Though the ultimate fate of the Hubble Space Telescope (HST) remains in doubt, yesterday an independent panel of experts formed by NASA at the request of Congress proposed three ways for Hubble to spend its final years.
The panel, chaired by John N. Bahcall (Institute for Advanced Study), was tasked "to evaluate the scientific impact of the current NASA plan for ending HST operations and beginning James Webb Space Telescope operations." (The larger, infrared Webb telescope will be Hubble's successor.) The group asked the scientific community for input and also held a public meeting on July 31st where it heard testimony from leading astronomers, astronauts, engineers, and administrators.
Yesterday the panel issued three recommendations — one for each of the possible scenarios for the telescope's future, from best to worst.
The future of Hubble depends on how many times space shuttle astronauts can visit it. Without these servicing missions, the telescope cannot survive. First, the instrument experiences drag from the uppermost fringes of the atmosphere. Left alone, it would reenter and crash uncontrolled — putting people at risk — sometime after 2013. Second, the most failure-prone part on Hubble has been its gyroscopes. Of the six gyros on board to keep control of its orientation — four are working today; three are required for normal operations (though the scope could likely observe with only two if necessary). The panel concludes that given the average gyroscope lifetime, Hubble has only a 50 percent chance of remaining operational five years after all six gyros are replaced.
Then there is the eventual issue of endgame. In the wake of the Columbia disaster, NASA decided that the telescope will not return to Earth in a shuttle cargo bay. Therefore, it must be brought down into the atmosphere in a controlled manner. This will require installing a rocket engine on the scope, either by shuttle astronauts or by remote control from the ground.
The panel's highest recommendation was for two future Hubble servicing missions (named SM4 and SM5), to occur around 2005 and 2010. The 2010 mission would give the telescope its final upgraded cameras, all new gyros, a boost to the highest altitude possible, and a reentry rocket to bring it down. Such a mission could extend Hubble's lifetime until 2020 or even later. This should allow a substantial overlap with the James Webb Space Telescope, currently set to launch in 2011, (but judging from past delays, likely to launch later).
Critics of this plan expressed concern that the money for the 2010 mission ($300 million all told) could be better spent on other NASA programs to get greater scientific returns. In response, the panel recommended that NASA call for proposals for other scientific missions that could compete with the best-case Hubble plan at the same cost. A peer-review forum would decide their relative merits.
If only one servicing mission is possible, the panel recommends it take place before 2007. The new gyros, orbital boost, and reentry rocket would need to be included at that time.
And if NASA decides not to service Hubble again at all, or if the committee assessing the Columbia disaster determines that servicing Hubble again isn't worth the risk of human life — the panel's "worst-case scenario" — engineers will need to devise a way of attaching a rocket to the telescope by remote control. Regardless of which path Hubble ultimately takes, the panel urges that the addition of the rocket should not limit the lifetime of HST in any way.
Hubble's current director, Steven Beckwith (Space Telescope Science Institute), says he is pleased with the Bahcall report. "I felt that the committee recognized what an important asset Hubble is, and that we should preserve that asset however possible."
The next step for Hubble depends on the recommendations in the upcoming Columbia Accident Investigation Board report. Assuming the Board gives NASA its blessing to visit the telescope again, Beckwith predicts that SM4 will happen. "I believe [NASA] will fund SM4, if they allow the shuttle to go. I don't know if they will fund SM5 however." But he remains hopeful. The Bahcall report "did say SM5 is a high priority."