Robots and Hubble: a bad idea?<< page 1: cost and technology challenges Alternative one: bring back the shuttleGiven that kind of assessment, there’s no wonder that there is some interest in alternatives to a robotic mission. The first, most natural alternative is to undo the SM4 cancellation and have the shuttle repair Hubble, as it has on four previous occasions. NASA has been opposed to this approach for the same reason it canceled SM4 to begin with: unlike shuttle missions to the ISS, there is no “safe haven” a shuttle crew can stay in if a problem strands the shuttle in orbit. In the post-Columbia environment, NASA is understandably jittery about exposing shuttle crews to any additional risk beyond the baseline of an ISS mission.
The assumption built into that rationale is that since there is no safe haven at Hubble now, there is no way a shuttle servicing mission can be safely performed in the future. However, there have been a number of suggestions of ways to provide that safe haven capability for a shuttle mission to Hubble. At least one company has proposed launching a module of some kind that would provide additional supplies to keep a shuttle crew alive in the event they were stranded in orbit for up to several months until a rescue mission can be launched. The AoA study, independent of those proposals, looked at another safe haven approach, using a module based on a spare of the Functional Cargo Block (FGB, using its Russian acronym) module developed for the International Space Station. (It’s not clear that the FGB-2 module itself would actually be available, given that the Russian government is now considering using it on the ISS.) Such a mission, the study found, could be carried out in 39 months—just eight months more a shuttle servicing mission alone—at which time Hubble would have a 59% chance of still being in serviceable condition. The cost of this safe haven mission, according to the study, is $2.8 billion. The report doesn’t explicitly state if this includes the shuttle mission as well, but if it does, it implies that a safe haven capability could be provided for as little as $600 million. Alternative two: replace HubbleThe second alternative, less frequently discussed, involves not trying to save Hubble at all. Instead, Hubble’s scientific capabilities are preserved by launching a replacement spacecraft. This approach has been suggested as a way to salvage the Cosmic Origins Spectrograph (COS) and Wide Field Camera 3 (WFC3), the two instruments that were to have been installed on Hubble on the SM4 shuttle mission. NASA officials have argued that COS and WFC3 work best when installed on Hubble. The instruments were designed for use on that telescope, and were intended to be used in combination with other instruments there, primarily the Advanced Camera for Surveys (ACS). The ACS, for example, is a better tool for detecting distant supernovae, which then can be studied in greater detail by WFC3 in order to measure the rate of expansion of the universe and the presence of dark energy. Those synergies can be further enhanced if the Space Telescope Imaging Spectrograph (STIS), an existing Hubble instrument that failed earlier this year, can be repaired during a servicing mission; such a repair is under “preliminary consideration”, according to NASA. However, a replacement mission is still attractive for other reasons. A new spacecraft can incorporate a generation’s worth of new technologies and other lessons learned in spacecraft and instrument development. A mission could include other instruments in addition to, or instead of, COS and WFC3, depending on scientific merit and feasibility. If NASA decided this new space telescope should be serviceable, it can be designed to be far more accommodating to future robotic servicing spacecraft than Hubble. On the other hand, if it is not intended to be serviced—as is the case with the Chandra, Spitzer, and Webb space telescopes—it can be placed in a better location than low Earth orbit, away from the frequent diurnal cycles, corrosive atomic oxygen, atmospheric drag, and other detrimental phenomena Hubble has to deal with.
As part of the AoA the Aerospace team looked at several options for “rehosting” either COS or both COS and WFC3 on a spacecraft in low Earth orbit or beyond. Their proposals included either using the spare Hubble primary mirror on the ground or developing new lightweight optics with the same 2.4-meter aperture as Hubble. They found that these rehosting missions would cost between $1.9 and $2.3 billion, around the same amount as a robotic servicing mission; that price tag includes a separate robotic deorbiting module for Hubble. The rehosting missions would take longer to develop than a robotic repair—over eight years versus nearly five and a half—but would have a much higher probability of success: 84-87 percent. Moreover, the new mission would not be under schedule pressure to launch before Hubble is no longer serviceable, although there will be a gap of several years between the end of Hubble and the launch of the new observatory. Picking the best alternativeWhich of these options is the best approach? In an effort to answer this, the AoA study plotted the various alternatives on a chart with cost on the x-axis and “expected value”—a combination of the scientific capability, development risk, and mission risk—on the y-axis. Missions that would only dispose of Hubble grouped at the lower-left corner: they had the lowest cost, but also offered little scientific value. The rehosting missions were in the upper-right corner: while costing over $2 billion, they offered the best combination of scientific value and minimized risk. Just below that was the SM4 shuttle mission itself (the safe haven option was not plotted separately). In the bottom-right corner was a cluster of robotic servicing mission options, costing about the same as the rehosting and shuttle mission options, but with an expected value not much higher than the deorbit-only alternatives because of the high risks entailed with robotic servicing. While a replacement spacecraft has the highest expected value in this study, it doesn’t necessarily mean it’s the right approach: there are good scientific reasons for keeping Hubble operating, not to mention the intangible benefits of repairing a spacecraft that has become an icon for the space program. What these studies do suggest, though, is that it is far too soon to exclude this and shuttle servicing options in favor of a risky robotic repair mission. The time may come when robotic servicing emerges as the best option, but, the administrator’s comments to the contrary, that time has not yet arrived. Home |
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