The Elusive Planet
is the planet closest to the Sun. Since it never strays far in
the sky from the Sun’s glare, early astronomers had a difficult
time viewing it, and considered it a "wandering star"
appearing just before sunrise or just after sunset.
is fitting that we know Mercury by the name of the swift
Roman messenger god, since it travels around the Sun faster
than any other planet. During one year on Earth, Mercury
makes over four orbits of the Sun. On the other hand, Mercury
rotates slowly on its axis – almost 60 times more
slowly than does our home planet. The amazing outcome is
that a single Mercury day takes two Mercury years. The symbol
for Mercury also represents the planet's swift orbit around
the sun. Click on Mercury's symbol to the left to learn
more about its origin.
is the second smallest planet in our Solar System, larger only
than Pluto and not much bigger than our own Moon. The surface
of Mercury is in fact very Moon-like, covered with large and ancient
craters, while its interior is like Earth’s, with a large
core of iron. Mercury has a very thin atmosphere, and no moons
of its own. It is a world of extreme temperatures in which the
surface can heat to over 400°C and cool to almost –200 °C.
of what we know about Mercury was discovered through flybys
of the planet during the Mariner 10 space mission in the
1970’s. But Mariner 10 photographed only half the
planet’s surface, and many questions remain. In 2004,
NASA will launch a spacecraft to make more detailed observations.
This planned mission is called MESSENGER, for “MErcury
Surface, Space ENvironment, GEochemistry
and Ranging”, and will reach orbit around
Mercury in 2011.