ASTEROIDS

Asteroids are small objects which orbit  around  the Sun and  which  show  no  cometary  activity  nor  have  the potential  for  it. The word ‘small’ is meant to exclude planets from the definition. All known asteroids have diameters less than 1000 km down to a few centimeters. 1 Ceres is the largest known asteroid, discovered  on  the  1  January  1801  by  Giuseppe  Piazzi  in Palermo, Italy with a diameter of 950 km. Asteroids are also  called minor planets. The newly discovered minor body  known  as  2001  KX76,  whose  diameter  makes  it  the  largest-known minor  body  in  the  solar  system   is  a Trans-Neptunian (TNO). Most  asteroids  orbit within  a confined  region  between  Mars  and  Jupiter,  called  the main Asteroid Belt.

Out of about 30 000 known objects, only 8500 asteroids have precisely determined orbits.  Each of these asteroids has a definitive identification, consisting of a number that indicates its order of entry and a name proposed by the discoverer (e.g. ‘3752 Camillo’ identifies the 3752nd asteroid discovered). Each is marked by its year of discovery and two letters indicating the date of first observation as temporary identification (e.g.  ‘1998 BC’ identifies the third asteroid (C) discovered during the Second half of January (B) 1998). A thousand asteroids have been detected by the Infrared Astronomy Satellite (IRAS) which provided a radiometric diameter and the albedo determination of most of them. About 700 aster- oids have the rotational period determined.  Less  than 1000 asteroids are larger than 30 km across, and of these, about 200 asteroids are larger than 100 km. Extrapolating the asteroid  size distribution down  to 1 km, an estimation  of  about  one million  asteroids whose  diameter  is larger than or equal to 1 km is obtained. The global mass of the population has been estimated to be of the order of one-thousandth of the Earth’s mass.

Asteroids were considered as uninteresting ‘vermin of the skies’ until it was understood that they store a rich variety of information.  In  fact, asteroids are believed  to be  the  remnants,  either  fragments  or  ‘survivors’  of  the swarm of Planetsimals from which the terrestrial planets were formed: the records of primordial chemical and physical processes can still be found frozen in the actual characters  of  asteroids.  The  study and analysis  of  these  objects  provides  information  on  the  nature  of  these  primordial swarms  even  if  differentiation  and/or  internal  activity affected in a different way their evolution.

The current knowledge on the asteroid population has been obtained mainly from the ground-based observations. Few large asteroids are included in the observational programme of  the Hubble Space Telescope (HST).NASA’s Galileo Mission approached for the  first  time  two  asteroids,  951 GASPRA and 243 IDA, offering to the planetary science community the opportunity to have a close look at two S-type main-belt asteroids.

Questions to Ponder

• Why are the compositions of comets and asteroids different?
• How dangerous are the asteroids?
• How are asteroid compositions and classifications determined?