NASA's decision to send a repair crew to the Hubble Space Telescope in 2008 is receiving seemingly universal praise. If all goes well, the world's best-known orbiting observatory should continue working until 2013, and perhaps even a few years longer than that. But then what? Hubble's ultimate fate is totally up in the air, so to speak.
Hubble is due to get fresh batteries, gyroscopes and observing instruments during the 2008 mission, but that's not all: The spacewalkers are also slated to install a "soft capture mechanism," so that future spacecraft will be able to use a straightforward docking maneuver to latch onto the telescope. Currently, Hubble has to be grappled by the shuttle's robotic arm and drawn onto a work platform in the open cargo bay. That procedure won't work once the shuttle fleet is retired in 2010.
The docking mechanism should allow for additional Hubble visits by NASA's Orion crew exploration vehicle, or by the private-sector spaceships being developed through NASA's Commercial Orbital Transportation Services program. Future crews could conceivably mount a sixth servicing mission, and a seventh, and so on. Or, less optimistically, an orbital disposal squad could attach a propulsion module that would push Hubble into a controlled plunge through the atmosphere. Such a plan would make sure the 12-ton telescope doesn't hurt anyone when it falls.
Several readers wondered whether that final, fiery trip would really be necessary. Here are a couple that encapsulate the two basic ideas:
William Bailey: "Heard yesterday that when Hubble dies, they are going to attach booster rockets to it and send it off into space. Well, use the booster rockets to get it to the space station. They could put two sets on - one to get it there, and one to stop or slow it way down to be 'lassoed' and tethered to the space station. Guess I just hate to give up and send it into deep space."
Unfortunately, changing Hubble's orbit from its current 28.5-degree inclination to the international space station's 51.6-degree inclination would take an incredible blast of rocket power. In this Space.com discussion thread, the required change in velocity is estimated to be as much as half the telescope's current orbital velocity - a speed-up that Hubble might not be able to weather, even if it were possible to attach a rocket that big. It's a lot easier to nudge Hubble down from its orbit, rather than pushing it sideways into an orbit to match the space station's.
Geoff: "Why not just load the Hubble into the shuttle bay and return it to Earth for the Smithsonian?"
"That was originally the plan," said Susan Hendrix, a spokeswoman for NASA's Goddard Space Flight Center. "But it's changed over the years."
In the beginning, Hubble was deployed from the space shuttle Discovery's cargo bay, but some of the parts that have been added since then - such as a new set of rigid solar panels and radiators - would have to be taken off the telescope before it could fit back in the shuttle, Hendrix said. Some estimate that the job would require three spacewalks, and the current thinking is that bringing Hubble back to Earth wouldn't be worth the risk.
"It wouldn't be safe," Hendrix told me. "It really wouldn't."
Of course, the 2008 servicing mission means that Hubble is likely to outlast the shuttle fleet. And once the shuttles are retired, it's not at all clear whether there'd be an orbital spacecraft big enough to bring Hubble back. The telescope is way too long to fit inside the European Space Agency's Automated Transfer Vehicle, for example.
Fortunately, there should be plenty of time to consider Hubble's ultimate fate. NASA's calculations indicate that if the telescope's batteries failed, the electronics would become irretrievably damaged in just two days - but even then, the telescope itself would likely remain in orbit until around 2021.
The Space Frontier Foundation's Rick Tumlinson, who was one of the leaders in the effort to save Russia's Mir space station, is very glad NASA Administrator Mike Griffin granted a reprieve for Hubble. "I applaud the administrator," he told me. "I give him great props."
In fact, Tumlinson sees no reason why Hubble should ever be sent down to its doom.
"I have a philosophical problem with the idea of deorbiting any large high-technology object from space," he said. "It's a frontier philosophy: You never throw out anything that you can repair, reuse or adapt for another use."
If there ever came a time when NASA was ready to hang up on Hubble, perhaps arrangements could be made for a public-private consortium to take charge of the telescope, Tumlinson said. Commercial orbital crews might well be in business sometime in the next decade or so - and willing to keep Hubble going as a profit-making venture or a public service.
Even a broken Hubble could have its uses. All that shiny hardware would be a gold mine for space-crazy collectors or orbital salvagers, circa 2020. Why let it go to waste?