The Sciences


By Tim FolgerApr 1, 1993 12:00 AM


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This December, three and a half years after its crushingly disappointing beginning, the Hubble Space Telescope may be fixed.

If all goes as planned, the space shuttle Endeavour, carrying seven astronauts, will rocket into orbit on December 2 for an eight-day mission to repair the defective Hubble Space Telescope. It is by far the most complex shuttle flight ever attempted, and one of the most eagerly awaited. In three or four six-hour space walks, the astronauts will face a challenging series of tasks. The most high-profile one will be the installation of a new camera and a set of small mirrors designed to compensate for the flaws in the orbiting telescope’s primary mirror. But several other demanding jobs are also on NASA’s wish list of repairs.

It’s a very ambitious schedule, and if we can get even part of it done I’ll consider the mission successful, says Jeffrey Hoffman, an astronaut and astronomer who will be doing at least one of the space walks.

The Endeavour will rendezvous with the Hubble on the third day of the flight. The shuttle’s 50-foot-long robotic arm will then try to snatch the $2 billion observatory and lower it onto a platform in the rear of the open cargo bay. Last May, on its first mission, the Endeavour had problems capturing a wayward satellite, and astronauts were forced to wrestle it into the cargo bay by hand. But NASA doesn’t expect similar problems with the Hubble. Unlike the balky satellite, the boxcar-size Hubble was designed with shuttle visits in mind: it is equipped with fixtures for the robotic arm to grab and 225 feet of handrails to make life easier for space-walking astronauts.

Once the Hubble is standing on end on a platform in the cargo bay, ground control will command the telescope’s solar panels to roll up. The next day the space walks begin.

All four of the astronauts scheduled for space walks (NASA calls them EVAs, for extravehicular activities) have logged time outside the shuttle on previous missions. Now they’re busy practicing weightless maneuvers in a 25-foot-deep water tank at NASA’s Johnson Space Center. The tank contains a mock-up of the shuttle’s cargo bay and the Hubble. We’re doing an unprecedented amount of training for this flight, says Hoffman. We have over 400 hours of water tank training scheduled for the EVA crew, which is more than any other flight has ever done.

How does their underwater training compare with the real thing? Some of the instruments the astronauts will be installing on the Hubble are very bulky--the corrective optics package, for example, is the size of a telephone booth--and moving them in the vacuum of space is a lot different from pushing them around underwater, says Hoffman.

When you’re dealing with large objects, the viscosity of the water makes a tremendous difference in how objects behave, he says. In water it’s difficult to get a large object moving; you have to push hard on it. And it’s very easy to get it to stop; you just stop pushing and the water will stop it. Well, it’s just the opposite in space. It’s very easy to get something going because there’s zero resistance. But you can’t just stop pushing and expect that it’s going to stop. You have to position yourself in a way that you can apply force in both directions, to start it and to stop it.

The other thing you have to worry about in space is that it’s usually very difficult to exert force exactly through the center of gravity of an object. So when you push and pull on large objects you generally start them rotating in one direction or another. We’re going to be moving very slowly and very deliberately. We’ll do our best to anticipate the problems we’ll run into. But training for them on the ground is not easy.

There are some problems, though, albeit mundane ones, that the astronauts are well prepared for. Four of them will be spending long days outside the shuttle, and since it takes two hours to put on their $2 million space suits and check them for safety, trips back inside for lunch or to visit the rest room are out of the question. So the space suits of the three male space walkers have strategically placed pouches to collect urine. Kathryn Thornton, the lone female, will make do with absorbent pads. As for meals, crumbless fruit bars (a few crumbs in your trousers could ruin your whole EVA) are stashed inside the suit’s helmet within easy biting distance. The astronauts can also sip water from straws.

One amenity the suits lack is a heating unit. Originally NASA was concerned about us getting overheated, says Hoffman. So the suits have wonderful cooling systems but all the heat comes from the body. When I went out on one of my first EVAs, during the night, I wasn’t moving around at all and my hands got really cold. It was sort of like going skiing on a 30-below day. My fingers got so cold I balled my fists. But on the shuttle the night only lasts about 35 minutes, and then you come into daytime again, and as soon as the sunlight gets on you, you warm right up. It’s a really wonderful feeling. Recently NASA has given us the option of wearing long underwear under our liquid cooling garments, and that’s made a big difference.

What’s more, the astronauts will be moving around so much on this mission that keeping warm shouldn’t be a problem. One of the two astronauts on each space walk--they’ll work in pairs on alternate days--will be attached to a platform at the end of the robotic arm and will carry the various instrument packages to and from the Hubble. The other astronaut, tethered to the shuttle, will climb onto the telescope. The two will work together to install the new instruments.

On the first space walk they will replace the Hubble’s solar panels. The existing ones suffer from a design flaw: they bend back and forth as much as two feet every time the telescope passes out of or into Earth’s shadow, causing the entire instrument to jitter. The European Space Agency has designed new, less flexible panels and is scheduled to deliver them to NASA in May.

On the day after the solar panels are replaced, the astronauts will get to the heart of the mission: restoring the telescope’s vision. They won’t touch its defective mirror but instead will install the $50 million corrective optics, called COSTAR (Corrective Optics Space Telescope Axial Replacement), between the primary mirror and three of the astronomical instruments. COSTAR consists of a series of mirrors that will refocus the slightly blurred starlight coming from the primary mirror. To make room for COSTAR, the astronauts will have to remove an identical-size instrument called the High-Speed Photometer, which measures rapid brightness variations in stars but which has turned out to be of interest to only a small percentage of astronomers.

Installing COSTAR should be comparatively straightforward, again because the Hubble is astronaut-friendly. To open up the big doors to put in COSTAR, we have to undo only five bolts, says Hoffman. If it were a normal satellite, there’d probably be a hundred bolts going all around the panel. But the Hubble was built with repair in mind, so that makes the task doable.

The corrective optics package will have to be very precisely aligned with the primary mirror, but the astronauts won’t have to worry about that. We slide the instrument onto little tracks, and the tracks guide it into the right position, says Hoffman. Then we close one latch on the panel at a time. Each latch cinches the instrument down and pushes it more and more into the proper alignment. If all those systems work correctly, we should achieve the desired alignment.

On that same day, though, the astronauts will attempt a far more delicate task: they will replace the 610-pound Wide Field Planetary Camera with a $100 million backup version that has built-in corrective optics. Unlike COSTAR, in which the mirrors will remain enclosed in a protective casing until they are inside the telescope, the new camera has a small mirror protruding from one end, where it can reflect light into the camera. The astronauts will have to avoid bumping the mirror as they maneuver the camera, which is roughly the size and shape of a grand piano. If we so much as touch that mirror, even brush against it, we’ll knock it out of alignment so badly that it won’t work, says Hoffman. So that requires extreme care. We think we can do it, but it certainly is very critical.

The third or fourth space walk will be devoted to the last of the high-priority jobs: the replacement of at least two gyroscopes that help point the telescope. the Hubble has six gyroscopes on board and needs at least three to point accurately enough to do all the science it was intended to do. Three of the gyroscopes have failed in the last two years. If NASA doesn’t replace them, and just one more gyroscope fails, the telescope will be seriously handicapped in spite of all the optical fixes.

On each space walk, after finishing the day’s priority jobs, the astronauts will try to fix a few other instruments aboard the Hubble as time permits. NASA would like to upgrade the memory of the telescope’s computer, repair the power supply of an ultraviolet-sensitive instrument called the Goddard High Resolution Spectrograph, and, lastly, fix a device called a magnetometer, which is involved in orienting the telescope.

If the astronauts do get to the computer, it too will prove to be a dainty task. It is one of the few instruments that wasn’t designed for in-orbit servicing. The astronauts will have to plug in connectors containing as many as 128 fragile pins (like the ones that plug into the back of a personal computer) while wearing gloves that are less suited than oven mitts are for detail work. We don’t have a lot of dexterity or feedback with the gloves, says Hoffman. You have to be very careful not to bend those pins.

Even if the astronauts complete all their assignments, it will take four to six weeks before anyone knows whether the repair mission was completely successful. After the Endeavour leaves the Hubble, ground control will begin making the final delicate adjustments of the optics. With luck, and assuming the new optics themselves don’t have any problems, the telescope should be restored to better than 90 percent of its intended capability.

I have bet my reputation on this mission, of course, says Joseph Rothenberg, associate director of flight projects for the Hubble mission at NASA’s Goddard Space Flight Center. But I’d be willing to bet my life, given the testing we’ve put into this program and the independent checks on every piece of optics. We’ve had enough of the aberrated Hubble. Let’s get the real thing up there.

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