Asteroids

Of numerous small rocky called bodies asteroids are present in the solar system, an important part of them circulate in a ring, between the orbit of Mars and that of Jupiter (2 - 4 UA ( Symbol: UA) The distance averages of the Earth in the Sun. An UA is worth 149 597 871 km. It is a unity often used for the distances in the solar system, or for the space of two stars in a double system. ), In what the astronomers call the belt of asteroids, otherwise called main belt. It so marks the limit between the telluric planets and the gaseous giants. An asteroid is a not observable heavenly object in the bare eye because of its small size which varies some tens of meters in several hundreds kilometers in diameter. They are a part of our solar system and turn around Him. The objects of less than 50 m of diameter are called by meteorites. Those are not satellites of planets but fragments of the disk proto-planets which did not manage to group together in planets during their formation. Asteroids have a big importance in the understanding of the formation of the solar system, it is for that reason that the astronomers show a strong interest in the study of these objects. The belt of asteroids was not discovered as such, we looked for the other thing, it is the effect serendip Neologism stemming from serendipity English, from Persian old man Serendip, name of Sri Lanka. Walpole (1754): the fact of discovering something accidentally and ingenuity while we are in search of something else. Merton ( 1945 ): the discovery luckily or ingenuity of relevant results for which we did not look. She relates to the rather common fact of observing an unexpected, absurd and major datum which gives the occasion to develop a new theory or to spread an existing theory. Charles Darwin ( 1953 ): quality which consists in looking for something and, having found the other thing, in recognizing that what we found has more importance than for what we looked.. The first asteroid was accidentally discovered 31 in December, 1800 by Giuseppe Piazzi, director, in the time, of the look-out

observatory of Palermo (Sicily). While observing the constellation of the Taurus, it perceived a not identified object moving very slowly in the night-sky. His colleague, Carl Friedrich Gauss determined the exact distance of this unknown object and placed this celestial body between planets Mars and Jupiter. Piazzi named him Ceres, of the name of the Greek goddess who makes take out the sap of the earth and witch grows the young shoots in spring. Between 1802 and 1807, three other bodies were discovered: Pallas, Junon and Vesta. In 1868, 100 asteroids were known. The 1000th approved discovery took place in 1921 and 10 000th in 1989. In March, 2006, there were 129 436 approved asteroids. The NEAR Shoemaker probe (Near Earth Asteroid Rendezvous) was thrown on February 17th, 1996 by the NASA to study in detail Eros, one of the biggest asteroids earth cruisers. This probe gave a complete cartography of Eros in 2000, it finally settled without damage on the asteroid, February 12th, 2001 and sent its last signal on February 28th, 2001. The space is far from being empty, it is sprinkled with dusts and with matter dating the creation of the

solar system. Asteroids and comets, metal and rocky objects, move in a vertiginous speed around planets and around our Sun. Meteorites and comets bombard our planet since the birth of the solar system. Although they seem wisely installed on their orbits between Mars and Jupiter, they are sometimes destructive and we owe them credibly the appearance of the life on Earth. Asteroids being our most close neighbors, they are somehow, bound to our fate. The total mass of all the asteroids of the main belt revolving around the Sun between Mars and Jupiter is estimated at 4×1021 kilograms, what amounts to approximately 5 % of the mass of the Moon. We distinguish various categories: silicates (group s), the carbon (group c) and the metal (group m). The first ones consisting of minerals, the second of carbon and ice and the last ones of metals. Asteroids are classified by groups according to their place within the belt of asteroids. We can quote the following groups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas which are named according to the main asteroid in the group).

Galileo probes, Near, Deep Space

It has been not enough time since the man investigates closely these asteroids. Indeed, given their fast movements, probes would have had difficulty in approaching it. It is in 1991, October 29th, that the Galileo probe, heading for Jupiter, approached for the first time the asteroid : Gaspra . The probe approached in 1 600 km, enough near to establish these dimensions in a precise way (20x12x11km, revealing 50 meters details. Almost two years later, on August 28th, 1993, Galileo met a second asteroid, Ida. Being then situated to 3 UA, they crossed themselves in km/s 12.4, separated one of the other one by 2400 km. This meeting revealed something very surprising, the presence of a satellite glancing through it as height as 100 km, baptized Dactyl.
February 17th, 1996, on Cape Canaveral, sound NEAR (for " Near Earth Asteroid Rendezvous ") took off in the direction of the asteroid Eros, the purpose being to be on the orbit to study it under all the angles during one year. It glanced through in the passage Mathilde in

1200 km in height. After a first failure in 1999, the probe returned and was on the orbit on February 14th, 2000 to settle on February 12th, 2001.
About at the same time, the Deep Space 1 probe was thrown in October, 1998 towards the asteroid Squall, by approaching as height as 15 km only, taking a multitude of photos. The Cassini-Huygens probe, heading for Saturn, photographed as for her Masursky. The biggest asteroids known for the belt are Ceres, Pallas and Vesta. They are the only ones to have a diameter superior to 500 km, thus the only ones to have an almost spherical geometry. Indeed, from a 500 km diameter, a rocky celestial body can take a spherical shape. The mass of the celestial body is sufficient to exercise a strength of gravitation superior to the strengths of cohesion of the matter. The matter divides up thus naturally into ball under the effect of its own weight. Below 500 km in diameter, asteroids as Gaspra (15 km) or Ida (56 km), present much more irregular forms.

Zones with asteroids

Between Mars and Jupiter (from 300 to 600 million km of the Sun).

Asteroids are mainly situated in the main belt, between the orbits of Mars and Jupiter. We find several hundreds of thousand listed objects there. All its objects would have been able to form a planet in this zone but the gravitational disturbances of Jupiter did not allow it. Jupiter played defender's role of the life on our planet. Without Jupiter, the bombardment on Earth would be 1000 times as frequent.

In the belt of Kuiper (between 35 and 100 UA of the Sun).

This zone contains ice-cold objects, and are not thus strictly speaking asteroids. This belt is rather a crèche of comets. The first member is discovered in 1992. We count a little more than 1000 today. The British call the asteroids of this type of "cubewanos". The biggest identified until today is Quaoar (1280 km in diameter).

In the cloud of Oort (between 20000 and 150000 UA neighborhood).

This zone of the sky, the remainder of the original nebula, would contain billions of nuclei of comets and would be the source of most of the new comets which enter the central regions of the solar system. The Cloud of Oort constitutes the reservoir of comets in long period. It is one diameter 1000 time superior in that of the solar system that we know with the 8 planets.

Vesta, the asteroid the most glittering with all, carries the name of the Roman goddess of the health. It is the only visible asteroid in the bare eye. Discovered on March 29th, 1807 by Heinrich Olbers, Vesta is the fourth " telluric planet " to be discovered. Among asteroids, it is the second en masse and the third in size. It turns around the Sun in 3,6 ground years and its average diameter is about 520 km. The composition of its surface is of basalt nature. Vesta possesses an enormous crater near its south pole. Vesta is thus a big rock of irregular shape, without track of water with a nucleus formed by iron. The enormous crater in Vesta's south pole (460 km wide and 13 km deep) is the result of a collision, The astronomers think that 5 % of meteorites found on Earth result from this gigantic shock.

The image opposite shows Vesta seen by the spatial telescope Hubble. Vesta, with its 520 km in diameters average is much more massive than any other body of this region of the belt of asteroids.
Dimensions : 578 × 560 × 458 km
Masse : 2,7 × 1020 kg
Density : 3,4 g/cm3
Period de rotation : 5,342 h

Vesta should be visited by the mission of the NASA Dawn left for 2007.

Gaspra discovered in 1916 by Grigoriy N. Neujamin and observed from 1991 till 1993 by Galileo during its journey towards Jupiter. It was the first meeting moved closer with a planetoid. Its surface (20x12x11 km) reflects 20 % of the solar light. Gaspra was glanced through at a 1600 km height. The Galileo probe activated its cameras 7 hours before this historic meeting and collected details of hardly 50 meters, so confirming that it was about an asteroid of type S, consisted of rocks, iron and common chondrites. The asteroid Gaspra, is not the one witch worries most the scientists, it is not a part of potential earth-cruisers.

This image is built from two photos of Gaspra realized by the probe Galileo through clear filters, since a distance of 5300 km, on October 29th, 1991 and 10 minutes, before the passage in the closest point. It presents Gaspra with the most high resolution (approximately 50 m by pixel). More of 600 craters of more than 100 m of diameter can be seen. The visible portion here is approximately 18 km in its biggest width. Gaspra is a body of irregular shape about 19 x 12 x 11 km. The north pole is at the top to the left. Gaspra makes a tour on itself counterclockwise in 7 hours.
Credit: NASA

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Astronomy - october 15th 2007