designation.

Some Interesting Comets

Notes

1P/Halley is named for Edmund Halley, the first person to recognise the periodicity of this comet. After analysing a list of historic comets, he noticed that several of the comets appeared to have nearly identical orbits. These apparitions were separated by 75 year intervals so on this basis, Halley predicted that the comet would return in 1758. Sadly, Halley died before he could see if his prediction was correct but in late 1758, an amateur astronomer named Paltizsch recovered the comet which now bears Halley's name. Astronomers and historians have since found records mentioning this comet from all over the world, with the earliest dating from 240 BCE in China.

2P/Encke has the shortest period of all known periodic comets. Like 1P/Halley, it is not named for its discoverer but for the person who first recognised it as a periodic comet and successfully predicted its return. During its 2007 apparition, it had an encounter with a coronal mass ejection, resulting in its tail being ripped away!

Austrian astronomer von Biela first suggested in 1826 that the comet which was to bear his name, 3D/Biela, was periodic. Twenty years later, the comet split into two pieces and it was not seen again after its next apparition in 1852. However, twenty years after that a major meteor storm occurred about the time the comet would have been expected and it is likely that this storm was caused by the remains of 3D/Biela. However, neither comet nor associated meteor shower have been observed since the nineteenth century.

A number of other meteor showers are linked to the passage of comets through the inner solar system:

• C/1861 G1 Thatcher gives rise to the Lyrids;
• 1P/Halley is responsible for two meteor showers, the η Aquariids and the Orionids;
• 7P/Pons-Winnecke has left a debris field known as the June Boötids;
• 8P/Tuttle is the progenitor of the Ursids;
• dust from 21P/Giacobini-Zinner rains to Earth during the Draconids;
• 26P/Grigg-Skjellerup is the parent body of the π Puppids and Puppid-Velids;
• 55P/Tempel-Tuttle provides the fuel for the Leonids in November;
• 96P/Machholz may be responsible for both the daytime Arietids and the Southern δ Aquariids;
• C/1911 N1 Keiss is a long-period comet that is the parent body of the Aurigids;
• the most famous shower of them all, the Perseids, originates with 109P/Swift-Tuttle;
• 2P/Encke gives rise to the Taurids in November;
• 289P/Blanpain is mostly likely the parent of the Phoenicids. Originally discovered in 1819, the comet was lost afterwards and eventually designated D/1819 W1 Blanpain. However, in 2003, the orbital elements of newly discovered asteroid 2003 WY25 were found to match the long-lost comet. A faint coma was detected two years later but it was not until Pan-STARRS caught the object during an outburst that Blanpain was given a periodic comet designation.

17P/Holmes unexpectedly flared into naked-eye visibility in October 2007, despite being well outside the orbit of Mars. It has a history of unusual behaviour, including outbursts. During the 2007 event, the coma swelled to more than the diameter of the Sun!

Such is our fascination with comets that a number of space missions have been dispatched to study these objects:

• NASA's International Cometary Explorer passed through the plasma tail of 21P/Giacobini-Zinner in 1985. After this encounter, ICE joined a fleet of other spacecraft observing 1P/Halley.
• NASA's venerable Pioneer 7, launched in 1966, was still going strong twenty years later when it observed the interaction between the tail of 1P/Halley and the solar wind.
• Pioneer Venus 1, launched in 1978 by NASA to explore the atmosphere and surface of Venus, spent some time observing 1P/Halley in the ultraviolet during the comet's 1986 apparition.
• In 1986, Europe's first deep space mission, Giotto, sent back the first close-up images of a comet, in this case, the famous 1P/Halley. Having unexpectedly survived the encounter, the spacecraft was sent on to a 1992 rendezvous with 26P/Grigg-Skjellerup.
• 1P/Halley was also visited by the two Soviet spacecraft, Vega 1 and Vega 2, and two Japanese spacecraft, Sakigake and Suisei.
• 'The little spacecraft that could', Deep Space 1, flew through the coma of 19P/Borrelly after completing its successful asteroid mission. Its cutting edge ion engine set a record for the longest operating time for a propulsion system in space on its way to the comet.
• 81P/Wild 2 was visited by the Stardust space mission in 2004. Samples from the comet were returned to Earth for analysis two years later, the first collection of extraterrestrial samples from beyond the Moon.
• In July 2005, NASA's Deep Impact spacecraft rendezvoused with 9P/Tempel 1. A heavy mass was flung into the comet whilst the flyby spacecraft measured the outflow of the resulting collision. The ejected material was so bright that it was clearly visible from Earth. With the spacecraft still operable, the decision was made to target another comet, 85P/Boethin. However, this object was not recovered and so 103P/Hartley 2 was chosen for a November 2010 encounter.
• 9P/Tempel 1 was visited again, in February 2011, this time by the renamed Stardust-NExT spacecraft. The purpose of the mission was to examine the crater left in the comet's nucleus by the Deep Impact projectile.
• 46P/Wirtanen was the original target of the Rosetta space mission but a launch delay meant that the spacecraft visited 67P/Churyumov-Gerasimenko instead. The lander Philae landed on the icy object in November 2014 whilst Rosetta remained in orbit around the comet. Although the landing didn't go exactly to plan, the landing craft was able to send back valuable data for three days before its batteries went dead. It came back to life as the comet approached the Sun and sent back more data in June and July 2015. Rosetta itself continued to observe the comet throughout the mission.

95P/Chiron is the prototype of the Centaur class of comets which inhabit the dark and distant regions between Saturn and Uranus. Although discovered in 1977, its cometary nature was not revealed until 1989 when a coma developed.

96P/Machholz 1 has the smallest perihelion distance of any known periodic comet. It passes so closely that the SOHO spacecraft imaged it as it went around the Sun in October 1996.

206P/Barnard-Boattini was the first comet to be discovered through the use of photography. It was lost after its first apparition and designated D/1892 T1 Barnard. However, it was accidentally rediscovered by 2008 by Boattini and renamed.

The Kreutz sungrazers are comets from a particular family which are believed to be fragments of one huge comet, X/1106 C1 'Great Comet of 1106', which was observed in 1106 and subsequently broke up. These comets pass extremely close to the Sun at perihelion. The smaller ones do not survive perihelion passage and the larger ones, which can become very bright, often fragment further.

• The brilliant C/1843 D1 'Great March Comet of 1843' was visible during the day and developed the longest known tail until C/1996 B2 Hyakutake came along.
• Another member of this family was C/1880 C1 'Great Southern Comet of 1880'.
• C/1882 R1 'Great September Comet of 1882' was visible during the day. The nucleus eventually split into three pieces.
• C/1887 B1 'Great Southern Comet of 1887' also belongs to this family of interesting comets.
• Orbital studies of C/1945 X1 du Toit place it amongst this group of comets.
• Some experts think that C/1965 S1 Ikeya-Seki is the same object as X/1106 C1 'Great Comet of 1106'. It too was visible during daylight hours, reaching an estimated apparent magnitude of −10.

C/1680 V1 'Great Comet of 1680' was the first comet to be discovered telescopically. Sir Isaac Newton used this comet's motion around the Sun to test Kepler's laws of planetary motion.

C/1729 P1 'Great Comet of 1729' is the brightest comet ever recorded, at an absolute magnitude of −3.0. C/1577 V1 'Great Comet of 1577', observed by the great astronomer Tycho Brahe (and so determining that comets travel above the atmosphere of the Earth), comes third at −1.8 and C/1746 P1 is fourth. In second place? Why, the mighty C/1995 O1 Hale-Bopp of course!

The bright D/1770 L1 Lexell passed less than 0.02 AU from the Earth in July 1770, the closest flyby of a comet in recorded history. It was never seen again.

C/1811 F1 'Great Comet of 1811' was visible to the naked eye for an amazing nine months. Not until the advent of C/1995 O1 Hale-Bopp was this record broken.

The spectacular C/1858 L1 Donati was one of the brightest comets in the nineteenth century and was famous for its complicated coma and prominent dust tail. It is thought to be the first comet to be photographed.

The Earth passed through the tail of C/1861 J1 'Great Comet of 1861' during its passage through the inner solar system.

C/1910 A1 'Great January Comet of 1910' was mistaken by members of the public for the far more famous 1P/Halley which appeared later that same year.

The most famous photographs of C/1956 R1 Arend-Roland show a pronounced anti-tail.

C/1969 Y1 Bennett was a naked-eye object for several months in early 1970.

Thanks to a media frenzy, great things were expected from C/1973 E1 Kohoutek but the comet never brightened the way theory suggested it might. However, it was observed by the crew aboard Skylab, making it the first comet to be observed by humans in space.

C/1975 V1 West was the first comet since C/1965 S1 Ikeya-Seki to be visible during the day. The nucleus split into at least four fragments after its perihelion passage.

The collision of D/1993 F2 Shoemaker-Levy 9 with the planet Jupiter in July 1994 was watched with intense interest by the astronomical community and the world at large. It was the first time an impact of one solar system object upon another had been seen. The resulting scars on the face of Jupiter were visible for months afterwards. The Galileo spacecraft, en route to Jupiter, provided the only direct observations of the impact.

1996 and 1997 were excellent years for comets. Not only was C/1996 B2 Hyakutake bright but it had an extremely long ion tail as the Ulysses spacecraft found out. The cometary tail was approximately 500 million kilometres long, twice the length of the previous record holder. C/1995 O1 Hale-Bopp became very bright on its journey away from the Sun, its progress avidly followed by many amateurs and interested members of the public. The separate gas and dust tails were clearly visible in photographs.

C/2006 P1 McNaught was the brightest comet in decades and could be seen during the day. As with C/1996 B2 Hyakutake, the solar observing spacecraft Ulysses passed through the tail. This unexpected bonus continues to add to our knowledge of comets, their composition and origin.