|Special Issue : 15/03/2004
A scientific publication by SGF and NEODyS
Comets FAQs ...
What are comets?
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases.
They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.
Where do comets come from?
The Oort Cloud was first proposed by Jan Oort in 1950. This "land of comets" contains comet-forming nuclei left over from the formation of the solar system. It is currently thought that this is the location where all comets originate. The way they enter the inner solar system is by gravitational pushes, usually by a passing star.
The Oort Cloud is believed by most scientists to exist, but it is still only theoretical. There have been no direct sightings of any Oort Cloud members, unlike the Kuiper Belt.
Recently, theory postulates that there exists a very large belt of asteroids and comets beyond Pluto that extends outwards for several A.U., that is also suspected of being the source of many short period comets.
KBO's are very difficult to detect due to their vast distances from the sun and very low surface reflectivity. Once identified, it is also very difficult to determine anything about them, even their size. The sizes in the table below are estimates that are based upon guesses as to how much light they reflect.
Which are the most famous comets?
|In the images , from left to right, three comets that are widely studied and that result to be very different: comet wild, comet Borrelly and comet Halley.|
What do we know about Comet 67P/Churyumov-Gerasimenko?
The NASA/ESA Hubble Space Telescope was used to make precise measurements of the size, shape and rotational period of the comet. As well as ESO's telescope that captured on the same day of Rosetta's launch the images on the left.
Observations made by Hubble in 2003 revealed that Comet 67P/Churyumov-Gerasimenko is approximately five kilometres by three kilometres in size and shaped like a rugby ball
A total of 16 nights of observations were obtained at Lowell Observatory during the 1982/3 and 1995/6 apparitions.
Production rates were determined for OH, NH, CN, C3, and C2, along with a measure of the dust production. All species exhibit larger production rates following perihelion, with water having a ~2\times pre/post-perihelion asymmetry, while minor species and dust have a larger asymmetry.
Peak water production (which occured about 1 month after perihelion) was ~1.0\times1028 mol s-1 and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ~2~km2
The peak dust production was ~450 cm, while the color of the dust is slightly reddened. In comparison to original ROSETTA target Comet 46P/Wirtanen, Comet Churyumov-Gerasimenko has essentially the same peak water production and a peak dust production about 3\times greater than does Wirtanen (assuming that the properties of the dust grains are similar)
What will Rosetta discover about comets?
Comets are observed since a long time, but the knowledge of the nucleus structure is very poor.
ROSETTA will clarify many of these open points!