Radio interference wreaks havoc with telescopes
Technology makes life easier for most – but not so much for radio astronomers. Belinda Smith explains.
Short mysterious bursts of radiation picked up by the Parkes radio telescope in New South Wales have baffled astronomers for years. But it turns out some, at least, were not intergalactic messages or a new kind of star. Wayward radiation from a microwave oven in a nearby building, released when the door was opened before the countdown finished, was picked up when the dish pointed in a specific direction. Emily Petroff from Melbourne’s Swinburne University and colleagues uploaded their discovery to the online archive arXiv in April.
Radio astronomy telescopes are big expensive pieces of equipment. Who knew a humble microwave oven could cause them so much trouble? It turns out these manmade interruptions are the bane of radio astronomers everywhere – and microwave ovens are only a tiny part of the problem.
Like sunshine and X-rays, radio waves are a form of electromagnetic radiation. But whereas X-rays have a wavelength so short they can interact with individual atoms that make up the bones in your body, radio waves have the longest wavelengths of any electromagnetic radiation. Some are as long as 100 kilometres. They can travel huge distances and skirt around objects such as buildings, making them ideal for radio and television broadcasts, radar and satellite communications. Radio waves around a centimetre long are called microwaves – such as those that reheat your leftovers.
Out in space there are plenty of sources of radio waves, such as vibrating molecules and electrons whizzing around magnetic fields. Some stars, like constantly exploding hydrogen bombs, churn through fuel and spew electromagnetic energy into space. And radio astronomers pick up those faint, ancient ripples of radiation from billions of light-years away.
How do radio astronomers make sure they’re listening to what’s coming from space, and not the kitchen down the road?
Even in 1960, radio telescopes could pick up 14-billion-year-old echoes of the Big Bang. That year American astronomers Arno Penzias and Robert Wilson pointed the Holmdel horn antenna in New Jersey into the sky and detected low-level microwave radiation from all directions. Their accidental discovery of cosmic background radiation – a key piece of evidence for the Big Bang theory – snared them the 1978 Nobel Prize for Physics.
Today’s generation of radio telescopes are much more sensitive and can easily be swamped by the cacophony of radio waves generated on Earth. So how do radio astronomers make sure they’re listening to what’s coming from space, and not the kitchen down the road?
International agreements have set aside a handful of frequencies for radio astronomers to observe. Hydrogen gas accounts for around 90% of the atoms in the Universe and provides star fuel. So the radio frequency generated by hydrogen – around 1,420 megahertz – is protected, meaning no one on Earth can generate waves in that part of the spectrum.
But most protective measures are local. As optical telescopes are built high on mountain plateaus, away from the glow of city lights, so radio telescopes are built in valleys, shielded by mountains. And just as dark zones are imposed around optical telescopes, radio telescopes have quiet zones, where radio transmissions are all but banned.
The largest enforced quiet zone is the United States National Radio Quiet Zone, 34,000 square kilometres straddling the Virginia, West Virginia border. It was created in 1958 to protect the Green Bank and Sugar Grove telescopes. Within its boundaries, there’s one AM radio station and mobile phone towers are pointed well away from the telescopes. Emergency services only use specific radio frequencies for communications. “It makes all the difference in the world,” says Jay Lockman, principal scientist at the Green Bank telescope.
Closer to the Green Bank telescope, controls are even stricter. Digital cameras are a strict no-no. And if you drive to the telescope, you’d better arrive in a diesel vehicle. Explosions from spark plugs in petrol cars emit little bursts of – yes – radio waves.
But what about telescopes in more heavily populated areas that can’t place a blanket ban on radio transmissions?
The 26-metre Mount Pleasant telescope sits in a valley 20 kilometres east of Hobart, Tasmania. As the Hobart airport is only a few kilometres away and towns are scattered around the valley and its surrounds, it’s impossible to keep the area completely radio quiet.
But copping a blast of terrestrial radio waves can have big repercussions. The Mount Pleasant telescope is responsible for monitoring Space X capsules as they deliver supplies to the International Space Station. And in a technique called very-long-baseline interferometry, it teams up with telescopes around the world to focus on a far-flung spot to get a fix on Earth’s orientation in space. This is vital to the global positioning system – to calibrate satellites, measure changes in the Earth’s orbit and even map tectonic plate movements to the millimetre.
So when John Dickey, director of the Mount Pleasant telescope, spots strange blips in his data, he and other astronomers from the University of Tasmania drive around the area with a portable radiometer trying to track its source. He once found a lumberyard drying logs with microwaves but worked out a compromise with the business. “There’s a lot you can do with goodwill in the community,” he says.
Sometimes, though, goodwill doesn’t cut it. Astronomers at the 45-metre telescope at the Nobeyama radio observatory in Nagano, Japan, have tried to detect waves emitted by vibrating complex organic molecules – which may be related to the formation of life. Unfortunately, the radio frequency those molecules emit exactly matches the frequency used by car-mounted anti-collision radar systems.
“Anti-collision radar is quite powerful by radio astronomy standards and may even damage our equipment,” Lockman says. “It’s much stronger than the incidental radiation that leaks out of a microwave oven.”
“Of course, the car industry is super big in Japan so literally nobody’s supporting us!” adds Masao Saito, director of the Nobeyama radio observatory. “It’s a real problem.”
Radio-quiet areas can still be found in remote parts of the world. The Murchison Shire in rural Western Australia, which houses the Murchison Widefield Array, has around 110 people living in an area half the size of Tasmania. And the new gold standard for radio astronomy will be the Square Kilometre Array, to be built in remote radio quiet zones in Western Australia and South Africa.
But construction of the Square Kilometre Array isn’t billed to start until 2018. So until then, Dickey and his team of astronomers will continue driving round the Tasmanian countryside searching for stray radio waves.