Hubble Space Telescope

Making observations with an orbital telescope is not a simple process. The telescope must be instructed where to point to acquire a new target, how to move in order to avoid light contamination from the Sun and Moon, how long to observe and with what instruments, what data format to use for transmission of result, how to orient its radio antennas to send and receive future commands, and so forth. All commands must be written in computer code and relayed to the Hubble by radio during a point in its orbit where it can communicate with antennas on the ground.

How does the Hubble know where to find a given target object? Like a person trying to find his or her way in unfamiliar territory, the telescope searches for stellar landmarks termed guide stars. The position of any star, planet, or galaxy can be specified in terms of particular guide stars—bright, easily found stars located near the object of interest. (The guide stars are not literally close to the objects they are used to locate, but appear to be near them in the sky.) Sky surveys performed by ground-based telescopes have mapped many of these stars, so the Hubble merely points itself to the appropriate coordinates, then uses the guide stars to maintain its position.

In early 1997, astronauts aboard the space shuttle Discovery performed another servicing mission, this time to swap out instruments. The HRS was replaced by the Space Telescope Imaging Spectrograph (STIS). Unlike the older instrument, the STIS collects light from hundreds of points over a target area instead of just one point. The servicing crew removed the FOS and in its place installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which allows the telescope to gather images and spectroscopic data in the infrared spectral region (0.8 and 2.5 micrometers), which in effect allows the Hubble to see through interstellar clouds of gas and dust that block visible light.

The crew also made repairs to the telescope's electrical, data storage, computer, and pointing systems, as well as to its battered thermal insulation blanket, which had been severely damaged by collisions with small bits of space debris. The final task of the repair mission was to nudge the observatory to an orbit six miles higher than previously, to enhance its longevity and stability. Altitude affects longevity because the orbit of any near-Earth object, including the Hubble, is degrading all the time due to friction with outlying traces of the Earth's atmosphere. Therefore, unless it is boosted out of Earth orbit or brought back to Earth by a space shuttle, the Hubble will eventually burn in the atmosphere. Because the Hubble is so massive, it would not vaporize entirely on reentry, but would shower some part of the Earth's surface with chunks of metal and glass. NASA is presently debating whether to (a) retrieve the Hubble intact after it is scheduled to go out of service in 2010, (b) guide it to a chosen crash zone on Earth, or (c) push it right out of Earth orbit with a specially-built rocket.

The Hubble Space Telescope has revolutionized astronomy by bringing a whole new understanding of the Universe to mankind. The following list highlights a few of the Hubble's achievements:

• Imaged comet Shoemaker-Levy 9 crashing into Jupiter in 1994.
• Showed that protoplanetary dust disks are common around young stars.
• Proved that Jupiter-size planets are uncommon in globular clusters.
• Shown that quasars reside in galaxies, many of which are colliding with each other.
• Shown that supermassive black holes reside at the centers of many galaxies.
• Permitted more accurate measurement of the Universe's rate of expansion than ever before.
• Observed distant supernovae which give evidence that the expansion of the Universe is actually accelerating, prompting a major revision of cosmological thought.
• Imaged large numbers of very distant galaxies distances with its Deep Field study, greatly increasing our estimate of how many galaxies there are in the Universe.

The Hubble will eventually be decommissioned. Work is already under way on its replacement, the James Webb Space Telescope (JWST, named for a former NASA administrator), due for launch in 2010. Unlike the Hubble, which travels around Earth in a moderately low orbit, the JWST will be located some 930,000 mi (1.5 million km) away, to avoid glare from the Earth. The JWST will make observations only at near- and mid-infrared wavelengths, seeking to study the early history of the Universe. Optical and ultraviolet wavelengths will not be observed by the new telescope.