The Future of the Hubble Space Telescope
By Stephen P. Maran
Assistant Director of Space Sciences for Information and Outreach NASA's Goddard Space Flight Center
Astronomers using the Hubble Space Telescope are at the dawn of a new era in exploring the universe. With two powerful new instruments, added to Hubble in February 1997 by the crew of the Space Shuttle Discovery, we can literally peer light years beyond what was possible before.
Our deepest look into space so far is the 10-day exposure by this telescope, called the Hubble Deep Field, that is celebrated in the first of the three "Mysteries of Deep Space" programs. That image is believed to reveal galaxies back to about 10 per cent of the present age of the universe, meaning to within about a billion years after the Big Bang. But now we can look further back still; back in time and far out in space. Light from the very distant and ancient galaxies is shifted towards very long wavelengths by the famous red shift of the expanding universe, as explored by Edwin P. Hubble himself. Light that was blue or yellow when it left the distant galaxies has been shifted to the invisible infrared region of the spectrum by the time it nears the Earth. This makes the Hubble Space Telescope's new Near Infrared Camera and Multi-Object Spectrometer - the ideal instrument to receive it. With NICMOS, it should be possible to peer into the Hubble Deep Field to the distance where galaxies (if such exist that far back) are seen when the universe had attained only five percent of its present age.
A second new instrument on the Hubble Space Telescope will multiply its scientific output. Until recently, each of the two original spectrographs on board (both of them removed from the Telescope in February by the Discovery crew) could record the spectrum of only one star at a time, or the spectrum of only one location in a galaxy or nebula at a time. Yet it is spectra which are the astronomer's most powerful diagnostic tool: from them we learn the chemical composition, temperature, density, pressure, and other properties of celestial objects. With the new Space Telescope Imaging Spectrograph the spectra of as many as 500 adjacent locations on the sky can be observed simultaneously. This huge so called multiplex advantage will enable projects that were impractical before. This is especially important in the hunt for giant black holes near the centers of galaxies. Previously, Hubble Telescope users had to make multiple successive observations of the spectra at different locations around a galaxy center in order to determine the rate at which the stars and gas are orbiting the center. The more powerful the source of gravity, the faster they whirl. Where speeds are high enough, the presence of a very massive black hole is deduced.
Now, a systematic census of galactic centers is possible with STIS, so that the Hubble Telescope can tell us whether giant black holes are standard features of normal galaxies, or rare oddball components of the universe. With its more sensitive detectors, the STIS will also permit spectroscopic analysis on more distant objects than previously possible with the Hubble Telescope.
In a year or two's time, the Telescope may complete a second Hubble Deep Field, this time in the southern sky, for comparison with the existing observation, which looked far to the north. In 1999, a new Advanced Camera for Surveys will be added to the Telescope's instrument complement. This camera will enable us to photograph a larger area of the sky at one time than Hubble could do before, and its images will be even sharper than those we now obtain. Beyond the new Millennium, an additional instrument to make Hubble's explorations more rewarding yet is to be added in the year 2002. Scientists at many laboratories are submitting proposals to NASA this month for the possible forms that the 2002 instrument might take. By current plans, the mission of the Hubble Space Telescope will end in 2005, but astronomers everywhere are hoping that it may continue even longer into the 21st Century. Some day, a Next Generation Space Telescope, now in the study phase, may become a reality. It would dwarf the Hubble Space Telescope and represent a quantum leap in capability to explore the most remote regions of the universe.