If you’re an astronomer, “Twinkle, twinkle little star” isn’t a cute bed-time song for children, it’s a nightly nightmare. Stars twinkle (and daytime skies are blue) because we live at the bottom of a thick soup of atmosphere that distorts our view of the heavens. Ever since Galileo, this has played havoc with observations of stars and planets.
Thinking about this, in 1946, American astronomer Lyman Spitzer published a study that described how astronomy would benefit if you could place an observatory above the Earth’s atmosphere. Forty-four years later, his dream became a reality, when NASA launched the Hubble Space Telescope.
The 13.1-metre-long, 4.27-metre wide, 11,000 kg Hubble boasted a 2.4-metre mirror that was the most perfect reflective surface ever created–so smooth that, if it were the size of the Gulf of Mexico, there would be no waves in it more than a millimetre wide. There was just one teensy little problem that became apparent shortly after Hubble was launched in 1990 (four years later than intended because of the Challenger space shuttle explosion): that perfect mirror was flawed–it was two microns (.0002 millimetres) too thin at the edge. It’s hard to believe, but that incredibly tiny flaw was enough to make Hubble nearsighted: the images that were supposed to be 10 times better than anything obtainable from Earth were blurry.
Hubble limped along, performing valuable science but not living up to expectations (and bearing the brunt of a lot of jokes) until 1993, when NASA performed corrective surgery. During a five dramatic space walks, shuttle astronauts installed new optics and instruments that corrected the problem with the mirror–and since that time, it seems that hardly a week goes by without some stunning new Hubble image being released, or new information about our universe based on Hubble data. Hubble has provided information about the births of stars and galaxies, about the existence of black holes, about our own solar system and the existence of planets around other stars. It has redeemed itself many times over…and the best is yet to come.
Over the past few days, astronauts aboard the space shuttle Discovery have been carrying out the second in-flight servicing of the Hubble Space Telescope, replacing two of its aging instruments with new state-of-the-art devices, and carrying out general repairs and maintenance.
Hubble has always been intended to be serviced in orbit, once the initial idea that it would be actually plucked out of orbit, returned to Earth for servicing and then re-launched proved to be unfeasible (due to problems with vibration and contamination).
Hubble is considerably more than just a big version of the backyard telescope you had when you were a kid. It contains an array of scientific instruments, each designed to provide different information about the universe.
Since 1993, the instrument package has consisted of two cameras, two spectrographs, and “fine guidance sensors.” The first camera is called the Wide Field/Planetary Camera 2 (it’s number two because the original camera was replaced as part of the corrective surgery performed in 1993). It records images at two magnifications, and is used to explore our own solar system and the farthest reaches of space. It’s the heart and soul of the Hubble Space Telescope.
The second camera is the Faint Object Camera. Built by the European Space Agency (Hubble is a cooperative program of the ESA and NASA), it records images of extremely faint objects by intensifying an image and then recording it. (As one scientist put it, it could see a firefly in Tokyo from Washington, D.C.) It’s the only instrument still aboard from the original package launched in 1990.
Spectrographs separate light into its component wavelengths, similar to the way a prism splits sunlight into a rainbow of colors. By analyzing which wavelengths are present, scientists can learn a lot about stars and other celestial objects, including their temperature, chemical composition and pressure.
The main purpose of the current servicing mission is to replace the two existing spectrographs, the Faint Object Spectrograph and the Goddard High Resolution Spectrograph, with new, state-of-the-art instruments that will dramatically enhance Hubble’s capabilities.
One of the new instruments is called New Infrared Camera and Multi-Object Sepctrometer (NICMOS, for short). NICMOS’s specialty is viewing the universe at near-infrared wavelengths. Infrared light isn’t absorbed or scattered by the clouds of gas and dust that block our view of much of the universe. NICMOS, which consists of three cryogenically cooled camera-detectors that are a thousand times more sensitive than any existing infrared detectors, will allow astronomers to see newly forming stars buried deep in gas clouds; peer into the centers of galaxies, including our own, to study exotic objects like quasars; and possibly even measure the properties of the disks of dust from which planets are believed to for. They will also be able to observe some of the universe’s oldest, most distant objects, which could help answer questions about the size, structure and future of the universe.
The other new instrument being installed is the Space Telescope Imaging Spectrograph (STIS), the most complex scientific instrument ever built for space science. Its new technology will allow it to gather data 30 times faster than the old spectrographs, and to look at up to 500 adjacent locations at once. The old spectrographs could only look at one place at a time. By allowing scientists to measure the movements of many different stars at once, STIS will make it possible to locate the super-massive black holes believed to lie at the centers of most galaxies, and begin to understand how one black hole differs from another.
Both instruments were installed successfully during the first spacewalk Thursday. Subsequent space walks replaced a guidance sensor (which keeps the telescope still so it the instruments can get unblurred images–the new one is so accurate it would allow you to, in the words of one scientist, hit a golf ball in Washington, D.C., and score a hole-in-one in Denver), replaced some of Hubble’s reel-to-reel tape recorders with new ones and added a solid-state digital recorder (important because the digital recorder can hold far more data, and the new instruments will produce vast amounts of information), and carried out various other minor repair and maintenance tasks.
Tonight, the astronauts will take an extra fifth spacewalk to repair the thermal insulation which is beginning to peel off of the Hubble. Without this insulation, the Hubble’s electronic innards could overheat, and fragments of it could contaminate the telescope’s optics. It’s not too surprising it’s starting to come off, since it’s subjected to huge temperature variations–from 200 degrees Celsius in the sun to -240 in the Earth’s shadow–every 97 minutes (the time it takes to orbit the Earth once). Since replacing the insulation wasn’t part of the original plan, the astronauts will be installing makeshift shielding created using spare material in the shuttle, Velcro fasteners, duct tape, alligator clips and wire.
If all goes well, the Hubble will be released back into space tomorrow and the shuttle will be home on Friday.
This won’t be the last time astronauts call on the Hubble; the next maintenance mission, scheduled for 1999, will install a more advanced camera in place of the Wide Field Planetary Camera, one 80 percent more efficient at detecting light. Hubble will also be boosted into a higher orbit on that mission; drag from the outermost edges of the atmosphere is already starting to pull it down. Then in 2002, astronauts will call again, although exactly what new instruments will be ready for installation then isn’t known yet.
