Long-dead satellite emits strong radio signal, puzzling astronomers

CNN

6 hours ago

Astronomers in Australia picked up a strange radio signal in mid-June — one near our planet and so powerful that, for a moment, it outshined everything else in the sky. The ensuing search for its source has sparked new questions around the growing problem of debris in Earth's orbit.
At first, though, the researchers thought they were observing something exotic.
'We got all excited, thinking we had discovered an unknown object in the vicinity of the Earth,' said Clancy James, an associate professor at Curtin University's Curtin Institute of Radio Astronomy in Western Australia.
The data James and his colleagues were looking at came from the ASKAP radio telescope, an array of 36 dish antennas in Wajarri Yamaji Country, each about three stories tall. Normally, the team would be searching the data for a type of signal called a 'fast radio burst' — a flash of energy blasting forth from distant galaxies.
'These are incredibly powerful explosions in radio (waves) that last about a millisecond,' James said. 'We don't know what's producing them, and we're trying to find out, because they really challenge known physics — they're so bright. We're also trying to use them to study the distribution of matter in the universe.'
Astronomers believe these bursts may come from magnetars, according to James. These objects are very dense remnants of dead stars with powerful magnetic fields. 'Magnetars are utterly, utterly insane,' James said. 'They're the most extreme things you can get in the universe before something turns into a black hole.'
But the signal seemed to be coming from very close to Earth — so close that it couldn't be an astronomical object. 'We were able to work out it came from about 4,500 kilometers (2,800 miles) away. And we got a pretty exact match for this old satellite called Relay 2 — there are databases that you can look up to work out where any given satellite should be, and there were no other satellites anywhere near,' James said.
'We were all kind of disappointed at that, but we thought, 'Hang on a second. What actually produced this anyway?''
NASA launched Relay 2, an experimental communications satellite, into orbit in 1964. It was an updated version of Relay 1, which lifted off two years earlier and was used to relay signals between the US and Europe and broadcast the 1964 Summer Olympics in Tokyo.
Just three years later, with its mission concluded and both of its main instruments out of order, Relay 2 had already turned into space junk. It has since been aimlessly orbiting our planet, until James and his colleagues linked it to the weird signal they detected on June 13.
But could a dead satellite suddenly come back to life after decades of silence?
To try to answer that question, the astronomers wrote a paper on their analysis, set to publish Monday in the journal The Astrophysical Journal Letters.
They realized the source of the signal wasn't a distant galactic anomaly, but something close by, when they saw that the image rendered by the telescope — a graphical representation of the data — was blurry.
'(T)he reason we were getting this blurred image was because (the source) was in the near field of the antenna — within a few tens of thousands of kilometers,' James said. 'When you have a source that's close to the antenna, it arrives a bit later on the outer antennas, and it generates a curved wave front, as opposed to a flat one when it's really far away.'
This mismatch in the data between the different antennas caused the blur, so to remove it, the researchers eliminated the signal coming from the outer antennas to favor only the inner part of the telescope, which is spread out over about 2.3 square miles in the Australian outback.
'When we first detected it, it looked fairly weak. But when we zoomed in, it got brighter and brighter. The whole signal is about 30 nanoseconds, or 30 billionths of a second, but the main part is just about three nanoseconds, and that's actually at the limit of what our instrument can see,' James said. 'The signal was about 2,000 or 3,000 times brighter than all the other radio data our (instrument) detects — it was by far the brightest thing in the sky, by a factor of thousands.'
The researchers have two ideas on what could have caused such a powerful spark. The main culprit was likely a buildup of static electricity on the satellite's metal skin, which was suddenly released, James said.
'You start with a buildup of electrons on the surface of the spacecraft. The spacecraft starts charging up because of the buildup of electrons. And it keeps charging up until there's enough of a charge that it short-circuits some component of the spacecraft, and you get a sudden spark,' he explained. 'It's exactly the same as when you rub your feet on the carpet and you then spark your friend with your finger.'
A less likely cause is the impact of a micrometeorite, a space rock no bigger than 1 millimeter (0.039 inches) in size: 'A micrometeorite impacting a spacecraft (while) traveling at 20 kilometers per second or higher will basically turn the (resulting) debris from the impact into a plasma — an incredibly hot, dense gas,' James said. 'And this plasma can emit a short burst of radio waves.'
However, strict circumstances would need to come into play for this micrometeorite interaction to occur, suggesting there's a smaller chance it was the cause, according to the research. 'We do know that (electrostatic) discharges can actually be quite common,' James said. 'As far as humans are concerned, they're not dangerous at all. However, they absolutely can damage a spacecraft.'
Because these discharges are difficult to monitor, James believes the radio signal event shows that ground-based radio observations could reveal 'weird things happening to satellites' — and that researchers could employ a much cheaper, easier-to-build device to search for similar events, rather than the sprawling telescope they used. He also speculated that because Relay 2 was an early satellite, it might be that the materials it's made of are more prone to a buildup of static charge than modern satellites, which have been designed with this problem in mind.
But the realization that satellites can interfere with galactic observations also presents a challenge and adds to the list of threats posed by space junk. Since the dawn of the Space Age, almost 22,000 satellites have reached orbit, and a little more than half are still functioning. Over the decades, dead satellites have collided hundreds of times, creating a thick field of debris and spawning millions of tiny fragments that orbit at speeds of up to 18,000 miles per hour.
'We are trying to see basically nanosecond bursts of stuff coming at us from the universe, and if satellites can produce this as well, then we're going to have to be really careful,' James said, referring to the possibility of confusing satellite bursts with astronomical objects. 'As more and more satellites go up, that's going to make this kind of experiment more difficult.'
James and his team's analysis of this event is 'comprehensive and sensible,' according to James Cordes, Cornell University's George Feldstein Professor of Astronomy, who was not involved with the study. 'Given that the electrostatic discharge phenomenon has been known for a long time,' he wrote in an email to CNN, 'I think their interpretation is probably right. I'm not sure that the micrometeoroid idea, pitched in the paper as an alternative, is mutually exclusive. The latter could trigger the former.'
Ralph Spencer, Professor Emeritus of Radio Astronomy at the University of Manchester in the UK, who was also not involved with the work, agrees that the proposed mechanism is feasible, noting that spark discharges from GPS satellites have been detected before.
The study illustrates how astronomers must take care to not confuse radio bursts from astrophysical sources with electrostatic discharges or micrometeoroid bursts, both Cordes and Spencer pointed out.
'The results show that such narrow pulses from space may be more common than previously thought, and that careful analysis is needed to show that the radiation comes from stars and other astronomical objects rather than man-made objects close to the Earth,' Spencer added in an email.
'New experiments now in development, such as the Square Kilometre array Low frequency array (SKA-Low) being built in Australia, will be able to shed light on this new effect.'