Thursday, 18 December 2014

Visiting Two Pieces of the Hubble Space Telescope

The Hubble Space Telescope's Wide Field and Planetary Camera 2,  on display at the National Air and Space Museum. The holes in its radiator mark impact craters that were removed for study. Chris Gainor photos

My work on the history of the Hubble Space Telescope's operations in space began last week with a visit to NASA's Goddard Space Flight Center in Greenbelt, Maryland, where Hubble is controlled, NASA Headquarters in Washington, D.C., and the Space Telescope Science Institute at Johns Hopkins University in Baltimore, Maryland. There I met some of the talented people who keep HST operating who I will call on to help me tell the story of Hubble's nearly 25 years on orbit.

I also made another stop in Washington at the Smithsonian National Air and Space Museum. In my last blog entry, I included a photo from a previous visit to that museum where I stood in front of the Structural Dynamic Test Vehicle that was used as a stand-in for Hubble. Last week I returned to that spot to see two instruments that were part of the space telescope for fifteen years. 

At the heart of Hubble's history is the fact that its instruments could be changed out by astronauts who flew to the telescope aboard the space shuttle and serviced it. After those two instruments were returned to Earth, the Smithsonian put them on display next to the Hubble test article.

Both were placed on board HST in the historic first servicing mission in December 1993 that restored Hubble's eyesight after it was discovered that Hubble's main 2.4-meter mirror was defective. 

One instrument is known as COSTAR (for Corrective Optics Space Telescope Axial Replacement), which acted much like a pair of glasses in that it changed the path of light going to Hubble's instruments to correct for the main mirror's defect. Instead of lenses, COSTAR used a set of ten sophisticated mirrors to do the job. Not only did the mirrors, each of which are about the size of a dental mirror, have to be ground to the correct shape, COSTAR had to move each mirror to the right place to direct light to the correct path for each instrument.

Close-up of the pick-off mirrors from HST's COSTAR instrument, now on display at the National Air and Space Museum.

The other instrument is the Wide Field and Planetary Camera 2 or WFPC2. It too was installed on Hubble in December 1993, and this instrument took many of Hubble's most famous images of the heavens, including the Hubble Deep Field image which when released in 1996 gave us our deepest view of the universe, which took us 12 billion years back in time. 

Both COSTAR and WFPC2 were removed from Hubble and returned to Earth on the final shuttle HST servicing mission in May 2009. By then, all new instruments on Hubble carried their own corrective optics, which meant that COSTAR was no longer needed, and the spaces occupied by COSTAR and WFPC 2 were taken by more advanced and up-to-date instruments. 

On show at the Smithsonian, COSTAR looks unremarkable, rather like a metal box the size of a phone booth. COSTAR's most interesting exterior feature is found on one corner, where  one can see those small and sophisticated mirrors on mechanical arms that extended to allow the mirrors to "pick off" the light from HST's main mirror. 

WFPC2 is a different story. Shaped something like a baby grand piano, the camera includes a radiator on one side that was exposed to space throughout WFPC2's 15 years on Hubble. Today that radiator is full of holes. When the instrument was returned to Earth, its radiator was pocked with scores of craters from micrometeoroids and small pieces of space debris. Each crater area was removed from the radiator for study of the growing problem of impacts on orbiting space vehicles. 

Both these instruments have stories to tell about the Hubble Space Telescope, its development, and the environment where HST still flies. 

Hubble is supposed to be directed to a destructive re-entry over an isolated part of the Earth once its work is done. More than five years after the last shuttle servicing mission, Hubble is still going strong, so the question of how much longer it will be able to operate remains a matter of speculation. Even though much of this historic telescope will be lost after HST ends its operational career, instruments such as the two I visited at the Smithsonian will remain on display for people to see and consider.

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