End of the line for Hubble? Astronomers ponder space telescope's final years.
Citing safety concerns that had come to light after the Columbia space shuttle tragedy in February 2003, O'Keefe told the gathering that NASA wasn't going to send any more shuttle missions to upgrade or repair the orbiting observatory. With its 3-decades-old batteries in danger of petering out and its spacecraft-stabilizing gyroscopes vulnerable to failure, the telescope could cease to function as early as 2007. Scientists had hoped to use shuttle missions to maintain it until as late as 2013.
The decision to end Hubble's life prematurely has set off a public furor. From snapshots of ultraviolet auroras shimmering above Jupiter to pictures of exploding stars and images of dozens of the most distant galaxies so far detected, Hubble hasn't only revolutionized astronomy. Around the world, its images have become icons of scientific discovery.
On June 1, O'Keefe announced that NASA is proposing to send robots, rather than astronauts, to repair and upgrade Hubble. Although many astronomers, including Beekwith, say they are heartened by this proposal, they also point out that an unmanned servicing mission has never before been attempted. The scientists are skeptical that even state-of-the art robots can deliver batteries and gyroscopes, let alone carry out current plans to install a new infrared camera and ultraviolet spectrograph.
Asked by Congress to consider the future of the Hubble Space Telescope, a National Academy of Sciences panel came out with a preliminary recommendation on July 14. It urges that NASA not preclude the use of the shuttle for a repair mission while the space agency considers the feasibility of robotic servicing.
"My advice to young astronomers is to treat each night on [any] telescope like it was the last night" says John Huchra of Harvard University. "In this ease, this may be the last years on the Hubble Space Telescope."
"It's definitely a race with the clock," adds John Tonry of the University of Hawaii in Honolulu. Which prompts the question, What observations should take priority in Hubble's last few years?
"We're not going to do all the good things that everybody has in mind before the batteries wear out," says Robert Kirshner of Harvard University. "We're going to have to make some very difficult, unpleasant choices."
DO OR DIE In May, scientists gathered at the Space Telescope Science Institute to discuss priorities. They agreed that they couldn't count on the success of a robotic mission. The astronomers expressed diverse opinions on what problems Hubble should tackle next, but they found a consensus on some issues. "The big question is, Should we dedicate a lot of Hubble time to one or two compelling problems, and the answer at the meeting was 'no," says Beckwith.
Much of the telescope's time is assigned almost a year in advance, but the space telescope director reserves some Hubble time for special projects or on-the-fly follow-up of new discoveries.
Last fall, Beekwith gave away more than 11 days of this discretionary time for astronomers to take the most detailed snapshot yet of the distant universe. As astronomers plumb this goldmine of imagery and spectrographic data, known as the Hubble Ultra Deep Field, they are asking whether the images in this narrow slice of the universe are representative of the rest of the cosmos.
To find out, Hubble should image at least two other deep fields before its time runs out to more accurately reveal the distribution of the colors, shapes, and clusterings of the first galaxies, says Marijn Franx of Leiden Observatory in the Netherlands.
Other astronomers at the May meeting urged that Hubble continue its pioneering work in determining the size and atmospheric composition of planets that lie beyond the solar system. By carefully gathering a star's light, Hubble can provide spectrographic signatures of closely orbiting planets that circle it. Astronomers don't have images of those extrasolar planets, so Hubble has provided the only direct data on them. Other telescopes have insufficiently clear vision and lack Hubble's spatial resolution.
Hubble examines extrasolar planets as they periodically pass in front of their stars as seen from Earth--a process known as a transit. At that time, the planets block a small but detectable amount of starlight. A precision measurement of the drop in brightness can indicate the radius of the transiting planet (SN: 12/1/01, p. 346).
An even smaller amount of the starlight filters through the transiting planet's atmosphere, whose constituent atoms and molecules absorb specific wavelengths. By comparing the spectra of starlight taken during and after a transit, astronomers can in principle determine the abundance of a variety of elements and compounds in a planet's atmosphere.
So far, David Charbonneau of the California Institute of Technology in Pasadena, CA, Ron Gilliland of the Space Telescope Science Institute, and their colleagues have used Hubble to find the atmospheric constituents of the just one planet (SN: 11/20/99, p. 324). But with two other transiting planets recently announced, and some 20 other searches underway, the orbiting observatory may soon have a bonanza of orbs to study. And that roster may soon go up precipitously. Last February, Hubble searched for transiting planets by surveying 50,000 stars that lie toward the center of the Milky Way, and astronomers plan to announce the results this summer. "There's time pressure to do the unique things that can only he done with Hubble," Gilliland notes.
SUPER STUDIES At the May meeting, Kirshner proposed that astronomers emphasize two types of projects in Hubble's remaining time. One would be a final set of images or spectra to document changes in a planet, star, or galaxy that Hubble has already examined over a long period of time. The other would focus on making observations for which even a small amount of additional data might have a huge payoff.
Only half jokingly, Kirshner noted that two of his own pet projects fit these criteria perfectly. He and his colleagues have been methodically documenting supernova 1987a ever since sky watchers witnessed its explosion 17 years ago. Astronomers have predicted that an expanding shock wave racing out from this blast will plow into a ring of debris cast off by the star some 30,000 years before it exploded. Hubble has already imaged bright knots within the ring, which represent the first places that the shock wave has reached. Over the next few years, the entire ring should light up. The spectacle will not only make a striking image, Kirshner says, but it's likely to reveal more about the 1987 explosion and the contents of nearby interstellar space.
Another sort of supernova study using Hubble is yielding a more cosmic perspective. By training Hubble on the most distant type la supernovas known in the universe and measuring their brightness, Kirshner and his colleagues have found evidence for dark energy, a mysterious cosmic push that is causing the universe to rev up its rate of expansion (SN: 5/22/04, p. 330). Searching for additional supernovas may be the best way to determine the basic properties of dark energy, including its strength and density. Even a few more years of Hubble studies could provide vital clues to the nature of dark energy, notes Kirshner.
ALL TOGETHER NOW Weighing in on the priorities debate, Beckwith points to the value of coordinating Hubble's observations with those of other telescopes. Combining Hubble's visible-light images of a patch of the southern sky with pictures taken by X-ray. infrared, and radio telescopes, for example, has provided a rich story line about galaxy evolution and the growth of supermassive black holes.
In another example, Jay Gallagher of the University of Wisconsin-Madison and his colleagues recently compared Hubble images of the galaxy M82 with those taken by the WIYN Telescope on Kitt Peak in Arizona. M82 is called a starburst galaxy because its core churns out clusters of young stars. The furious rate of star formation in these galaxies, which are close enough to study in depth, resembles the frenzied activity common in the earliest, most distant galaxies in the cosmos, Gallagher and other astronomers contend.
Aside from its star-birthing activity, M82 is note worthy because it shoots out winds of hot gas tens of thousands of light-years long. Gallagher's team set out to look for a connection between these winds and the rate of star birth. The team overlaid the ultrasharp Hubble images, which homed in on tiny details at the center of the galaxy, with the less detailed but broader WIYN images, which show the widely dispersed winds. The team found that the winds are driven by hot gas expelled from the central areas of the galaxy. Gallagher first reported the findings at the May meeting.
Several astronomers noted that when Hubble dies, astronomy will lose its only sharp, ultraviolet eye on the universe. Most ultraviolet light can't penetrate Earth's atmosphere to reach ground-based detectors, and only a few other space observatories have any ultraviolet capability. Ultraviolet spectra reveal the abundance of elements in nearby regions of the cosmos, while ultraviolet images record the energetic light from newborn stars in nearby galaxies. There are no plans for an ultraviolet-sensitive space observatory for at least the next 15 years.
Ultraviolet telescopes are less useful for studying distant bodies. That's because ultraviolet light from distant galaxies gets shifted into the near-infrared region of the spectrum as a result of the universe's expansion. Indeed, the proposed space observatory often referred to as Hubble s successor, the James Webb Space Telescope, will search for the first stars and galaxies by examining the distant universe at infrared wavelengths.
Hubble was originally expected to be around long enough to overlap with the new observatory, peering at some of the same targets as the James Webb, but in visible and ultraviolet light. By the time that telescope is launched, no earlier than 2011, Hubble's last observations could well be history.
NEVER AGAIN--Astronauts perform repairs on the Hubble Space Telescope during a servicing mission 2 years ago. NASA has canceled additional shuttle-borne repair missions to the telescope but is now considering a robotic mission.
HUBBLE BONANZA--The Cat's Eye planetary nebula features shells, jets, and knots of gas generated by a dying star (top). Deepest visible-light and near-infrared image of a patch of the universe (middle). Light from the outburst of a normally faint star called V838 Monocerotis reflects off eddies in a surrounding dust cloud (bottom).
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|Date:||Jul 24, 2004|
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