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What Is An Astronomical Unit?
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The solar system broadly includes the sun and the celestial bodies
orbiting the sun, including the nine planets and their satellites;
the asteroids, comets, and meteoroids; and interplanetary dust and
gas. The term may also refer to a group of celestial bodies orbiting
another star.
An Astronomical Unit or AU is the unit of distance used in the
measurement of orbits and trajectories within the solar system. One
AU is the average distance between the earth and the sun. Its value
is roughly estimated to be 149,600,000 kilometers, by means of
radar-ranging studies of nearby celestial objects such as Venus or
passing asteroids; these studies have enabled astronomers to
determine the scale of the solar system with great accuracy. Based
on these studies, scientists say that the most distant known planet,
Pluto, has an orbit at 39.44 AU from the sun. The boundary between
the solar system and interstellar space, also called the heliopause,
is estimated to occur near 100 AU. |
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Let’s take a look at some celestial bodies
in the solar system. Asteroids are small rocky bodies that move in
orbits primarily between the orbits of the planets Mars and Jupiter.
They are present in thousands of numbers and range in size from
being microscopic in size to one called the Ceres, which has a
diameter of a 1000 kilometers.
Some asteroids are pulled into eccentric orbits by forces other than
their attraction to the sun. If the orbits of such bodies intersect
that of the earth, then they are called meteoroids. When they appear
in the night sky as streaks of light, they are known as meteors, and
recovered fragments are termed meteorites.
Scientists have studied and concluded that the surfaces of Mercury,
Mars and several satellites of the planets (including Earth’s moon)
show the effects of an intense bombardment by asteroidal objects
early in the history of the solar system. They say that some meteors
and interplanetary dust may also come from comets, which are
basically aggregates of dust and frozen gases about five to ten
kilometers in diameter.
Comets circle the sun in two main groups. A ring of debris that
orbits the sun beyond the planet Neptune is known as the Kuiper
belt. Several comets with periods of less than 500 years are said to
be members of the Kuiper belt. A spherical cloud of comets extending
to the edge of the solar system is said to be the Oort cloud.
Astronomers believe that comets with very long periods reside in the
Oort cloud.
Many of the objects that do not fall into the asteroid belts, the
Kuiper belt, or the Oort cloud are believed to be comets that will
not return to the sun. The surfaces of the icy satellites of the
outer planets are scarred by impacts from such bodies. It is said
that the asteroid-like object Chiron, with an orbit between Saturn
and Uranus, may be an extremely large inactive comet. Similarly,
some of the asteroids that cross the path of Earth’s orbit may be
the rocky remains of burned-out comets. |
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