Techniques d'observation spectroscopique d'astéroïdes

CHAPTER 1. WHY ASTEROIDS? 35
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their images were located in the same relative positions on all photographic films. Because
a moving asteroid would be in a slightly different position on each picture exposure and the
background stars and galaxies were not, it could be identified.
Nowadays the charge coupled devices (CCD) are used instead of photography. While, the
CCD technology is more sensitive and accurate than the older photographic methods, the modern
discovery technique itself is rather similar. Separated by several minutes, three or more
CCD images are taken of the same region of the sky. These images are then compared to see
if any asteroid has systematically moved to different positions on each of the separate images
(Fig. 1.1).
For a newly discovered object, the separation of the asteroid location from one image to
another, the direction it appears to be traveling, and its brightness allow to estimate its orbital
characteristics and roughly its size. For example, an object that appears to be moving very
rapidly from one image to the next one, is almost certainly very close to the Earth. Computeraided
analyses of the CCD images have replaced the older, manual techniques for all the current
asteroid search programs.
Fig. 1.1 shows a field obtained with INT-WFC (Isaac Newton Telescope, Wide Field Camera)
on the night of February 28, 2012. Ten asteroids could be identified in this field by taking
consecutive images at an interval of five minutes. At the moment of the observation only three
of the identified asteroids were known.
Thanks to this technological development, during the last decades the total numbers of the
asteroids discovered had grown exponentially. Among the most important surveys dedicated to
asteroid detection (particularly to Near Earth Asteroids) are those leaded by the United States
(CSS, LINEAR, Spacewatch, LONEOS and NEAT) which have been using large field, mostly
1m class telescopes. In the Europe the most important programs were: ASIAGO/ADAS in
Italy and Germany, CINEOS in Italy, KLENOT in the Czech Republic, NEON in Finland.
An example of successfully observing run is the one performed by [Boattini et al., 2004].
During two short runs at ESO (European Southern Observatory) LaSilla, they employed the
MPG (Max Planck Gesellschaft) 2.2m telescope as a search facility, and the NTT (New Technology
Telescope) 3.5m as a follow-up telescope to survey faint asteroids beyond 22 magnitude,
for three observing nights. The authors observed about 700 Main Belt asteroids as faint as V
22 magnitude. They exposed between 60s and 150s in the R(red) band.
To conclude this section, as of August 13, 2012 there are 588,219 observed asteroids from
which 333,841 have the orbits well determined (as a consequence they were numbered) 2 .
1.3 Distribution and diversity of asteroids
Asteroids are often grouped according to their orbital parameters. Fig. 1.2 shows the distribution
of the asteroids as a function of their heliocentric distance. The majority of asteroids are
2 http://www.naic.edu/~nolan/astorb.html