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

Considering Hubble’s fate: a followup

When the HST-JWST Transition Plan Review Panel, an independent group of six astronomers commissioned by NASA to study the fate of the Hubble Space Telescope, held a public hearing in late July, they planned to issue their final report to NASA around October 1. (See “Considering the fate of Hubble”, August 11, 2003.) However, in a surprising move, the panel completed its report within a week of the July 31 public hearing, and NASA published the report on August 14.

“A lot of astronomers and NASA officials were astonished when we said our report was ready just one week after our public meeting,” said John Bahcall, the chairman of the review panel. “This was possible because we reached unanimous agreement on our conclusions very quickly; remarkable when you consider there were six independent-minded scientists on the panel. Our secret is we did our homework very thoroughly. Many people helped to educate us.”

While the timeline of the review panel’s final report was surprising, its contents were less so. The panel offered three options for the future of Hubble. The highest-ranked option was to conduct two shuttle servicing missions, SM4 in 2005 or 2006 and SM5 around 2010, which would keep the telescope operational well into the next decade. The second option would be to conduct only SM4 by the end of 2006 so that Hubble could continue working through the end of this decade. The final option would be to have no more shuttle servicing missions, and instead send an automated propulsion module that would attach to Hubble and safely deorbit it once its mission ended.

The last option is certainly the least desirable, although the panel admitted that NASA must prepare for this worst-case scenario should shuttle missions to destinations other than the International Space Station be considered too risky in the post-Columbia era. (Sources within NASA suggest that the agency currently feels uncomfortable about sending a shuttle mission to Hubble, but those perceptions could change as the return-to-flight activites proceed.) The panel suggested that, if this is the case, NASA consider developing a “fast-track” two-meter telescope that could be launched by the end of the decade. Such a telescope would use two instruments, Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC-3), that would be installed on Hubble on SM4.

The panel proposed developing a new orbiting telescope to replace Hubble by the end of the decade if no future Hubble servicing missions are planned.

One key question the panel grappled with is whether an additional servicing mission, SM5, is worthwhile from a scientific standpoint. A number of astronomers submitted comments to the panel calling for such a mission, proposing a number of specific surveys that Hubble could perform during its extended mission. On the other hand, however, some astronomers expressed concerns that funding used for SM5 and extended Hubble operations, which could be several hundred million dollars, could drain money away from other astronomy spacecraft programs.

The panel didn’t make a judgement on the value of performing SM5, but did suggest a way to determine its merit. The panel said that NASA should issue an announcement of opportunity (AO) for scientific work to be performed using Hubble after SM5, using either existing or new instrumentation. The winning proposal would then be evaluated against other science missions in the Discovery, Explorer, and similar programs to determine its overall merit. This method, the panel concluded, would ensure good science, either using Hubble or other spacecraft, would be funded.

Another way to carry out SM5 would be when the time comes to plan deorbiting the telescope. NASA is currently considering two ways to do this: have an automated propulsion module attach to the telescope, or have a shuttle servicing mission carry out the task. The latter is preferable from the standpoint of the panel, since the same mission could also be used to perform final upgrades to the telescope to permit it to continue operating for several more years.

“When Hubble finally reenters the atmosphere, it will be kind of a sad day for a lot of us Hubble-huggers,” said Grunsfeld.

One interesting section included in the report dealt not with Hubble but with its successor, the James Webb Space Telescope (JWST). Based on past history, the panel expressed considerably skepticism about NASA’s plans to launch JWST in 2011. The panel noted that for NASA’s four “Great Observatories”—Hubble, Compton, Chandra, and SIRTF—the average time between issuing the AO for instruments to the launch of the spacecraft is nearly16 years. With the AO for the JWST instruments published in late 2001, the panel noted that if JWST meets this average, the spacecraft will not launch in 2017. Indeed, the spacecraft’s launch has already slipped a few years from late this decade until 2011. This poses a concern for astronomers who desire to have an overlapping period where both Hubble and JWST are simultaneously operating.

John Grunsfeld, a NASA astronaut and self-proclaimed “Hubble-hugger”, told attendees of the annual International Mars Society Conference in Eugene, Oregon, last week that he thinks that, one way or another, Hubble will be kept operational until 2010 to 2012. “When Hubble finally reenters the atmosphere,” he said, “it will be kind of a sad day for a lot of us Hubble-huggers, but you can’t deny that it has performed a tremendous amount of science.”