Fundamental Astronomy

Fig. 7.63. Meteors are easy to capture on film: one just leaves
a camera loaded with a sensitive film on a tripod with the
Some of the long period comets are put into short
period orbits by the perturbations of Jupiter and Saturn,
whereas some others can be ejected from the solar
system. However, there are no comets that have been
proven to come from interstellar space, and the relative
abundances of several isotopes in the cometary matter
are the same as in other bodies of our solar system.
The origin of the Oort cloud and Kuiper belt is different.
The Oort cloud objects were formed near the giant
planets and have been ejected to the outer edge of the
solar system by gravitational perturbations soon after
the formation of the solar system. Small objects beyond
the orbit of Neptune had no such interactions and they
remained near the accretion disk.
Meteoroids. Solid bodies smaller than asteroids are
called meteoroids. The boundary between asteroids and
7.17 Minor Bodies of the Solar System
shutter open for an hour or so. Stars make curved trails on the
film. (L. Häkkinen)
meteoroids, however, is diffuse; it is a matter of taste
whether a ten metre body is called an asteroid or a meteoroid.
We could say that it is an asteroid if it has been
observed so often that its orbital elements are known.
When a meteoroid hits the atmosphere, an optical
phenomenon, called a meteor (“shooting star”) is seen
(Fig. 7.63). The smallest bodies causing meteors have
a mass of about 1 gram; the (micro)meteoroids smaller
than this do not result in optical phenomena. However,
even these can be observed with radar which is able
to detect the column of ionised air. Micrometeoroids
can also be studied with particle detectors installed in
satellites and space crafts. Bright meteors are called
bolides.
The number of meteoroids increases rapidly as their
size diminishes. It has been estimated that at least 10 5 kg
of meteoritic material falls on the Earth each day. Most
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