Servicing of the Hubble Space Telescope

Abstract

1.What is HST?The Hubble Space Telescope was proposed by Lyman Spitzer in 1947, approved and designed by NASA in the 1970s, built in the 1980s, and launched in 1990. It has been operating successfully ever since.HST's useful wavelength range is from 0.12 to about 2.4 microns, encompassing the ultraviolet, visible, and near-infrared.HST's design (especially its size) was driven by the need to fit it in the cargo bay of the space shuttle.Deployment by the shuttle limited the altitude of HST's orbit to about 600 km above the earth's surface.At this altitude there remains enough residual atmosphere to cause gradual but significant drag on a large object such as HST. This is particularly a problem near the peak of the Sun's 11-year activity cycle, when its ultraviolet radiation increases, heating the earth's upper atmosphere and causing it to expand. This orbital degradation can be compensated by regular reboosts of the orbit with the space shuttle.Despite this drag, having HST in a low-earth orbit (LEO) has advantages. First, HST orbits well within the earth's magnetic field, which shields the telescope and its detectors from the stronger radiation experienced at higher altitudes. Second, the earth's magnetic field also provides a reservoir into which HST can dump excess angular momentum that builds up in the reaction wheels from its many slews.Satellites at higher altitude cannot do this and rely on chemical thrusters. Such thrusters are an expendable that limits the operational lifetime. Even more importantly for HST, any chemical outgassed can be deposited on optical surfaces within the telescope and its instruments. Even extremely minute amounts of such contaminants can destroy optical performance in the ultraviolet, one of HST's key capabilities. Finally, the critical LEO advantage for HST has been the ability for regular visits by the space shuttle with a crew trained to replace failed hardware, add new capabilities, and to replace existing scientific instruments with new ones that exploit the full capabilities of evolving technology. The key result of this last advantage is that the HST of today is substantially better than it was at launch, and that each new servicing mission to the observatory makes it much more powerful, both scientifically and functionally.LEO comes with costs too. HST orbits the earth every 96 minutes. In all but a few favored directions on the sky, the object HST is viewing gets occulted by the earth on every orbit. This limits a single viewing of a celestial object to roughly 50 minutes and causes the overall efficiency of HST to be limited to about 50%. However, it is possible (and routine) to observe an object or a field for many orbits by adding up many exposures. The effects of cosmic rays limit the maximum useful exposure time for most detectors Downloaded by 171.234.246.208 on June 22, 2016 | http://arc.aiaa.org |