Key Instrument For X-Ray Mission Delivered to Japan for Installation into Astro-E Spacecraft
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453February 11, 1999
Greenbelt, Md. -- NASA's High Resolution X-ray Spectrometer (XRS) -- the first space instrument to exploit an entirely new approach to X-ray detection -- arrived in Japan last week to be installed on Astro-E, the latest in a series of Japanese observatories devoted to studying celestial X-ray sources.
"The precision for measuring the energies of X-rays will be 10-times better than current X-ray satellites," said Dr. Richard Kelley, XRS principal investigator at NASA's Goddard Space Flight Center, Greenbelt, Md. "This should allow fundamental breakthroughs in our understanding of essentially all types of X-ray emitting sources, especially material very close to black holes and the X-ray emitting gas in the vast spaces between the individual galaxies that make up clusters of galaxies."
The XRS instrument uses microcalorimeter detectors that sense the energies of individual X-ray photons as heat, as opposed to converting X-rays to electrical charges and then collecting that charge, which has been the mechanism in other X-ray detectors. The instrument was developed jointly by the Institute of Space and Astronautical Science (ISAS) in Japan and Goddard, utilizing microcalorimeter detectors originally developed at Goddard.
Dr. Kelley said that, for the first time, the XRS will provide both high spectral resolution and high throughput in one instrument. High spectral resolution allows measurements at many different wavelengths and provides much finer detail in the light spectrum. High throughput means the XRS has a large collecting area combined with the ability to measure most of the X-rays that reach the detector, an efficiency crucial for analyzing faint and distant X-ray sources.
For the XRS to measure the heat produced by a single photon, its microcalorimeter array must be cooled to an extremely low temperature, only about -460 degrees Fahrenheit (0.060 degrees Kelvin). (The absence of all heat, called absolute zero, is 0.0 degree Kelvin.) The XRS team and their Japanese associates accomplished this by creating a three-stage cooling system capable of maintaining these low temperatures for about two years in orbit -- yet another innovation for the Astro-E mission.
The XRS instrument will analyze X-ray photons focused by an X-ray telescope, also being built at Goddard by a team led by Dr. Peter Serlemitsos. That group has already delivered four similar telescopes -- a lightweight foil mirror system -- to Japan to be used with Astro-E's four X-ray Imaging Spectrometer (XIS) instruments. The XIS detectors, a collaboration among Japanese universities and institutions and the MIT Center for Space Research, utilize Charge Coupled Device (CCD) arrays, built by MIT. Rounding out the Astro-E instrumentation, the University of Tokyo is creating a Hard X-Ray Detector system that will extend the "vision" of Astro-E to higher energies.
In this way, according to Dr. Nicholas White, Head of the X-ray Branch at Goddard, "the Astro-E mission continues a successful ongoing cooperation between Japanese and U.S. scientists."
In regards to the innovative cooling system: First, there is an outer tank of solid neon at -429 degrees Fahrenheit (17 degrees Kelvin), being built by the Japanese team and which intercepts heat from the relatively warm interior of the Astro-E satellite. Inside this is a tank of liquid helium at -457 degrees Fahrenheit (1.3 degrees Kelvin) intercepting heat from the neon tank. Suspended from the helium tank and attached to the microcalorimeter detector is a capsule of paramagnetic crystals surrounded by a superconducting magnet, capable of generating a field of 20,000 Gauss. (The Earth's magnetic force, in comparison, is about 0.5 Gauss.) The magnet is used in conjunction with a cryogenic heat switch to extract heat from the paramagnetic crystals, allowing them to cool to -460 degrees Fahrenheit (0.06 degrees Kelvin) for up to about 35 hours.
Over the next couple of weeks, the XRS team will install the microcalorimeter detector and inner cooling system into the outer neon tank, which will be integrated later this year into the Astro-E satellite.
The Astro-E launch is planned for February 2000. The mission's scientific goals are to study clusters of galaxies and the nucleosynthesis history of the Universe; supernova remnants and the enrichment of the interstellar medium; active galactic nuclei and possible supermassive black holes; and stellar coronae of stars 10,000-times more active than our Sun.
For further information on Astro-E check the following Internet address: http://rxte.gsfc.nasa.gov/docs/astroe/.
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