How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 25. Comets

25.1 A brief history of comets

Thousands of years ago, the motion of comets seemed erratic and unpredictable compared to the motion of the Sun, Moon, planets, and stars, and the erratic behaviour of comets led people to assume that they originated from inside of the Earth’s atmosphere. Tycho Brahe disproved this in the late 16th century (discussed in Chapter 3). 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.[1]

A photograph of comet Hale-Bopp over a field. Its two tails are visible.

Figure 25.1
Image credit

Comet Hale-Bopp as seen from Earth in 1997.

Individual comets reappear because they are orbiting the Sun in elongated elliptical orbits, but this was not proven until the 18th century. The German astronomer Georg Samuel Dörffel first suggested this idea in 1681.[2]

Isaac Newton showed how this was possible six years later when he published his laws of gravitation[3] (discussed in Chapter 5). Newton believed that comets were rocky objects that contain ice that vaporises when it’s heated by the Sun, creating the comet’s tail.

In 1705, Edmond Halley looked at all of the documented appearances of comets and tried to derive their orbital parameters using Newtonian physics. 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.[4] 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.[5]

The link between comets and meteor showers was proven in the late 19th century when the 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.[6] This led people to think of comets as having surfaces covered in small rocks, below a layer of ice.

In the 1950s, the American astronomer Fred Lawrence Whipple suggested that comets are actually composed of more ice than rock, and contain frozen water, carbon dioxide, and ammonia.[7-9] Whipple’s theory was supported by observations made by spacecraft launched in the latter half of the century.

Over 5000 comets have now been observed orbiting the Sun,[10] and 11 comets have been observed orbiting stars outside of the Solar System.[11,12] We now know that the nuclei of comets are mostly composed of ice that vaporises when the comet is close to the Sun. This forms a bright atmosphere of vapour that 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.[13]

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.[14]

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.[15]

A diagram of a comet showing the dust tail and the ion tail.

Figure 25.2
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A diagram of a comet, showing the coma, nucleus, and ion and dust tails.

A diagram of a comet orbiting the Sun. Both tails get bigger the closer they are to the Sun and both point away from the Sun. The ion tail points directly away, and the dust tail is curved towards the path of the orbit.

Figure 25.3
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A diagram of a comet, the ion tail always points directly away from the Sun.

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.[16] Volatile material has also been observed on objects in the asteroid belt, with water vapour detected on the dwarf planet Ceres in 2014.[17]

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[18] (discussed in Chapter 26). The Kuiper Belt exists beyond the orbit of Neptune and was hypothesised by the Dutch-American astronomer Gerard Kuiper in 1951.[19] It’s now thought to contain about 1000 billion comets.[18]

Comets with periods longer than 200 years are thought to originate from the Oort Cloud (also discussed in Chapter 26). 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.[20]

The Estonian astronomer Ernst Öpik first suggested that long-period comets may originate from the Oort Cloud in 1932,[21] and this idea was extended by Jan Oort in 1950.[22] The Oort Cloud is thought to contain hundreds of billions of comets, which may contain thousands of times the amount of water found in the Earth’s oceans.[20]

How much water is in the Solar System?

Number of comets in the Kuiper Belt = 1×1012

Number of comets in the Oort Cloud = 2×1012

Total number of comets in the Solar System = 3×1012

Total mass of water in a comet = 1013 kg

Total mass of water in Earth’s oceans = 1.4×1021 kg

Water in the Kuiper Belt/Water in Earth’s oceans = 1×1012 × 1013/1.4×1021 = 7143

Water in the Oort Cloud/Water in Earth’s oceans = 2×1012 × 1013/1.4×1021 = 14,286

Water in all the comets in the Solar System/Water in Earth’s oceans = 3×1012 × 1013/1.4×1021 = 21,429

25.1.1 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 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.

Halley’s Comet, an image from Vega 1.

Figure 25.4
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Halley’s Comet, an image from Vega 1.

Halley’s Comet, an image from Vega 2.

Figure 25.5
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Halley’s Comet, an image from Vega 2.

The Institute of Space and Astronautical Science (ISAS), 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 latter 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.

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.[23]

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 in 2001.

Halley’s Comet, an image from Giotto.

Figure 25.6
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Halley’s Comet, an image from Giotto.

Comet Borrelly.

Figure 25.7
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Comet Borrelly, an image from Deep Space 1.

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 many organic compounds including glycine, an amino acid.[24] 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’ (discussed in Chapter 17). The Miller-Urey experiment, which was conducted by the American chemists Stanley Miller and Harold Urey in 1953, had previously shown that amino acids can be produced relatively easily in nature.[25]

Comet Wild.

Figure 25.8
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Comet Wild, an image from Stardust.

Comet Tempel.

Figure 25.9
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Comet Tempel, an image from Stardust.

Three more missions to comets were launched in the 2000s. The first, NASA’s CONTOUR (COmet Nucleus TOUR), launched in 2002. CONTOUR was due to visit at least two comets, but NASA lost contact with it soon after launch and the mission was a failure.

NASA’s Deep Impact spacecraft launched in 2005 and reached the comet Tempel six months later. 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. This is comparable in size to the crater produced by the Chicxulub object, which may have been responsible for the extinction of most of the dinosaurs on Earth (discussed in Chapter 17). 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 organic material like charcoal. The surface is covered in a fine layer of dust that has the consistency of talcum powder. Frozen water exists about one metre beneath the surface and frozen carbon dioxide exists beneath this.[26] Deep Impact also came within 700 km of the comet Hartley in 2010.

Comet Hartley.

Figure 25.10
Image credit

Comet Hartley, an image from Deep Impact.

Finally, the ESA’s Rosetta spacecraft was launched in 2004 and flew past Mars and two asteroids before reaching comet Churyumov-Gerasimenko, and becoming the first spacecraft to orbit a comet, in 2014. On 12 November 2014, Rosetta’s lander Philae achieved the first-ever soft landing on a comet nucleus.[27]

Comet Churyumov-Gerasimenko.

Figure 25.11
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Comet Churyumov-Gerasimenko, an image from Rosetta.

Close-up of Churyumov-Gerasimenko.

Figure 25.12
Image credit

Comet Churyumov-Gerasimenko, an image from Rosetta.

25.2 References

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