Hubble Space Telescope

Launch and Servicing Missions

The HST was launched by the space shuttle Discovery on April 24, 1990. Hubble initially was equipped with five science instruments: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the High-Resolution Spectrograph, and the High-Speed Photometer. In addition, Hubble was fitted with three fine guidance sensors used for pointing the telescope and for doing precision astrometry—the measurement of small angles on the sky.

After Hubble was launched, scientists discovered that its primary mirror was misshapen because of a fabrication error. This resulted in spherical aberration: the blurring of starlight because the telescope could not bring all the light to a single focus. Using image-processing techniques to reduce the blurring in HST images, scientists were able to do significant research with Hubble until an optical repair could be developed.

In December 1993, the first HST servicing mission carried replacement instruments and supplemental optics aboard the space shuttle Endeavour to restore the telescope to full optical performance. A deployable optical device, called the Corrective Optics Space Telescope Axial Replacement (COSTAR), was installed to improve the sharpness of the first-generation instruments. The COSTAR was outfitted with pairs of small mirrors that intercepted the incoming light from the primary mirror and reconstructed the beam so that it was in crisp focus. In addition, the original Wide-Field Planetary Camera was replaced with a second camera, the Wide-Field Planetary Camera 2, which has a built-in correction for the aberration in the primary mirror.

In March 1997 the space shuttle Discovery returned to the HST for a second servicing mission. Two advanced instruments, the Near Infrared Camera and Multi-Object Spectrometer and the Space Telescope Imaging Spectrograph were installed to replace two first-generation instruments. Astronauts also replaced or enhanced several electronic subsystems and patched unexpected tears in Hubble's shiny, aluminized, thermal insulation blankets, which give the telescope its distinctive foil-wrapped appearance and protect it from the heat and cold of space.

In December 1999 a third servicing mission replaced a number of subsystems but added no new instruments. About a month before the mission a critical gyroscope had failed, leaving Hubble with only two operational gyros out of a total of six onboard. This had left the telescope incapable of precision pointing. The December mission restored Hubble to six fully functioning gyroscopes. The telescope's main computer was upgraded from a 1960s computer with 48 kilobytes of memory, to an Intel 486 microprocessor.

In March 2002, the next and most ambitious serving mission in the series, involving five exhausting six-hour space walks by pairs of astronauts, took place. They installed a high-efficiency camera called the advanced camera for surveys. The mission also performed "heart surgery" by replacing a complex power control unit, which required completely shutting off the telescope's electrical power. The telescope also got stubby new solar panels that increased the power enough for all of the instruments to operate simultaneously.

In 2004, the last servicing mission will install the wide-field planetary camera 3 and the cosmic origins spectrograph. Hubble will be on its own until 2010, when NASA stops the observing program and must decide whether to retrieve Hubble and install a rocket propulsion system that will put it into a safe higher orbit or let it reenter the atmosphere and largely burn up over the ocean.

How Hubble Operates

Hubble is controlled at the Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STSI), located at the Johns Hopkins University in Baltimore, Maryland, directs the science mission. Space telescope research and funding engages a significant fraction of the worldwide community of professional astronomers. Astronomers compete annually for observation time on Hubble.

Observing proposals are submitted to peer review committees of astronomers. The STSI director makes the final decision and can use his or her own discretionary time for special programs. Accepted proposals must be meticulously planned and scheduled by experts at STSI to maximize the telescope's efficiency.

The space telescope is not pointed by "real-time" remote control but instead automatically carries out a series of preprogrammed commands over the course of a day. This is necessary because the telescope is in a low Earth orbit, which prevents any one ground station from staying directly in contact with it. Instead, controllers schedule intermittent daily linkups with the space observatory via a series of satellites in geosynchronous orbit.

A date "pipeline," assembled and maintained by STSI, ensures that all observations are stored on optical disk for archival research. The data are sent to research astronomers for analysis, and then made available to astronomers worldwide one year after the observation.

By the turn of the twenty-first century, Hubble had looked at over 13,000 celestial targets and stored over 6 gigabytes of data onto large optical disks. The telescope had made nearly one quarter million exposures, approximately half of these were of astronomical targets and the rest were calibration exposures.

Hubble Provides New Insights

The HST has made dramatic inroads into a broad range of astronomical frontiers. Astronomers have used Hubble to look out into the universe over distances exceeding 12 billion light-years. Because the starlight harvested from remote objects began its journey toward Earth billions of years ago, the HST looks further back into time the farther away it looks into space (as do all large telescopes). Hubble has seen back to a time when the universe was only about 5 percent of its present age.

The Hubble Deep Field.

Hubble's deepest views of the universe, made with its visible and infrared cameras, are collectively called the Hubble Deep Field. These "long exposures" of the universe reveal galaxies that existed when the universe was less than 1 billion years old. The Hubble Deep Field also uncovered hundreds of galaxies at various stages of evolution, strung along a corridor of billions of light years. The high resolution of Hubble images enables astronomers to actually see the shapes of galaxies in the distant past and to study how they have evolved over time.

Expansion and Age of the Universe.

Another key project for the HST has been to make precise distance measurements for calculating the rate of expansion of the universe. This was achieved by measuring distances to galaxies much farther out than had previously been accomplished in decades of observing.

Determining the exact value of this rate is fundamental to calculating the age of the universe. In 1998, a team of astronomers triumphantly announced that they had accurately measured the universe's expansion rate to within an accuracy of 10 percent. This brought closure to a three-decade-long debate over whether the universe is 10 or 20 billion years old. The final age appears to be between 13 and 15 billion years, but this estimate is also affected by other parameters of the universe.

The HST was also used to find out if the universe was expanding at a faster rate long ago. This was done by using Hubble to peer halfway across the universe to find ancient exploding stars called supernovae. These stars can be used to calculate vast astronomical distances because they are so bright and shine at a predictable luminosity, which is a fundamental requirement for measuring distances.

Hubble observations, as well as other observations done with ground-based telescopes, show that the universe has not decelerated. In fact, to the surprise of astronomers, the expansion of the universe is accelerating, and therefore will likely expand forever. This realization offers compelling evidence that there is a mysterious repulsive force in space, first theorized by German-born American physicist Albert Einstein (1879-1955), which is pushing the galaxies apart—in addition to the original impetus of the Big Bang.

This idea was bolstered in 2000 when Hubble astronomers accidentally discovered a supernova so far away, it exploded when the universe was actually decelerating. This supernova happened about 7 billion years ago, just before dark energy began accelerating the universe, like a car accelerating through a traffic light that has just turned green.

Black Holes.

The HST has provided convincing evidence of the existence of supermassive black holes that are millions or even a billion times more massive than the Sun. Hubble's exquisite vision allows astronomers to zoom in on the environment around a black hole and make critical measurement of the motion of stars and gas around the hole, to precisely measure its mass. The measurements show that there is far more mass at the core of galaxies than can be accounted for by starlight. This unseen mass is locked away inside black holes.

HST observations of both quiescent and active galaxies, the latter of which pours out prodigious amounts of energy, have shown that supermassive black holes are commonly found at the hub of a galaxy. A Hubble census of black holes also showed that the mass of a black hole corresponds to the mass of the central bulge of a galaxy. Therefore, galaxies with large bulges have more massive black holes than galaxies with smaller bulges. This suggests that supermassive black holes may be intimately linked to a galaxy's birth and evolution.

Quasars.

Hubble's keen ability to discern faint objects near bright objects allowed for definitive observations that showed the true nature of quasars, which are compact powerhouses of light that resemble stars and that reside largely at the outer reaches of the universe. HST observations conclusively showed that quasars dwell in the cores of galaxies, which means they are powered by supermassive black holes that are swallowing material at a furious rate.

Gamma-Ray Bursts.

Hubble played a key role in helping astronomers resolve questions regarding the nature of mysterious gamma-ray bursts. Gamma-ray bursts are powerful blasts that come from random directions in the universe about once per day. Hubble observations found host galaxies associated with some of these blasts. This places the bursts at cosmological distances rather then being localized phenomena within our galaxy. Hubble also showed that the blasts occur among the young stars in the spiral arms of a host galaxy. This favors neutron star collisions or neutron star-black hole collisions as the source of the bursts.

Stellar Environments.

The HST has unveiled a wide variety of shapes, structures, and fireworks that accompany the birth and death of stars. HST images have provided a clear look at pancake-shaped disks of dust and gas swirling around and feeding embryonic stars. Besides helping build the star, the disks are also the prerequisite for condensing planets. Hubble images also show blowtorch-like jets of hot gas streaming from deep within the disks. These jets are an "exhaust product" of star formation.

In dramatic images, HST has shown the effects of very massive young stars on their surrounding nebulae. The astronomical equivalent of a hurricane, the intense flow of visible and ultraviolet radiation from an exceptionally massive young star eats into surrounding clouds of cold hydrogen gas, laced with dust. This helps trigger a firestorm of star birth in the neighborhood around the star.

The HST has produced a dazzling array of images of colorful shells of gas blasted into space by dying stars. These intricate structures are "fossil evidence" showing that the final stages of a star's life are more complex than once thought. An aging star sheds its outer layers of gas through stellar winds. Late in a star's life, these winds become more like a gale, and consequently sculpt strikingly complex shapes as they plow into slower-moving material that was ejected earlier in the star's life.

The most dramatic star-death observation for the HST has been tracking the expanding wave of debris from the explosion of supernova 1987A. HST observations show that debris from the supernova blast is slamming into a ring of material around the dying star. The crash has allowed scientists to probe the structure around the supernova and uncover new clues about the star's final years.

Extrasolar Planets.

Even Hubble's powerful vision is not adequate to see the feeble flicker of a planet near a star. Nevertheless, Hubble was still very useful for conducting the first systematic search for a special type of planet far beyond our stellar neighborhood. For ten consecutive days Hubble peered at the globular cluster 47 Tucane to capture the subtle dimming of a star due to the eclipse-like passage of a Jupiter-sized planet in front of the star. Based on extrasolar planet discoveries in our own stellar neighborhood, astronomers predicted that seventeen planets should have been discovered. However, Hubble did not find any, which means that conditions favoring planet formation may be different elsewhere in the galaxy.

Aiming at a known planet 150 light-years away, Hubble made the first-ever detection of an atmosphere around a planet. When the planet passed in front of its star, Hubble measured how starlight was filtered by skimming through the atmosphere. Hubble measures the presence of sodium in the atmosphere. These techniques could eventually lead to the discovery of oxygen in the atmospheres in inhabited terrestrial extrasolar planets.

SEE ALSO ASTRONOMY, KINDS OF (VOLUME 2); EXTRASOLAR PLANETS (VOLUME 2); GYROSCOPES (VOLUME 3); HUBBLE, EDWIN P. (VOLUME 2); OBSERVATORIES, SPACE-BASED (VOLUME 2).

Ray Villard

Bibliography

Chaisson, Eric. The Hubble Wars. New York: Harper Collins, 1994.

Smith, Robert W. The Space Telescope. Cambridge, UK: Cambridge University Press,1993.

Internet Resources

The Hubble Space Telescope. Space Telescope Science Institute. <http://hst.stsci.edu/>.