The infrared instrument re-quires super-cooling for its detector to cryogenic temperatures. NICMOS’s original design called for the detectors to be encased in a solid block of super-cooled nitrogen at a temperature of -333 degrees F. The nitrogen would gradually boil off over a five-year period, while NICMOS viewed the infrared universe.
A problem developed shortly after NICMOS was installed in 1997. A “heat short” was detected where one cold surface was touching a warm surface. The nitrogen was boiling off far faster than planned, limiting the useful lifetime to just two years.
So astronomers immediately doubled up on infrared observations, giving NICMOS higher priority than Hubble’s other scientific instruments. NICMOS ran out of nitrogen in January 1999.
Engineers always wanted to develop some kind of replacement cooling system for NICMOS, even before the heat short was detected. Engineers had vague ideas for a “refueling” system, where a tank of liquid helium could be installed, or a mechanical refrigerator.
NO VIBRATIONS, PLEASE
NASA selected a reverse Brayton-cycle refrigerator for development. Some of the funds to develop the NICMOS Cooling System (NCS) were supplied by the Department of Defense, which uses infrared sensors in space to detect missile launches.One of the folks intimately involved with NICMOS, principal investigator Rodger Thompson, was very wary about whether or not a mechanical refrigerator would work with his instrument. Hubble has to remain rock-steady to focus on distant stars and galaxies. Any vibration or jitter would destroy the quality of the images. In its early years, Hubble had major problems with its solar arrays flexing back and forth, in addition to the well publicized mirror with the wrong prescription.
The challenge for the NCS engineers was to design a mechanical refrigerator that could work without any noticeable vibration. The solution was to use extremely tiny turbines, spinning as quickly as possible. Anything spinning extremely rapidly tends to cancel out vibrations. For example, dentist drills are designed to spin extremely rapidly to minimize vibration and pain when drilling cavities.
Creare Systems of Hannover, NH, was able to micro-manufacture small turbines, smaller than a pencil. They’re carved by vaporizing tiny pieces of metal through a process similar to arc welding. The turbines spin at 400,000 RPM, about 25 to 50 times as fast as a car engine.
The NCS uses a reverse turbo-Brayton cycle, with neon as the working gas. The neon enters the turbine at a pressure of 34.5 pounds per square inch and exits at 21.3 pounds per square inch. The decrease in pressure reduces its temperature from -334 to –343 degrees. Technically speaking, with that temperature range, the refrigerator is a cryocooler.
NASA astronomer David Leckrone noted the technology improvement. “It’s like going from an icebox to a refrigerator,” he said.
NASA decided to evaluate the NCS and several other future Hubble components before committing them for installation on Hubble. The test-flight would act as a “dress rehearsal” to make sure the components would work as planned in space and give the engineers some additional time to work out any kinks or bugs before the final installation in Hubble.
NCS flew on the STS-95 shuttle mission in October 1998, which also featured the return to space for pioneering astronaut and Senator John Glenn. The performance on the STS-95 test flight was excellent, assuring astronomers that the technology was sound.
Thompson was pleased with the results. “I’ll admit I was wrong,” he said.
Other tasks — including installing new solar arrays, a new science instrument, and other improvements to Hubble — took priority over installing the cooler for NICMOS. It was the lowest priority on the STS-109 Hubble servicing mission.
Five spacewalks were planned with the NCS installation on the final spacewalk. NASA emphasized that the NCS was an experimental device to prove the engineering and technology. If it was successful in cooling the instrument, even if other problems prevented scientific images from being returned, it would still be considered a success.
NASA scientist 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.”
The STS-109 spacewalks went well, although one spacewalk was delayed over an hour due to a leak in a spacesuit. On the fifth and final spacewalk, the astronauts installed the NCS and its radiator. As astronaut John Grunsfeld carried the large radiator over to its proper position, another crewmember commented, “Looks like a big surfboard, John.”
The cooler had clamps which attached to Hubble’s handrails and it took some manual adjustments by the astronauts to get it in place.
All together, the five spacewalks set a new space shuttle record — 35 hours and 55 minutes, exceeding the previous record set on the first Hubble servicing mission.
After the five spacewalks were finished, mission control radioed up, “The words are inadequate to describe what you have accomplished. Simply put, if you would allow me to turn a phrase, you have enabled mankind to look where no one has looked before.”
ALL SYSTEMS GO
On March 18 engineers at the Goddard Spaceflight Center sent commands to activate the cooler. It took three weeks to cool down NICMOS to the optimum operating temperature. (One of the key advantages to a refrigerator vs. a block of ice is the temperature can be fine-tuned for the best operating temperature for the detectors.)The first images with the newly restored NICMOS were released at the recent American Astronomical Society’s spring meeting.
“It is fantastic that we have restored Hubble’s infrared eyesight,” said Thompson. “NICMOS has taken us to the very fringes of the universe and to a time when the first galaxies were formed. We can’t wait to get back out there.”
NASA paid about $21 million to develop NCS. It restored operations to a $110 million scientific instrument, which would otherwise have to be written off. In addition, it verified that a mechanical refrigerator would work in space, opening the possibility for using it on future missions.
The tradeoff is NICMOS is an old instrument, developed using 1980s technology and launched in 1997. The original plan called for a CCD detector with only 64-by-64 pixels and when NICMOS was launched it had 256-by-256 detectors, roughly the equivalent of an old 1985 EGA computer monitor. Nevertheless, infrared astronomers are ecstatic about the capability to use NICMOS again.
Chien is with Earth News. He can be reached at kc4yer@amsat.org (e-mail).
Publication date: 06/24/2002
