Dec 05 2022

Square Kilometer Array

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Construction begins this week on what will be the largest radio telescope in the world – the Square Kilometer Array (SKA). This project began more than 30 years ago, in 1991, as an idea, with an international working group forming in 1993. It took three decades to flesh out the concept, create a detailed design, secure the land rights, and secure government funding. The first antennas will go online by 2024 with more added through 2028 (which will complete the first phase – about 10% of the total planned project). This will result in a radio telescope array with a total area of one square kilometer.

There are actually two components to the total array. One is being built in Australia, the SKA-Low, for low frequency. These will use antennas that look like two-meter tall metal Christmas trees. There will be 500 arrays of 256 antennas for a total of 131,000 antennas. This will be the low frequency array, able to detect radio waves between 50 megahertz and 350 megahertz. There will also be SKA-Mid in South Africa, which will be an array of 197 dishes sensitive between 350 megahertz and 15.4 gigahertz. The whole thing will be connected together, with the bulk of the computing power located in the UK.

Why do astronomers connect radio receivers together? This has to do with interferometry – the ability to combine two signals so that they can simulate a single receiver with a diameter equal to the distance between the two receivers.  It’s not the same as having one giant dish, however. An array increases the resolution of the received image, but the sensitivity is still a function of the total receiving area (not the distance). The Very Large Array (VLA) in New Mexico has radio dishes on rails, so that they can be moved into different configuration. By moving the dishes apart you can achieve greater resolution, but by moving them closer together you get greater precision – so there is a trade-off from moving receivers farther apart. There is no substitute for total collecting area, which is why the SKA will have so many individual receivers.

The SKA is being built on land that is already used for radio astronomy. The advantage to this is because radio astronomy requires radio silence. The area cannot have any technology emitting radio waves. You cannot even use your cell phone in the area. The one disadvantage is that during construction the work will interfere with the radio antenna already in use, but that’s a small price to pay.

What are we going to use the largest radio telescope array in the world for? The SKA will be able to look in a wide range of radio frequencies with greater resolution and sensitivity than any other existing instrument, about 50 times more sensitive than the next largest. In astronomy more sensitive and higher resolution instruments means we can peer farther into the universe, which also means farther back in time, and receive higher resolution images. The SKA will allow us to take a look, therefore, at the early universe with greater precision than before.

Part of the reason the SKA was placed in the Southern Hemisphere is because it has a better view of the Milky Way (our own galaxy). So we will be able to make measurements in the radio spectrum of the Milky Way in greater detail. We can also use it to image other galaxies, even distant ones. Other projects including imaging neutral hydrogen in the early universe, imaging black holes, pulsars, and cosmic magnetism.

Interestingly, one of the items on the short list for the SKA is fast radio bursts – as the name implies, these are short duration bursts of radio frequency energy, as much energy as our sun puts out in a year over just milliseconds. The first FRB was discovered in 2007, after the SKA was conceived and designed.

But what about SETI – the search for extraterrestrial intelligence? The earliest SETI projects used radio telescopes or arrays dedicated to the project. This proved unsustainable financially. Current SETI projects, learning from this experience, also do a lot of non-SETI radio astronomy. This makes sense in terms of efficiently using limited resources, and the time and careers of the radio-astronomers. They can still do good astronomy and get papers published, even if we go decades without detecting ET signals. The SKA will do the same thing, although instead of piggybacking other radio astronomy projects onto a primary SETI mission, the SKA will piggy back some SETI observations onto a schedule mostly dedicated to other radio astronomy projects.

The idea of SETI is to look for radio frequency signals where a terrestrial origin has been ruled out and a natural origin has also been ruled out. We are therefore left with an extraterrestrial, non-natural origin of the signals, with the assumption being these would likely originate from a technological source. Ruling out a natural origin, of course, is very difficult. So far every time we have detected anything in the universe with no known natural origin it has turned out to be of a previously unknown natural origin. This is why SETI meshes so well with regular radio astronomy – essentially SETI is looking for anomalous signals, ones we can’t explain, and then sets out to explain them. Whether or not we can come up with a natural explanation, it’s a win-win. Either we learn something new about the universe, or we have a really good candidate signal for ET technological origin.

Of course, not every radio astronomy observation fits this pattern. Often we are looking to gather more data about known phenomena, to understand them better. But any observation that has the potential to see something new, can still function to advance SETI.

With the SKA we will be able to detect fainter signals. It’s likely that ET radio signals will be extremely faint. It takes a lot of energy to produce radio signals that can be detected light years away – especially if you are broadcasting in every direction. SETI researcher use a lot of techniques to help narrow a bit what is otherwise a massive search. They think about which radio frequencies ET might use – such as relative clear windows that would have less interference. They also consider if any ET civilization would have a reason to send a focused beam aimed at us – perhaps, for example, from their perspective the Earth transits the sun, which means they could see oxygen in our atmosphere.

Regardless, the more sensitive our radio astronomy that larger our viewing bubble in which we may be able to detect a faint ET signal of technological origin. This significantly increases the volume of space we can explore, and the statistical probability of finding new anomalies – whether or ET or natural origin.

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