A comet is an icy body that releases gas or dust. They are often compared to dirty snowballs, though in recent years, some scientists refer to them as "snowy dirtballs." Comets contain dust and ice, and organic materials such as carbon dioxide, ammonia, methane and more. Some researchers think comets might have originally brought some of the water and organic molecules to Earth that now make up life as we know it.
Comets orbit the Sun in elliptical orbits and can be inclined to the plane of the solar system at any angle. Comets can sprout "tails" extending many tens of millions of miles, during their closest approach to the sun. All comets have a nucleus, which is the hard rock/ice object. When a comet nucleus nears the sun, solar energy begins to heat the ice and vaporize it. The gas flies off the comet, sometimes violently enough to break the nucleus apart, and throws dust up with it. The gases form a cloud around the nucleus called the coma. Some of the gas is stripped of electrons and blown back by the solar wind. This forms a bluish colored ion tail. The dust particles are pushed away from the comet by solar radiation, forming a dust tail that is the easiest to see with the unaided eye, but occasionally the ion tail is visible as well. Each time a comet passes close to the sun, it loses more of its ice. Eventually, after many passes, the comet may no longer have enough material to form tails. Its surface will be covered by dark dust and it will look more like an asteroid. Short period comets are thought to come from the Kuiper Belt on the outskirts of Neptune's orbit and further, and longer period comets are thought to come from The Oort cloud, a vast spherical shell that surrounds the solar system. The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU to as far as 50,000 AU from the Sun. Occasionally a comet streaks through the inner solar system; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have will never forget the celestial show.
Comets are pristine remnants from the formation of the solar system. Recent spacecraft, equipped with special instrumentation, have encountered comets and have revealed some surprising findings and have raised even more questions about these mysterious icy bodies. Today's comet hunters and missions into space are deepening our understanding of the origins and morphology of comets as products of a dynamic process of gravity acting on celestial bodies in motion.
Comets, Meteoroids and Meteorites: What's the Difference?
In space, smaller particles in orbit around the Sun are referred to as meteoroids. Once a meteoroid enters the Earth's atmosphere and vaporizes, it becomes a meteor, or as it is commonly known, a "shooting star." If a small asteroid or large meteoroid survives its fiery passage through the Earth's atmosphere and lands upon the Earth's surface, it is then called a meteorite.
Debris from comets is the source of most small meteoroids. Many comets generate meteoroid streams when their icy cometary nuclei pass near the Sun and release the dust particles that were once embedded in the cometary ices. These meteoroid particles then follow in the wake of the parent comet. Collisions between asteroids in space create smaller asteroidal fragments and these fragments are the sources of most meteorites that have struck the Earth's surface.
Because they are readily available for study, many meteorites have already been subjected to detailed chemical and physical analyses in laboratories. If particular asteroids can be identified as the sources for some of the well-studied meteorites, a detailed knowledge of the meteorite's composition and structure will provide important information about conditions and the chemical compositions from which the parent asteroid formed more than 4.6 billion years ago.
Meteor showers happen when Earth passes through the orbital path of a comet that left a lot of dust behind. Earth plows through the dust, and the particles form meteors as they hit the atmosphere. Occasionally a small rock may fall through the atmosphere, causing an extremely bright and colorful streak across the sky called a fireball.
Fireballs are often mistaken for comets, but comets do not streak across the sky quickly; they are usually visible for many days.) Sometimes fireball rocks are not completely vaporized, and they impact Earth's surface. The threat of a catastrophic impact from a meteor or comet is a staple of popular culture. If there was a dinosaur killer in Earth's past, is there a human killer in our future? What are the chances and how do we assess the risks?
Missions to Comets:
There have been a surprising number of spacecraft sent to study comets - both past and present.
The first probe to visit a comet was the ISEE-3/ICE probe. Launched in 1978, the probe was sent to study comet Giacobini-Zinner. The mission proved the "dirt-snowball" theory, that comets are composed of mixed rock and ice. The mission ended in 1981 and is now in a heliocentric orbit that will bring it close to Earth in 2014. There are plans to capture the probe for display in the Smithsonian Institution.
The second mission was comprised of two spacecraft, VEGA 1 and VEGA 2. These Soviet missions were launched December 15 and 21, 1984, respectively. After carrying Venus entry probes to the vicinity of Venus (arrival and deployment of probes were scheduled for June 11-15, 1985), the two spacecraft were re routed using Venus gravity field assistance to intercept Comet Halley in March 1986. The first spacecraft encountered Comet Halley on March 6, 1986, and the second three days later.
The next probe to visit a comet was Japan's Sakigake probe. Launched in 1985, the probe reached comet Halley in 1986. Its closest approach was 7 million kilometers.
The European Space Agencies first deep space mission was Giotto. This remarkable probe was able to study two comets: comet Halley in 1986 and comet Grigg-Skjellerup in 1992. Its closest approach to a comet was only 200 km.
Japan's second probe, Suisei, was also sent to study comet Halley in 1986. This mission was a direct result of the success of Sakigake.
While not a cometary probe, the Galileo spacecraft did capture the impact of comet Shoemaker-Levy 9 in 1994.
The Ulysses probe - designed for Solar studies - did pass close to comet Hyakutake in 1996. The result was the study of the longest tail ever - composed of ions, gas and dust.
NASA's Deep Space 1 was a probe designed both a comet and an asteroid. Launched in 1998, the probe studied comet Borrelly in 2001.
On July 3, 2002, the CONTOUR probe was launched to study two comets. Contact to the probe was lost after only a month in the mission.
The Stardust mission was a return sample mission. Launched in 1999, the probe encountered comet Wild 2 in 2004 and captured particles surrounding the comet. The Stardust mission ended in 2006.
NASA's Deep Impact probe launched on January 12, 2005. Its purpose was to send a projectile to Comet Tempel 1, creating an impact crater so the main craft could perform spectroscopy of the internal structures.
The European Space Agency's Rosetta probe was launched in 2004 to study comet 67P/Churyumov-Gerasimenko. It will attempt a first ever landing on a comet when it reaches its target in November of 2014.
Links to Information about Comets:
Near Earth Objects Dynamic Site (NEODyS): Web-based listing by Andrea Milani of the known orbital and physical properties of all Near-Earth objects (NEA's), together with information for calculating their future closest approaches to Earth.
NASA NEO Program Office website at JPL: This Program Office was established in mid-1998 to help coordinatethe study of those comets and asteroids that can approach Earth's orbit
Comet ISON: "The Comet of the Century"