Pimp my scope: Revamping Hubble
Pimp my scope: Revamping Hubble
Jeff Hecht - New Scientist - 13 September 2008
2004 WAS a bad year for astronomer Michael Shull. His problems began when a key instrument on board the Hubble Space Telescope failed, abruptly ending his promising observations of the wispy ionised gas that drifts between the galaxies. Things might not have seemed so bad had NASA not been forced to ground its shuttles following Columbia's fatal crash the year before. One of the cancelled missions was to Hubble to install a new instrument, partly developed by Shull, that would have had him back up and running. But with the instrument firmly grounded, Shull, who works at the University of Colorado in Boulder, has been stuck analysing old data ever since.
That's why Shull will be watching keenly when the space shuttle Atlantis blasts off on 10 October or thereabouts from Kennedy Space Center in Florida for one last visit to Hubble before the fleet is retired in 2010. During the 11-day mission, Atlantis's crew will make five space walks to fix a handful of broken instruments, service worn components and install brand new ones. The visit is intended to give the world's favourite telescope brand new vistas onto the universe, and prolong its already extended life for at least another five years.
For the past 15 years Hubble has been charming us all with stunning images, and thrilling astronomers with an avalanche of spectroscopic measurements that have revolutionised our understanding of the universe. However, since its last service in 2002, the telescope has been showing its age. Three of the six gyroscopes whose job it is to align it with target stars have failed. A camera used in 70 per cent of its observations failed in January 2007. The spectrograph Shull was using has died. And the telescope's batteries are steadily fading. In short, Hubble needs a major makeover.
This is a mission that almost didn't happen. After the Columbia disaster, a safety review panel recommended that astronauts should be able to take refuge at the International Space Station if a shuttle ever became unsafe for re-entry. But the shuttle does not carry enough fuel to reach the station from Hubble's orbit. NASA's administrator at the time, Sean O'Keefe, was unwilling to make an exception for a visit to Hubble, and cancelled the 2004 service mission.
Outraged by what amounted to a death sentence for Hubble, astronomers put pressure on NASA. Following a 30-month hiatus in shuttle flights, and once it was clear that the surviving craft were back and flying well, O'Keefe's successor Mike Griffin decided a mission to Hubble was worth the risk. The astronauts were more than willing; Hubble is their favourite destination. The prospect of doing science and taking lengthy space walks has astronauts clamouring to make the flight.
Because Hubble was designed at a time when NASA expected shuttle flights to be routine, its scientific instruments were packaged in modules that astronauts can swap in orbit (see diagram). On their first space walk in the forthcoming mission, astronauts John Grunsfeld and Andrew Feustel will remove the Wide-Field Planetary Camera (WFPC2) installed in 1993. It is famed for its iconic images, including the Hubble Deep Field, which shows some 10,000 young galaxies near the edge of the universe. Yet its four camera chips can claim no more than 640,000 pixels each - fewer than most cellphones. Nevertheless, WFPC2 showed what can be revealed by staring into deep space for long periods.
In its place, Grunsfeld and Feustel will insert Wide-Field Camera 3, which sports the latest optics. Its single 16-megapixel chip will snap images over a wider range of wavelengths, from the ultraviolet to the near infrared. It also carries a 1-megapixel chip that can pick up light from distant galaxies red-shifted to much longer infrared wavelengths. All the gyros will be replaced on this first walk, and two sets of batteries will be installed during the first two excursions.
In the second space walk, the astronauts will also fit a new instrument called the Cosmic Origin Spectrograph in a slot now occupied by a module that corrected a flaw in Hubble's mirror, and which is no longer needed because all the instruments now incorporate their own fixes. Ground controllers will spend about six weeks testing and calibrating the new spectrograph before the observations can begin. "I hope we'll have our first data in November or December," says Shull, who helped to develop it.
The trickiest part of the mission will take place during the third and fourth space walks. NASA was caught off guard by the instrument failures of 2004 and 2007. The new wide-field camera and Cosmic Origins Spectrograph were both designed to work with the Space Telescope Imaging Spectrograph (STIS) - which broke down four years ago - and the Advanced Camera for Surveys (ACS) which failed last year. Happily NASA scientists know exactly what is wrong with STIS. "We are replacing one circuit board," says lead scientist for the servicing mission David Leckrone of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Fixing a failed circuit on terra firma is usually a simple matter of opening the case and replacing the defective component. But nobody has tried it in space. Getting access to the circuit board will require removing the 111 screws that secure the access panel. And floating 600 kilometres above Earth, in zero gravity and wearing a bulky spacesuit, will make the task both delicate and tedious. A single stray screw could disable STIS or even Hubble itself. So astronauts Michael Massimino and Michael Good will fix a transparent "capture plate" over the access panel, then insert a special tool through holes aligned with each screw. The idea is that as each screw is removed, it will be trapped in the holes in the capture plate. Massimino and Good will then remove the panel together with the capture plate, along with the screws held between the two. With the circuit boards exposed, they can remove the failed board and snap a new one into place. They will then install a new cover that attaches with a couple of flip levers. No screws required.
The ACS poses a different problem. Its power supply failed catastrophically last year, cutting off power to its two most important camera chips and leaving it all but redundant. The astronauts will give it a new power supply that will fully restore its main camera and possibly the second one too on their third and fifth space walks. The good news for the astronauts is that the job requires removing only 32 screws.
If all goes well, Hubble's revamp will open up a raft of new possibilities for astronomers. For Shull it will mean he can get on with tracking down missing baryonic matter - similar to the ordinary atoms that make up stars and galaxies but which has so far eluded observation. He thinks the missing matter lies between galaxies, where it should reveal its presence by absorbing ultraviolet light from distant quasars. In May, Shull reported finding about 40 per cent of the missing material but could only give a sketchy picture from the painstaking analysis of old data from STIS and a NASA satellite called FUSE, which was launched in 1999 and suffered a series of problems before failing in July 2007. The new Cosmic Origins Spectrograph is 20 times as sensitive as STIS, so he expects it to find more of the intergalactic will-o'-the-wisp at greater distances, as well as tracing how it flows.
Meanwhile Richard Ellis of the University of Oxford and the California Institute of Technology in Pasadena will use the new wide-field camera to hunt for galaxies near the edge of the visible universe. His search relies on gravity to concentrate light from the distant galaxies enough for Hubble to detect it. In July, he reported finding 10 objects that appear to have red shifts of at least 7, dating them at about 900 million years after the big bang. But those observations were only just possible using the combined abilities of Hubble and NASA's Spitzer Space Telescope, and Ellis says only five of these objects "are particularly convincing".
With Hubble's new Wide-Field Camera 3 able to see further into the infrared than its existing instruments, Ellis hopes to find many more distant objects. "More important," he says, "the quality of data will be better." The camera should be able to measure red shifts up to 10, corresponding to about 500 million years after the big bang.
A second group hunting distant galaxies, led by Garth Illingworth of the University of California, Santa Cruz, will fix the new camera's gaze on the same patch of sky as the Hubble Ultra Deep Field image. By staring for a total of 200 orbits, the camera will be able to record still younger galaxies and push our knowledge closer to the edge of the universe. We can expect Illingworth's team to produce another iconic Hubble image.
Plenty more is in the works for the revived Hubble, including searches for planets passing in front of other stars. Astronomers have their fingers crossed that Hubble can keep going long enough to overlap with the James Webb Space Telescope. Scheduled for launch in 2013, it will observe much deeper into the infrared than Hubble. With two telescopes operating, astronomers will be able to study the same objects in different parts of the spectrum simultaneously, a technique that has been very productive in studying supernovae, for example.
Hubble won't last forever though. The service mission also will install a grapple so when the inevitable failure comes, a robotic spacecraft can grab the hulk and dump it harmlessly into the Pacific Ocean. Astronomers are already planning a successor to Hubble called ATLAS. With a mirror measuring 8 metres across, ATLAS will dwarf Hubble's 2.4-metre mirror. "It would be fantastic to have something that big in the ultraviolet and optical," says Shull. "It's probably 10 to 15 years off, but you have to start planning now."