The Hubble Space Telescope
Published on Jul 23, 2007 at 10:28 pm.
3 Comments.
Filed under astronomy, space telescopes, wonders.
My first choice for one of the Seven Wonders of Astronomy has got to be the Hubble Space Telescope. The HST has been one of the premier instruments of astronomy since its launch in 1990. It wasn’t the first telescope in space, nor is it the only one up there right now. However, it was the biggest and most ambitious orbital observatory ever launched. It will be replaced by an even larger telescope, the James Webb Space Telescope, launched hopefully in 2013.
Any list of observatory wonders would be incomplete without HST. Anyone who knows anything at all about astronomy has heard of the Hubble Space Telescope. It is NASA’s showpiece. The images that it collects are unparalleled. Of course, astronomers are more interested in data than images, and the images contain data, but the other instruments on board HST have been used to make great discoveries, too. But, the images are what fascinate the public. And, whenever anyone says “the space telescope,” it is generally assumed that the Hubble Space Telescope is the one that is being referred to, even if there is more than one space based telescope in orbit.
One problem that astronomers have faced since we first looked up into the sky has been that the Earth’s atmosphere gets in the way. Besides the obvious problem of weather, the atmosphere dims and distorts the light passing through it. It has only been very recently that active and adaptive optics have permitted astronomers to overcome some of the distortion effects caused by the atmosphere. But the dimming effect, called atmospheric extinction, is another matter. Many wavelengths of light are altogether blocked from reaching the ground by our atmosphere. So, the only way to compensate for those effects is to not look through the atmosphere. That means that you need a telescope outside of Earth’s atmosphere. And, of course, such a telescope would also be immune to all of the distorting effects of the atmosphere.
The first really serious proposal to build a telescope outside of Earth’s atmosphere is usually attributed to Lyman Spitzer in a report that he wrote in 1946. However, this was over a decade before the very first Earth orbiting satellite. During those years, there was some debate on whether such a space based telescope would best be place on a solid surface, such as the Moon, or in a space station in Earth orbit. Virtually all suggestions called for astronauts to operate the telescope, pointing it and taking pictures on film plates like their counterparts at observatories on Earth. However, by the late 1960s, technology had advanced to the point that scientists began to wonder about a completely automated space observatory. Work on what was to become the Hubble Space Telescope began even before the Apollo program was over. Incidentally, there actually was a telescope placed on the Moon. In 1972, Apollo 16 carried a far ultraviolet camera that used a 3 inch telescope set up on the lunar surface. And, in 1973, a space station based telescope was carried on Skylab, America’s first space station.
But, by far the most ambitious space based telescope attempted was HST. When construction began on the Hubble Space Telescope, it was designed for the Space Shuttle. At that time, NASA had no other heavy launch vehicle capable of delivering a satellite like HST to orbit. NASA has come under some fire for designing payloads that can only be launched by one type of launch vehicle, such as the Space Shuttle. And, HST itself proves the point that this it would be advantageous to have more than one possible launch vehicle. The Space Telescope was originally set for launch in October of 1986. However, the Challenger explosion earlier that year put a halt to Space Shuttle Operations for a while. When flights resumed, there was a backlog of other missions that needed to be launched. HST did not finally launch until April 24, 1990.
Since atmospheric effects would not limit HST the way that they would an equivalent sized ground based telescope, extra care was taken to figure the mirror into a perfect shape. It has been said that Hubble’s mirror was the most perfectly ground mirror ever made. Unfortunately, it was ground wrong. It may have been perfectly made, but the instrument used to test the mirror had a small flaw, so the mirror was made to perfectly match that flaw. Funding was not available for additional tests, so the flaw was not determined until the first images were sent back from orbit. By then, it was too late to fix it. HST was the butt of many jokes, and it quickly became a public relations nightmare for NASA. However, despite media reports to the contrary, even with the flawed mirror, HST was producing images better than any ground based telescope. The images simply were not as good as they could have been. The biggest problem was with faint sources. Many science objectives were not impacted at all by the mirror problem. There were other instruments on board that did not need perfect imaging capability. Nonetheless, a solution was developed to fix the problem. During the first servicing mission, COSTAR (Corrective Optics Space Telescope Axial Replacement), a series of mirrors placed in the light path, was installed to correct for the misshapen mirror.
STS-61, the Space Shuttle Mission that installed COSTAR, was billed by the press as a “repair mission” for HST. It was made to sound as if the only reason for the mission was to fix the mirror. In reality, Hubble was designed from the beginning to be serviced in orbit. In fact, it was designed for a servicing mission every three years. These missions would replace worn components and would upgrade computers and instruments with new and better computers and instruments. Most parts of the spacecraft were modular, designed for easy replacement by astronauts in space suits. Such satellite repair and servicing was one of the original goals for the Space Shuttle, though after the Challenger mission, a decision was made to do such missions only when absolutely necessary, and primarily with HST. After the loss of the Columbia, NASA for a while had decided not to even service Hubble any more. However, plans have changed, and a servicing mission to HST is now planned for September, 2008.
HST was launched with two cameras, two spectrographs, and two photometers. One of the photometers was replaced by COSTAR. The cameras have also been replaced, with one being replaced even on the first servicing mission. The first servicing mission was STS-61 in December, 1993. Another servicing mission was flown by STS-82 in February, 1997. The third serving mission was moved up when four gyroscopes (of the 6 on board HST) that are used to position the telescope failed. That mission was STS-103 in December 1999. However, there were still routine servicing and equipment replacement tasks that were not yet ready in 1999, so the second half of the third servicing mission was flown in March, 2002, by STS-109. That was the last servicing mission. Since that time, there have been additional equipment failures, including the main camera and a spectrograph. Though many regard the spacecraft as crippled, it continues to make observations and to record data with the remaining instruments. Of great concern, though, is the fact that the spacecraft is down to four operating gyroscopes, two of which seem to be having problems and likely will not last until the next servicing mission. HST uses three gyroscopes to point, but some work can still be done with two gyroscopes (though this limits the science missions that are possible). In 2005, a decision was made to switch to two gyroscope operations in a hope to possibly extend the life of the gyroscopes until the next servicing mission.
NASA had originally wanted the final Shuttle mission to HST to bring it back to Earth to be displayed in the Smithsonian. However, with the loss of both Columbia and Challenger, it has become apparent that Space Shuttle flights are risky, and NASA administrators decided that it would not be worth the risk to astronaut lives to go get the Hubble Space Telescope simply to display in a museum. And, if there is another significant accident with a Space Shuttle mission between now and when the final servicing mission happens, then that mission may not occur. Without it, then HST will come crashing back to Earth a few years later as its orbit decays. In fact, HST is already at a much lower than normal orbit, and if the servicing mission is delayed long enough, then it will be too low for the Shuttle to safely rendezvous with. And, it has been a while since servicing, so there are a lot of things that need to be done. It is not clear if there is enough time in a single mission to get to everything.
Whatever happens to HST, though, it has been a great ride, and it definitely warrants inclusion in any list of wonders of astronomy.
-Astroprof
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