The Hubble Space Telescope (HST), in orbit since 1990, will have two of its instruments replaced during a ten-day mission of the space shuttle Discovery scheduled to begin at 2:59 a.m. on Feb. 11. Even though the HST was launched in 1990, its detectors and technology date from the late 1970's, when the satellite was designed. In order to keep the telescope at the cutting edge of astronomical research the instrumentation must be upgraded, just as Earth-based telescopes are constantly improved to maintain their ability to do up-to-date research.
In the first servicing mission, in 1993, optical devices were installed to correct for the spherical aberration of the primary mirror. This aberration was the result of disastrous errors made in the original grinding and polishing of the mirror when it was manufactured. The first servicing mission was successful and the Hubble has been meeting its original expectations.
The two new instruments are the Space Telescope Imaging Spectrograph, called STIS, and the Near-Infrared Camera and Multi-Object Spectrometer, called NICMOS. These two instruments will replace the Faint Object Spectrograph and the Goddard High Resolution Spectrograph which will be returned to Earth. Because the new instruments must include corrective optics to overcome the spherical aberration of the primary mirror, the size and number of detectors in these new instruments has been cut back to stay within budget.
STIS contains a two-dimensional array of detectors for the simultaneous spectroscopic analysis of several objects or of several points within an extended object such as a galaxy or nebula. Observation of the swirling motions of stars in the central portions of galaxies should make it possible to determine the presence of black holes there. This device will have two types of detectors installed making it sensitive to radiation from the ultraviolet portion of the spectrum, wavelengths shorter than those in visible light, to the near infrared portion of the spectrum, wavelengths longer than visible light.
NICMOS is designed to examine the part of the electromagnetic spectrum from the near infrared, at wavelengths slightly longer than visible wavelengths, to wavelengths of 2.5 micrometers. Some portions of this wavelength region are visible from the ground, but our atmosphere absorbs much radiation in this range. The original instruments on the HST were designed primarily for examination of ultraviolet wavelengths as well as visible. This instrument extends the Hubble's range of spectral sensitivity into the longer, infrared wavelength region, opening the telescope's view to regions of star formation that are shrouded in dust and now invisible.
In addition to these two new optical instruments, the astronauts will replace one of the original tape recorders with a solid state recorder that is much faster and has a greater capacity than the tape recorder. With this new recorder it will be possible to record simultaneously data from the two new instruments, STIS and NICMOS, as well as from the Wide Field/Planetary Camera 2 that was installed during the 1993 servicing mission. Replacement of one of the Fine- Guidance Sensors with corrective optics (to compensate for the spherical aberration) and of a faulty gyroscope will improve the accuracy with which the telescope can be pointed. A new cover will be installed on one of Hubble's magnetometers, replacing a "jury-rigged" cover put in place in 1993 when astronauts noticed the that the original was torn loose. Two other electronic control devices will be replaced and the Hubble will be boosted about 20 miles to its original altitude of 380 miles. Although in no danger of falling, the next service mission, planned for 1999, will use the older, heavier space shuttle Columbia which is not expected to be able to boost the HST as readily as Discovery.