The Rochester Review, University of Rochester, Rochester, New York, USA
Bill Forrest, Judy Pipher, and Dan Watson are designing and testing the instrument's infrared detectors--in sum, its "eyes."
he last in NASA's chain of four orbiting Great Observatories--SIRTF, the Space Infrared Telescope Facility --is scheduled for launch from Cape Canaveral on December 14, 2001. Its mission: nothing less than to cast light on the birth and evolution of the universe.
Taking a more than casual interest in the launch will be a team of Rochester astronomers whose work is at the core of the discoveries.
Headed by the University's three experimental astronomers, Judy Pipher, William Forrest, and Dan Watson, the Rochester group is designing and testing the instrument's "eyes"--the infrared detectors that can perceive a slice of the spectrum 30 times wider than the narrow slit occupied by the light we normally consider visible.
Once launched, SIRTF (sort of rhymes with "sort of") will train its detectors at all kinds of phenomena obscured to the most advanced technology currently available: stellar nurseries, dark, mysterious "brown dwarfs," the earliest galaxies at the edge of the universe--and much more that we don't even know about.
There's a stealth universe out there, and we've been missing out on the view. Or as Forrest puts it: "You don't know what you're missing until you find out you've been missing it."
This new eye on the sky will perceive light from a region of the electromagnetic spectrum that we don't see but encounter every day nonetheless.
Every object on Earth emits some infrared energy. Our own bodies give off IR radiation in the form of heat (that's how rattlesnakes find us). Our homes and offices are bathed in infrared beams that zip across our living rooms from our remotes to our televisions, carry our voices across the oceans, and enable our copiers to produce readable images and our CDs to play great works of music.
The SIRTF Observatory -- consisting of a meter-class telescope and three supercooled payload instruments packaged in a kind of giant thermos bottle -- will follow the Earth in orbit around the sun, moving, as one scientist puts it, "rather like a duck trailing its mother."The infrared is "invisible" only because our own photodetectors--our retinas--aren't equipped to see it.
As the military has long known, among the great advantages of infrared technology is not only its ability to detect objects by sensing their heat but also the capability of peering unimpeded through dust. The Gulf War made us all familiar with the Army's use of "night vision" to poke through desert dust and nighttime darkness to pinpoint tanks and soldiers.
As far back as the 1960s, a small group of pioneering astronomers recognized the potential of similar technology in space exploration, where an infrared probe, cutting through the clouds of dust and gas that pepper the universe, can pick up on the heat of vastly distant objects.
Rochester's Judith Pipher, then a graduate student at Cornell, became one of the first students to work in the field, designing and building her own single-pixel detectors--a bit akin, on a minute scale, to crafting a half-cylinder automobile.
Pipher's further advances got a boost in the early 1980s, when a former Rochester scientist at what is now Raytheon sent to Pipher (by then installed at the University) one of the first infrared arrays--that is, a collection of many pixels on a single chip.
SIRTF's quarry: Among the celestial sights SIRTF will seek out is the elusive brown dwarf, seen here in an artist's conception. Barely visible to traditional modes of detection, these planet-like objects show up nicely in the infrared. It is believed that brown dwarfs may account for some of the long-sought "dark matter" that is thought to permeate the universe.
At about this time Pipher recruited Forrest, and together they developed a 32 x 32 array--the first multi-pixel IR array used for astronomical observations, and a milestone in creating a new view of the heavens. The array saw "first light" atop the Wilmot Building on campus in late 1983.
"We mounted the camera in the late afternoon, and before midnight we were smoothly taking data," says Pipher. "It was a first-of-its-kind instrument and it was working immediately. We're very proud of that."
Others were impressed, too, and the Rochester pair were recruited to a geographically widespread group of scientists who were proposing an orbiting infrared telescope--a proposal that would lead, eventually, to SIRTF.
("Orbiting" is an important point here. Earthbound scopes, even of the IR variety, suffer from impaired vision when attempting to cut through our dense terrestrial atmosphere.)
Pipher and Forrest signed on, as did Watson, at the time a researcher at Caltech who joined the University in 1988.
In those early days the SIRTF working group envisioned a telescope in orbit by 1990. But the Hubble Space Telescope, one of the other great observatories launched during the last decade, turned out to be far more expensive than planned (with a price tag of somewhere between $2.5 and $9 billion), while SIRTF's own estimated cost ballooned to more than $2 billion. In the meantime, NASA's space station program picked up momentum in Congress and siphoned funds from other projects.
A Different Way of Seeing
Looking into space at a different wavelength is a lot like tuning your radio in search of your favorite station. Every wavelength carries unique information, and it's just a matter of finding the right channel to hear the music. For infrared astronomers, the music to their ears will be unrivaled shots of stellar nurseries and planet-like objects.
The infrared isn't a universal tool to find all hidden objects, though. One of Bill Forrest's favorite stories involves a failed search for polar bears in the Arctic.
"The beasts blend in so well with the snow that one year some scientists decided to find them by looking in the infrared," he says. "They did the experiment flying all over the Arctic and pointing an infrared camera at the ground, and all they found were piles and piles of steaming-hot polar bear poop. A polar bear has such good insulation that it blends in with the ambient temperature; they're invisible in the infrared."
Ultimately biologists did find a way to pinpoint the bears by looking in the ultraviolet portion of the spectrum.
When the wavelength of choice makes such a difference in earthbound applications, one can only imagine the ramifications when NASA plunks down hundreds of millions of dollars for a telescope designed to whiz through space searching the cosmos.
Most of us have been dazzled by the photos coming back from the Hubble Space Telescope, and rightly so; now SIRTF offers to beam back images of celestial events hidden to Hubble. It's like receiving information on a whole new channel that's been broadcasting for eons but which we've never heard before.
Years passed. Then a series of discoveries including that of more than a dozen planets beyond our own solar system and the fantastic images sent back by Hubble, along with the well-televised landing on the red planet by the Mars Pathfinder boosted public interest in space exploration.
inally, a little over a year ago, NASA formally began work on the new telescope, capping its cost at $458 million and putting it on a fast track for development.
Noting its advantages of high sensitivity, low cost, and long lifetime (from two and a half to five productive years), NASA is looking at SIRTF as a winner:
"The Space Infrared Telescope Facility will do for infrared astronomy what the Hubble Space Telescope has done in its unveiling of the visible universe, and it will do it faster, better, and cheaper than its predecessors," declares Wesley Huntress, former associate administrator for space science.
nce aloft, the space observatory will orbit the sun and follow the Earth, "rather like a duck trailing its mother," as one scientist puts it. It will be equipped with a mirror nearly a meter wide to soak in and focus the light hitting the telescope. A solar panel pointed toward the sun will power the craft, while a heat shield will keep the Earth's infrared light from mucking up experiments. The 1,600-pound craft will aim its antenna toward Earth twice a day to beam its storehouse of information back to headquarters: the SIRTF Science Center at the Caltech-NASA Jet Propulsion Laboratory in Pasadena.
The telescope's vital scientific cargo will include three instruments: the Infrared Array Camera, or IRAC (on which Pipher and Forrest are working), the Infrared Spectrograph (in which both Watson and Forrest have a hand), and the third, a device called the Multiband Imaging Photometer. The three instruments are packed inside a cryostat like
a giant thermos bottle to keep them bathed in liquid helium just this side of absolute zero.Forrest and Pipher are playing a crucial role in the development of IRAC--the infrared camera--working closely with industrial contractors to design and test two of the four detectors that will fly as part of the device.
Each camera "eye" is actually a finger-nail size array of more than 65,000 detectors that create an electrical signal when hit by infrared light.
Since the team joined the SIRTF group 16 years ago, the arrays have grown dramatically--from Pipher and Forrest's original 32 x 32 to today's 256
x 256 detectors. And electronic noise, which can swamp valuable incoming info, has been reduced to less than 1 percent of previous levels.In another advance, the team also has developed a new multiplexer, the device that keeps straight all of the faint signals from the heavens funneling in from each of those 65,536 detectors. The arrays are so sensitive, and the view from space such an advantage over the highest ground-based observatory, that SIRTF is capable of seeing thousands more objects, and infinitely fainter ones, than any of its predecessors.
SIRTF will be anywhere from hundreds to a million times more sensitive than NASA's previous infrared orbiting effort of 16 years ago. While the earlier facilities could have detected a cow jumping over the moon, SIRTF will be able to detect a cow jumping over one of Jupiter's moons.
Eye in the sky: The infrared detector that enables SIRTF's camera to "see." This 256 x 256 indium antimonide array is of the type being tested at the University.
Such results don't come easily. The SIRTF arrays were made possible by the combined resources of NASA and several aerospace companies, the ingenuity of astronomers around the world, and years of back-and-forth between manufacturers and users like the Rochester team.
Once SIRTF is aloft, one-fifth of its observing time will be reserved for the scientists who have designed and built it, assuring the Rochester group direct access to many weeks' worth of priceless data. (Telescope time is as crucial to observational astronomers as lab space is to any experimental scientist.)
Throughout the past winter the three University astronomers, two postdocs, and six graduate students were busy testing the detectors, either by observation or by tinkering with the electronics.
"But now, finally," says Pipher, "we're getting to spend extensive periods of time thinking about how we're going to do what we've always wanted to do"--deciding, that is, which cosmic questions to ask, and where to point the telescope for answers.
They certainly have a lot of options: Astronomers guess that the number of galaxies in the universe is something like the number of stars estimated to be in the Milky Way--tens of billions. No matter which way they turn the camera, they're likely to see interstellar
gas and dust, which fill more than 99 percent of the universe and are the building blocks for everything else. Because of its ability to see right through dust, the telescope is perfect for witnessing events usually curtained from view, including the birth of stars and even of entire solar systems.
Graduate student Robert Benson tests an infrared detector, housed inside a thermos-like dewar vessel. Because infrared radiation is actually heat radiation, astronomers must reduce the thermal pollution of the surrounding environment when observing weak celestial signals. Measuring devices are refrigerated to temperatures near absolute zero (about minus 460 degrees Fahrenheit).
How that dust and gas become the vast molecular clouds that are the progenitors of star systems is one of the big mysteries of stellar formation. Gravity somehow pulls this gangly stretch of raw material together into a swirling disk of matter, and from that stars and planets are spun off and solar systems created. Until now scientists couldn't witness the action directly because of the dusty cocoon within which it takes place.
"We'll be able to watch the disks go from their earliest stages--when they're still depositing material onto a central star--to the development of holes in the gas and dust distribution," Watson says. "We can then observe an environment like that from which our own solar system probably descended, with the rocky terrestrial planets inside and the giant gas planets toward the outside."
Then there's the hunt for brown dwarfs, planet-like objects believed to be floating throughout space. These objects are dozens of times more massive than our solar system's largest planet, Jupiter, but too small to sustain the nuclear-fusion reactions that power the stars. Thus they are cool and perfect for infrared viewing. And it is possible that they may account for much of the universe's famous "missing mass."
Some of SIRTF's most intriguing observations will be deep-sky surveys, where its eyes will peer backward in time toward some of the earliest and most distant galaxies in the universe, some 14 billion years old. These are roughly 100 quadrillion (or a thousand trillion) times farther away from Earth than the sun.
"These earliest galaxies seem to have surprisingly large amounts of heavy elements, and even dust, formed within the first billion years of existence," says Watson. "That's a very fast pace for a galaxy to generate enough stars to make much in the way of heavy elements, and it's hard to explain. Where we look the furthest back in time is where we meet our biggest challenges."
ven as the SIRTF arrays are being readied for their mission, the team is already involved in its next project: developing new,
bigger arrays for the Next Generation Space Telescope (NGST), an eight-meter telescope planned for launch sometime in the next decade. NGST is central to NASA's new Origins program, a 20-year master plan to explore the origins of everything from the Big Bang to the creation of life.
SIRTF will give astronomers a good look at the protoplanetary disks of dust and gas surrounding nearby stars. By observing disks of various ages, SIRTF can trace the evolution of solar systems like our own.
The technical tinkering involved in all this falls a long way from public view. The stunning images from today's telescopes betray none of the fancy electronics necessary to capture the majesty of space. In most quarters, the public's image of an astronomer is of someone captivated by the beauty of the skies, standing over a telescope long into the night.
Not quite. When discoveries do come, it's usually on a computer, crunching numbers. Says Pipher bluntly: "We don't look at the sky; we're in a control room looking at computer monitors." That's partly the nature of much observation today, and partly because the University's team is at the forefront of detector development.
"Detector technology has been somewhat of an orphaned field," says Watson. "Everyone wants to hold the baby, but no one wants to go through labor."
Until, that is, you start thinking about what your infant prodigy will be able to accomplish.
Tom Rickey's most recent story for Rochester Review described the Seychelles Islands study on mercury and fish-eaters. He is senior science editor for the Office of University Public Relations. For more on SIRTF, check out its Web site at http://sirtf.caltech.edu.(SIRTF photos, courtesy of NASA; infrared detector photo, Astronomy Group, Raytheon Systems.)
Copyright 1999, University of Rochester
