flying telescopes - sofia science center · flying telescopes this boeing 747sp, shown during a...

he airplane looks like anyother 747, until you notice thelarge, gaping hole in its leftside, just behind the wing. Thehole is no accident. It will giveastronomers an unobstructedview of the universe through a2.5-meter telescope mounted

inside the plane. When the airplane,nicknamed SOFIA, finally takes flightin 2004, it will become the latest in asmall but distinguished series of flyingobservatories.

For nearly 40 years, astronomershave battled air turbulence, enginevibrations, and complicated aerody-namics of wind blowing past a hole inan airplane’s side to gain a view of theuniverse impossible to achieve fromthe ground. Airborne astronomy hasshown scientists the secrets of starformation, planetary rings, and theenergetic cores of galaxies.

But why go to all the trouble?Ground-based telescopes work wellfor studying the sky in visible light.But Earth’s atmosphere blocks mostother forms of light from reaching theground. This is good when it comes toultraviolet light, X-rays, and gamma

rays, which would otherwise damageskin and other cells in our bodies. Butit’s bad when it comes to infraredlight, also known as thermal radia-tion or heat.

Astronomers are interested ininfrared light because, unlike visible

T

FLYING TELESCOPES

This Boeing 747SP, shown during a 1998test flight, will house the SOFIA infraredtelescope. The painted dark rectangulararea on the rear of the fuselage marks thetelescope’s location.This aircraft is namedthe Clipper Lindbergh. It was christened in1977 by Anne Morrow Lindbergh on the50th anniversary of her late husband’s solotransatlantic flight. Courtesy of L3 Com-munications, Integrated Systems.

by Sally Stephens

Following nearly four decades of bigger and better airborne observatories, NASA’s SOFIA mission will

carry infrared astronomy to new heights.

FLYING TELESCOPES

Reprinted by permission of the Astronomical Society of the Pacific, from the May/June 2002 issue of Mercury magazine , ©ASP, 2002

light, its wavelengths are too long to be scat-tered by tiny dust particles. That means aninfrared telescope can see what is happen-ing deep inside large dust clouds thatappear opaque when viewed in visible light.Such clouds house the nurseries where starsare born and they hide the center of theMilky Way Galaxy from view.

Unfortunately, water vapor in Earth’satmosphere absorbs infrared light, keepingmost of it from reaching even the tops ofmountains. But water vapor is concentratednear the bottom of the atmosphere. That’swhere airplanes come in. At 41,000 to45,000 feet (12,500 to 13,700 meters), theair contains only about 20% of the mole-cules present at sea level, but that’s stillenough air for the wings to generate lift,enabling an airplane to fly. But the amountof infrared-absorbing water vapor has gonedown by a factor of a thousand. “It’s kind ofa happenstance of our atmosphere that youcan fly an airplane and still see in theinfrared,” says UCLA astronomer Eric Beck-lin, chief scientist for SOFIA (StratosphericObservatory For Infrared Astronomy).

Airborne Astronomy Takes FlightThe idea of using airplanes to carry tele-scopes above the water vapor crystallized inthe mid-1960s. NASA had a Convair CV 990,

named Galileo, that scientists were using tostudy Earth’s atmosphere and ionosphere. Itwasn’t really designed for astronomy, how-ever. Any onboard telescopes had to peerthrough window glass, severely reducing theamount of infrared light received. Despitethe problems, a team led by University ofArizona astronomer Gerard Kuiper flew onGalileo to study planets in our solar system.From Galileo they made the first detectionof water ice in Saturn’s rings and provedthat Venus’s clouds were not made of watervapor, as had been thought. Tragically, inApril 1973, while testing an instrument totrack migratory sea mammals, Galileocollided with a Navy patrol plane in mid-airabove a runway. Both aircraft crashed,killing all 11 onboard Galileo, and 5 of the 6people on the Navy plane.

Kuiper’s Galileo observations convincedastronomers that airplanes could provide auseful platform for the study of the infrareduniverse. In 1967, NASA scientists and engi-neers, led by Arizona’s Frank Low, modifieda twin-engine, six-passenger Lear Jet specif-ically for infrared astronomy. They removedthe jet’s back seats and replaced them withcramped racks for electronic equipment,tools, notebooks, and a vacuum pump.They designed a 12-inch telescope to fit inplace of one of the aircraft’s emergency exitwindows. Sealed off from the rest of the

cabin, the telescope was open to the atmos-phere, with no glass to block infrared light.

Conditions aboard the Lear Jet Observa-tory (LJO) were primitive at best. Becausethe plane was not fully pressurized,astronomers and pilots had to wear bulky,uncomfortable oxygen masks. The cabinwas often either too warm or too cold.“While observing, you sat on the floor,”recalls Al Harper of the University of Chi-cago’s Yerkes Observatory. “One personwould sit by the front door of the planeoperating the electronics. The telescopeoperator would sit with knees tucked upunder the telescope. It was rather intimate.”

Despite the hardships, astronomersusing the LJO made significant discoveries.They confirmed that Jupiter and Saturnradiate more energy than they receive fromthe Sun, indicating both have substantialinternal heat sources. They began to peerinside vast interstellar clouds whose dusthad blocked observations in visible light.Indeed, LJO took the first far-infrared spec-trum of the Orion Nebula, a large cloud ofgas and dust where stars are being born.

Kuiper: A Giant Leap ForwardThe LJO was limited by its small telescopeand the fact that it could only stay aloft for2.5 hours, half of which was spent

The center of the Milky Way Galaxy appears in visible (left) and infrared (right) light. Infrared light penetrates the thick interstellar dust seen in thevisible light image, giving us a nearly unobstructed view of the galactic core. Courtesy of Howard McCallon, Gene Kopan, Robert Hurt, and 2MASS.


climbing to and descending from altitude.Astronomers dreamed of an airplane witha larger telescope that could fly longermissions. In 1974 they got it, when NASA’sKuiper Airborne Observatory (KAO),named for the astronomer who first sawthe potential of airborne astronomy, beganits flights.

The KAO was a civilian version of a U.S.Air Force C-141 Starlifter transport. Scien-tists and engineers cut a hole (they call it a“port”) just in front of the left wing, largeenough to accommodate a 36-inch (91.5-

cm) telescope. The open cavity enclosed thetelescope but was sealed off from the cabinwhere the astronomers worked. Duringflight, the telescope was open to the stratos-phere; during takeoff and landings, a slidingdoor protected the telescope cavity. Infraredlight collected by the telescope bounced offmirrors and into detectors mounted on thecabin side of a pressurized bulkhead.

Inside the cabin, oxygen masks hung likestalactites from the ceiling, a constantreminder that in case of a sudden decom-pression, everyone would have just a few

seconds to don a mask before passing outfrom lack of oxygen. Astronomers and tech-nicians sat in front of several rows of mon-itors and computer consoles. Researcherscould even begin to analyze the data whilestill in flight. “The LJO was about the size ofa Volkswagen bus,” Harper recalls. “TheKAO was more like a Greyhound bus. Itgave you more elbow room.”

Although less cramped, the KAO couldhardly be described as plush. It lacked thethermal insulation found on commercialairliners. As a result, over the course of a 7-

After Galileo’s success, the next step in airborne astronomy was a 12-inch infrared telescope (inset) mountedinside a Lear Jet (above). Using this telescope, astronomers confirmed that Jupiter and Saturn radiate moreenergy than they receive from the Sun, meaning both planets have substantial internal energy sources.

Airborne astronomy took flight in the mid-1960s whenastronomers installed a small infrared telescope inside aConvair CV 990 called Galileo (above). All images courtesy ofNASA/Ames Research Center unless otherwise indicated.

Inset: University of Arizona astronomer Gerard Kuiperpioneered airborne astronomy when he mounted a telescopeinside Galileo. Courtesy of Yerkes Observatory.


hour flight, the cold of the stratosphere,where temperatures plummet to –50° C,seeped into the plane. Astronomersdressed in heavy sweaters and down jack-ets to ward off the chill. Those seated nearthe plane’s outer skin often huddled underblankets as they worked. Soda froze solidinside cans left on the cabin’s metal floor.

The roar of the KAO’s engines madeconversation difficult. Everyone woreheadphones and spoke into microphones.With everyone tuned to the same channel,private conversations were practicallynonexistent.

Harper recalls that when the KAO firstbegan flying, the camera used to guide thetelescope wasn’t very good. Looking at thecamera’s monitor, he was lucky to see 4th-magnitude stars (the unaided eye can seestars six times fainter than that). To betterguide the telescope, he would sit by it on afolding stool, looking through the eyepieceof a small guiding scope attached to thelarger research telescope. A photographer’sblack hood covered his head to block outthe cabin light. “I was always afraid of get-ting a black eye,” he recalls, as turbulencebounced the plane — and the telescope’seyepiece — this way and that. The guid-ance system eventually improved, much toHarper’s relief.

Left: Susan Stolovy, a Cornell University instrument specialist (she now is a Staff Scientist at the SIRTF Science Center, Caltech), gives her colleagues“high fives” during a July 18, 1994 KAO flight. She and her colleagues had just realized they had detected a whopping signal from the impact of CometShoemaker-Levy 9’s G fragment on Jupiter. Right:Technician Terry Richardson performs maintenance work on the KAO’s 36-inch telescope.

KAO discovered 9 rings around Uranus dur-ing a March 10, 1977 stellar occultation. Therings blocked some of the star’s light, causingnoticeable drops in its brightness. Courtesy ofJames Elliot (MIT) et al. Inset: In 1986 Voyager2 took this false-color image of Uranus’s 9known rings. Courtesy of NASA/JPL.


Innovation was a hallmark of KAO, andof airborne astronomy in general. Duringone KAO flight to observe a solar eclipse,astronomers used the hole in a snack crack-er as a pinhole to project a solar image ontoa piece of paper. It was an easy way to mon-itor the progress of the eclipse, while theirinstruments recorded the telescope’s view.

“You have to be doubly prepared whenyou fly,” says Becklin.“There’s so much goingon and things are happening. You not onlyhave to worry about the telescope and theinstruments, you also have to worry aboutflying the aircraft in the correct direction.”Changing direction slightly to fly around athundercloud, for example, while no big dealfor a passenger flight, could be disastrous forastronomers observing a specific object. Anychange in aircraft direction altered where inthe sky the telescope pointed. Move too far,and the telescope could no longer see theobject astronomers wanted to study.

During the KAO’s 20 years of flights, itsurpassed nearly everyone’s expectations.Astronomers discovered a ring of gas orbit-ing the supermassive black hole at the cen-ter of our galaxy. They studied powerfulinfrared emissions from other galaxies anddetermined that spiral galaxies emit asmuch energy at far-infrared wavelengths asall other wavelength regions combined,making them more luminous thanastronomers had thought. They revealednewborn stars inside Bok globules, thickinterstellar dust clouds that appear com-pletely black in visible light. They probedthe distribution of water and organic mole-cules — materials thought to be necessaryfor life — in the spaces between stars and instar-forming clouds.

Inside the solar system, KAOastronomers detected water molecules incomets, the first proof that they really are“dirty snowballs.” They discovered that

Pluto has a thin methane atmosphere. Andthey made observations that revealed asystem of narrow rings around Uranus.

SOFIA and Space TelescopesFor years, astronomers had dreamed of fly-ing an even larger telescope to observe inthe infrared. A bigger telescope collectsmore photons and can, therefore, viewfainter objects. It also provides sharperimages, especially at longer infrared wave-lengths. In 1986, NASA began to study thetechnical challenges involved in modifyingan even bigger airplane to accommodate atelescope significantly larger than the KAO’s36-incher. Work on SOFIA began in 1996.Unfortunately, it was necessary to retire theKAO in 1995 in order to provide adequatefunding to develop SOFIA.

Today, an international consortium ofscientists, engineers, and technicians is hard

The KAO telescope imaged Comet Halley crossing the Milky Way in April 1986.This photo was taken with equipment designed, mounted, and operatedby a Charleston, South Carolina, County School District project. KAO astronomers detected water molecules, proving comets really are “dirty snowballs.”


at work on SOFIA, the next generation inairborne telescopes. They’re modifying aBoeing 747SP to carry a 2.5-meter (8-foot)telescope — the largest telescope ever toleave the surface of Earth. The GermanAerospace Center (DLR) is building themirror and telescope. Both will then beflown to Waco, Texas, where L3 Communi-cations, Integrated Systems is carrying outthe airplane modifications. USRA (Univer-sities Space Research Association), SOFIA’sprime contractor, will operate the observa-tory for NASA, managing Americantelescope allocation time (DLR will control20% of the time) and maintaining theobservatory. USRA has teamed with UnitedAirlines, whose personnel will fly SOFIAand perform aircraft maintenance work.

“SOFIA will have a sharper infrared viewof the universe than any previous tele-scope,” says Becklin. And that includesspace-based telescopes, which tend to besmaller because of weight restrictions.

Size matters, but so does accessibility. Ifan instrument fails on any airborne obser-vatory, technicians already onboard theplane can work on the instrument until it’sfixed, most of the time before the flightends. “We can install the latest and greatestdetectors and instrumentation at any time,”adds Becklin. That means SOFIA will carrystate-of-the-art instruments throughout its20-year scheduled lifetime. Space-basedtelescopes may begin with the latest tech-nology, but, by the time their missions end,their equipment is usually obsolete.

SOFIA is also more flexible than spacetelescopes such as IRAS or SIRTF (see“SIRTF: NASA’s Next Great Observatory,”page 32). Think of SOFIA as the world’slargest portable telescope. Airplanes can flyanywhere in the world to view an event inany part of the sky at any time. When KAOdiscovered Uranus’s rings, for example, itflew out of Australia (where it needed to be)to record what happened when Uranusocculted a distant star, an event not visibleeverywhere on Earth. Such flexibility isimpossible for telescopes anchored to theground and is difficult for tightly scheduledspace-based telescopes.

This is not to say that airborne tele-scopes are always better. Telescopes in theextreme cold of space experience fewer

KAO represented a giant leap forward forairborne astronomy (note the telescopeport in the top image). Right inset: KAO’sMission Director’s console exemplifiesthe airplane’s crowded interior.


problems with background noise. “Theinfrared background,” explains Becklin, “isdetermined by photons from the surround-ing thermal environment — the telescope,the sky. On SOFIA, we still have the residualtemperature of the atmosphere. It’s typi-cally –40° C in the stratosphere. But thatstill generates a lot of unwanted infraredphotons. Space is much colder.” Eliminatingbackground noise is critical when one viewsfaint objects, the light of which can be lostamid noise, or when one looks for faintfeatures in a bright spectrum.

Space-based and airborne telescopes eachhave their advantages and disadvantages. Anairplane can observe for no more than 7 or 8hours at a time, constrained by fuelconsumption. Space telescopes have no suchtime restrictions. It’s not an either/or situa-tion; the two complement each other. “Theairborne program is an essential part of thesupport for the space program,” says Harper.“It provides ready access and a place todevelop and test instruments and techniquesthat will fly on future space telescopes.”

Pointing the TelescopeOf course, any tests or astronomical observa-tions made with an airborne observatoryrequire that the telescope remain steady. Theairplane itself plays a major role in keepingthe telescope pointed at a desired object. Anairborne telescope can only look out an openport on one side of the airplane (larger open-ings that would allow the telescope to lookout both sides would leave the plane struc-turally unsound; traditionally, the holes havealways been cut on the left). If astronomerswant to look at an object visible in the west,for example, the plane has to fly north. Theairplane’s flight path must be carefully plot-ted and flown to keep the telescope pointedin the right direction.

Anyone who’s ever flown knows that airturbulence and engine vibrations can shakean airplane severely and unpredictably.Recalling one particular incident of turbu-lence, Yvonne Pendleton of NASA’s AmesResearch Center says, “It was amazing to methe first time I noticed the back of the

SOFIA Technical Details

Weight of Telescope, including Science Instrument ............about 20,000 kg

Configuration............Cassegrain telescope

with Nasmyth focus

Primary Mirror ..................2.7 m diameter;

2.5 m effective aperture

System Focal Ratio ............................f/19.6

Wavelength Range ....0.3 to 1,600 microns

Field of View ........................8 arcminutes

Temperature of Telescope...........240 Kelvins

Operating Altitude ........12,500 to 13,700 m

Observation Time at Operating Altitude............................8 hours

Number of Flights Per Year ..........about 160

Lifetime........................................20 years

or the past several years, NASA’s Office of Space Science hasmandated that every mission devote a small percentage of

its budget to education and public outreach. These initiativesprovide immediate educational benefit to the American publicthat go beyond the long-range benefits of scientific research.SOFIA will devote its education effort to using the naturalattraction of a “telescope in an airplane” to help educators teachbasic concepts of science, astronomy, and critical thinking.

The Astronomical Society of the Pacific, in partnershipwith the SETI Institute, is responsible for the SOFIA Educa-tion and Public Outreach (E/PO) program. When fully opera-tional, educators and the general public will be able to getinvolved in a variety of ways. The program will eventuallytouch millions of students of all ages.

In the most visible aspect of the program, up to 200 educa-tors per year will be trained as “Airborne Astronomy Ambas-sadors.” The educators will fly in groups of 2 or 3 aboardSOFIA missions, learning first-hand what scientists do andhow they think. As important as helping the Airborne Ambas-sador teach more effectively, the prestige and enthusiasm theseteachers will gain from such an experience will last for yearsand will have an impact on thousands of students.

The SOFIA E/PO staff will help teachers all over the countryform educational partnerships with SOFIA-related scientistsand will teach the scientists how to conduct effective classroomvisits. Teachers will also have access to many specially developed

classroom activities built around SOFIA science, available bothon the Internet and in classroom-ready printed form.

For more information on SOFIA, or to learn more about theE/PO programs (including how to subscribe to the free Educa-tors E-newsletter), visit www.sofia.usra.edu. — Mike Bennett

Education and Outreach with SOFIA

F

Teachers flew aboard the Kuiper Airborne Observatory, and they’llfly aboard SOFIA as well.This image shows teachers at work duringa 1993 KAO flight. Courtesy of NASA/Ames Research Center.


KAO’s telescope turning this way and that.Then I looked at the monitor that depictedour stellar image and realized the image wasquite stationary. It was suddenly obvious tome that the plane was moving around thetelescope and not the other way around.”

How does the telescope avoid all thebumps and shakes? The secret is pressurizedair bladders that isolate the telescope fromaircraft vibrations. SOFIA’s telescope isshaped somewhat like a well-balanceddumbbell, with the telescope on one endand the instruments and counterweights onthe other. After bouncing off the telescope’sseries of mirrors, infrared light passesthrough the “bar of the dumbbell,” called aNasmyth tube, on its way to the detectors.Twelve air bladders, also called air springs,are arranged symmetrically around the bar,and they absorb nearly all of the plane’svibrations, keeping the telescope’s imagerelatively steady.

In addition, gyroscopes will help keepSOFIA’s telescope pointed properly. Theycounter sudden gusts swirling from outsidethe 4-meter-wide opening into the tele-scope cavity (that’s wide enough to drive an18-wheel truck through). In flight, air flowsalong the airplane’s side at more than 800kilometers per hour. While observations invisible light will be somewhat blurred bythis stream of air, infrared light, with itslonger wavelengths, passes through essen-tially unscathed.

When the air stream reaches the telescopeopening, it flows into the cavity, where it hitsa small curved ramp near the top of theopening. The ramp deflects most of the airback outside. Indeed, wind tunnel tests show

that turbulence in the airflow downstreamfrom the opening is only slightly differentfrom that of an unmodified airplane.

But some air does make it down to thetelescope. It isn’t a steady wind, however,but contains turbulence and gusts, whichcan continually nudge the telescope awayfrom its intended target. If any or all ofthree gyroscopes attached to the telescopeframe detect movement from one of thesewind gusts, a computerized control systemdrives electromagnetic motors that nudgethe telescope back in place. All of this hap-pens automatically. According to PatrickWaddell, SOFIA Missions OperationsDirector, “Actions of the control system areessentially invisible to astronomers and thetelescope operator.”

The gyroscopes, along with the air blad-ders and the plane’s heading, will provideSOFIA with unprecedented pointing accu-racy. Imagine holding a laser pointer to illu-minate a penny 16 kilometers away and notletting the beam waver from that penny.That’s how steady SOFIA’s pointing will be.

The Future of Airborne Astronomy“I cannot wait for SOFIA to become opera-tional,” says Pendleton. “I never failed to getdata from a KAO flight, and I am lookingforward to an equally productive experienceon SOFIA.” When the modifications arecompleted, SOFIA will fly out of NASA’sAmes Research Center in California’sSilicon Valley. Three to four flights a week

Left:A technician inspects SOFIA’s 2.7-meter primary mirror. Image courtesy of SAGEM-REOSC. Right: Compared with previous airborne observatories, SOFIA’s Mission Control area is downright spacious and luxurious. Illustration courtesy of L3 Communications.

Both SOFIA’s airplane and mirror (bottom left) dwarf their KAO counterparts.


are planned, each of which will spend 8hours at altitudes of 12,500 to 13,700meters, slightly higher than the cruisingaltitude of commercial jet airliners.

Astronomers plan to use SOFIA’s tele-scope to further their understanding of howgalaxies, stars, and planets form and evolve.SOFIA’s high spatial resolution will enableastronomers to resolve and then study dis-tinct features in star-forming regions that,until now, have appeared as indistinct blobs.Scientists will be able to map the gaseousoutflows found around both young and oldstars in greater detail, providing more cluesabout what happens as stars mature.Astronomers will be able to resolve featuresin the atmospheres of planets in our solar

system. For example, they will be able totrack seasonal movements of carbon dioxideand water vapor between polar ice caps andequatorial regions in Mars’s atmosphere.

In addition, astronomers hope to useSOFIA to find distant galaxies, visible at longinfrared wavelengths, which are thought tobe in early stages of their evolution. SOFIA’shigh resolution will enable scientists toseparate infrared emission in galactic nucleifrom emission in the rest of a galaxy, givingthem more information about the energeticprocesses occurring around supermassiveblack holes. Astronomers will be able tospatially resolve and investigate nearbymerging galaxies and the bursts in starformation that typically ensue.

SOFIA’s telescope and scientific instru-ments will also provide unprecedentedspectral resolution, especially at longerinfrared wavelengths. Astronomers hope tostudy the detailed chemistry of the interstel-lar medium, which emits most of its energyin the infrared. They will search for thepresence of complex organic molecules andother atoms that are thought to be neces-sary for life and determine their distribu-tion within giant molecular clouds andstar-forming regions. Astronomers will lookfor clues about conditions in the early solarsystem and how life began on Earth by ana-lyzing the composition of comets and aster-oids that have remained unchanged sincethey formed billions of years ago.

Astronomers will fly on SOFIA, just asthey did with the KAO. Conditions should bemore comfortable, however, with more spaceand better noise and thermal insulation.Looking into the future, Becklin envisions anera when only a few technicians will actuallyfly on the plane. Astronomers will follow theflight and record data from the ground,either in a control room or in their offices.“This is not going to happen right away,” saysBecklin, “but it will in 10 years.”

As SOFIA moves closer to its first scienceflight, one fact stands out: At roughly 20,000kilograms, SOFIA’s telescope alone weighs asmuch as the entire Lear Jet Observatory!How far airborne astronomy has come.

Former Mercury editor SALLY STEPHENS isnow a freelance science writer in San Fran-cisco. Most recently, she has worked on a

high-school science curriculum being devel-oped by the SETI Institute and on material

for the SOFIA website.

Left:The SOFIA 747 (rear) rests in a Waco,Texas hangar along with a cut-up section of a “donor” 747 housing a full-scale dummy telescope. Courtesyof L3 Communications. Right:Technicians test the cabin side of the telescope’s suspension assembly and Nasmyth tube. Courtesy of MAN-Technologies.

Among its many science targets, SOFIA will zoom into prolific star-forming regions such as theOrion Nebula, seen in this 2MASS infrared image. Courtesy of UMass/IPAC-Caltech/NASA/NSF.



by Michelle Thaller

nfrared astronomy is truly coming of age. Many of NASA’s mostimportant and ambitious upcoming missions, including SOFIA,

the Next Generation Space Telescope, and Terrestrial Planet Finder,feature infrared telescopes, so the stage is being set for infraredscience to play a leading role in the study of the universe for thecoming decades.

The next major infrared mission is the Space Infrared TelescopeFacility (SIRTF), a cryogenic space telescope set to launch in early2003. SIRTF was originally conceived to be the fourth and finalinstallment of NASA’s Great Observatories, a fleet of space telescopesdesigned to give astronomers a view of the universe across the entireelectromagnetic spectrum. With the Compton Gamma-Ray Obser-vatory having completed its mission, and with the other two GreatObservatories (the Hubble Space Telescope and Chandra X-rayObservatory) still in place, SIRTF stands poised not just to roundout the program, but also to connect that program solidly intoNASA’s new initiative, the search for humanity’s cosmic origins.

In fact, the search for our origins best illustrates why infraredlight has become such a popular wavelength region. While celestialbodies must have temperatures of around 10,000° C to radiate anysignificant amount of visible light, anything with a temperatureabove absolute zero emits infrared radiation. That includes a wholelot of stuff that has evaded thorough study, or even detection, up tothis point.

In human terms, we tend to think of infrared light as heat radi-ation. With the example of infrared night-vision cameras in mind,it’s easy to understand how astronomers hunt for extrasolar planetswith infrared light. Just as infrared detectors can be used to find awarm person hiding in the dark, they can be used to image planetsthat emit no visible light of their own. Even around young starswhere planets haven’t fully formed, infrared light can be used todetect the warm disks of dust that will someday coalesce into

planets. Other warm but dark objects include the still-mysteriousbrown dwarfs. Some brown dwarfs exist in extrasolar planetarysystems, while still more seem to wander through space alone,kicked out of their star systems by gravitational interactions.

Infrared light is also a hot topic (or cool, depending on your tem-perature preference) when it comes to more exotic celestial speci-mens. Recent studies suggest that supermassive black holes lurk inthe centers of most, if not all, large galaxies. Galactic cores, includingour Milky Way, are obscured by vast clouds of dust and debris,making visible-light observations nearly impossible. Infrared lightconveniently cuts through intervening material, allowing directviews of the warm, active regions close to the giant black holes.Infrared light’s penetrating ability also allows astronomers to peer

SIRTF: NASA’s Next Great Observatory

I

Lockheed Martin Space Systems (the main contractor) integrates theCryogenic Telescope Assembly (built by Ball Aerospace and TechnologiesCorporation) with the rest of the spacecraft.Courtesy of Lockheed Martin.

SIRTF’s orbit is similar to Earth’s, but SIRTF will trail behind our planet.Courtesy of NASA/JPL/Caltech.


into the hearts of stellar nurseries, wherethey can bear witness to the early stages ofstar and planet formation. Turning to theouter reaches of the universe, infrared tele-scopes probe epochs when cosmic expan-sion has redshifted all of the light out of thevisible range entirely. Astronomers will beable to probe the Dark Ages and find outhow and when the very first stars turned on.

So, if the infrared universe has so manyfascinating offerings, how come we haven’texplored it in detail yet? The current age ofinfrared astronomy is being ushered in byan amazing leap in technology, largelyfueled by the declassification of sensitiveinfrared detectors developed by the U.S.military. The last major infrared space tele-scope, the Infrared Astronomical Satellite(IRAS), gave astronomers a wonderful firstglimpse of the infrared sky back in 1983and serves as a great example of how tech-nology has advanced in 20 years. IRAS wasbuilt before the era of large array cameras,and it had 60 separate infrared detectorscovering the wavelength range of 8 to 120microns. SIRTF, by comparison, will carrydetector arrays with tens of thousands ofpixels, enabling both imaging and spec-troscopy over a 3 to 160 micron range.

The sensitivity of SIRTF’s new infrareddetectors is amazing. Have you ever turnedyour face up to the sky on a summer dayand felt the warmth of the Sun on your skin?Now picture trying to feel the heat from dis-tant stars and galaxies, and you have someidea what a telescope like SIRTF can do.

Another clever engineering trick was todiscard the traditional thermos-bottleapproach to infrared telescopes. Infraredtelescopes must be cooled to within a fewdegrees above absolute zero, becauseastronomers are interested in heat fromspace, not from the telescope itself. In thepast, space telescopes like IRAS and themore recent European Space AgencyInfrared Space Observatory (ISO) solvedthe cooling problem by encasing the entirespacecraft in a liquid-helium refrigerator(called a cryostat). The liquid helium tanksadded both mass and bulk, both of whichmake launch difficult and expensive. InSIRTF’s case, the spacecraft has beendesigned to cool passively once it’s exposedto the near-vacuum of space. Only the mostsensitive instruments will be cooled withliquid helium, now reduced to a relativelytiny volume of 378 liters (which should lastat least 2.5 years, possibly up to five). Thereduced mass allows SIRTF to be launchedon a relatively small Delta II booster,making the project even more economical.

With no liquid helium thermos, NASAmust launch SIRTF beyond Earth orbit intoa more distant, and therefore cooler, regionof space. SIRTF will be launched withenough velocity to drift slowly but continu-ally away from our warm planet. SIRTF willfall into a solar orbit similar to that of Earth,trailing behind at the rate of 0.1 astronom-ical unit (15 million kilometers) per year.

This orbit will enable the telescope to oper-ate more efficiently, with less of its viewblocked by the bright Earth and Moon.

In the end, the most exciting thingsabout SIRTF will be all the stuff we haven’teven thought of yet. In an era of increas-ingly specific, scientifically focused spacemissions, SIRTF has a shot at serendipity.Astronomers have never seen the infraredsky in anything approaching the detail andsensitivity that SIRTF offers, and in somecases, we’ve never even had a chance to lookat the universe in some of the wavelengthsthat SIRTF will cover. We will see things thatwe don’t expect, and can’t explain (at leastnot right away). And that’s the best reasonof all to welcome SIRTF’s upcoming launch.

MICHELLE THALLER([email protected]) is an astronomer

at the California Institute of Technology andthe manager of the SIRTF Office of Education

and Public Outreach. Besides fulfilling hermanagement duties at Caltech, Michelle

travels the country promoting NASA’seducation program and even tries to get a

paper or two published about massive binarystars when time permits.

Technicians inspect SIRTF’s infrared telescope,whose mirror is 0.8 meter in diameter.Courtesy of NASA/JPL/Caltech.

Technicians at the Ball Aerospace and Tech-nologies Corporation work on SIRTF’s cryo-genic tank.The tank is filled with liquid heliumto keep critical components chilled to just a fewdegrees above absolute zero. Courtesy ofBall/NASA/JPL/Caltech.

Author Michelle Thaller appears in visible light(left) and in infrared light (right). Before theinfrared image was taken, she rubbed an ice cubeabove her lips.The cold ice cube created the dark“mustache.” Courtesy of Michelle Thaller.

flying telescopes - sofia science center · flying telescopes this boeing 747sp, shown during a...

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