Comets have been documented for thousands of years[1a], yet there's still a lot we don't know about them. We don't know what their surface is composed of, how thick their crust is, and how much frozen water is contained beneath. We also don't know what types of organic chemicals comets contain.
Scientists are particularly interested in these questions since comets often collide with planets, and so it's theorised that they could 'seed' planets with oceans and amino acids, the 'building blocks of life'.
These questions could be answered by landing a spacecraft onto the surface of a comet, something the European Space Agency (ESA) plans to do in less than two weeks.
1. Comets throughout history ↑
Thousands of years ago, the motion of comets seemed erratic and unpredictable compared to the motion of the Sun, Moon, five visible planets, and the stars. This may be why they were often associated with bad luck or messages from God[1b].
The erratic behaviour of comets also led people to assume that they originated from inside of the Earth's atmosphere. Danish astronomer Tycho Brahe disproved this in the late 1500s.
Tycho measured the parallax of the Great Comet of 1577, and calculated that the comet was at least four times further away than the Moon. The fact that individual comets can reappear, because they are orbiting the Sun in elongated elliptical orbits, was not proven until the 1700s. German astronomer Georg Samuel Dörffel first suggested this idea in 1681.
English natural philosopher Isaac Newton showed how this was possible six years later, when he published his laws of gravitation. Newton believed that comets were rocky objects that contain ice, which vaporises when it's heated by the Sun, creating the comet's tail.
In 1705, English astronomer Edmond Halley looked at all of the documented appearances of comets, and tried to derive their orbital parameters using Newton's method. This led him to predict that the comets of 1531, 1607, and 1682, were actually all the same object, which would reappear about 75 years after its last appearance.
Halley became the first person to successfully predict the return of a comet when the comet reappeared in 1759. This comet has since been known as Halley's Comet.
The Mawangdui silk, showing the shapes of comet tails and the different disasters associated with them, compiled in around 300 BCE. Image credit: NASA/JPL/Public domain.
Halley's Comet depicted in the Bayeux Tapestry, completed in the 1070s. Image credit: Myrabella/Public domain.
The link between comets and meteor showers was proven in the late 1800s, when Italian astronomer Giovanni Schiaparelli showed that the Perseid meteor shower, which occurs every August, is caused by the path of the Earth travelling through debris left by the comet Swift-Tuttle. This led people to think of comets as having surfaces covered in small rocks, below a layer of ice.
In the 1950s, American astronomer Fred Lawrence Whipple suggested that comets are actually composed of more ice than rock, and contain frozen water, carbon dioxide, and ammonia. Whipple's theory was supported by observations made by spacecraft launched in the latter half of the century.
We now know that the nuclei of comets are mostly composed of ice, which vaporises when the comet is close to the Sun. This forms a bright atmosphere of vapour, which is made of charged particles called ions, and dust particles, which can be composed of silicates, hydrocarbons, and ice. This atmosphere is known as a coma.
The nuclei of observed comets range from tens of metres to about 60 km in length. The coma creates a shell around the nucleus that can be millions of km wide, and is surrounded by an even larger shell composed of hydrogen.
The tails of a comet are also produced by interactions between the comet and the Sun, with the dust and vapour creating two separate tails. Both tails always point away from the Sun, but the charged particles react more strongly to the Sun's magnetic field and the solar wind, making it point directly away from the Sun. Dust particles are less affected by the Sun, and so the direction of the dust tail is curved by the orbit of the comet. The tails of a comet can extend for hundreds of millions of km.
Asteroids can be distinguished from comets because they do not have enough material capable of vaporising when they are close to the Sun, and so do not produce a coma, but the line between asteroids and comets is ambiguous. This is because comets will eventually lose all of their volatile material. They are then known as 'extinct' comets. Volatile material has also been observed on objects in the asteroid belt, with water vapour detected on the dwarf planet Ceres in January this year.
The origin of comets can be determined from their orbital parameters. Comets that take less than 200 years to orbit the Sun are thought to originate from the Kuiper Belt[20a]. The Kuiper Belt exists beyond the orbit of Neptune and was hypothesised by Dutch-American astronomer Gerard Kuiper in 1951. It's now thought to contain about 1000 billion comets[20b].
Artists' impression of the Kuiper Belt and Oort Cloud. Image credit: NASA/JPL/Public domain.
Comets with periods longer than 200 years are thought to originate from the Oort Cloud. The Oort Cloud is a spherical cloud of comets that orbit the Sun from over 1.5 light-years from the edge of the Kuiper Belt. This is a third of the distance to the closest extrasolar star, Proxima Centauri[22a].
Estonian astronomer Ernst Öpik first suggested that long-period comets may originate from the Oort Cloud in 1932, and this idea was extended by Dutch astronomer Jan Oort in 1950. The Oort Cloud is thought to contain hundreds of billions of comets[22b].
2. Missions to comets ↑
NASA and the ESA launched the first spacecraft to fly past a comet, ICE (International Cometary Explorer), in 1978. Its primary mission was to study the interaction between the Earth's magnetic field and the solar wind. ICE's mission was extended, and it flew through the tail of the comet Giacobini-Zinner, almost 8000 km from the comet's nucleus, in 1985. It flew through the tail of Halley's Comet, from a distance of about 28 million km, the following year.
There were five missions launched in the 1980s that also observed Halley's Comet in 1986. The Soviet Unions' Vega 1 and Vega 2 were launched in 1984 and, after completing their primary mission to Venus, they flew past Halley's Comet from a distance of about 9000 km.
The Institute of Space and Astronautical Science, now a division of the Japanese Aerospace Exploration Agency (JAXA), launched the Sakigake and Suisei spacecraft in 1985. The former came within 7 million km of Halley's Comet, and the later passed within 150,000 km.
Finally, the ESA's Giotto spacecraft, which was also launched in 1985, travelled within 600 km of Halley's Comet. Its mission was extended in 1992, when it came within 200 km of the comet Grigg-Skjellerup. Its camera had been destroyed during its first mission and so it didn't obtain any new images, but it did measure the magnetic field strength of both comets.
Halley's Comet, image from Vega 1. Image credit: NASA/Public domain.
Halley's Comet, image from Vega 2. Image credit: NASA/Public domain.
The observations of Halley's Comet in 1986 confirmed Whipple's theory. They showed that the surface of Halley's Comet is mostly composed of rock and dust, and the atmosphere is mostly composed of dust and water, as well as carbon dioxide, and ammonia.
In the 1990s, two more spacecraft were launched that would go on to observe comets. These were NASA's Deep Space 1 and NASA's Stardust spacecraft. Deep Space 1's primary mission was to observe an asteroid, but its mission was extended and it flew past the comet Borrelly, from about 2000 km from the nucleus, in 2001.
Stardust's primary mission was to collect samples of cosmic dust, and dust from the comet Wild, which it did in 2004, coming within about 200 km of the comet's nucleus. It also travelled within about 200 km of the nucleus of the comet Tempel in 2011.
Stardust's dust samples were returned to Earth in 2006. These were found to contain a number of organic compounds, including glycine, an amino acid. Amino acids are important to all life forms on Earth, as chains of amino acids make up proteins. DNA contains information about which particular amino acid chains to build, and this is why amino acids are sometimes referred to as the 'building blocks of life'. The Miller-Urey experiment, which was conducted by American chemists Stanley Miller and Harold Urey in 1953, had previously shown that amino acids can be produced relatively easily in nature.
Comet Borrelly, image from Deep Space 1. Image credit: NASA/Public domain.
Comet Wild, image from Stardust. Image credit: NASA/Public domain.
Comet Tempel, image from Stardust. Image credit: NASA/Public domain.
Three more missions to comets were launched in the 2000s. The first, NASA's CONTOUR (COmet Nucleus TOUR) spacecraft, was launched in 2002, and was planning to visit at least two comets, but NASA soon lost contact with the spacecraft and the mission was a failure.
NASA's Deep Impact spacecraft was launched in 2005. Deep Impact reached the comet Tempel six months after its launch. It then released almost 400 kg of copper, which crashed into the comet at just over 10 km/s (37,000 km/h). This created the same amount of energy as almost 5 tonnes of TNT, and produced a 150-metre wide crater. The resulting explosion was imaged by the spacecraft from about 500 km away.
The Deep Impact mission showed that the nucleus of the comet Tempel is spongy, with lots of holes, and parts of the surface are very weak. The surface of Tempel is extremely black, providing one of the least reflective surfaces in the Solar System. This means that it easily absorbs heat, and is probably made of an organic material, like charcoal. The surface is covered in a fine layer of dust, which has the consistency of talcum powder. Ice exists about one metre beneath the surface, and is mostly composed of frozen water, and beneath this, frozen carbon dioxide.
Deep Impact also came within 700 km of the comet Hartley in 2010.
Comet Tempel during the impact event caused by Deep Impact, image from Deep Impact. Image credit: NASA/JPL/Paul Stephen Carlin/Public domain.
Comet Hartley, image from Deep Impact. Image credit: NASA/JPL-Caltech/UMD/Public domain.
Finally, the ESA's Rosetta spacecraft was launched in 2004, and flew past Mars and two asteroids before reaching the comet Churyumov-Gerasimenko earlier this year. It's currently in orbit, making it the first spacecraft to orbit a comet.
The Rosetta's lander, Philae, is scheduled to detach from the Rosetta spacecraft on the 12th November, landing about 7 hours later. It will approach the comet at less than 4 km/h and detach two harpoons to prevent it bouncing off the surface, before securing itself with drills. It can then begin transmitting data from the comet's surface.
It will remain attached to the comet as it passes by the Sun, from November this year until December 2015. At this point, the mission will end and the spacecraft will return samples of the comet's surface to the Earth.
You can track Rosetta's progress here.