Comets
Comets are small irregularly shaped objects thought to originate from the Kuiper belt, Scattered disc and Oort cloud. They are composed of a mixture of ice, dust and rock. When they are close enough to the Sun, they heat up, partially vaporise and develop a tail. The solar wind causes the tail to always point away from the Sun. Comets that are bright enough to be seen, cause much excitement when they appear in our night sky.
Comets are different to asteroids because they have an atmosphere surrounding their central nucleus and have a different origin. Most comets come from the outer regions of the solar system.
They usually orbit the Sun in a highly elliptical path and have a wide range of orbital periods ranging from several years to millions of years. Short-period comets generally originate in the Kuiper belt or the Scattered disc. Longer-period comets are thought to come from the Oort cloud.
Short period comets revolve around the Sun usually in the same direction as the planets and within 30° of the orbital plane of the planets. Long period comets are often found orbiting the sun at any inclination and may not orbit in the same direction as the planets.
Sometimes a long-period comet passes so close to a planet that the planet’s gravitational force changes the comet’s orbit, slowing it down and keeping it in the inner solar system. The comet then becomes a short period comet.
Some comets are found in circular orbits within the inner solar system.
Parts of a Comet
The solid core of a comet is known as the “nucleus”. The nucleus is made up of rock, dust, and water ice with frozen gases such as carbon dioxide, carbon monoxide and ammonia. The surface of the nucleus is generally dry, dusty or rocky and appears dark. Comet nuclei with radii up to 30 km have been observed, but most are smaller. Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes. Not visible to the human eye is the hydrogen envelope, a sphere of tenuous gas surrounding the nucleus and measuring as much as 20 million km in diameter.
When a comet comes to within 3 AU from the Sun, heating causes streams of dust and gas to be released. The released material forms an atmosphere around the comet called the “coma”. Gas and dust from the coma are always carried away from the Sun by the solar wind and radiation to form a “tail”. Comets often display two distinct tails, depending on the comet’s composition. One type of tail is made up of dust particles and appears yellow or white from reflected sunlight. The dust particles are very small (about 1 μm across) and move away from the nucleus into an arc that may be millions of kilometres long. The other type of tail is composed of ions (particularly carbon monoxide ions) and electrons—this tail is often straight, blue in colour, and extends about ten times farther than a dust tail. The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Ionised particles are more affected by the Sun’s magnetic field (see Figs. 13.10 and 13.11).
Fig. 13.10Comet Hale-Bopp (C/1995 O1) as seen in 1997. The comet became one of the largest comets ever observed, with a nucleus measuring over 40 km in diameter. It is a long-period comet with an orbital period of 2537 years. Its greatest distance from the Sun is 370 AU. Notice the two parts to its tail (Credit: NASA).
Fig. 13.11Comet McNaught (C/2006 P1) was discovered by astronomer and comet-hunter Robert McNaught. This image of the comet was taken in January 2007 as both the comet and the Sun were setting over the Pacific Ocean. The comet has a very long period of 92,600 years and it reaches a maximum distance of 4100 AU from the Sun (Credit: European Southern Observatory).
A comet is brightest and its tail is generally longest at about the time it passes perihelion (the closest point in its orbit to the Sun). As the comet moves away from the Sun it’s tail, head and nucleus receive less solar radiation and gradually fade. The comet may be lost from view until its next return, which could be as short an interval as 3.3 years (as for Enke’s Comet) or as long as 80,000 years (as for Comet Kohoutak) (Fig. 13.12).
Fig. 13.12The solar wind and radiation pressure from sunlight blow a comet’s dust particles and ionised atoms away from the Sun. Consequently, a comet’s tail always points away from the Sun.
Comets that survive passage around the Sun and return on a regular basis are said to be “periodic comets”. However, periodic comets lose their mass reappearance after reappearance, and they eventually disappear. A typical comet loses between 1/60 and 1/100 of its mass with each pass of the Sun.
In October 1995, the comet Hale-Bopp was observed to eject some mass when it was still beyond Jupiter. Astronomers believe the ejection resulted from evaporation of surface ice with an assist from the comet’s rapid rotation. Comets can also be destroyed when they come too close to a planet, a moon, or the Sun. A spectacular example of this was comet Shoemaker-Levy 9 which fragmented under the tidal force from Jupiter in 1992. Two years later, astronomers observed the pieces return and crash into Jupiter. The fact that the comet came apart in the first place, suggests that its nucleus was very weakly held together.
Other comets that have been observed to break up during their passage around the Sun, include Comet West and Ikeya-Seki.
If a comet passes across the Earth’s orbit, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between 9 August and 12 August, when Earth passes through the orbit of Comet Swift-Tuttle. Material from comet Halley is the source of the Orionid meteor shower in October.
Many comets and asteroids have collided with Earth in the past. Some scientists believe that comets that hit Earth about 4 billion years ago brought the vast quantities of water that exists in Earth’s oceans. The detection of organic molecules in some comets has also caused some scientists to speculate that such comets may have brought life to Earth.
Probing Comets
The famous Halley’s comet originates from the Kuiper belt and has a period of about 76 years. In 1986 five spacecraft were directed towards Halley as it approached Earth. Two of these missions were launched by the Soviets (Vega 1 and 2), two by Japan (Susei and Sakigake), and one by the European Space Agency (Giotto). The probes found Halley’s nucleus was irregular in shape (16 km long and 8 km wide). The dust-rich surface was darker than coal and contained small hills and craters. The comet’s inner coma consists of a mixture of about 80 % water vapour, 10 % carbon monoxide, and 3.5 % carbon dioxide, and some complex organic compounds. Most of the particles in the tail are composed of a mixture of hydrogen, carbon, nitrogen, oxygen and silicates (see Table 13.4).Table 13.4List of comets visited by spacecraft
Comet name
Year discovered
Spacecraft
Year of visit
Closest approach (km)
Giacobini-Zinner
1900
ICE
1985
7800 km
Halley
Ancient times
Vega, Giotto
1986
8900 km
Grigg-Skjellerup
1902
Giotto
1992
200 km
Borrelly
1904
Deep Space 1
2001
2171 km
Wild 2
1978
Stardust
2004
240 km
Tempel 1
1867
Deep Impact
2005
Blasted a crater
Hartley 2
1986
EPOXI/DI
2010
700 km
Tempel 1
1867
Stardust
2011
181 km
Churyumov-Gerasimenko
1969
Rosetta
2014
In orbit, lander
In 2001, the Deep Space 1 probe obtained high-resolution images of the surface of Comet Borr
elly. The probe found the dark surface of this comet was hot and dry, with a temperature of between 26 °C and 71 °C.
In July 2005, an impactor from the Deep Impact spacecraft blasted a crater on Comet 9P/Tempel 1 to study its interior. The results of the mission suggested the majority of the comet’s water ice was below the surface and that jets of vaporised water produced the coma of the comet. The probes spectrometer detected dust particles finer than human hair, and discovered the presence of silicates, carbonates, metal sulfides, amorphous carbon and polycyclic aromatic hydrocarbons (see Fig. 13.13).
Fig. 13.13Image of comet 9P/Temple 1 taken 67 s after it was hit by Deep Impact’s impactor in 2005 (Credit: NASA/JPL-Caltech/UMD).
Stardust was a 300 kg space probe launched by NASA on 7 February 1999. Its primary mission was to collect dust samples from the coma of comet Wild 2 and return them to Earth. The probe successfully completed its mission when it returned a canister containing the dust samples on 15 January 2006. The analysis showed that the coma/tail of comet Wild 2 contained a wide range of organic compounds (including the amino acid glycine) and crystalline grains that had been heated to a temperature around 1000 °C. In April 2011, scientists from the University of Arizona discovered iron and copper sulfide minerals that must have formed in the presence of water. The discovery shatters the existing paradigm that comets never get warm enough to melt their icy bulk.
The most exciting probe to reach a comet was the ESA’s Rosetta probe in 2014. The three tonne spacecraft carried 11 experiments and a lander called Philae, which has another nine experiments. Rosetta obtained its power from two huge solar panels, each 14 m long. Since its launch in 2004, Rosetta had to make three gravity-assist flybys of Earth and one of Mars to help it on course to its rendezvous with comet 67P/Churyumov-Gerasimenko. In August 2014 Rosetta went into orbit around the 4 km wide comet. At this point Rosetta was 405 million km from Earth, about halfway between the orbits of Jupiter and Mars, and moving at nearly 55,000 km/h. A lander called Philae was placed on the comet on 13 November 2014. The lander collected data with its suite of instruments, sniffing, hammering, drilling, and even listening to the comet. The data was sent back to Earth before the lander’s battery ran out of power. Fortunately, Philae came out of hibernation on 13 June 2015 and was able to contact ESA scientists again. Philae found the comet is built up of fluffy layers of dust and ice with only half the density of water, and contains organic molecules necessary for life. The comets surface is riddled with unusual cylindrical pits that have walls covered in what scientists called “goosebumps”.
Shaped like an enormous rubber duck, comet 67P has a large body that is joined to a smaller ‘head’ by a narrower ‘neck’ region. Scientists are unsure whether the comet gained its shape through uneven erosion, or as a result of two bodies fusing together after an ancient collision early in the solar system.
In early 2015, scientists announced that the deuterium to hydrogen ratio in water molecules surrounding the comet, was different to that of water molecules on Earth, suggesting comets like 67P were not the source of water on Earth.
Rosetta measured the comet’s average temperature at −70 °C, and found the comet was losing 11 kg of gas and dust a second as it approached the Sun. Scientists have not found any ice on the comet’s surface. The dark exterior is covered with complex carbon-rich organic molecules.
The comet is in an elliptical 6.5 year orbit that takes it from beyond Jupiter at its furthest point to between the orbits of Mars and Earth at its closest to the Sun. Rosetta will accompany the comet for over a year as it swings around the Sun and back out toward Jupiter again (see Fig. 13.14).
Fig. 13.14Comet 67P/Churyumov-Gerasimenko taken by the Rosetta probe on 6 August 2014 from a distance of 20 km. The image shows the comet’s “head” at the left of the frame, which is casting shadow onto the ‘neck’ and ‘body’ to the right (Credit: ESA/Rosetta/MPS for ORIRIS Team/NASA).
A comet is generally named after its discoverer, the first person to see it (or the first two or three people if they independently find it at much the same time). Comets are also assigned letters and numbers. Many discoverers of comets are amateur astronomers who examine a particular area of the sky at night. Some comets are discovered not by the eye but by examination of photographs taken through telescopes. The European Space Agency’s (ESA) SOHO spacecraft has been used to discover thousands of small comets that fly very close to the Sun. Many of them crash into the Sun or are pulled apart by its strong gravity.
Because new comets come from places in the solar system that are farthest from the Sun and thus coldest, they probably contain matter that is unchanged since the formation of the solar system 4.6 billion years ago. So the study of the constituents of comets is important for understanding the early stages of solar system formation.
Meteoroids
The solar system is also strewn with rocky debris called meteoroids, which are smaller than asteroids. Most meteoroids are less than a few 100 m across. The ones larger than pebbles probably broke off when asteroids collided.
Meteorids are often pulled by gravity into the Earth’s atmosphere. Air friction heats them up and their surface begins to vaporise. As the meteoroid penetrates further into the atmosphere, it leaves behind a trail of dusty gas, and we see it glowing as a meteor or shooting star. Some meteors even explode with a bright flash. Some even survive to hit the ground and are found by collectors as meteorites (see Table 13.5).Table 13.5Types of meteorites
Composition
Seen falling (%)
Finds (%)
Irons
6
66
Stony irons
2
8
Stones
92
26
Meteorites are grouped in three major classes according to their composition: iron, stony-iron, and stony meteorites. Rare stony meteorites called carbonaceous chondritis may be relatively unmodified material from the primitive solar nebula. These meteorites often contain carbon material and may have played a role in the origin of life on Earth.
Most meteorites that hit the Earth are stony in nature and are often referred to as simply stones.
Large meteorites that hit earth’s surface produce a crater (similar to those we see on the Moon). The largest meteorite crater may be a depression over 400 km across deep under the Antarctic ice pack. This is comparable with the size of lunar craters. Another very large crater, in Hudson’s Bay in Canada, is filled with water. Most meteorite craters on Earth are either disguised in such ways or have eroded away. A large crater that is obviously meteoritic in origin is the Barringer Crater in Arizona, USA. It is the result of what was perhaps the most recent large meteor to hit Earth, for it was formed only 25,000 years ago.
The Future
There is no doubt that more discoveries will be made in the Kuiper belt, the Oort cloud and Scattered disc. Any new objects discovered will be necessarily faint, cold and far away from Earth. It is also possible that the IAU will change their definition of what constitutes a planet in the future.
In August 2006, the IAU reclassified a number of objects in the solar system as ‘dwarf planets’. Currently five dwarf planets are recognised by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. Dwarf planets share similar characteristics to normal planets, but they are not dominant in their orbit around the Sun. The dwarf planets classified so far are members of larger populations. For example, Ceres is the largest body in the Asteroid belt, while Pluto is a large body in the Kuiper belt, and Eris is a member of the Scattered disc. Some objects might be considered to be dwarf planets, but their shape appears to deviate from hydrostatic equilibrium mainly because of massive impacts that occurred after they solidified. The definition of dwarf planet does not address this issue.
Under the 2008 IAU definition of planet, there are currently eight planets
and five dwarf planets in the solar system. There has been some criticism of the new definition, and some astronomers have even stated that they will not use it. Part of the dispute centres around the idea that dwarf planets should be classified as normal planets. For now, the reclassification of Ceres, Pluto and Eris has attracted much media and public attention.
Further Information
https://solarsystem.nasa.gov/planets/profile.cfm (check out dwarf planets, comets, Kuiper belt, and Oort cloud)
www.space.com/planets/
www.pluto.jhuapl.edu
www.nasa.gov/mission_pages/newhorizons/
About the Author
John Wilkinson is a science educator with over 30 years experience in teaching science, physics and chemistry in secondary colleges and universities in Australia. He is author of over 100 science textbooks. He completed his Masters degree and PhD in science education at La Trobe University, Australia. Throughout his life he has been interested in Astronomy and operates his own observatory from his backyard. His main astronomical interests include the Moon, Sun and Solar System objects.
John is also author of “New Eyes on the Sun” and “The Moon in Close-up” both published by Springer.
Glossary
AlbedoA measure of the amount of light reflected from a planet, asteroid or satellite.
The Solar System in Close-Up Page 24