4.03.2025
When it starts taking scientific data in 2028, the Square Kilometre Array Observatory promises to be the world’s largest and most sensitive radio telescope. But as Sarah Wilddiscovers, its development has been shaped by a suite of smaller experiments that are already resetting the agenda – and flagging problems that the giant telescope could face.
From its sites in South Africa and Australia, the Square Kilometre Array (SKA) Observatory last year achieved “first light” – producing its first-ever images. When its planned 197 dishes and 131,072 antennas are fully operational, the SKA will be the largest and most sensitive radio telescope in the world.
Under the umbrella of a single observatory, the telescopes at the two sites will work together to survey the cosmos. The Australian side, known as SKA-Low, will focus on low-frequencies, while South Africa’s SKA-Mid will observe middle-range frequencies. The £1bn telescopes, which are projected to begin making science observations in 2028, were built to shed light on some of the most intractable problems in astronomy, such as how galaxies form, the nature of dark matter, and whether life exists on other planets.
Three decades in the making, the SKA will stand on the shoulders of many smaller experiments and telescopes – a suite of so-called “precursors” and “pathfinders” that have trialled new technologies and shaped the instrument’s trajectory. The 15 pathfinder experiments dotted around the planet are exploring different aspects of SKA science.
Meanwhile on the SKA sites in Australia and South Africa, there are four precursor telescopes – MeerKAT and HERA in South Africa and Australian SKA Pathfinder (ASKAP) and Murchison Widefield Array (MWA) in Australia. These precursors are weathering the arid local conditions and are already broadening scientists’ understanding of the universe.
“The SKA was the big, ambitious end game that was going to take decades,” says Steven Tingay, director of the MWA based in Bentley, Australia. “Underneath that umbrella, a huge number of already fantastic things have been done with the precursors, and they’ve all been investments that have been motivated by the path to the SKA.”
Even as technology and science testbeds, “they have far surpassed what anyone reasonably expected of them”, adds Emma Chapman, a radio astronomer at the University of Nottingham, UK.
MeerKAT: glimpsing the heart of the Milky Way
In 2018, radio astronomers in South Africa were scrambling to pull together an image for the inauguration of the 64-dish MeerKAT radio telescope. MeerKAT will eventually form the heart of SKA-Mid, picking up frequencies between 350 megahertz and 15.4 gigahertz, and the researchers wanted to show what it was capable of.
Like all the SKA precursors, MeerKAT is an interferometer, with many dishes acting like a single giant instrument. MeerKAT’s dishes stand about three storeys high, with a diameter of 13.5 m, and the largest distance between dishes being about 8 km. This is part of what gives the interferometer its sensitivity: large baselines between dishes increase the telescope’s angular resolution and thus its sensitivity.
Additional dishes will be integrated into the interferometer to form SKA-Mid. The new dishes will be larger (with diameters of 15 m) and further apart (with baselines of up to 150 km), making it much more sensitive than MeerKAT on its own. Nevertheless, using just the provisional data from MeerKAT, the researchers were able to mark the unveiling of the telescope with the clearest radio image yet of our galactic centre.
Now, we finally see the big picture – a panoramic view filled with an abundance of filaments…. This is a watershed in furthering our understanding of these structures
Farhad Yusef-Zadeh
Four years later, an international team used the MeerKAT data to produce an even more detailed image of the centre of the Milky Way (ApJL 949 L31). The image (above) shows long radio-emitting filaments up to 150 light–years long unspooling from the heart of the galaxy. These structures, whose origin remains unknown, were first observed in 1984, but the new image revealed 10 times more than had ever been seen before.
“We have studied individual filaments for a long time with a myopic view,” Farhad Yusef-Zadeh, an astronomer at Northwestern University in the US and an author on the image paper, said at the time. “Now, we finally see the big picture – a panoramic view filled with an abundance of filaments. This is a watershed in furthering our understanding of these structures.”
The image resembles a “glorious artwork, conveying how bright black holes are in radio waves, but with the busyness of the galaxy going on around it”, says Chapman. “Runaway pulsars, supernovae remnant bubbles, magnetic field lines – it has it all.”
In a different area of astronomy, MeerKAT “has been a surprising new contender in the field of pulsar timing”, says Natasha Hurley-Walker, an astronomer at the Curtin University node of the International Centre for Radio Astronomy Research in Bentley. Pulsars are rotating neutron stars that produce periodic pulses of radiation hundreds of times a second. MeerKAT’s sensitivity, combined with its precise time-stamping, allows it to accurately map these powerful radio sources.
An experiment called the MeerKAT Pulsar Timing Array has been observing a group of 80 pulsars once a fortnight since 2019 and is using them as “cosmic clocks” to create a map of gravitational-wave sources. “If we see pulsars in the same direction in the sky lose time in a connected way, we start suspecting that it is not the pulsars that are acting funny but rather a gravitational wave background that has interfered,” says Marisa Geyer, an astronomer at the University of Cape Town and a co-author on several papers about the array published last year.
HERA: the first stars and galaxies
When astronomers dreamed up the idea for the SKA about 30 years ago, they wanted an instrument that could not only capture a wide view of the universe but was also sensitive enough to look far back in time. In the first billion years after the Big Bang, the universe cooled enough for hydrogen and helium to form, eventually clumping into stars and galaxies.
When these early stars began to shine, their light stripped electrons from the primordial hydrogen that still populated most of the cosmos – a period of cosmic history known as the Epoch of Reionization. The re-ionised hydrogen gave off a faint signal and catching glimpses of this ancient radiation remains one of the major science goals of the SKA.
Developing methods to identify primordial hydrogen signals will be the Hydrogen Epoch of Reionization Array (HERA) – a collection of hundreds of 14 m dishes, packed closely together as they watch the sky, like bowls made of wire mesh (see image below). They have been specifically designed to observe fluctuations in primordial hydrogen in the low-frequency range of 100 MHz to 200 MHz.
Understanding this mysterious epoch sheds light on how young cosmic objects influenced the formation of larger ones and later seeded other objects in the universe. Scientists using HERA data have already reported the most sensitive power limits on the reionization signal (ApJ 945 124), bringing us closer to pinning down what the early universe looked like and how it evolved, and will eventually guide SKA observations. “It always helps to be able to target things better before you begin to build and operate a telescope,” explains HERA project manager David de Boer, an astronomer at the University of California, Berkeley in the US.
MWA: “unexpected” new objects
Over in Australia, meanwhile, the MWA’s 4096 antennas crouch on the red desert sand like spiders (see image below). This interferometer has a particularly wide-field view because, unlike its mid-frequency precursor cousins, it has no moving parts, allowing it to view large parts of the sky at the same time. Each antenna also contains a low-noise amplifier in its centre, boosting the relatively weak low-frequency signals from space. “In a single observation, you cover an enormous fraction of the sky”, says Tingay. “That’s when you can start to pick up rare events and rare objects.”
Hurley-Walker and colleagues discovered one such object a few years ago – repeated, powerful blasts of radio waves that occurred every 18 minutes and lasted about a minute. These signals were an example of a “radio transient” – an astrophysical phenomena that last for milliseconds to years, and may repeat or occur just once. Radio transients have been attributed to many sources including pulsars, but the period of this event was much longer than had ever been observed before.
New transients are challenging our current models of stellar evolution
Cathryn Trott, Curtin Institute of Radio Astronomy in Bentley, Australia
After the researchers first noticed this signal, they followed up with other telescopes and searched archival data from other observatories going back 30 years to confirm the peculiar time scale. “This has spurred observers around the world to look through their archival data in a new way, and now many new similar sources are being discovered,” Hurley-Walker says.
The discovery of new transients, including this one, are “challenging our current models of stellar evolution”, according to Cathryn Trott, a radio astronomer at the Curtin Institute of Radio Astronomy in Bentley, Australia. “No one knows what they are, how they are powered, how they generate radio waves, or even whether they are all the same type of object,” she adds.
This is something that the SKA – both SKA-Mid and SKA-Low – will investigate. The Australian SKA-Low antennas detect frequencies between 50 MHz and 350 MHz. They build on some of the techniques trialled by the MWA, such as the efficacy of using low-frequency antennas and how to combine their received signals into a digital beam. SKA-Low, with its similarly wide field of view, will offer a powerful new perspective on this developing area of astronomy.
ASKAP: giant sky surveys
The 36-dish ASKAP saw first light in 2012, the same year it was decided to split the SKA between Australia and South Africa. ASKAP was part of Australia’s efforts to prove that it could host the massive telescope, but it has since become an important instrument in its own right. These dishes use a technology called a phased array feed which allows the telescope to view different parts of the sky simultaneously.
Each dish contains one of these phased array feeds, which consists of 188 receivers arranged like a chessboard. With this technology, ASKAP can produce 36 concurrent beams looking at 30 degrees of sky. This means it has a wide field of view, says de Boer, who was ASKAP’s inaugural director in 2010. In its first large-area survey, published in 2020, astronomers stitched together 903 images and identified more than 3 million sources of radio emissions in the southern sky, many of which were new (PASA 37 e048).
Because it can quickly survey large areas of the sky, the telescope has shown itself to be particularly adept at identifying and studying new fast radio bursts (FRBs). Discovered in 2007, FRBs are another kind of radio transient. They have been observed in many galaxies, and though some have been observed to repeat, most are detected only once.
This work is also helping scientists to understand one of the universe’s biggest mysteries. For decades, researchers have puzzled over the fact that the detectable mass of the universe is about half the mass that we know existed after the Big Bang. The dispersion of FRBs by this “missing matter” allows us to weigh all of the normal matter between us and the distant galaxies hosting the FRB.
By combing through ASKAP data, researchers in 2020 also discovered a new class of radio sources, which they dubbed “odd radio circles” (PASA 38 e003). These are giant rings of radiation that are observed only in radio waves. Five years later their origins remain a mystery, but some scientists maintain they are flashes from ancient star formation.
The precursors are so important. They’ve given us new questions. And it’s incredibly exciting
Philippa Hartley, SKAO, Manchester
While SKA has many concrete goals, it is these unexpected discoveries that Philippa Hartley, a scientist at the SKAO, based near Manchester, is most excited about. “We’ve got so many huge questions that we’re going to use the SKA to try and answer, but then you switch on these new telescopes, you’re like, ‘Whoa! We didn’t expect that.’” That is why the precursors are so important. “They’ve given us new questions. And it’s incredibly exciting,” she adds.
Trouble on the horizon
As well as pushing the boundaries of astronomy and shaping the design of the SKA, the precursors have made a discovery much closer to home – one that could be a significant issue for the telescope. In a development that SKA’s founders will not have foreseen, the race to fill the skies with constellations of satellites is a problem both for the precursors and also for SKA itself.
Large corporations, including SpaceX in Hawthorne, California, OneWeb in London, UK, and Amazon’s Project Kuiper in Seattle, Washington, have launched more than 6000 communications satellites into space. Many others are also planned, including more than 12,000 from the Shanghai Spacecom Satellite Technology’s G60 Starlink based in Shanghai. These satellites, as well as global positioning satellites, are “photobombing” astronomy observatories and affecting observations across the electromagnetic spectrum.
ASKAP, MeerKAT and the MWA have all flagged the impact of satellites on their observations. “The likelihood of a beam of a satellite being within the beam of our telescopes is vanishingly small and is easily avoided,” says Robert Braun, SKAO director of science. However, because they are everywhere, these satellites still introduce background radio interference that contaminates observations, he says.
In 2022, the International Astronomical Union (IAU) launched its Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. The SKA Observatory and the US National Science Foundation’s centre for ground-based optical astronomy NOIRLab co-host the facility, which aims to reduce the impact of these satellite constellations.
Although the SKA Observatory is engaging with individual companies to devise engineering solutions, “we really can’t be in a situation where we have bespoke solutions with all of these companies”, SKAO director-general Phil Diamond told a side event at the IAU general assembly in Cape Town last year. “That’s why we’re pursuing the regulatory and policy approach so that there are systems in place,” he said. “At the moment, it’s a bit like the wild, wild west and we do need a sheriff to stride into town to help put that required protection in place.”
In this, too, SKA precursors are charting a path forward, identifying ways to observe even with mega satellite constellations staring down at them. When the full SKA telescopes finally come online in 2028, the discoveries it makes will, in large part, be thanks to the telescopes that came before it.
Quelle: physicsworld