![[Gaspra]](gaspra.gif)
The image is a false color view of the asteroid 951 Gaspra taken by the
Galileo spacecraft.
On January 1st 1801, Giuseppe Piazzi discovered an object he thought was a new comet. But after its orbit 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, and Juno). By the end of the 19th century, several hundred were known.
Several hundred thousand asteroids have been discovered and given provisional designations so far. Thousands more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. 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 mass of all the asteroids is less than that of the Moon.
The image is a false color view of the asteroid 951 Gaspra taken by the
Galileo spacecraft.
14 comets and asteroids have so far been explored by spacecraft:
Below is a table of information on selected asteroids, and at the bottom of the page are comments on the asteroids. All of the values for mass and many of the diameter values are only rough estimates. The links are to pages on the NSSDC site.
Asteroid Diameter ~Mass Rotation Orbital Spectral Semimajor Orbital Orbital Number Number and Name (km) 1015 kg Period Period Class Axis Eccentricity Inclination and Name --------------- -------- ------- -------- ------- -------- --------- ------------ ----------- -------- 1 Ceres 960 x 932 870,000 9.075 hrs 4.60 yrs C 2.766 AU 0.0779 10.58 deg 1 Ceres 2 Pallas 570 x 525 x 482 318,000 7.811 hrs 4.61 yrs U 2.776 AU 0.2309 34.85 deg 2 Pallas 3 Juno 240 20,000 7.210 hrs 4.36 yrs S 2.669 AU 0.2579 12.97 deg 3 Juno 4 Vesta 530 300,000 5.342 hrs 3.63 yrs U 2.361 AU 0.0903 7.13 deg 4 Vesta 45 Eugenia 214 6,100 5.699 hrs 4.49 yrs FC 2.721 AU 0.083 6.61 deg 45 Eugenia 140 Siwa 110 18.5 hrs 4.52 yrs 2.732 AU 0.2009 1.97 deg 140 Siwa 243 Ida 58 x 23 100 4.633 hrs 4.84 yrs S 2.861 AU 0.0451 1.14 deg 243 Ida 253 Mathilde 66 x 48 x 46 103.3 417.7 hrs 4.31 yrs C 2.646 AU 0.2660 6.71 deg 253 Mathilde 433 Eros 33 x 13 x 13 7.2 5.270 hrs 1.76 yrs S 1.458 AU 0.2230 10.83 deg 433 Eros 951 Gaspra 19 x 12 x 11 10 7.042 hrs 3.29 yrs S 2.209 AU 0.1738 4.10 deg 951 Gaspra 1566 Icarus 1.4 0.001 2.273 hrs 1.12 yrs U 1.078 AU 0.8268 22.86 deg 1566 Icarus 1620 Geographos 2.0 0.004 5.222 hrs 1.39 yrs S 1.246 AU 0.3354 13.34 deg 1620 Geographos 1862 Apollo 1.6 0.002 3.063 hrs 1.81 yrs S 1.471 AU 0.5600 6.36 deg 1862 Apollo 2060 Chiron 180 4000 5.9 hrs 50.7 yrs B 13.633 AU 0.3801 6.94 deg 2060 Chiron 2530 Shipka 5.25 yrs 3.019 AU 0.1237 10.10 deg 2530 Shipka 2703 Rodari 3.25 yrs 2.194 AU 0.0572 6.04 deg 2703 Rodari 3352 McAuliffe 2 - 5 2.57 yrs 1.879 AU 0.3686 4.77 deg 3352 McAuliffe 3840 Mimistrobell 3.38 yrs 2.249 AU 0.0831 3.92 deg 3840 Mimistrobell 4179 Toutatis 4.6 x 2.4 x 1.9 0.05 130. hrs 3.98 yrs S 2.512 AU 0.6339 0.47 deg 4179 Toutatis 4660 Nereus 2 1.82 yrs 1.490 AU 0.3603 1.42 deg 4660 Nereus 4769 Castalia 1.8 x 0.8 0.0005 1.10 yrs 1.063 AU 0.4831 8.89 deg 4769 Castalia 4979 Otawara 18.9 3.19 yrs 2.168 AU 0.1170 0.41 deg 4979 Otawara 9969 Braille 2.2 x 1.0 3.58 yrs 2.341 AU 0.4336 29.0 deg 1992 KD
New masses calculated for 3 largest asteroids - U.S. Naval Observatory press release, 7 January 1998