DOMOV Asteroidi

   On the first day of January 1801, Giuseppe Piazzi discovered an object which he first thought was a new comet. But after its orbita was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, in Juno). By the end of the 19th century there were several hundred.

   Several hundred thousin asteroids have been discovered in given provisional designations so far. Thousins more are discovered each year. There are undoubtedly hundreds of thousins more that are too small to be seen od planeta the Earth. There are 26 known asteroids larger than 200 km in premer. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in premer. Of those in the 10 to 100 km range we have cataloged about half. But we know very few of the smaller ones; there are probably considerably more than a million asteroids in the 1 km range.

   The total masa of all the asteroids is less than that of the Moon.

   243 Ida in 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathilde (left) on 1997 June 27 returning many images. NEAR (now renamed "NEAR-Shoemaker") entered orbita around 433 Eros (right) in January 1999 in has so far returned a wealth of images in data. They are the only asteroids which have been studied closely so far.

   The largest asteroid by far is 1 Ceres. It is 933 km in premer in contains about 25% of the masa of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta in 10 Hygiea which are between 400 in 525 km in premer. All other known asteroids are less than 340 km across.

   There is some debate as to the classification of asteroids, comets in moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos in Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, in perhaps some of the newly discovered moons of Saturn, Uranus in Neptune are all more similar to asteroids than to the larger moons. (The composite image at the top of this page shows Ida, Gaspra, Deimos in Phobos approximately to scale.)

   Asteroids are classified into a number of types according to their spectra (in hence their chemical composition) in albedo:

Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)

   There is little data about the densities of asteroids. But by sensing the Doppler effect on radio waves returning to Earth od planeta NEAR owing to the (very slight) gravitational tug between asteroid in spacecraft, Mathilde's masa could be estimated. Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a compacted pile of debris.

   Asteroids are also categorized by their position in the solar system:

Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps. These are regions where an object's orbital period would be a simple fraction of that of Jupiter. An object in such an orbita is very likely to be accelerated by Jupiter into a different orbita.

   There also a few "asteroids" (designated as "Centaurs") in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbita between Saturn in Uranus; the orbita of 5335 Damocles ranges od planeta near Mars to beyond Uranus; 5145 Pholus orbita od planeta Saturn to past Neptune. There are probably many more, but such planet-crossing orbita are unstable in they are likely to be perturbed in the future. The composition of these objects is probably more like that of comets or the Kuiper Belt objects than that of ordinary asteroids. in particular, Chiron is now classified as a comet.

   4 Vesta has been studied recently with HST (left). It is a particularly interesting asteroid in that it seems to have been differentiated into layers like the terrestrial planets. This implies some internal heat source in addition to the heat released by long-lived radio-isotopes which alone would be insufficient to melt such a small object. There is also a gigantic impact basin so deep that it exposes the mantle beneath Vesta's outer crust.

   Though they are never visible with the unaided eye, many asteroids are visible with binoculars or small telescopes.

Asteroid table

A few asteroids in comets are listed below for comparison. (oddaljenost is the mean oddaljenost to the Sun in thousins of kilometers; masaes in kilograms). 

 No.  Name      oddaljenost  Polmer     masa  Discoverer   Date
---- ---------  --------  ------  -------  ----------  -----
2062 Aten         144514       0.5   ?      Helin       1976
3554 Amun         145710       ?     ?      Shoemaker   1986
1566 Icarus       161269       0.7   ?      Baade       1949
 433 Eros         172800      33x13x13      Witt        1989
1862 Apollo       220061       0.7   ?      Reinmuth    1932
2212 Hephaistos   323884       4.4   ?      Chernykh    1978
 951 Gaspra       330000       8     ?      Neujmin     1916
   4 Vesta        353400     265  3.0e20    Olbers      1807
   3 Juno         399400     123     ?      Harding     1804
  15 Eunomia      395500     136  8.3e18    De Gasparis 1851
   1 Ceres        413900     466  8.7e20    Piazzi      1801
   2 Pallas       414500     261  3.18e20   Olbers      1802
 243 Ida          428000      35     ?      ?           1880?
  52 Europa       463300     156     ?      Goldschmidt 1858
  10 Hygiea       470300     215  9.3e19    De Gasparis 1849
 511 Davida       475400     168     ?      Dugan       1903
 911 Agamemnon    778100      88     ?      Reinmuth    1919
2060 Chiron      2051900      85     ?      Kowal       1977

More about asteroids

Open Issues