Still Cooling after All These Years

Fifteen years is a long time for any high-tech cryogenic device to continue working, but that is just what an M77 cryocooler aboard NASA’s RHESSI satellite has accomplished. Designed by Sunpower, Inc. (CSA CSM), the unit’s piston has cycled over 26 billion times over this period. And, as periodic oil changes can’t happen in space, the M77 has done so without oil to provide lubrication between the piston and piston wall. Instead, it relies on gas bearings to provide a thin protective layer of gas to prevent collisions.

The M77 is single stage, integral Stirling cycle cooler designed to provide up to 4W of cooling at 77 K with an input of 100W. NASA tested the unit under vacuum for more than 10,000 hours at Goddard Space Flight Center before the mission and even noted in mission documents that Sunpower had engineered the cooler for a lifetime greater than 50,000 hours (around six years). Nevertheless, the space agency planned for only a two-year mission.

“Sunpower is proud to have contributed towards the RHESSI project and the significant science that has been generated with this instrument,” says Jimmy Wade, business development manager at Sunpower. “The fact that this cryocooler lasted over 15 years is a testament to our technology and the manufacturing processes used for our cryocoolers.”

The satellite’s detectors, which allow researchers to collect and analyze valuable scientific data, have also aged well. Four of the nine still have a low energy threshold f ~3 keV and an X-ray spectral resolution of ~1.5 keV FWHM. In February of last year, NASA ran the satellite through an annealing process, the fifth of its lifetime, to remove accumulated radiation. The system returned to anticipated operational parameters.

Today, the M77 continues to provide cooling power to the satellite’s detectors, albeit in a diminished capacity. Its cold plate has slowly increased in overall temperature, moving from 77 K to around 140 K. To counter the increase in temperature, and subsequently extend the overall life of the satellite, NASA operates only three of the nine RHESSI detectors at any one point.