The Hubble Space Telescope seen after the completion of the SM3B servicing mission in 2002. (credit: NASA)

Considering the fate of Hubble

by Jeff Foust
Monday, August 11, 2003

The Hubble Space Telescope is arguably the most famous scientific instrument in history. Launched in 1990 after decades of deliberation and delay, it immediately made it onto the front pages of newspapers and the top of nightly newscasts, in the worst possible way. The flawed mirror that made the telescope the butt of comedians and cartoonists initially diminished its science, but a successful repair mission in 1993 put it back into the public limelight in a positive way. Since then, the telescope has observed phenomena ranging from disintegrating comets in our solar system to the most distant, oldest galaxies ever seen. Its data have reshaped numerous fields of astronomy, and its images have become popular with people around the world.

Yet, as the saying goes, nothing lasts forever. Hubble was launched with a planned lifetime of 15 years; it has been in orbit now for over 13 years. NASA is currently planning to keep the telescope operational through the end of the decade, but after that its fate is unclear. The agency is hard at work at a successor, the James Webb Space Telescope (JWST), an ever larger telescope scheduled for launch in 2011. Stationed in the shade of the Earth-Sun L2 point, 1.5 million kilometers away, this six-meter telescope promises to look deeper into the cosmos, perhaps better explaining the nature and origins of the Universe.

Astronomers, however, are reluctant to let go of Hubble. Some don’t want to decommission a working spacecraft that is still providing good data. Others, noting the emphasis of JWST on infrared wavelengths, don’t want to lose the optical and ultraviolet capabilities of Hubble. Some want to exploit Hubble in its final years to carry out surveys not possible with the telescope today. Meanwhile, NASA is left trying to contemplate not only when to decommission the telescope, but how to dispose of it at the end of its mission.

In the final version of NASA’s fiscal year 2003 budget, Congress inserted a provision asking the space agency to examine the future of Hubble. Specifically, Congress directed NASA to perform an in-depth study whether another servicing mission, in addition to the mission currently planned for 2004 or 2005, should be undertaken to keep Hubble operating at least until the JWST is launched. In response, NASA created the HST-JWST Transition Plan Review Panel, a group of six independent astronomers tasked with developing a plan to determine how long Hubble should be kept operating. The panel held a public meeting on July 31 in Washington, and plans to submit its final report to NASA at the beginning of October.

Hubble’s near future

Hubble was not only designed to be a human-tended spacecraft, it requires regular visits by astronauts to keep the spacecraft from falling into disrepair. Four shuttle crews have already visited the spacecraft since its 1990 launch, replacing instruments, electronics, solar panels, gyroscopes, and other equipment. One more servicing mission, dubbed Servicing Mission 4 (SM4), is currently scheduled. (The mismatch between the names and the number of missions stems from splitting the original Servicing Mission 3 into two parts, SM3A and SM3B, to speed up repairs of Hubble’s failing gyros in late 1999.)

The loss of Columbia has made the schedule for future shuttle missions, including SM4, uncertain. While project officials had planned for SM4 to take place in 2004, it appears more likely that the mission will take place in 2005: the project is currently using a baseline launch date of May 5, 2005. A firm launch date isn’t likely until the shuttle returns to flight.

There has been some question about whether SM4 would every fly. It is the only shuttle mission in the foreseeable future that will not travel to the International Space Station, and thus would not have the opportunity to be visually inspected for damage by the station crew, as recommended by the Columbia Accident Investigation Board. NASA is, however, developing alternative means for inspecting the orbiter, using a camera mounted on one end a boom attached to the shuttle’s robot arm.

SM4 is critical for Hubble to continue operating. Two of the six gyros on Hubble have failed. The spacecraft needs three to operate, meaning that the spacecraft is two failures away from shutting down, much like it did in late 1999 shortly before the SM3A mission. A reliability model maintained by the Aerospace Corporation for NASA predicts that there is a 30 percent chance of such a shutdown by May 2005, increasing to 50 percent by the end of 2005 and 70 percent by mid-2006. Gyro reliability is the limiting factor in Hubble’s immediate future.

Assuming the SM4 does take place and successfully replaces all six gyros on Hubble (in addition to other tasks), the question then becomes how long Hubble can operate without needing another shuttle visit. The same reliability model estimates that Hubble has a 30 percent chance of continuous scientific observations five years after SM4, dropping to 18 percent after six years. Assuming that SM4 takes place in 2005 as currently planned means that there is a 70 percent chance that it will have already shut down by 2010, the time NASA currently plans to cease operations.

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