Friday, November 20, 2015

4 Pi SETI - Why Not Build the Argus Radio Telescope Full Scale?

Robert Dixon
Update: we need to digest this first. All-sky there could be 2000 FRBs per day.

This  post - like nearly all the others - is about the questions I have. I am not a radio astronomer.I want to know how and how much and when about the radio telescope we will discuss below. This was stimulated by my interview with Robert Dixon last year, in which he discussed the virtues of the Argus concept. I lay awake that night thinking about it, and wondering why it hadn't been built yet.
 
The Search for Extraterrestrial Intelligence (aka SETI) is just that - a search. Like any search, it becomes much more difficult when more dimensions are added to the search space, and far easier when one or more of those dimensions are removed.

For example, if you are looking for an obscure little store with the best barbeque ribs in the West, and all you know is that it's in Los Angeles County, then your search space is quite large. However, if you have the additional information that it's on Santa Monica Boulevard, then you you still have some searching to do, but much, much less. You might even find the joint before it closes.

The problem with the SETI search is that it uses telescopes, and telescopes like the Allen Telescope Array or the Arecibo Observatory, only look at a small part of the sky at any one time. In fact, all the radio observatories in the world put together are only looking at a tiny sliver of the sky. If an ET beacon is transmitted for a short time while you are looking at another part of the sky (which you almost certainly are), you would completely miss it. We can dream about adding more telescopes to the search, but we would need a ridiculous number to cover the whole sky at once - or would we?




LOFAR stations in the Netherlands
There is an alternative telescope design that uses a combination of a large number of simple, cheap receiving elements and high speed processing to cover the entire sky at once. At longer wavelengths, these telescopes already exist: like LOFAR in Europe, Owens Valley in the US, the Long Wavelength Array in New Mexico,  and soon Phase 1 of the low frequency array as part of the Square Kilometer Array in Australia, which should work as a faster, more sensitive LOFAR.

The ASKAP in Australia, currently under development, will reach frequencies up 1.8 GHz (well above the 21 cm Hydrogen line, near where the Wow! Signal was observed), but with 30 square degrees of FOV, which is hardly the whole sky.

A prototype of a full-sky radio telescope that covers more wavelengths of SETI interest, but which is too small to be adequately sensitive, is called Argus, and was built on a rooftop at Ohio State University with a tiny budget. However, Argus is mainly built out of computers, and it scales up well and benefits directly from Moore's Law. Just a decade or so ago ago, a large Argus array required too much computing power, but now is easily within reach with inexpensive computers. We could build a large one with simple, easily maintained elements and a building full of computer racks for a fraction of what the Square Kilometer Array would cost.

Experimental Argus array at Ohio State
In addition to the building full of computing racks and routers, you need a fiber optic connection, and a flat open space of land filled with a large number of simple receiving elements. The number of antenna elements you would need at 1500 MHz (the radio astronomy band most commonly searched), would be about 10000 elements to detect short transients at a strength of 150 Jansky, or roughly comparable in strength to the Wow! Signal (which was a longer transient). This would not detect most of the Fast Radio Bursts so far observed, so a case might be made for an even larger array, depending on what progress is made understanding the FRBs in the interim. These elements could be produced inexpensively at such quantities - perhaps a few hundred dollars apiece. The exact number needed will depend on how sensitive we want the array to be - if we want it to able to detect events like the 5 millisecond Lorimer Burst, we may need quite a bit more than 10,000 elements.

I am going to roughly cut a number of $5 million for land and buildings, $10 million for the antenna elements, electronics and cabling ($1000 for each element including installation cost) , $10 million for engineering, software and project management, and $10 million for the baseband equipment (computer, networking, etc.). That's $35 million. And, since I am probably wrong by at least a factor of two, let's say $70 million for the entire facility, and we may want another, essentially duplicate telescope in the opposite hemisphere, so just under $150 million for the complete, deluxe, capability covering nearly all of the celestial sphere at any time. Compare this to the more than $2 billion the Square Kilometer Array is projected to cost at completion.

These facilities would filter their data down to actual transients, removing obvious sources of interference such as satellites, and and forward the results to a science center somewhere that would have their finger on the button to turn more sensitive telescopes across the electromagnetic spectrum onto transient sources, similar to Oxford's 4 Pi Sky organization.

Unlike the SKA, performing sensitive astronomy on relatively small fields of view, the big ARGUS would survey the entire sky over a wide range of frequencies, looking for unexpected transients. Such events are mostly going to be natural, and would be of interest to respectable radio astronomers doing real science, but if ET beacons, as I posit, are transmitted for only a short duration in our direction, we would have a much better chance of finding them.

The value of doing this depends on your hypothesis on how frequently and for what duration clear ET beacons appear in the sky. If you posit that they happen frequently, then in time you will happen upon one by chance while doing commensal SETI with the SKA or Arecibo or the next generation Very Large Array (presently in the conceptual design stage), or targeted SETI searches as part of the Breakthrough Initiative. If you posit, as I do, that these events are rare, then the money spent on a bigger, more sensitive ARGUS would be a good investment, and there are transient natural phenomena we would catch as well.

So, the question I have is, why not? Spread over several years and multiple phases, construction would only cost a few million per year and could be architected to exploit improving computer technology. However, it appears that little work has been done on Argus, since the 2008 paper by Ellingson, Hampson, and Childers.

Radio telescope arrays are under construction as mentioned above, but focus primarily on long wavelength arrays for measuring heavily redshifted signals from the early universe. The next generation VLA currently in planning is a radio telescope similar in architecture to the Square Kilometer Array - many apertures but fairly narrow fields of view. Although I'm not aware of a cost estimate for the ngVLA, it would have to be comparable to the SKA, possibly even more. For a fraction more, we could colocate a large Argus with it, and spot transients it could potentially target.

With an affordable full-scale Argus, we would not completely remove two spatial dimentsons from the search - instruments like the SKA are far more sensitive in the narrow beams that they search within than the Argus can be without approaching these instrument in cost.

I need help at this point. What am I missing? Is the overall science case weaker than I think, with or without SETI as a priority? Or would it cost far more than I estimate? Or is it just not as sexy as an array designed for unprecedented sensitivity? What else? If you're knowledgeable on these matters, I'd love to hear from you.