Spitzer Space Telescope

Spitzer was the fourth and final member of NASA's Great Observatories, joining Hubble, Compton, and Chandra as the infrared counterpart to their visible, gamma-ray, and X-ray eyes. To keep its cryogenically cooled telescope away from Earth's heat, it was placed not in orbit around the planet but trailing behind it around the Sun, slowly drifting away year by year. The flight marked the 300th Delta launch and the first use of a Delta II Heavy without a third stage.