The square kilometre array: spread across two continents, the SKA will be the largest astronomical facility ever built.
Still, there's ample sign of human activity. Silhouetted against the colorful clouds are the dishes of the South African MeerKAT radio telescope array, which is still very much under construction during my visit in late November 2016. "By the end of 2017, all 64 MeerKAT antennas will hopefully be in place," says my host Angus Flowers, the project's media liaison, as he drives us back to the Losberg Lodge, a former farmstead close to the observatory's central area now operated by radio astronomers.
A few kilometers away from the budding MeerKAT is a broken ring of low mountains that help to keep out unwanted radio interference from distant farms and towns. Later that night, when we are outside under the stars, enjoying our braai dinner (the popular South African version of barbecue), I imagine what the site may look like some 15 years from now. If you could fly up, high above the flat-topped mesas, you'd be looking down at the largest collection of radio dishes ever built: the South African part of the Square Kilometre Array (SKA). Consisting of 2,000 antennas, it will spread out over much of the Karoo, way beyond the horizon, and even into other African countries.
And that's only part of the story. While the African array (called SKA-mid) will focus on mid-frequency radio waves, its Australian counterpart (SKA-low), made up of more than a million simple antennas, will study the low-frequency radio universe. So far, only the first phase of the ambitious, two-continent project (SKA1) has been funded. SKA1, cost-capped at 650 million euros (nearly $700 million U.S.), will consist of 200 dishes in South Africa and many tens of thousands of antennas in Australia. The hope is to complete the second phase (SKA2) in 2030, with a whopping tenfold increase in observing power.
The SKA is an unprecedented multi-phase, multi-wavelength, multi-continent endeavor. "It never ceases to impress me," says Flowers.
Radio astronomy is a young discipline. After Karl Jansky's 1933 discovery of radio waves from the Milky Way, it took until the late 1950s before giant dishes were erected, like the venerable 76-meter Lovell Telescope at the Jodrell Bank Observatory in northern England. But only with the advent of the techniques called radio interferometry and aperture synthesis in the 1960s did astronomers succeed in obtaining detailed "images" of the radio sky. These are the techniques behind famous facilities like the Westerbork Synthesis Radio Telescope in the Netherlands, the Karl G. Jansky Very Large Array (VLA) in New Mexico, and the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile (S&T: Nov. 2013, p. 22).
As most amateur astronomers know, a telescope's aperture determines both the instrument's light-gathering power ("sensitivity") and its angular resolution. But angular resolution also depends on wavelength. While a 4-inch optical telescope provides a resolution of approximately one arc-second, you would need a 50-kilometer dish to "see" the same amount of detail at a radio wavelength of 21 centimeters, equivalent to a frequency of 1420 megahertz.
However, an array of smaller dishes spread out over an area 50 kilometers across works almost as well. By combining the signals detected by the individual antennas (the process called radio interferometry), you can synthesize a virtual telescope (aperture synthesis) that's as large as the distance between the dishes, albeit with a much lower sensitivity than a single huge instrument would have.
MeerKAT, with its 13.5-meter dishes, uses this same technique. More than five years ago, South African radio astronomers and engineers completed a test facility, known as KAT-7 (Karoo Array Telescope, with seven 12-meter dishes). The main goal was to demonstrate the country's ability to design, build, and operate such high-tech instruments, in support of South Africa's bid to eventually host the Square Kilometre Array. MeerKAT is now one of the four official SKA precursor telescopes. The observatory's first-light image, obtained with only 16 operational dishes, was released in July 2016. It shows more than 1,300 remote galaxies--most of them never observed before--in an area of sky measuring approximately 2[degrees] on a side.
The preliminary results bode well for the completed MeerKAT array, but even more so for SKA1-mid--the mid-frequency part of the SKA's first phase. Construction will start in 2018, using Chinese-built antennas to expand the existing array. SKA1-mid will provide a maximum baseline of 150 km and a total collecting area of some 33,000 [m.sup.2], equivalent to 126 tennis courts. It will have four times higher resolution and five times higher sensitivity than the VLA.
Prospects and Challenges
With its huge frequency range (50 to 350 megahertz for SKA-low, and 350 megahertz to 14 gigahertz for SKA-mid), the first phase of the Square Kilometre Array is already going to be an incredibly versatile instrument. According to British radio astronomer Phil Diamond, Director General of the SKA Organisation, the project is similar in scale to other big observatories such as ALMA, the James Webb Space Telescope, and the European Extremely Large Telescope, and will contribute to a vast number of science topics. "It will cover almost everything, from fundamental physics to extraterrestrial life," he says, adding, "Yes, SETI is in the science case."
The SKA will reveal the cosmic distribution of hydrogen gas throughout space and time, able to reach back to the universe's first 100 million years. This distribution will shed light on the evolution of galaxies and the history of star formation, since both of these processes use hydrogen as a building block--and, when enough radiation is involved, can ionize its atoms on large scales, transforming the hydrogen landscape (see page 30). Studying cosmic evolution and the growth of the universe's large-scale structure should also provide more information on the role and nature of dark matter and dark energy.
On a less cosmic scale, the SKA is sensitive enough to detect the faint radio waves that are emitted by rotating carbon-bearing molecules, both in large molecular clouds and in protoplanetary disks, giving insight into prebiotic chemistry. Its data will enable astronomers to map intergalactic magnetic fields by measuring the fields' effects on the polarization of radio waves, or by observing synchrotron radiation from electrons spiraling around magnetic field lines.
As for fundamental physics, SKA1 will be a superb pulsar observatory. By meticulously measuring the pulse arrival times of dozens of millisecond pulsars all over the sky for many years on end, astronomers hope to finally detect extremely low-frequency gravitational waves from binary supermassive black holes in remote galaxies. Just like the recent detections of higher-frequency spacetime ripples from merging black holes (S&T: May 2016, p. 10), such observations would provide stringent tests of Einstein's theory of general relativity and might lead the way to a successful description of quantum gravity. The gravitational waves will (hopefully) appear in SKA1-mid's data, while SKA1-low's observations will provide a baseline to clean out noise.
Building the SKA is not going to be an easy catch, though. The Great Karoo is a desert-like, almost uninhabited area. Tens of kilometers of gravel roads and tracks had to be sealed to allow easier access to the observatory from Carnarvon. Nearby farmers need to be convinced that MeerKAT (and SKA after it) is not producing harmful radiation, and conversely that they need to keep sources of radio interference to an absolute minimum. The local population turns out to be very suspicious of the ever-growing facility, with some worried it will devolve into a land grab and destroy the agricultural economy.
Then there's the technical challenge. Building the dishes, pedestals, and sensitive receivers is one thing, but hooking every single antenna up to a central supercomputer through optical fiber connections is another. SKA1-mid's total raw data output amounts to 2 terabytes per second, or over 60 exabytes (60 x 1018 bytes) per year--more than 5% of the total internet data traffic in 2016. This all has to be processed in real time by dedicated correlators and data processors to produce the final high-resolution radio images that astronomers are after. The required number-crunching power is on the order of 350 petaflops--350 thousand trillion calculations (or floating point operations, hence "flop") per second; the expected yearly output of archived science data products would fill 7 billion DVDs. As Australian astronomical computing expert Andreas Wicenec (University of Western Australia) says, "The deluge continues."
I met Wicenec in June 2016 in Perth, Western Australia, where he's involved in the Australia SKA Pathfinder (ASKAP). Nearing completion at the Murchison Radio-astronomy Observatory, some 800 kilometers north of Perth, ASKAP is another SKA precursor telescope. It's a six-kilometer wide, 36-antenna array. The 12-meter dishes are each equipped with phased array feeds, receivers capable of detecting multiple radio beams simultaneously, providing the observatory with an unprecedented 30-square-degree field of view--150 times the apparent area of the full Moon in the sky.
According to Wicenec, ASKAP produces some 250 terabytes per day of raw data, more than 15 times the nightly data forecast for the upcoming Large Synoptic Survey Telescope (S&T: Sept. 2016, p. 14). A dedicated supercomputer called Galaxy at the Pawsey Supercomputing Centre in Perth processes the flood. At present, it's the fastest radio observatory in the world in terms of survey speed. Astronomers expect ASKAP to eventually map some 70 million radio sources--a thirtyfold increase over the current number.
Like MeerKAT, the project was largely developed to support an SKA bid--in the first decade of this century, both South Africa and Australia were hoping to host the future array, back when the plan was to have it in only one place. But contrary to MeerKAT, ASKAP will not become part of the SKA: SKA-low will be at the same site, but it will use a completely different kind of antenna. Instead, says CSIRO's Astronomy and Space Science director Douglas Bock, ASKAP will be a great observatory on its own. Bock expects the array to be completed in 2018.
In May 2012, after a lot of political tugs of war, it was finally decided to build the Square Kilometre Array on both continents. "That wasn't necessarily the cheapest option," admits SKA director Diamond, "but it was a nice way to exploit the best qualities of both sites."
The original goal was to expand ASKAP to a 96-dish array. With its huge field of view, it would have become the survey part of the Square Kilometre Array--a full-fledged third observatory next to SKA-mid and SKA-low. But two years ago that plan was scrubbed--at least for the foreseeable future --because of budgetary issues. Instead, the first phase of the Australian part of the SKA will just contain about 130,000 simple dipole antennas, grouped in stations of 256 each. The relatively cheap, mass-produced antennas will look a bit like small, slender Christmas trees and stand almost two meters tall. A few years from now, the Australian outback will be decorated with some 500 patches of metal forest.
Spiders and Christmas Trees
The technique of using huge numbers of dipole antennas to create maps of the low-frequency radio sky has been pioneered over the past decade by the Low Frequency Array (LOFAR) in the Netherlands, which was inaugurated in the summer of 2010. The concept is pretty straightforward. LOFAR consists of some 50 individual "stations," each about 100 meters across and containing dozens of dipole antennas. Radio waves from a source at the zenith, right above your head, will arrive at all the antennas in a particular station at exactly the same time. But for every other position in the sky, there will be tiny differences in the wavefront's arrival time. By combining the signals from individual antennas according to the specific pattern of time-of-arrival differences that corresponds to a particular sky position, a LOFAR-like array can be virtually "pointed" in any direction--a process called beam forming.
In fact, smart computer processing allows LOFAR's dipoles to "look" in eight different directions at once. To further enhance resolution, astronomers then combine observations from the individual stations interferometrically.
The required computer processing power for this technique is huge: SKA1-low will produce an incredible 157 terabytes of raw data per second, or five times the estimated global internet traffic in 2016. Because of the limited capacity of optical fibers, part of the data reduction will be done by smaller processors at the actual antenna stations.
The Murchison Radio-astronomy Observatory feels even more remote than MeerKAT in South Africa: Murchison Shire spreads about 110 inhabitants across an area larger than the Netherlands. A small propeller aircraft takes me from Perth to Boolardy Station, a former sheep farm east of the settlement. It's a spectacular 90-minute flight over an orange-red semi-desert covered with low, shrubby vegetation. From Boolardy, it's another 30 minutes by 4WD truck to the actual observatory. Eagles soar high in the sky; a young kangaroo hops across the bumpy gravel road. In the harsh sunlight, the white dishes of ASKAP are almost too bright to look at. Technicians are using small mobile cranes to install new phased array receivers.
Not far from the core region of ASKAP is the Murchison Widefield Array (MWA), the third SKA precursor instrument. More or less similar to LOFAR, it consists of 128 "tiles" of 16 small, spider-like antennas each. By the time you read this, the number of tiles will probably have grown to 256--they're really cheap and easy to deploy. For SKA1-low, the antennas will have a different design, and the tiles, or stations, will be bigger and more numerous. Eventually, a total of 512 stations will be spread out over an area 65 kilometers across. As each station will hold 256 antennas, the total number of "Christmas trees" is 131,072. In the proposed second phase of the Square Kilometre Array, the plan is to increase the number of antennas to well over a million, and to have outlier stations all over the Australian continent and even in New Zealand.
With 25% better resolution, eight times higher sensitivity, and 135 times faster survey speed than LOFAR, SKA1-low is poised to finally solve the riddle of cosmic dawn: when and how the cosmic dark ages came to an end, and what kind of objects--massive stars or fledgling quasars--were the first to make the universe shine. Similar observations will be carried out in South Africa's Great Karoo by the HERA telescope (Hydrogen Epoch of Reionization Array), which is the fourth and final SKA precursor instrument. Next year, HERA will consist of some 350 simple stationary wire-mesh dishes, each 14 meters across and sitting shoulder-to-shoulder to catch low-frequency radio waves from the zenith.
By the end of this year, the detailed design and the construction proposal for the first phase of the Square Kilometre Array will be completed. Diamond expects construction to start next year. In fact, the first 12-meter antenna has already arrived on site in South Africa.
"We'll have early science results in 2021, and SKA1 will be fully operational in 2024," Diamond says. "It's a huge challenge. We're developing and constructing something no one has ever built before." Asked about the prospects for the second phase of the project, the SKA director admits that none of the 12 member countries has committed themselves to building SKA2 yet. "There's always the risk of cancellation, but it doesn't particularly keep me awake at night," he says.
Nor me. What does keep me awake, both in the South African Karoo and in the Australian outback, is the unbelievable view of the star-studded night sky--a universe filled with wonder. Ten years from now, many outstanding cosmic mysteries may have been solved by the sheer power of humankind's largest and most sensitive astronomical observatory ever. And there's little doubt that the Square Kilometre Array --even if only phase 1 will ever be realized--is going to present us with a lot of scientific surprises, too. Says Diamond: "When the Hooker telescope on Mount Wilson started operations in 1917, no one expected the discovery of the expansion of the universe. I have really no idea what the SKA will be famous for a century from now."
* S&T Contributing Editor GOVERT SCHILLING lives in the Netherlands but enjoys visiting astronomical facilities all over the world. In August 2017 Harvard University Press will publish his new book, Ripples in Spacetime: Einstein, Gravitational Waves, and the Future of Astronomy.
The Epoch of Reionization
MILESTONES IN COSMIC HISTORY After the Big Bang, the universe was filled with a soup of photons and subatomic particles. After 370,000 years, it cooled enough for atoms to form, mostly neutral hydrogen. (This is also when the light of the cosmic microwave background was released to fly freely through the universe.) The universe remained in a neutral state until light from the first stars and galaxies started to ionize their surroundings in the Epoch of Reionization. After several hundred million years, the gas in the universe was completely ionized.
AFTER THE VIOLENCE of the Big Bang fully subsided, some 370,000 years later, the universe was filled with cool, neutral hydrogen gas. But those cosmic dark ages came to an end when the energetic radiation of the first stars and quasars started to ionize their surroundings. Over time, these growing bubbles of hot gas became ever more numerous. Eventually, they started to overlap each other and, in the end, intergalactic space became ionized.
Astronomers can learn a lot about the early evolution of the universe by studying this Epoch of Reionization (it's called re-ionization because the gas started out in an ionized state right after the Big Bang). Neutral hydrogen atoms naturally emit at a radio wavelength of 21.1 cm (a frequency of 1420.4 MHz), but the radiation from these very early epochs has been redshifted by the expansion of the universe to wavelengths of a couple of meters. This range falls in the low-frequency regime. By mapping the sky at various wavelengths (corresponding to various redshifts and look-back times), cosmologists will be able to reconstruct the details of how neutral hydrogen disappeared from the scene.
WHAT SQUARE KILOMETER?
The Square Kilometre Array (British spelling) is named after the planned total collecting area of its second phase (SKA2). However, the first phase of the project (SKA1) will be much smaller. SKA1-mid, consisting of a couple hundred dishes in South Africa, will end up with a total collecting area of some 33,000 [m.sup.2] (0.033 [km.sup.2]); SKA1-low, in Australia, will have a collecting area of 0.4 [km.sup.2], basically due to the large concentration of antenna fields in the core region of the array (so the collecting area won't grow by a factor of ten when SKA2 is realized). Combined, SKA1 will have a collecting area of less than half a square kilometer. For comparison: the recently completed Chinese Five-hundred-meter Aperture Spherical Radio Telescope (FAST; S&T: Feb. 2017, p. 26), the largest single-dish instrument in the world, has a collecting area of 0.2 [km.sup.2].
Caption: SUNSET OVER THE KAROO MeerKAT dishes stand sentinel at dusk in South Africa's growing radio observatory.
Caption: HOW INTERFEROMETRY WORKS (Above) When two radio antennas observe a source that's not at the zenith, the signal travels slightly different distances to reach each of them (here, the path to Dish A is longer than to Dish B). A supercomputer correlator then compares the signals and determines how much of a shift is necessary to make the signals constructively interfere. That shift corresponds to the time delay, and thus the path length difference and angle to the source. Combining this information from many pairs of antennas improves the position's accuracy.
Caption: FIRST LIGHT Each bright dot in the image at far left represents a distant galaxy, detected with the first 16 dishes of the MeerKAT array in 2016. The close-up (near left) zooms in on a few of these galaxies, revealing radio-bright outflows powered by the galaxies' supermassive black holes.
Caption: ON THE MAP Shown are the preliminary locations for SKA1 -mid and SKA1 -low. Each South African icon represents a single dish; each Australian icon represents a station of 256 individual antennas. The insets give a slightly clearer picture, although the sheer scale is still hard to grasp. Designers chose the spiral configuration because it provides a variety of baseline distances and angles between antennas, enabling very high-resolution imaging with interferometry. (The best layout would be random, but construction considerations make that undesirable.)
Caption: A RADIO SPIDERS About knee height, the antennas of the Murchison Widefield Array stand together in "tiles" of 16 antennas. They cover a similar frequency range as SKA1-low will but use a different antenna design.
Caption: A HERA One of four SKA precursor projects, HERA detects radio signals from neutral hydrogen, studying the large-scale cosmic structure that existed before and during the Epoch of Reionization.
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|Title Annotation:||RADIO REVOLUTION|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2017|
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