The European Space Agency has just announced the end of the Herschel mission. The telescope, which cost in excess of a billion dollars to fly, spent four years in space. With a primary mirror 3.5 meters in diameter, Herschel was the largest infrared telescope ever flown and more than a meter larger than Hubble. Its discoveries include fundamental insights on star formation and the the nature of the interstellar medium.
If Herschel was so successful, why shut it down? The problem is, the telescope is going blind. Infrared telescopes face a difficulty unique in astronomy: things which are warm emit infrared light. You and I emit energy most strongly in the infrared. So do planets and the atmosphere. Unfortunately, so do the telescope and its instruments. The light emitted by the instruments themselves could easily drown out the astronomical sources scientists want to observe. To counteract this, the entire instrument package had to be cooled to nearly absolute zero. This is only possible by using liquid helium, the second coldest known substance. Herschel carried more than 2,000 liters of liquid helium, but it slowly evaporates over time. As the instruments began to warm up, their sensitivity dropped. Now the helium is entirely gone, which renders the observatory useless.
So what's next for infrared astronomy? The James Webb Space Telescope will probably be the next major infrared observatory. At nearly twice the size of Herschel, James Webb will be the largest space telescope ever constructed. Building such a behemoth is no easy task - the telescope will have cost nearly $9 billion when it (hopefully!) launches in 2018. Until then, there is a mountain of Herschel data just waiting to be analyzed!