The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is an HST instrument providing the capability for infrared imaging and spectroscopic observations of astronomical targets. NICMOS detects light with wavelengths between 0.8 and 2.5 microns - longer than the human-eye limit.
The sensitive HgCdTe arrays that comprise the infrared detectors in NICMOS must operate at very cold temperatures. After its deployment, NICMOS kept its detectors cold inside a cryogenic dewar (a thermally insulated container much like a thermos bottle) containing frozen nitrogen ice. NICMOS is HST's first cryogenic instrument.
The frozen nitrogen ice cryogen in NICMOS was exhausted in early 1999, rendering the Instrument inoperable at that time. An alternate means of cooling the NICMOS was developed and installed in the March 2002 servicing mission. This device uses a mechanical cooler to cool the detectors to the low temperatures necessary for operations. The technology for this cooler was not available when the instrument was originally designed, but fortunately became available in time to support the reactivation of the instrument.
Since late 2008, the NICMOS Cooling System (NCS) has experienced difficulties maintaining the instrument’s nominal scientific operating state, in which the detectors are maintained at ~ 77K. Repeated restart attempts have demonstrated that it is not possible to restart the NCS in a cold state immediately following safing events. The main culprit for the problems is believed to be water ice in the primary (circulator) loop of the NCS. An inefficient approach to this problem would be to put the NCS through a several-month warm-up/cooldown cycle and hope that there is an opportunity for science prior to the next payload safing event.
The only feasible path towards satisfactory operation of NICMOS is to remove the putative water by venting the existing contaminated Ne coolant and replacing it with a fresh charge, which is available onboard but has never actually been used on-orbit. Based on the Cycle 18 proposal review results, STScI and Goddard HST Project, with the concurrence of NASA Headquarters, have decided that NICMOS will not be available for science in Cycle 18. A decision on the availability of NICMOS beyond Cycle 18 has not yet been made and awaits further discussion.