Why go into Space

Planck will observe the Cosmic Microwave Background from space, 1.5 million km (1million miles) from Earth.  But why go into space?  There are two main reasons.

Clear skies

The Earth's atmosphere can be a problem for ground-based telescopes.  Firstly, it restricts the wavelengths that can be observed, due to absorption and emission of specific elements and molecules.  Even in the clear wavelength "windows", there is a background signal which is constantly changing due to turbulence in the atmosphere.  Having to look through this background signal limits the sensitivity of any experiment.  Planck will avoid all of these problems by observing from space.

Seeing the bigger picture

From any point on Earth's surface, it is impossible to see the entire sky - the Earth is in the way.  As the Earth rotates, the visible patch of sky moves from East to West, but there is a limit to how far north or south we can see.  For example, here in the UK we can only see a small amount of the sky in the southern hemisphere.  An observer on the equator can in principle see the whole sky over the course of a year, but the poles are always right on the horizon and so very hard to see through all the dust, murk and atmospheric turblence.

Space telescopes in orbit around the Earth, such as the Hubble Space Telescope, fly in a low earth orbit, a few hundred kilometres above the surface.  They orbit the Earth once in around 90 minutes, in an orbit tilted so that they see both the northern and southern skies.  However, such telescopes are still relatively close to the Earth and affected by it's glow.  They also pass in and out of the Earth's shadow, which causes huge changes in their temperature and power generated by their solar panels.  Being in such a low orbit would prevent Planck from controlling its temperature as accurately as is needed.

Being so close to the Earth, which is 13,000 km across, satellites in low-Earth orbit can only see half the sky - the planet blocks the other half.  Being so far from Earth, as Planck is, means that there it can not only see the sky unimpeded by the Earth, but is also away from the glow of the Earth.  Planck's specific method of observing, mapping a ring around the sky every hour, means that it takes just over 6 months to see the entire sky.

The L2 point is special because the Earth, Sun and Moon all lie in roughly the same direction, so providing Planck avoids that one direction, it won't see the glare from any of them.