Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/projects/askap/archive/Irion1.pdf Дата изменения: Mon May 27 03:55:59 2002 Дата индексирования: Sat Sep 6 07:32:55 2008 Кодировка: Поисковые слова: microwave background

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If astronomers and agencies from six continents can work together, the world's largest telescope may rise in the next decade--with a radical new design

Tuning In the Radio Sky
during the half-hour drive to Hat Creek RaHAT CREEK RADIO OBSERVATORY, CALIFOR- intelligent life to tens of millions of stars. Plans call for SKA to reap this scientif ic dio Observatory from the nearest airf ield. A NIA--From the controls of his Cessna, astronomer Jack Welch points to a clearing in har vest by spreading its detectors across $60 million, decade-long project to improve Lassen National Forest. There, in this isolat- more than 1000 kilometers of land. But de- VLA has now begun, he notes, "but it's ed spot in nor theaster n Califor nia, ten spite the economy of mass production, it clear that ultimately we will need a new 6-meter telescopes stare at the sky--a mod- may cost $1 billion to build. That's why a general-purpose telescope with much more est array by the standards of radio astrono- formal international consortium of radio as- collecting area." Such wave-gathering prowess would give my. But by 2005, something more g rand tronomers is pushing SKA as a global project will take its place: a cluster of 350 radio from the outset. Scientists on six continents SKA the sensitivity and breadth of the best dishes, acting in unison to view the universe have contributed ideas for its design and lo- optical telescopes, says Welch, who concation, sparking a creative outburst that the ceived the ATA project at UC Berkeley. Toand watch for signs of life elsewhere. Two volcanoes, Lassen Peak and Mount f ield hasn't seen in 30 years. "Radio wave as- day, astronomers can combine signals from Shasta, overlook the site of this project, tronomy has been so productive, but there is widely separated radio dishes to make sharp called the Allen Telescope Ar ray (ATA). still much to do," says Peter Dewdney of images, but the objects must be bright and can't cover much area on the sky. That's f itting, for the ar ray's As astronomer Alyssa Goodman concept--many receivers linked by of Har vard University puts it: cheap electronics--is r u mbling "Radio astronomy leads to Nobel through radio astronomy. By cutPrizes but not pretty pictures. It ting the str uctural costs of huge would be nice if it led to both." dishes and instead combining sigSome efforts are already under nals from many small detectors, raway. At millimeter wavelengths-- dio astronomers aim to explore the the high-frequency end of the cosmos electronically at a nonradio spectrum--construction will astronomical price. soon begin on the long-awaited "In a metaphorical sense, we're Atacama Large Millimeter Array lear ning how to build telescopes (ALMA), a joint U.S.-European out of computers, not metal," says effort to build 64 12-meter dishes physicist Kent Cullers of the SETI high in the Chilean desert. The [Search for Extraterrestrial IntelliU.S. National Radio Astronomy gence] Institute in Mountain View, Observatory (NRAO) opened its California. "This is the future of radio astronom y," ag rees as- New-wave radio. The Allen Telescope Array, a planned network of 350 100-meter Green Bank Telescope in West Virginia last ye a r, a n d tronomer Leo Blitz of the Univer- radio dishes in California, relies on low mass-production costs. astronomers at the 300-meter sity of California (UC), Berkeley, which is building ATA with the SETI Insti- Canada's National Research Council in Pen- Arecibo Observatory in Puerto Rico and in tute. "If you want to build a large telescope ticton, British Columbia. "We think SKA Australia, France, India, Italy, and the Unitfor a f i xed amount of money, I can no deserves a place among the world's great ed Kingdom have recently constructed or substantially upgraded their facilities. longer think of any reason to build a single telescopes of the next decade." Still, many note that the average hair collarge dish." or at radio astronomy meetings is becoming Cullers and Blitz are conf ident that 12 to Skating toward SKA 15 years hence, this trend will produce a When proponents make the case for includ- grayer. "A huge number of us entered the telescope of breathtaking scale: the Square ing SKA in their future, they often point to f ield in the 1960s," says astronomer Donald Kilometer Ar ray (SKA). As its name im- radio astronomy's past. For example, three of Backer of UC Berkeley, noting that Arecibo plies, this instr ument would gather radio the f ive Nobel Prizes in astronomy have re- and Jodrell Bank Observatory near Manwaves with a combined detecting area of a warded work at radio wavelengths. Moreover, chester, U.K., were vital destinations. "That's full square kilometer--making it 100 times the instrument responsible for the second- not happening now. ATA and SKA are excitmore sensitive than any existing array. SKA highest annual rate of pub lications in ing, but they have yet to pull in students." If SKA does not enjoy the same cachet would expose the now-invisible era when hy- astronomy--behind only the Hubble Space drogen f irst clumped together, tracing the Telescope--is the Very Large Array (VLA), among astronomers as major plans in the "cosmic web" of dark matter that underlies a network of 25-meter radio dishes in optical, infrared, x-ray, and other wavelengths, it may be because such a huge and all structure in the universe. Other studies Socorro, New Mexico. That record hides a small sur prise: radical concept still seems so foreign. "I unique to SKA's radio window include mapping magnetic f ields in and among galaxies VLA's 27 dishes run on 25-year-old technol- w as so ignorant of this subject that I in exquisite detail, f inding thousands of pul- ogy. "The VLA was completed in 1980, but thought the telescope was built square to sars and using them to track gravitational it hasn't been upgraded at all," says Welch, make the Fourier transforms easier," joked ripples in space, and extending the search for who pilots a faded yellow Plymouth Valiant astrophysicist Roger Blandford of the Cali3 MAY 2002 VOL 296 SCIENCE www.sciencemag.org

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100 megabits per second, "the vir tual exploration of planets could take place," he says. "Instead of a rather coarse image on a Radio astronomers aren't the only ones enamored of huge arrays newspaper page, we could have real-time television, even highof cheap receivers. The concept has also caught the eyes of NASA definition video." Eac h mission could also send bac k far more engineers, who long to overhaul the agency's aging Deep Space data at other wavelengths for deeper analysis of atmospheres Network (DSN). and surfaces. More collecting area on Earth would also mean Consisting of thr ee 70-meter smaller transmitters on spacecraft, reducing their weight and dishes and several smaller antennas size and perhaps eliminating the need for orbiters to relay in Spain, Australia, and Goldstone, data from planetary landers. Calif ornia, DSN is NASA's link to NASA is exploring two options to realize those gains by next probes that explore the solar sysdecade, Geldzahler sa ys. tem. Since the mid-1970s, its capacOne is to move communicaity has gone up only modestly. This tions to the high data rates factor, along with basic transmitters of optical light, with lasers and computers on spacecraft, has on spacecraft and 10-meter stuck space exploration on snapshot telescopes on the ground. mode in a video age. But if telescope costs prove For a near-term boost, NASA has pr ohibitiv e, NASA ma y upgraded its Goldstone receiver to work at a frequency of mimic the approach of the 32 g i g aher tz r ather than 8 g i g aher tz, sa ys Barry Allen Telescope Array. WeinGeldzahler, p rog ram executive for space operations at reb and his colleagues at JPL NASA headquarters in Washington, D.C . That conversion will soon submit a proposal throughout the DSN system--to be completed by 2006-- to NASA for a pr ototype will quadruple data transmission above the current rate of Deep dish. Data rates have gone up slowly DSN array of 100 12-meter about 100 kilobits per second for a probe at the distance since 1976 ( g raph) for NASA's Deep Space radio antennas. Beyond that, of Jupiter. Other plans include more efficient software on Network. Weinreb says, "DSN and the spacecraft and refurbishing the ground antennas. "The inSquar e Kilometer Arr a y frastructure has been allowed to go fallow," Geldzahler acknowl- [SKA] could go hand in hand. There is a lot of common technology." edges. "In our budget priorities, missions have come first." Radio astronomers think the solution is clear. "I see the connecIt's vital to hike DSN's capacity much further, says electrical tion between DSN and SKA as completely obvious," says Alyssa engineer Sander Weinreb of NASA's Jet Propulsion Laboratory Goodman of Harvard University in Cambridge, Massachusetts. (JPL) in Pasadena, California. With a data-transfer rate of 10 to ­R.I.

Space Communication for the Video Age

fornia Institute of Technology in Pasadena. But after he looked into it, Blandford became a conve r t. "In a 1° f ield of view, you'll see 300,000 extragalactic sources," he says. "It will be a splendid probe for cosmology, and it will discover completely new sources as well."
Venture capital

Blandford's latter point is one that SKA's organizers would like to sell. "It's quite possible that the main thing this instrument will do is to show us something that no one expected," says the SETI Institute's Jill Ta r ter, chair of the U.S. SKA consor tium. But Tar ter and her collaborators acknowledge that the lure of the unknown won't sway funding agencies. "In today's climate, you need a shar ply focused scientif ic case," Backer says. "We won't get $1 billion to build the next bigger thing just because we can." So, SKA's partners focus their pitch on the web Due for a tune-up. The productive but aging Very Large Array is of primordial hydrogen, getting a decade-long upgrade.
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which Backer calls "as rich a prize as the cosmic microwave background itself." Hydrogen suffused the dark era between the origin of the microwave background--when atoms f irst formed and light streamed freely into the cosmos--and the birth of the f irst stars and galaxies. A neutral hydrogen atom emits radiation at a wavelength of 21 centimeters when its lone electron flips its spin. As space expands, this weak signal from the early universe stretches into tenuous wisps of meters-long radio waves. They will pene-

trate through everything, giving astronomers a clear view of mass concentrations in the infant universe. "A square kilometer is not a random size," says astronomer Harvey Butcher of the Netherlands Foundation for Research in Astronomy (ASTRON) in Dwingeloo. "If you put the Milky Way at the very beginning of galaxies, you need a square kilometer to be able to detect it, given the sensitivity of receivers today." Various components of our galaxy's ancestors will pop into focus for SKA. Radio signatures of carbon monoxide and other molecules will trace the history of heavy elements in early galaxies. The whirlings of coherent microwave emissions from vast clouds of w ater v a por , called me gamasers, promise to expose some of the most distant supermassive black holes at the cores of activ e galaxies. Closer to home, SKA will resolv e the magnetic f ields that lace through galaxies and the cradles of stars within them. Within the Milky Way, SKA should f ind at least 10,000 pulsars, the dense, spinning remnants of exploded stars. The telescope will track the relative rotation speeds of the fastest of these beacons with an accuracy of better than a millionth of a second. Albert

CREDITS: (TOP TO BOTTOM) NASA JPL; COURTESY OF DAYTON JONES/NASA JPL; NRAO

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Seeking Peace in a Radio-Loud World
Although radio astronomers adore the technology the communications industry has spawned, they detest some of its byproducts: blaring antennas, swarming satellites, and chatter-filled airwaves that threaten to wash out dim sources in the sky. For years, treaties have shielded key parts of the radio spectrum from commercial interference. For example, at the 2000 World Radiocommunication Conference in Istanbul, Turkey, regulators preserved big chunks of the spectrum at millimeter wavelengths to benefit the planned Atacama Large Millimeter Array in Chile and other high-frequency observatories. Some facilities--such as the 100-meter Green Bank Telescope in West Virginia--also sit within "radio preserves," where terrestrial signals are curtailed. However, such measures aren't cure-alls. Commercial pressures squeeze broadcasters and satellite operators into frequencies next to those in which astronomers try to work, and their signals often bleed into the "protected" bands. Satellites, which don't turn off above radio preserves, will only become more numerous. And as astronomers build more-sensitive radio telescopes to peer into deep space, many observations will shift into commercial wavebands--thanks to the expansion of the universe, which stretches all emissions to longer wavelengths. Planners for the Square Kilometer Array (SKA) crave a true radio-quiet zone where satellites might not transmit, such as the sparsely populated Australian outback. The Global Science Forum of the Paris-based Organization for Economic Cooperation and Development is sponsoring a task force to study this notion--and to convince industry that tight control over signal leakage makes smart business sense. "The satellite community is certainly much more aware of the radio astronomy problem" than in the past, says Tomas Gergely, program manager for electromagnetic spectrum management at the National Science Foundation. "But to have total access to the spectrum at any one place on Earth is impossible in my view." Fortunately, the arrays of smaller elements in most SKA designs offer a way to cope. By delaying some of the signals r elativ e to other s, astronomers can suppress radio waves in arbitrary patterns on the sky-- just as a light beam passing through slits creates an interference pattern of bright fringes and dark spots. As- Null and void. Manipulating waves from the Allen tr onomer Geof fr e y Bo w er of the Telescope Array will suppress radio interference in University of California (UC), Berke- any pattern on the sky (blue). ley, has shown that a seven-antenna prototype of the Allen Telescope Array (ATA) can beat down signals from satellites by a factor of 1000. This tec hnique, called interferometric nulling , will be even more powerful on the full 350-telescope array. "The radio sky over ATA will look like Swiss cheese," says Jack Welch of UC Berkeley. "Each satellite will have a little nulled horizon around it. Otherwise, their emissions would go off-scale." A combination of more efficient satellites and clever nulling just may make SKA feasible, says astronomer Michael Davis of the SETI Institute in Mountain View, California, former director of the Arecibo Observatory in Puerto Rico. "We are asking satellite engineers and operators to be technically innovative and creative," Davis says. "We have no standing if we don't also do that ourselves." ­R.I.

The wide f ield of view also will make SKA ideal to search for blips from other civilizations. Electronic processing will let Ta r ter and other SETI Institute astronomers monitor many stars in the same viewing area as another object being studied. Even if such signals are merely "leakage" of alien radio or TV equivalents, SKA could pick them up from deep within our Milky Way. That's a far cry from the slow pace of searching today, says Welch. "Even with Arecibo, we couldn't hear Howdy Doody beyond Alpha Centauri," the closest star, he observes.
Global by design

Einstein's theorized gravitational waves-- ripples in the fabric of spacetime caused by massive disturbances, such as coalescing b lack holes in the centers of distant galaxies--may flutter the apparent motions of the pulsars enough for SKA to detect. SKA's ability to focus on much larger patches of the sky than ALMA also will make it an ideal radio survey tool, says astronomer Jim Cordes of Cornell University

in Ithaca, New York. Cordes is eager to obser ve what he calls the "transient radio sky"--bursts and new objects that may come and go in days or weeks. "We are very behind our colleagues at other wavelengths in exploring the transient sky," Cordes says. For instance, SKA might see the afterglows of distant gamma ray bursts, only 1% as bright as any seen so far, or even bursts that appear only in radio waves.
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Welch and Tar ter, who are mar ried, and their colleagues at the SETI Institute and UC Berkeley are building ATA to deepen their own research, but they clearly view it as the forerunner to SKA. At a projected $26 million, ATA will provide roughly the same collecting area as the single-dish Green Bank Telescope at one-third the cost. Its oddly shaped 20-foot aluminum dishes (the manufacturer disdains metric units) will be pressed out by a satellite-dish outf it in Idaho Falls, Idaho, over the next 2 years at the rate of one every other day. "We're light, we're agile, and we're quick," says UC Berkeley's Blitz. ATA also represents a sociolo gical breakthrough for the f ield. Its funding is entirely private--nearly all of it from technologist Paul Allen, the co-founder of Microsoft and a big SETI Institute fan. "We run this like a skunk works," says project scientist John Dreher of the SETI Institute. "There are minimal reviews and no ponderous government management structure. We just have to keep one panel happy." Scaling ATA up to a SKA-sized network would take thousands of dishes, each one perhaps 12 meters across. Making that f inancially feasible will require fur ther cost reduction by a factor of 3 or 4, Welch estimates. India also is pursuing a similar concept for SKA, so the two countries may exchange ideas about how to bridge that cost gap. The leading alternative to the many-dish idea, in the minds of most observers, would look like f ields full of simple looped wire antennas or wires embedded in tiles. Spearheaded by ASTRON, this concept may arise within 5 years as a $75 million project called the Low-Frequency Array, or LOFAR. "The basic element of LOFAR is really cheap: It's just a long string of wire," says physicist Joseph Lazio of the Naval Research Laborator y in Washington, D.C., a par tner in LOFAR with the Massachusetts Institute of Technology. "It's essentially a big FM radio." LOFAR would focus on wavelengths between 0.5 and 10 meters. About 10,000

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CREDIT: GEOFFREY BOWER/UC BERKELEY RADIO ASTRONOMY LABORATORY


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CREDITS: (CLOCKWISE FROM TOP) EN SIMONS/SYDNEY VISLAB; NRC-CNRC; BAO; ASTRON

the y fund their science." One or more countries in the consortium may be loath to invest outside their borders, some astronomers say privately. The rough f iscal menu calls for the United States and Europe each to f inance one-third of SKA, with the remaining onethird from other countries. The initial commitment for major constr uction funding will be the toughest row to hoe, says Paul Vanden Bout, director of the National Radio Astronomy Obser v ator y in Charlottesville, Virginia. He points to the U.S.European ALMA project in Chile as an example. "We might have talked for a very long time indeed if the National Science Foundation [NSF] had not been willing to fund the millimeter array for design and development work, thus signaling that they were serious about contemplating this for real construction," he says. "Unless one of the par ties steps up and throws some real cash at SKA, the conversation about it could go on for a long time." Ekers smiles gently when he hears such comments. "That's a typically U.S. view," he sa ys. "There are other models to follow. We have always looked at this decade as time for research and development and next decade as the funding one. We know that ALMA will tak e manpowe r and resources from the U.S. and Europe, and that gives us time to build prototypes." Individual countries in the for mal SKA consor tium are each kicking in about $500,000 to $2 million per year for R&D within their borders. Apart from the private ATA, the U.S. is at the low end. NSF recently granted the U.S. SKA consortium $1.5 million for 3 years via a grant to Cornell University, less than one-third of its request. Last year, a national panel of astronomers recommended that SKA receive $22 million in total funding for technology development in this decade. "A half-million dollars per year is what we can do for now," says G. Wayne Van Citters, director of the Division of Astronomical Sciences at NSF. "We hope to ramp it up as the decade goes on." Design ideas may differ, but SKA enthusiasts tend to agree on one thing: The flow of the money stream, whether trickle or torrent, simply will alter the year in which SKA f irst scans the heavens. "I view these developments as inevitable," says ASTRON's Butcher. "If they don't happen in my generation, then my generation has failed."
­ROBERT IRION

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