Considering the fate of Hubble
The prospect of Hubble failing late this decade—potentially several years before JWST is launched—has raised interest in a sixth servicing mission, SM5. The mission, tentatively slated for around 2007 or 2008, would perform final maintenance on the telescope as well as possibly install new instruments for Hubble’s final years. SM5 could also allow Hubble to continue to operate well past 2010, overlapping with JWST.
The astronomical community is divided about SM5 and a Hubble life extension. John Huchra, an astronomer who serves as chairman of the board of directors of the Association of Universities for Research in Astronomy (AURA), notes that the servicing missions have the effect of rejuvenating not just the telescope but the community as well. “In effect, a brand new observatory has been delivered to the community on each such mission and the community has responded with a full intellectual commitment to exploiting this resource,” he said in a written statement. “Unlike our early expectations, HST is not gradually declining in its productivity and scientific appeal—it is increasing.”
Huchra also notes that Hubble supports a large number of astronomers. “Our best estimate at the time [of a recent report] was that HST provided core support for a quarter of all US astronomers,” he said. With no large space telescopes planned to observe at optical and ultraviolet wavelengths until at least 2020, a long gap created by the early retirement of Hubble could be detrimental to the community, Huchra warned. “Such a gap would have an enormous negative impact on American astronomers and astronomy and would eliminate the grand synergy we have seen between HST’s high-resolution imaging and observations at so many other wavelengths and with so many other facilities and missions.”
Huchra calculates that extending Hubble’s lifetime though 2013, including providing new instruments to Hubble on SM5, would cost about $700 million total from 2006 through 2013. (He excludes the cost of the shuttle mission itself, arguing that NASA would conduct a final end of mission (EOM) to Hubble in any case.) “Is the science and scientific capability that would be enabled by using the EOM visit to HST to refurbish it one more time, including providing overlap with JWST and maintaining UV/Optical spectroscopic and imaging capability for an additional ~4 years, worth augmenting NASA’s budget by less than 1%?” he asked.
Not everyone would answer in the affirmative. George Rieke, an astronomer at the University of Arizona, believes that revolutions in astronomical theory—paradigm shifts, as described by philosopher Thomas Kuhn—accompany the introduction of new telescopes and instruments. “A major effort in new instrumentation would be required for HST to have much potential for paradigm shifting,” he concludes in a statement. “Given the quality of the instruments currently in use, and the advanced state of optical and near-ultraviolet detectors, even new instruments will generally only bring moderate improvements in speed, not qualitatively new capabilities.”
The money required to develop those instruments, Rieke believes, could jeopardize other astronomer spacecraft. Since Congress specified that any additional funding for another Hubble servicing mission should not come at the expense of JWST, Rieke worries that future missions currently in the earliest planning stages could be at risk, such as the Single Aperture Far Infrared Observatory (SAFIR) and the Space Ultraviolet Optical Telescope (SUVO). “The money spent for an extended HST mission could advance one of these other missions substantially toward reality,” he said. “It is possible that the ultimate consequence of extending the life of HST would be that SUVO or SAFIR would never be built.”
Former astronaut Bruce McCandless believes that Hubble is a “National Asset” and should be maintained as long as it is feasible to do so and as long as the telescope is in demand by astronomers. One way to measure demand is the “oversubscription ratio”, the ratio of the amount of time requested by astronomers versus the amount of time actually available on the telescope. For Hubble this ratio is currently about 6:1. “This indicator of its scientific viability could be used as the ‘switch’ for discontinuing further support,” McCandless suggested. “I would suggest that when it falls into the 2:1 range, it would show that the premier astronomers have found a newer, more capable facility for making their observations, or that the HSO [Hubble] can no longer deliver the quality of data sought irregardless of the availability of other facilities.”
“I would also observe that the HSO still commands front page space in popular news media, to a degree that no other astronomical observational system has ever done previously,” McCandless noted. “The public interest is alive here, even if it has become jaded with regard to other space endeavors.”
Disposing of Hubble
Whenever Hubble’s mission finally comes to an end, what will happen to the spacecraft itself? NASA had been planning to conduct a final shuttle mission to the telescope, placing the spacecraft in the shuttle’s cargo bay and return it to Earth. Ed Weiler, NASA’s associate administrator for space science, had previously talked about how he wanted to replace the partial replica of the Hubble currently on display at the National Air and Space Museum with the actual spacecraft.
In the post-Columbia world, however, there is less enthusiasm for a shuttle mission solely to retrieve a spacecraft that could still be operational. Critics of a shuttle mission to retrieve Hubble include John Grunsfeld, a veteran astronaut who served on the last two servicing missions and is also a professional astronomer.
“If there were to be a mission after the SM4 for the purpose of returning Hubble to earth in the Shuttle Payload bay, the Astronaut Office would have reservations supporting the mission,” Grunsfeld said in a statement. He noted that retrieving Hubble would not be as simple as grabbing it with the shuttle’s robot arm and berthing it in the cargo bay: up to four spacewalks could be required to jettison equipment and otherwise prepare the spacecraft. “In a sense this mission would be risking human lives, and a unique national resource (the Space Shuttle), for the purpose of disabling great science, albeit due to necessity at end-of-life.”
One alternative under consideration would attach a propulsion module to the spacecraft, either during a shuttle servicing mission to the telescope or on a separate unmanned launch. This module would then be used to deorbit the shuttle in a controlled fashion, bringing it down in much the same way the Compton Gamma Ray Observatory was deorbited in 2000. Planning for such a propulsion module is still in its earliest stages, though. Without the module, and assuming no shuttle missions to Hubble after SM4 in 2005 that would boost the station’s orbit, the telescope would deorbit naturally in late 2013 or 2014. That would carry the risk of debris surviving reentry and landing on populated areas.
While astronauts may not be in favor of a dedicated mission to bring back Hubble, Grunsfeld said that they would be willing to conduct missions to prolong the spacecraft’s life. “Looking back on these servicing missions I feel that playing a part in the success and scientific return from the Hubble Space Telescope is the most important and significant achievement in my life. I can say without hesitation that traveling to space to upgrade the instruments and ensure the future of the Hubble Space Telescope was worth the potential risk to my life,” he said. “If astronauts are going to risk their lives to service the Hubble Space Telescope, we should do it in order to enable great science.”