17 years in orbit
Published on Apr 24, 2007 at 4:23 pm.
1 Comment.
Filed under space telescopes.
April 24, 1990, 8:34am EDT, Space Shuttle Discovery lifted off from Pad 39-B at the Kennedy Space Center to deliver the Hubble Space Telescope into orbit. The launch had originally been planned for April 10, but it was scrubbed because of a problem with one of the shuttle’s auxiliary power units. Discovery again tried to lift off April 12, and then April 18, before finally getting off the ground on April 24.
The idea of a telescope in space can be traced all the way back to Lyman Spitzer in the mid 1940’s. However, serious plans for an orbiting telescope did not develop until the early 1970’s. Even before the end of the Apollo Program, NASA was looking into the possibility of an orbiting observatory. Budget woes, however, delayed the beginning of the project until the late 1970’s (at the time called the Large Space Telescope). The telescope was originally expected to be launched as early as 1986 or 1987. However, the Challenger accident of 1986 pushed the launch date back to 1990.
The Hubble Space Telescope is not the largest telescope ever constructed. It only has a diameter of 2.4 meters. It also isn’t any closer to the objects being studied, as it has an orbit that is only a few hundred kilometers above the surface of the Earth. But, it’s real advantage is that it does not have to look through the Earth’s atmosphere. That means no atmospheric distortions (though we are getting better at dealing with them with ground based telescopes), and no atmosphere to absorb or dim light. That means that HST can image in wavelengths that don’t make it to the ground very well.
Constructed in a modular manner, the primary scientific instruments and spacecraft systems can be accessed by astronauts in orbit. In fact, HST was designed to be serviced every three years. The modular design makes it easy to remove defective instruments or parts and replace them with new ones. It also allows for the easy upgrade of instruments as technology advances. Originally designed for a lifetime of 15 years in orbit, the Hubble continues to do a wonderful job.
It has had its problems, though. Shortly after launch, it was found to have a mirror that was ground to a slightly wrong shape. Even so, it produced better data than had been possible from ground based telescopes. Three years later, though, corrective optics were added to eliminate the effect of the mis-ground mirror. Gyroscopes used to orient the telescope have been a problem since launch. It seems that there was always one failing. However, the telescope was fitted with six gyroscopes, and only three were needed at any one time for normal operation. The first set of solar panels were found to flex whenever HST passed from dark to light, or light to dark, but those early solar panels were later replaced by far superior ones. The latest problem to hit Hubble is that one of its main cameras developed a fault that prohibits it from operating as it should. The other instruments, though, continue to produce wonderful data.
But, the most serious problem facing the Hubble Space Telescope is not one with the telescope itself. After the Columbia accident, the next planned servicing mission to HST was called off. NASA now has changed its mind to conduct one more servicing mission, but that is still some time in the future, and it will be the last one. HST does not have its own propulsion system, and it needs to be boosted to a higher orbit every few years by the Space Shuttle. A slow atmospheric drag causes and low Earth orbit satellite to gradually spiral downward in a decaying orbit. Without being boosted higher, HST would eventually come tumbling back to Earth in an uncontrolled fireball. But, even without the boost, the telescope has now already gone far beyond its design criteria since the last servicing mission of March 2002. There may be so many things to fix on the next mission that there will not be time to get to all of them. But, unless there is a catastrophic failure, HST should continue to provide useful data for some time to come, even if it was originally expected to last only until about 2005.
-Astroprof
(Images courtesy of NASA)
A Ler…-- Rastos de Luz on April 25, 2007 at 5:42 am: 1
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