The Hubble’s near infrared camera multi-object spectrometer (NICMOS) was supposed to work for four to five years. Infrared detectors need to be cooled for adequate performance. It’s similar to trying to view stars from the city — when you remove light sources (heat), you can see the stars more clearly.

When NICMOS was designed, the best engineering solution was to encase the detectors in a dewar the size of a beer keg, filled with solid nitrogen. Before launch, super-cold helium was circulated through the nitrogen to keep it solid. The plan was for the nitrogen to gradually boil off over its on-orbit lifetime while keeping the detectors cool. But an unexpected heat short in the instrument resulted in nitrogen boiling off faster than planned. NICMOS ran out of nitrogen in January 1999, rendering it useless.

Once engineers realized there was a heat short, they started to design a possible solution. Instead of trying to refill the dewar, why not add a refrigerator-like cooler?

FROM ICEBOX TO REFRIGERATOR

Hubble is one of the most precisely made spacecraft ever built. Its pointing accuracy has been compared with shooting a laser from New York to Washington, DC, and holding the laser on a target the size of a dime. So any cooling system would have to be vibration free.

Creare Systems of Hanover, NH, developed incredibly small turbines, about the size of small French fries. The turbines are masterpieces of precision miniature manufacturing. They’re carved by vaporizing the metal, similar to arc welding. An electrode generates sparks, which vaporize tiny pieces of metal until the proper shape is achieved. The turbine spins at 400,000 rpm, about 25 to 50 times as fast as a car engine. The high spin rate minimizes vibration to a level that it’s effectively undetectable.

The NICMOS Cooling System (NCS) uses a reverse turbo-Brayton cycle with neon as the working gas. The neon enters the turbine at a pressure of 34.5 lb/sq in. and exits at 21.3 lb/sq in. The decrease in pressure reduces its temperature from –334 to –343 degrees F. Technically speaking, with that temperature range, the refrigerator is a cryocooler.

A spacecraft is basically a dewar in a vacuum, and thermal balance is extremely important. So an important part of the NCS is a 13-by 3-ft external radiator. It uses a self-priming capillary loop to transfer the heat from the cooler to the radiator. As Grunsfeld moved the large radiator into place, Linnehan commented, “Looks like a big surfboard, John.”

NASA astronomer David Leckrone noted the technology improvement this way: “It’s like going from an icebox to a refrigerator.”

WILL IT WORK?

The cooler was tested on the STS-95 shuttle mission in 1998, which also featured the return to space for pioneering astronaut John Glenn.

One key advantage of a refrigerator vs. an icebox is the NICMOS cooler’s temperature can be adjusted for the best performance from the NICMOS detectors.

Engineers estimate it will take about a month for the cryocooler to cool down NICMOS’s detectors to their operating temperature and another month for tests before it’s declared opened for business. Astronomer Ed Cheng said, “We anticipate seeing some results from NICMOS in the May timeframe.”

Approximately one-fifth of the approved proposals from astronomers to use Hubble include the restored NICMOS instrument. Besides the cryocooler, the astronauts also installed a new pair of high-efficiency solar arrays, a new power control unit, a new flywheel, and a $76 million camera.

Grunsfeld and Linnehan performed three of the five spacewalks. The other two spacewalks were done by Jim Newman and Mike Massimino. The remainder of the crew consisted of commander Scott Altman, pilot Duane Carey, and flight engineer Nancy Currie.

The cryocooler is considered a fairly high-risk experiment, with no certainty that it will actually work as planned. NASA astronomer Anne Kinney said, “The way I have looked at this mission is if we get through our successful days, then we get an ‘A’ for the mission. If we also get the NICMOS cooler fixed and up and operating, we get an ‘A-plus’ for the mission.

“We’ve referred to it all along as an experimental activity. We very much plan to succeed, but it’s above and beyond the mission specifications.”

If the NICMOS cooler does work as planned, NASA will use it in the next generation of astronomical spacecraft.

NASA launched the Hubble Space Telescope in 1990. At that point, NICMOS was supposed to be installed on a maintenance mission in 1993. But the misshaped primary mirror, Hubble’s infamous spherical aberration, resulted in a three-year delay to NICMOS’ launch. NICMOS, then under construction at Ball Aerospace in Boulder, CO, was modified to include corrective mirrors, which compensated for Hubble’s incorrect optics. NICMOS was installed in Hubble in February 1997 with a planned lifetime of four to five years, but only worked for two years due to the heat short.

NASA is planning one final Hubble servicing mission in 2004. It will install additional radiators and cooling capabilities. In 2010, a shuttle will be sent to Hubble to retrieve it and return it to Earth for ultimate display in a museum.

Chien is with Earth News. He can be reached at kc4yr@amsat.org (e-mail).

Publication date: 04/01/2002