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MPC Theatre: Comet Chasers
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1995-05-24
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The two moons of Mars, Phobos and Deimos, are very small and difficult to detect from
Earth. Both of the satellites are dim asymmetrical bodies which appear to be captured
asteroids from the nearby main asteroid belt.
Phobos orbits less than 3,700 miles above the surface of Mars, and has an orbital period of
7 hours and 39 minutes. Nonetheless, it is a small body with low reflectivity, and appears
less than half the diameter of the Earth's Moon in the Martian sky.
Phobos is an irregular-shaped body, stretching 17 miles at its longest diameter. The moon
is heavily-cratered, as well as covered with grooves and ridges. It was accurately
measured by both of the Viking orbiters that Phobos is slowly spiraling down towards
Mars at a rate of 60 feet per century. The moon will crash into the red planet in about 40
million years.
Deimos is smaller and much further out than Phobos, with a diameter not greater than 9
miles at nearly twice the distance from Mars. This is also an irregular moon, with a dark
lumpy appearance and many impact craters. Unlike Phobos, Deimos seems to have a
stable orbit.
Deimos is a difficult object to see in the Martian sky. From the surface of Mars, Deimos
would have an apparent diameter only twice that of the planet Venus. Thus, neither of the
Martian moons provide much illumination during the night.
In July, 1988, the former Soviet Union launched two "Phobos" spacecraft to investigate
the innermost Martian moon. The first spacecraft was lost on its way to Mars, apparently
due to a command error which caused it to lose its communications link with the Earth.
The second probe was put into Mars orbit in January, 1989.
The space probe orbited Mars and was scheduled to deploy two landers to the surface of
the moon Phobos. The first lander was a stationary long-duration probe expected to
operate for about a year. The second lander was a mobile "hopper" with mechanical legs
to hop over the surface of the moon.
Unfortunately, the Phobos 2 spacecraft was lost before contact was made with the Martian
moon. The exact cause of the break in communications is unknown. Possibly the space
probe was hit by a micrometeoroid, or perhaps the dish antenna drifted away from Earth.