304 The Poetry of Physics and The Physics of Poetry
difference of opinion as to whether these objects are asteroids or not.
Some astronomers refer to them as KBOs (Kuiper Belt Objects). Over
1000 have been observed to date but it is estimated that there are many
many more. The Kuiper Belt is much larger than the Asteroid Belt with a
width 20 times as great and a mass 20 to 200 times greater.
It is believed that the Kuiper Belt formed in the original proto-
planetary disc but was too far out from the Sun and therefore there was
not enough material to form a planet. Other Kuiper Belt-like structures
have been observed around nine other stars.
Comets
Comets are small bodies usually a kilometer or two in diameter which
orbit the Sun in highly elliptical orbits. When they pass close to the Sun
they display the tails for which they are famous. More than a thousand
comets have been discovered so far, half with the naked eye. Each year
6 to 8 comets appear of which two or three are the return of periodic
comets. The majority of comets have orbits, which bring them beyond
the orbit of Neptune. Although they most likely have periodic orbits, the
length of their periods, which are centuries long, makes it impossible to
keep track of them and thus, they appear as one shot objects. A number
of comets have shorter periods ranging from five to 100 years. The first
periodic comet to be discovered was Halley’s comet, which has a period
of 76 years. Halley calculated the orbit of a bright comet, which appeared
in 1682, noted that its orbit corresponded to the appearance of the bright
comets of 1531 and 1607 and predicted the comet would appear again in
- It did appear in 1758 and again in 1835, 1910 and 1986 and is due
back in 2061. Its appearance was also first reported in 240 BCE.
The orbits of the “periodic” comets differ markedly than the “non-
periodic” comets. The periods of the “periodic” comets are naturally
much shorter enabling us to observe more than one of their visits. The
“periodic” comets orbit in the same direction as the planets and their
orbits are more in the plane of the planetary orbits than the non-periodic
comets whose orbits are more or less randomly distributed with respect
to the planetary plane. The revolutions of the non-periodic comets are
evenly divided between direct and retrograde rotation about the Sun.
It is believed that comets form in the debris of matter that lies in
either the Scattered Disc or the Oort Cloud. The Scattered Disc contains
icy planetoids with highly elliptical orbits that come no closer to the Sun
than 30 AU and travel out to distance in excess of 100 AU. The Oort