Re:Why not visible light? (Score 4, Insightful)

It's because it makes sense to use space telescopes to look at radiation that can't be observed with ground based telescopes, because the Earth's atmosphere absorbs all of it. Herschel with its three instruments (HIFI, PACS and SPIRE) operates in the submm and far infrared, a part of the spectrum inaccessible from ground, and will spend a lot of observing time e.g. to look at interstellar water, a molecule believed to play an important role for the cooling of star forming clouds.