Why study comets?
Comets are considered as primordial bodies of the formation of the solar system, 4.5 billion years ago. Studying them allows to go down in time and thus to reach the initial conditions. As impactors of the Earth in a later stage their melting could have formed a part of our oceans. Their nucleus appearing primarily composed of water and carbonaceous molecules, a link with the life on Earth is very likely.
What does a comet look like?
The last fly-by missions, particularly Deep Impact and Epoxy, confirm the concept of a nucleus formed of dust and water and carbon dioxide ices (which drove some scientists to speak of "dirty snow ball"). But each comet has its own particularities: shape, composition, outgassing activity, rotation, ...
What is the difference between a comet, a shooting star and a meteorite?
Shooting star name is given to any object (natural or not) which enters at high speed in the Earth's atmosphere and which, while volitilizing there, generates luminous phenomena.
After having crossed the Earth's atmosphere, any extraterrestrial body that can reach the ground is called a meteorite.
A comet as seen from the ground is a deep space phenomenon (many millions of km from the Earth) due to the gas stemming from the nucleus outgassing. Though the tail stretching over long distances (millions of km) is very very thin the Sun makes it luminous, then visible. This phenomenon takes place far outside the Earth atmosphere (thickness of few tens of km only). If a complete nucleus would enter the Earth's atmosphere, it would become a very bright shooting star, but also a catastrophic event. However small parts of nucleuses separated from the main body or remnants of old comets can enter the Earth atmosphere, survive its crossing, reach the ground and become meteorites rain.
What is the difference between a comet and an asteroid?
The comets are very small bodies which were formed at long distance from the Sun. Because of gravitational disturbances, some among them approach the sun where the received heat creates nucleus outgassing and makes them thus visible.
The asteroids are small inert bodies of the solar system, orbiting primarily between the orbit of Earth and the one of Jupiter. Some are suspected to be old comets having entirely outgassed.
Do comets present a danger to the Earth?
The current observation of the solar system but also the past history of the Earth show that many comets have orbits close by Sun. Fortunately the gravity fields of Sun and Jupiter effectively protect the Earth from such impacts. Thus the probability is very low today but not null (1 impact of object of 1 km diameter every a few hundreds of thousands of years, the probability decreasing considerably when the size increases).
Which are or were the main other cometary missions?
- Vega, Giotto (1986): Halley comet flyby
- Deep Space 1 (2001): Borelly comet flyby
- Stardust (2003): Wild 2 comet flyby, dust collection and return on Earth (2006)
- Deep Impact (2005): Tempel 2 comet flyby with an impactor in the nucleus
- Epoxy (2010): Hartley 2 comet flyby