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Comets
originate in the outer solar system; they are thrown inwards towards the Sun
by gravitational perturbations from planets or nearby stars. They have a
variety of different orbital periods, ranging from a few years, to 50 or 100
years, to thousands of years, while some are believed to pass through the
inner Solar System only once before being thrown out into interstellar
space. Short-period comets are thought to originate in the Kuiper Belt,
which lies beyond the orbit of Neptune. Some comets can very occasionally
collide with a planet or satellite. As comets approach the Sun they develop
enormous tails of luminous material that extend for millions of kilometers
from the head.
Many people thought that comets were a sign that something
bad was about to happen to them.The coma is the roughly spherical blob of
gas that surrounds the nucleus of a comet; it is about a million km across.
Comets are dirty snowballs, asteroids are rocks. A comet that has been
deflected into the inner Solar System from the Oort Cloud will have an
incredibly elongated path, perhaps with a perihelion of 1 astronomical unit
(AU) and an aphelion of 10,000 AU, so that it would take many centuries to
return again; it might even approach on a hyperbolic path that will
eventually carry it into interstellar space.The lose mass each time they
pass through the inner regions of the Solar System. Though dozens of new
comets are discovered each year, it is rare for naked eye comets to appear.
The Solar System consists of planets and their moons, stars (the sun), and
other objects such as asteroids, meteors, and comets. Comets have been
around since the beginning of the solar system, and they have been recorded
since written history began.
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