Since 1990, the Hubble Space Telescope has been transmitting images to scientists from its orbit 375 miles above the Earth.

From its perch in the Earth's upper atmosphere, Hubble can produce high-resolution images of astronomical objects that have ten times higher resolution than almost all those the best ground telescopes produce. The dust and gas in the Earth's atmosphere blurs visible light and make it hard for ground telescopes to see faint stars. The atmosphere also absorbs some wavelengths of light, making it difficult or impossible to observe infrared, ultraviolet, gamma rays and X-rays.

One of Hubble's unique characteristics is that it was the first space mission specifically designed for astronauts to periodically maintain and improve. As of December 2003, there have been three servicing missions, during which Hubble's design allowed astronauts to take it apart, replace worn out equipment and upgrade instruments.

Initial telescope trouble
After Hubble was first launched, it made headlines when a flaw in its main mirror made its images blurry. Despite the $1.5 billion spent to develop the telescope, scientists discovered that the main mirror was too flat. Although the amount of extra glass that had to be removed from the mirror was equivalent to just 1/50th the width of a human hair, that was enough to make Hubble no better than the ground telescopes, according to National Aeronautics and Space Administration scientist Ed Weiler.

NASA had planned for telescope maintenance missions once every few years. In 1993, during the first of these missions, astronauts were able to correct the mistake in Hubble's primary mirror by mounting other mirrors -- just as eyeglasses can improve people's vision without actually changing their eyes.

After that successful mission, "the telescope began making images of faint objects never seen before," according to the book Power to Explore, a history of NASA's Marshall Space Flight Center. Its unparalleled imaging capabilities then allowed astronomers to gain a better understanding of the universe by peering further into the cosmos, making many astronomers strong supporters of the project.

"Hubble's rate of discovery is simply unprecedented for any single observatory, but what may be even more important in the long tern is what Hubble has given to just about everyone on Earth. Hubble's spectacular images and discoveries of black holes, colliding galaxies and bizarre objects at the edge of the universe have been brought into millions of homes," NASA's Weiler said.

Hubble's key discoveries
In 1994, Hubble's observations clarified more than two centuries of scientific thinking about black holes. Using Hubble, astronomers found seemingly conclusive evidence for a massive black hole in the center of a galaxy 50 million light years away from Earth. The observation provided strong support for predictions made by Albert Einstein's in his general theory of relativity.

A black hole cannot be directly observed, so the scientists analyzed the hot gases orbiting around it. Astronomers used Hubble images to determine that the gases were moving at about 1.2 million miles per hour and then used that data to calculate the black hole's mass.

Beyond allowing scientists to verify the existence of black holes, the telescope transmits images that have enabled scientists to learn more about the history of the universe and to come closer to determining its age.

Hubble uncovered the oldest, burned-out stars in the Milky Way galaxy, which gave astronomers a new way to estimate the universe's age. Previous estimates had also been based on Hubble information that allowed scientists to calculate the universe's rate of the expansion.

Scientists determined that the ancient stars that Hubble viewed in 2002 were 12 billion to 13 billion years old. Since earlier Hubble observations showed that the first stars were formed less than 1 billion years after the universe's birth in the big bang, finding these stars moves astronomers much closer to calculating the universe's age.

"This new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value," said Harvey Richer of the University of British Columbia, Canada.

The end of Hubble
Periodic maintenance missions are not just a way of keeping Hubble outfitted with current technology -- they are also necessary to keep the telescope operating.

Hubble moves through the Earth's upper atmosphere, and the gases it travels through cause a drag that over many years saps its energy and moves it closer to Earth. To counteract this drag, Hubble needs to be boosted periodically into a higher orbit during a shuttle mission. According to scientists, Hubble could return to Earth as early as 2013 if it does not receive another shuttle boost.

While most systems in the Hubble spacecraft last a long time, its gyroscopes do wear out. To carry out observations, at least two, but ideally three, of the telescope's six gyroscopes need to be operating. As of December 2003, three are working gyroscopes, one is serving as a spare and two have failed.

In August 2003, a panel of independent astronomers issued a final report to NASA that outlined their recommendations for the transition between Hubble and its eventual replacement, the James Webb Space Telescope.

The panel, led by John Bahcall of the Institute for Advanced Study at Princeton, outlined several options but concluded that there should be two additional manned servicing missions, with the second occurring only if it was successful in a competition with other new astrophysics proposals.

Continuing the Hubble's mission would require another manned mission by 2006, the panel reported. Originally, five manned service missions were planned before the end of the telescope operations, which was slated for 2010. After the loss of the space shuttle Columbia, the likelihood of another manned mission to Hubble, and thus the telescope's fate, has become less clear.

After Hubble
Although the length of Hubble's mission is still unclear, NASA has been hard at work developing another telescope that will help astronomers amplify their understanding of the universe. The James Webb Space Telescope, scheduled to launch in 2011, differs from Hubble in several ways. NASA hopes the new telescope will help scientists look even deeper into space to see the "first stars and galaxies in the universe," according to the agency's Web site.

One key difference between the two telescopes is that the new one will have better instruments for seeing infrared light, which has a longer wavelength and is seen at the far reaches of the universe. Meanwhile, Hubble is better at detecting the shorter wavelengths of light that can be seen with the human eye. Because of these differences between the two telescopes, the NASA panel recommended that the two telescopes' operations overlap so scientists can study both types of images from certain objects.

The James Webb Space Telescope's mirror will be six times larger than the one on Hubble, giving it a greater ability to collect light. The new telescope will also operate much farther from Earth -- some 940,000 miles away -- making it easier to operate than Hubble. Once in place, it will orbit the sun, unlike Hubble, which orbits the Earth. Its distance from Earth will also make it impossible for astronauts to reach it and make repairs.

The NASA-commissioned panel reported it was "impressed by the progress that has been made by the JWST [James Webb Space Telescope] team." However, after siting the amount of time it took to develop other observatories, the panel cautioned that the launch date "might be delayed substantially beyond 2011."

-- By Karyn Schwartz, Online NewsHour