Late last week, the Square Kilometre Array (SKA) board chairman, John Womersley, announced that the future telescope will have more than one home: Australia/New Zealand and South Africa.
The announcement ends a protracted, at-times tense and ultimately surprising bidding process to attract the roughly £1.5 billion (more than A$2 billion) telescope to either Australia/New Zealand or South Africa.
The SKA telescope is being built to investigate fundamental questions about our universe and its origins. It will attempt to shed light on the origins of stars and galaxies in our universe and investigate the nature of gravity. To do this, it will rely on a massive collecting area - a square kilometre of it (or, in the case of the dual-site decision, two square kilometres).
The antennas of the telescope, and hence the science goals, will be split – more or less evenly – between South Africa and Australia/New Zealand. In this scenario, the entire frequency coverage, from 70 MHz to 10,000 MHz, will be split in two: half built in South Africa and half in Australia/New Zealand.
So here are some answers to questions you might have about the split-site decision:
What was the initial recommendation?
The executive summary of the report released by the SKA Site Advisory Committee (SSAC), actually states that either site could be used for the SKA. But when considering both the “13 technical, science and other Selection factors” and “Implementation plans and cost factors”, South Africa was presented to the SKA board as the preferred site.
It’s interesting to note that this evaluation included “socioeconomic and political considerations”.
It was generally seen that Australia’s bid was stronger because of our relative wealth and stability. Given this part of the evaluation consisted of 25% of the total (factor B in the table below), it was not a major factor, relative to the science case.
Indeed, according to the report (see table below) the “Political, socioeconomic and financial” considerations (a subset of factor B mentioned above) only represented 2% of the overall evaluation, with the Australian bid winning this part of the evaluation in any case.
Going by this, any implication that the decision was a purely political one is simply incorrect. (Of course, one can’t quantify the actual political influence wielded by representatives of the rival bids behind the scenes).
So why didn’t the SKA board take the SSAC’s advice?
At this stage we don’t know exactly why the SKA board decided to look into the dual-site option, given the South African recommendation. The above suggests political forces were not a major reason. All we do know for sure is the SKA will be hosted in both Australia/New Zealand and South Africa.
What exactly is being built where?
There’s no doubt the SKA is a complex telescope, and accordingly the decision to split it is itself a complex one, made with due care and process. But it was decided that the array can be split into two pieces: a SKA “lite” (SKAlite), if you will, and a SKA “full” (SKAfull). SKAlite will have about 10% of the capability of SKA full.
Explicitly, the SKAlite will consist of the low-frequency (70-200MHz) and survey parts being constructed in Australia, and the mid-frequency part being constructed in South Africa.
The mid-frequency (200-500MHz) part will consist of satellite dishes that will be added to the South African pathfinder instrument, MeerKAT.
In Australia, our part of SKAlite will consist of a low frequency (70-200MHz) array and a survey instrument that will be constructed by adding elements to the already in-construction observatory, the Australian SKA Pathfinder.
Construction of SKAlite, also known as Phase 1, will begin in 2016 with the first science due to get underway in 2020.
SKAfull, also known as Phase 2, will again consist of three parts, this time divided by observing frequency: low, medium, and high. Only the low-frequency part is being constructed in Australia.
This part of the telescope should be constructed by early next decade, with a higher-frequency extension potentially being constructed later next decade. This would increase the frequency range of the SKA above 10,000MHz.
Is the SKA split a good thing?
The response to the division of the SKA parts has been mixed. The negative responses have generally been about cost issues. On the plus side, some have argued that the dual-site selection will decrease costs, reduce the duplication of technology and infrastructure, and even increase the scientific benefit.
Given the world economic outlook is rather bleak at present, it’s unsurprising that some, including Nobel Laureate Professor Brian Schmidt, have contemplated the extra cost two square kilometres of collecting area (instead of the planned one) may generate.
As far the Site Options Working Group (SOWG) can determine, the dual-site option for SKAlite will likely introduce a (rough) cost increase of between €17 million (A$27 million) and €29 million (A$46 million).
This is smaller than the uncertainty (~8%) in the cost of SKAlite itself and includes other costs, such as the increased funding to the head offices of the SKA. These increased costs might well be offset by other cost savings anyway.
The report stops short of forecasting the economic costs or savings of the SKAfull, since the construction is so far away as to make any projections meaningless.
Indeed, the SOWG only states that the dual-site option for SKAfull is “preferred”. We will have to wait a few years for a complete analysis.
What will the dual-site SKA do better?
Though it may not cost any more to have a dual-site SKA, the SOWG said in its report that the science case is actually stronger for this option.
Specifically, the SOWG says, this will be due to the dual-site option increasing the sky survey sensitivity of the SKA, compared to the baseline considered previously.
This extra scientific bang-for-your-buck will be delivered by integrating the already-under-construction pathfinder instruments of MeerKAT and ASKAP. Under this scenario, 60 dishes will be added to the ASKAP telescope and 180 to the MeerKAT telescope, to provide the baseline specifications that the SKA demands.
One must note that this is not simply a case of increasing the dish numbers of either site, but completely replacing the dishes. That said, the infrastructure (i.e. the bases, the cables and so on) involved with these telescope will not be replaced.
Perhaps the biggest improvement, and one of the main motivations for the dual-site selection, is the inclusion of ASKAP into the SKAlite construction.
The dual-site philosophy will, in the view of the SOWG, increase the scientific merit of the SKAlite (from having a survey component), while not compromising the mid-frequency part, which will be constructed through the MeerKAT pathfinder.
Given this will require little additional cost, it represents the biggest scientific bang for our buck.
So where are we?
While some people may have wanted a single-site selection – indeed, as an Australian, I was hoping Australia would get the nod – there seem to be definite advantages to the dual-site selection as the SKA board have recommended.
My own personal view is that this decision maximises existing infrastructure, while minimising costs and leaving flexibility for the future (i.e. the positioning of the next phase of the SKA construction).
We can now look forward to the exciting prospect of one of the greatest achievements of human engineering and imagination being (at least partly) developed and built in Australia.
As Bryan Gaensler wrote in a recent article on The Conversation “I can’t wait … to get the data in my hands!”