Placing a telescope on the space shuttle has one big advantage
over a free-flying satellite telescope, such as the Hubble Space
Space shuttles and satellites trying to make astronomical observations
from low-earth orbit have an intrinsic problem: the earth keeps getting
in the way! Think of it this way: if you blow up a beach ball
(let's say about 16 inches in diameter) and let it represent the earth,
"low-earth" orbiting spacecraft are typically orbiting only 1/2 inch
above the beach ball! Hence, almost half the sky is blocked from view
at any one time.
Free-flying satellites that are on-orbit for years, such as the Hubble
Space Telescope, cannot carry sufficient fuel to allow the use of
rocket engines to point them quickly around the sky. (Rather, they use
so-called momentum wheels to slowly change the direction of
pointing on the sky.) When the earth gets in the way, Hubble basically
sits and waits for the earth to get out of the way, and then goes about
its business. After several years of fine tuning operations, the Hubble
telescope generally spends just over 30% of its time actually observing
objects of interest. Of course, Hubble can afford this "inefficiency"
given that it is expected to operate for 15 years or more on orbit,
instead of two weeks like the Astro Observatory!
Placing a telescope on the space shuttle has one big advantage: the
shuttle can change pointing directions quickly using its Reaction
Control System (basically a series of control jets used to maneuver the
shuttle). When the earth gets in the way, the Astro Observatory quickly
moves to a new target and keeps on going! Hence, constructing an
efficient timeline becomes a game of piecing together a sequence of
desired observations where the visibilities of the objects are out from
behind the earth. In this way, the Astro Observatory achieved about 75%
on-target efficiency during the Astro-2 mission, an unheard of number
for a low-earth orbit observatory!