Mechanical cryocoolers represent a significant enabling technology for NASA's Earth and Space Science Enterprises. Many of NASA's space instruments require cryogenic refrigeration to improve dynamic range, extend wavelength coverage, or enable the use of advanced detectors to observe a wide range of phenomena-from crop dynamics to stellar birth. Reflecting the relative maturity of the technology at these temperatures, the largest utilization of coolers over the last fifteen years has been for instruments operating at medium to high cryogenic temperatures (55 to 150 ¶K). For the future, important new developments are focusing on the lower temperature range, from 6 to 20 ¶K, in support of studies of the origin of the Universe and the search for planets around distant stars. NASA's development of a 20 ¶K cryocooler for the European Planck spacecraft and a 6 ¶K cryocooler for the MIRI instrument on the James Webb Space Telescope (JWST) are examples of the thrust to provide low-temperature cooling for this class of future missions. COOLERS ON NEAR-TERM EARTH AND SPACE SCIENCE MISSIONSSince 1991, which included the launch of the Upper Atmospheric Research Satellite (UARS) with the Improved Stratospheric and Mesospheric Sounder (ISAMS) instrument, NASA's Earth and Space Science Program has launched 18 long-life cryocoolers into space, 12 of which are still operating on multi-year missions. 1 By the late 1990s, 5-year-life space cryocoolers built on the Oxford-cooler compressor concept of the original ISAMS instrument were considered flight proven, and many long-life space cryocoolers were developed using this compressor concept together with both Stirling and pulse tube expanders.Starting in December 1999, three large NASA Earth Observing System platforms (Terra, Aqua and Aura) were launched. Together, they included nine long-life cryocoolers in five separate Earthscience instruments. A final category of cryocoolers are those associated with observing space physics and planetary phenomena. These include gamma-ray and infrared instruments to examine the Universe and the surfaces of other planets. Table 1 summarizes the status of long-life cryocoolers used on NASA space science missions. These, together with other short-term NASA cooler flights over the past 15 years, are detailed in the following paragraphs.