As astronauts work to repair and upgrade the Hubble Space Telescope this week, another large space telescope of NASA started a new life on its own. As expected, on May 15th, 2009, the Spitzer Space Telescope ran out of its liquid helium coolant that has enabled it to observe dark, cold objects both within and outside of the solar system with ultra-high sensitivity. This doesn't mean that the telescope is nearing the end of its life; rather, it merely switches the mission from the “cold” phase it has been in since launch to a new "warm" phase, with new scientific objectives.

This is actually a big week for infrared astronomy. The European Space Agency launched two space-based IR telescopes of its own, the Herschel Space Observatory (HSO) and the Planck mission. The HSO is armed with the largest mirror ever put into space, at 3.5m, approximately four times the size of Spitzer's. It's carrying three years' worth of coolant that will allow it to image deeper into the infrared, pushing close to radio astronomy wavelengths. With the Spitzer now shifting to the shorter-wavelength ranges of the IR, the HSO and Spitzer missions will largely complement each other. The Planck satellite also observes in the far-infrared, but its goal is to measure the cosmic microwave background radiation. Both HSO and Planck are on a four-month trip to the L2 Lagrange point, during which time ESA scientists should have the chance to calibrate their instruments.

The Spitzer telescope is part of NASA’s Great Observatories Program, which is made up of four space-based telescopes which together cover the full-spectrum of light from far-infrared to gamma-ray radiation. The other three are the Hubble Space Telescope (covering the visible and near-ultraviolet), Chandra X-ray Observatory (soft X-ray), and Compton Gamma Ray Observatory (hard X-ray and gamma ray),

The Spitzer Space Telescope covers the infrared part of the spectrum. Astronomical objects that shine primarily in infrared are mostly cold, dark objects. To observe these stellar bodies, a telescope’s detector must be as cold as possible, as the thermal noise within the detector itself can otherwise overwhelm the very photons that come from the objects of interest. The Spitzer has been peering at those cold cosmic targets by chilling its light detectors to just 5.5 Kelvin above absolute zero using liquid helium, the coldest cryogen that we have.

The telescope was launched on August 25th, 2003, carrying 360 litters (96 gallons) of the cryogen, which was originally meant to last for at least 2.5 years with a hope of stretching it to 5 years. Achieving almost 6 years of cold-phase mission is beyond these original expectations. Even without the cryogen, the Spitzer’s capabilities in the mid-infrared are not expected to degrade at all, as that detector is designed to maintain temperatures below 30 Kelvin even without the liquid helium. These capabilities will not be surpassed by NASA until the James Webb Space Telescope becomes operational sometime after 2013.

What the telescope will lose is its ability to see objects in far-infrared—it can no longer see some of the darkest objects it revealed during the cold phase. Unlike the Hubble, which is in low-Earth orbit, the Spitzer orbits around the sun just behind Earth, and cannot be reached by the Shuttle to replenish the supplies and upgrade the instruments.