The X-Ray Spectrometer (XRS) is an instrument designed to study x-rays emitted by black holes and other exotic astronomical objects. The first one was destroyed in a launch attempt from the Kagoshima Space Center in Japan in February 2000. A replacement was then built and launched in July 2005. Unfortunately, a problem developed with the liquid helium coolant supply, which suddenly evaporated only 19 days after the launch. A mishap investigation board is now being organized to find the cause of the unexpected loss of helium coolant. We hope that they will pinpoint a problem which can be avoided on future satellites. In the mean time, however, we know that many of the technologies used in XRS worked well, and we expect that they will be used in future space missions.
XRS shows how liquid helium cooling and an ADR can work together as part of a satellite cooling system. XRS is also interesting for another reason. Because the volume of liquid helium was so small, the system included some unusual design features. These features were intended to lengthen the lifetime of the liquid helium coolant supply by reducing the need for cooling.
To work properly, the x-ray astronomy sensors in XRS needed to be cooled to sixty thousandths of a degree above absolute zero. For this temperature range, we chose an Adiabatic Demagnetization Refrigerator (ADR). The ADR has been used in laboratories on the ground for years, and is thus a well-established technology.
Another commonly used laboratory cooler for this temperature range is the liquid helium dilution refrigerator. For satellite use, the ADR has 2 important advantages over the dilution refrigerator. First, the ADR is more efficient. Efficiency is important in a satellite, where electric power and all other resources are strictly limited. Second, the dilution refrigerator requires a complicated internal plumbing system. This plumbing would be difficult to adapt for a satellite. In one part of the plumbing, a lighter liquid floats on top of a heavier liquid. It would be difficult to design a replacement for this part of the system which would work in zero gravity.
All the really low temperature cooling systems have one thing in common. Unlike the refrigerator in your kitchen, none of these systems will work at room temperature. They all must be cooled to low temperatures in order to produce the even lower temperatures that we are aiming for. The XRS ADR was cooled by a tank of liquid helium at 1.3 Kelvin (1.3 degrees above absolute zero. For more information on the Kelvin temperature scale, see our temperature scales page.) For an explanation of how liquid helium coolant is used in satellites, see our Introduction to Liquid Helium and Liquid Helium in Space pages. Surrounding the liquid helium tank was a tank of solid neon at 17 Kelvin (17 degrees above absolute zero.) We at Goddard built the helium tank, the ADR, the x-ray sensors, and all the equipment that attached directly to them. The neon tank was built by our Japanese partners, who also built the satellite that XRS was to ride on.
Liquid helium cools by evaporating as it absorbs heat, just as, on a warm day, we are cooled by the perspiration that evaporates from us. For XRS, we had to design the system to have a tiny evaporation rate, much smaller than had been done before in a satellite. Because of the small space avialable in the satellite, the XRS helium tank could only carry 18 to 20 liters of liquid helium. (A liter is about the same as a quart.) This supply of liquid helium had to last for the 2 years of the mission. By comparison, in one laboratory cryogenic system I used recently, that much helium evaporated in a single day.
On the ground, some laboratories have machines that capture the helium vapor and recondense it to form liquid helium. Unfortunately, such helium liquifyers are much bigger than the space available for the entire XRS instrument. So we had to concentrate on making the helium evaporate as slowly as possible. In the next section, I'll focus on three things we did to reduce the evaporation rate of the helium bath.
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