AXAF - Credit: NASA. 14,526 bytes. 329 x 180 pixels.

The Chandra Advanced X-ray Astrophysics Facility was one of the four Great Observatories (along with HST, GRO, and SIRTF). AXAF will study the composition and nature of galaxies, stellar objects and interstellar phenomena as well as basic issues in theoretical physics using the most sensitive X-ray telescope ever built. International participants include the UK, Germany, and the Netherlands. Originally consisting of 2 separate spacecraft, the AXAF program was reduced to a single spacecraft in 1993 due to fiscal constraints. Satellite operations will be conducted by MSFC using NASA's Deep Space Network. Spacecraft: 3-Axis stabilised, zero momentum biased control system. Solar arrays generate over 2 kW. Light weight composite structure. Six reaction wheels. Four 2-degree-of-freedom dry tuned rotor gyros. Three 40 AHr nickel hydrogen batteries. Two 1.8 Gbit solid state recorders can store 18.8 hours of data per recorder. The Integral Propulsion System on Chandra used four TR-308 bipropellant thrusters for orbit changes. The thrusters use N2O4 (nitrogen tetroxide) oxidizer and N2H4 (hydrazine) fuel. They had a thrust of 472N and a specific impulse of 322.3s; the TR-308 was an upgrade of the TR-306 that was used on three Lockheed Martin Series 5000 satellites. The same fuel tanks also feed Marquardt 89N monopropellant hydrazine RCS thrusters. In addition to the IPS, the MUPS (Momentum Unloading Propulsion System) used 0.9N hydrazine thrusters to unload momentum from the gyro systems used to point Chandra.Payload: X-ray telescope consisting of four pairs of nearly cylindrical mirrors, ranging from 1.4 meters in diameter to 0.68 meters in diameter. These mirrors focus X-ray energy over a 10 m focal length onto two of the four science instruments, the High Resolution Camera (HRC) and the AXAF CCD Imaging Spectrometer (ACIS). Also carried are two non-focal plane instruments, the High-Energy Transmission Grating (HETG), and Low-Energy Transmission Grating (LETG).

Design Life: 5 years. Total Length: 12.2 m.

The Chandra Advanced X-ray Astrophysics Facility was one of NASA�s four Great Observatories (along with Hubble Space Telescope, Compton Gamma Ray Observatory, and the SIRTF). Chandra will study the composition and nature of galaxies, stellar objects and interstellar phenomena as well as basic issues in theoretical physics using the most sensitive X-ray telescope ever built. The IUS under-performed and placed Chandra in an orbit about 900 km lower than planned. Therefore Chandra's own IPS propulsion system had to be used to make up the difference. The first such manoeuvre was at 01:11 GMT on July 25 when the IPS engines fired for 5 minutes to raise perigee to 1192 km. Further perigee burns on July 31, August 4, and August 7 raised the orbit to its final 10,000 km x 140.000 km. Additional Details: Chandra.

• 4 - McDowell, Jonathan, Jonathan's Space Report (Internet Newsletter), Harvard University, Weekly, 1989 to Present. HTML when accessed: ftp://sao-ftp.harvard.edu/pub/jcm/space/news
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