Current plans are for the Soft X-ray Spectrometer, XSX, to fly on the Astro-H satellite, slated for launch in 2013. The SXS measures x-rays emitted by astronomical objects. Astronomers are especially anxious to study these x-rays, because they are emitted by processes that involve large amounts of energy. For example, astronomers expect that material falling into black holes will emit x-rays. The term "spectrometer" means that the SXS will measure the spectrum of the x-rays recieved, that is, it will record the wavelengths.
Earlier versions of the Soft X-ray Spectrometer (SXS), then called the X-Ray Spectrometer (XRS) had unsuccessful flights.
The renamed Soft X-ray Spectrometer (XSX) will have a modified cooling system design. The basic approach of the x-ray sensors will stay the same, however.
The Astro-E2 satellite, carrying XRS, was launched at 12:30 PM, Japan time, on July 10, 2005. After launch, the satellite was renamed Suzaku, after a mythical bird that brings good luck.
Unfortunately, the new name did not bring enough good luck to save the XRS-2 instrument. The XRS instrument operated for 19 days, demonstrating that the ADR (magnetic cooling system) was working properly. Then, the liquid helium coolant evaporated and was lost. Without the helium coolant, the instrument cannont function, so it was shut down. A mishap investigation board is being set up and will work to discover the cause of the loss of helium.
The one bright spot is that the mission demonstrated the usefulness of certain technologies in space. These include the Adiabatic Demagnetization Refrigerator, the high temperature superconducting leads, mounted to prevent damage during launch, and the microcalorimeter X-ray astronomy sensors. This flight experience will help the design and testing of the follow-on Soft X-ray Sensor (SXS) scheduled for the Astro-H mission in 2013.
In developing and building XRS - 1, Goddard engineers and scientists developed technology that will be used on other missions, not just on XRS. For example, an Adiabatic Demagnetization Refrigerator (ADR) is planned for use on HAWC and on SAFIRE, 2 instruments for use on SOFIA, the Stratospheric Observatory for Infrared Astronomy. (SOFIA is a Boeing 747 airliner converted to carry a telescope.) One of the XRS team's major accomplishments was adapting the ADR, a well established technology here on the ground, for use in space.
For a non-technical introduction to the purpose and workings of the XRS cooling system, see Introduction to XRS.
Goddard is supplied the central part of the XRS instrument, called the XRS insert. This section was inserted into a solid neon tank built in Japan. The rebuilt XRS-2 insert for the Astro-E-2 spacecraft was constructed to the same plans as the original.
The main parts of the insert are:
In ground-based tests, the first XRS achieved a resolution of approximately 12 eV over the energy range of 0.3 to 10 keV by using calorimeters cooled to 0.065 Kelvin. These low temperatures are produced by an Adiabatic Demagnetization Refrigerator (ADR), which in turn is cooled by superfluid helium. For non-technical background information on liquid helium cooling see introduction to liquid helium. The Japanese-supplied solid neon dewar, at a temperature of 17 Kelvin, is fitted around the helium tank to reduce the external heat load on the helium.
For the flight to Japan, the XRS - 1 insert's shipping container was packed with dry ice, to keep the ADR salt pill cool. (The salt pill could have been damaged it the temperature had risen too far above room temperature.) About 80% of the dry ice was still frozen when the insert was unpacked.
Originally, XRS was planned to fly on AXAF, the Advanced X-ray Astrophysics Facility, now known as Chandra. Plans changed, and XRS was removed from AXAF, but found a new home on Astro-E. Unfortunately, Astro-E had much less space available for XRS's liquid helium coolant. Specifically, we had to ensure that the liquid helium supply would last for two years, even though the volume was small (only 18 to 20 liters.) To achieve this long lifetime, we had to ensure that the heat flow into the helium tank was less than 800 microwatts. If the heat flow were higher than this, then the liquid helium coolant would evaporate completely before the end of the required 2 year lifetime.
Such a low heat flow into the helium tank is unprecendented in satellite cryogenics. Therefore, we had to study possible sources of heat which we had not bothered considering on previous missions. Fortunately, a number of these sources turned out to be too small to worry about. Here are some of those possible heat loads.
After looking at the possible heat loads came the challenge of designing and building the system with the required low heat flow. Among the innovations required to achieve the low heat flow, three deserve special mention:
To keep track of the amount of helium in XRS, we built a mass gauging system. For non-technical background information on liquid helium cooling see introduction to liquid helium.
XRS References , with some abstracts.
The XRS cryostat made the cover of the August 1999 Physics Today, in connection with the article "Quantum Calorimetry". For more details, see the Physics Today August contents (a non-NASA, non-US government link.)
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