Observing from Low-Earth Orbit

Placing a telescope on the space shuttle has one big advantage over a free-flying satellite telescope, such as the Hubble Space Telescope. 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!