Picking a site for a large telescope like the Square Kilometre Array is a bit like picking a host country for the Olympics. First, there's a long list of candidate sites – the SKA originally had six. Then detailed reports are compiled about each of them, taking into account their scientific, economic and environmental strengths. Eventually, the members of a committee are charged with voting one way or another.

Of the six candidate sites hoping to host the SKA, two were short-listed in 2006 – one in southern Africa, and another in Australasia. On the scientific front, what is wanted is a dry site, far away from human habitation. Water vapour absorbs radio waves – that's how a microwave oven works – and humans like carrying mobile phones around with them and living near radio transmitters, both of which generate the radio equivalent of light pollution. It's for that reason that none of the candidate sites were in Europe. Southern Africa and central Australia, on the other hand, are ideal. These are desert environments, and the lack of water means they are likely to remain unpopulated for many years to come.

Choosing between the two deserts was always going to be very tricky. Scientifically, both are ideal. In both cases, the principle source of radio interference is from aircraft and satellites; interference from ground-based sources is negligible. Logistically, both would be expensive to prepare: unsurprisingly, there are few roads and power lines in the desert, and these will need to be built from scratch. Manual labourers are cheaper to hire in South Africa, but the African Karoo desert is rockier and harder to work than the Australian sandy desert. The African site has more international development opportunities, but also more political instability. The SKA will require antennas to be spread over hundreds of kilometres, and in southern Africa, that means putting them in many different countries. One of South Africa's neighbours is Zimbabwe.

When the decision committee met in April but adjourned without decision, it looked likely that behind the scenes they were trying to form a political deal that would keep both candidate sites happy. Given the intense work that both sites have put into their bids, the large research groups that have grown up on both continents, and the pathfinder instruments that both are building, it would be a loss for either to lose momentum. And today, the Board of the SKA has confirmed that the SKA will be shared between the two sites.

Will it work? Personally I think it's the right decision. The SKA was always going to have two, and eventually three, different types of antenna working in different frequency bands. There is no very strong reason why both need to be in the same place since they share minimal hardware. And although the site preparation costs will now be much higher – two sites need preparing – much of the cost of this will be bourne by the host nations.

Moreover, having two sites will help with a looming logistical problem that the SKA faces. At present, pathfinder instruments are being constructed in both sites. In five years time, it is expected that the bulldozers will move in to begin building Phase I of the SKA. And in ten years time, they are expected to roll in once again when Phase II is built. How will it be possible to do any science with so much building work going on around the antennas? With two sites available, it will be much easier (though not entirely trivial) to leave one radio quiet while the other is down for maintenance.

But the people who will be disappointed are people studying transients. When a radio source flares up, what they'd most like to be able to do is to see the flare at as many different wavelengths as possible. But, with the high- and low-frequency components of the SKA on different continents, only a very small patch of sky will simultaneously be visible to both. As the sky rotates, it might be possible to observe the beginning of a transient with one, and the end with the other, but not both simultaneously. As with any decision, it's swings and roundabouts, but I think a lot of people will be out celebrating tonight.

The sky on 20 Apr 2024

Source

For more information, see the official press release.

Image credit

Artist's impression of the SKA

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