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Q:
I would like to know why the (Chandra) satellite is not orbiting
the moon?
A:
Chandra is not in an orbit around the moon for two reasons:
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It takes too much energy (meaning too large of a rocket)
to get to the moon, and more importantly:
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If it was in orbit around the moon, far too many sources
would be eclipsed by the moon. That is why a high Earth
orbit is better than a low Earth orbit, too.
What astronomers would like to do someday is have observatories
on the moon. It would be a stable platform, and the extremely
sparse lunar atmosphere would make it possible to use even an
X-ray telescope. A popular choice for future missions is to
place the spacecraft in orbit around the Sun-Earth L2 point
(approximately 1.5 million kilometers from Earth in the
anti-solar direction). The L2 orbit is attractive because it has
few observing constraints, an optimum thermal environment, the
lowest radiation environment, and simplifies spacecraft
communications and operations. Transfer to L2 would be achieved
by a lunar flyby to minimize propulsion requirements and obtain
a compact lissajous orbit around the L2 point.
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