If they are somehow deflected towards the inner solar system, their surfaces are exposed to intense solar radiation and the ices start to vaporize. Since this interaction with solar radiation is very probably the first chemical processing in billions of years, comets provides us with information about the composition of matter at the time of their formation. Their different compositions seem to represent different regions of the pre-solar nebula. Most of them seem to be rich in water. Spectroscopic information of the most important volatile components of comets like H 2 O and CO 2 or organic C-X stretch bands cannot be obtained from the ground since the absorption from water and carbon dioxide in the lower atmosphere blends out those bands.
Most asteroids orbit in the main belt between the orbits of Mars and Jupiter.
Asteroids are thought to be remnant material from the processes of formation and initial development of planets and therefore an important source of information on conditions in the early solar system. The composition of the planets is directly linked to the distribution of matter and volatiles during the formation of our solar system. The radial partitioning of matter during the formation of planets should reflect itself in the different compositions of the planets. The satellites of the outer planets may retain surface spectral signatures of the primordial partitioning of these constituents. Stellar occultations of planets and satellites have been a very valuable source of information on planetary atmospheres and rings during the KAO flights.