How and when to bring the Hubble Space Telescope home

image Two University of California, Berkeley, physicists joined colleagues on a NASA advisory panel last week in unanimously recommending that NASA extend the life of the Hubble Space Telescope to maximize the amount of science it does before plummeting to Earth.

"We are urging them to keep it going as long as practical," said Nobel Laureate, physicist and panel member Charles Townes, a Professor in the Graduate School at UC Berkeley.

Fellow panel member Christopher McKee, chair of UC Berkeley's physics department, said the panel's recommendations reflect the consensus of the astronomical community, but must be weighed against other new space astrophysics proposals.

"The Hubble Space Telescope has revolutionized astronomy, and will continue to do great astronomy," McKee said. "But NASA must take many factors into account in addition to science in reaching a final decision about the telescope."

The six-member panel, formed in January to advise NASA on the future of the telescope, or HST, issued its final report last Thursday, providing the agency with three preferred options, depending on the ultimate fate of the shuttle fleet.

NASA originally planned to service the telescope for the fourth time in about 2005, then send up a fifth shuttle mission around 2010 to bundle the telescope up and haul it back to Earth. This would provide a nice museum exhibit and remove the admittedly small risk of injury should the telescope plummet to Earth.

One example of the astronomical riches provided by Hubble: an image of a stellar explosion that occurred in the Large Magellanic Cloud, a nearby, small companion galaxy to the Milky Way visible from the southern hemisphere. The February disintegration of the Columbia shuttle, however, threw a wrench into these plans, promising fewer shuttle flights and making NASA less willing to risk astronauts' lives to bring the telescope home.

"Astronaut John Grunsfeld addressed the panel and said that he and his colleagues realize that, whenever they go up, they are risking their lives, and they are perfectly willing to do that in order to enable Hubble to do more science, but they were not ready to risk their lives just to bring it back to stick in a museum," McKee said.

NASA has proposed strapping a rocket to the butt of the telescope to guide it safely through the atmosphere to a mid-ocean plunge, but it's unclear whether a shuttle trip will be required to do this or if it can be performed by an unmanned robotic mission.

"Right now, there is no propulsion module that can be taken up in a shuttle, and it's possible that, after review, that may be regarded as too dangerous," McKee said. "Then you would have to go to a robotic propulsion module that would latch onto the shuttle and bring it down, but such a module does not exist, either."

The panel, chaired by John Bahcall of the Institute for Advanced Study in Princeton, N.J., argued to keep Hubble aloft because it can contribute great science for another decade or more and it complements its successor, the planned James Webb Telescope. Hubble provides great images in the visible and ultraviolet wavelength bands, while the Webb telescope will be optimized for infrared observations to probe the heart of dust clouds and the red-shifted light from the earliest days of the universe.

"The James Webb Telescope is going to provide a much greater leap in our capabilities than Hubble will, offering the potential for making new revolutionary discoveries," McKee said. "But you still have the option of putting new instruments on Hubble, making it even more powerful than it is now."

Another of the panel's concerns was that, given past experience with other space telescopes, the Webb telescope could be delayed substantially beyond its planned 2011 launch date. Launch could possibly be as late as 2017, which means astronomers would have no observatory in space for up to seven years, stopping instrument development and eroding observational teams.

The panel's preferred option was two more shuttle flights the next in 2005 to install new gyros, replace insulation and boost it higher to avoid orbital decay; the second in 2010 to again reboost and replace gyros and perhaps to attach a rocket for its eventual guided plunge into the ocean. The gyros, needed to point the telescope in the right direction, tend to fail after about five years of operation, which makes the telescope wobble. Three previous shuttle missions have not only installed new gyros, but also upgraded scientific instruments on the telescope.

"We are also clear that if they select our favored option, which is to have a fifth shuttle mission to install new gyros, a propulsion module, and possibly new instruments, that this should be done in peer-reviewed competition with potential new missions, not with funds for currently planned missions," McKee said. This scenario could extend the orbital life, if not operational life, of Hubble until 2020 or beyond.

If two additional shuttle missions are impossible, a second option is a single shuttle flight by 2006 at the latest to add new gyros that will keep the telescope in continuous operation, perhaps until 2011, the proposed launch date of the Webb telescope. The same shuttle would attach a propulsion module, if one were available by then. Otherwise, a robotic mission would be needed.

The least favored option is no further shuttle missions and a robotic mission to strap on a rocket for a controlled descent before atmospheric drag brings it down on its own in about 2013. By then it would have been out of operation because of failed gyros for more than five years.

The Hubble Space Telescope has been a boon to astronomers since it first started snapping pictures of the heavens in 1993. Since then, the public has feasted on colorful photos of dust clouds, startlingly sharp pictures of distant galaxies, and hauntingly deep images showing the wealth of galaxies in the universe. These images have had a major scientific impact in nearly all areas of astronomy.

"One of the most important discoveries in the physical sciences in recent years was the discovery of acceleration of the universe," McKee said. "That was done with ground-based telescopes, but observations with Hubble were crucial in helping to both establish that result and show that the acceleration hypothesis was the correct explanation of the data."

"By any standards the HST has been a spectacular success one of the most remarkable facilities in the entire history of science," the panel's report concluded.

Other panel members were Barry Barish of the California Institute of Technology, Jacqueline Hewitt of the Massachusetts Institute of Technology, and Martin Rees of the University of Cambridge.

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