When the Spitzer Space Telescope was first designed, it was going to cost over two billion dollars. Some very smart scientists and engineers redesigned the spacecraft and its planned orbit to cut the cost to less than half a billion dollars. Now Spitzer will cost about the same as building a new sports stadium.
In the first plan, the Spitzer Space Telescope was going to orbit the Earth, as many satellites do. The special thing about Spitzer is that it needs to use liquid helium (cryogen) to stay cold. Since liquid helium is pretty heavy, and it is expensive to launch a heavy spacecraft, scientists tried to use less liquid helium in their new plan.
To still keep the Spitzer Space Telescope very cold with less liquid helium, the scientists decided to change Spitzer's orbit to a much colder part of space. Spitzer is now going to share Earth's orbit around the Sun, but slowly drift away from the warm Earth. With this new orbit, Spitzer will have a very different view of the sky than before. The Spitzer Space Telescope has to be careful when looking at some parts of the sky. It can't look right at the Sun (it would be bad for Spitzer's infrared eyes, just like looking right at the Sun is bad for our visible light eyes).
The Spitzer Space Telescope also has to be careful to keep its solar panels pointed towards the Sun, since that's where it gets its energy. Even though Spitzer has to be careful, and can't look at some parts of the sky at some times, it will be able to see every part of the sky at least every six months during its five-year life.
What is the Spitzer Space Telescope? What is the Purpose of the Spitzer Space Telescope? What Will the Spitzer Space Telescope Do?