SETI pushes ahead
Posted: 24 May 2011
As Mark Twain might have said, the reports of SETI’s death are greatly exaggerated. As a key instrument in SETI, the Search for Extraterrestrial Intelligence, the Allen Telescope Array (ATA) has hit the buffers. A network of forty-two 6.1-metre radio dishes at Hat Creek in northern California, run by the SETI Institute and the University of California, Berkeley, the ATA was placed into ‘hibernation’ in April and staff laid off as funding to maintain its operation dried up in the California sunshine. The media proclaimed the end of SETI, but a new survey of planets discovered by NASA’s Kepler spacecraft is showing that there is life beyond the ATA.
This May astronomers from the University of California, Berkeley procured the Robert C Byrd Green Bank radio telescope, which at 90.4-metres is the largest steerable radio dish in the world, to pursue potential alien signals from 86 Kepler candidates. The survey will acquire 24 hours – amounting to about 60 terabytes – of data in total that, after a rough analysis, will be passed onto the one million SETI@home users around the globe for their computers to analyse. Although the total search time is limited on the Green Bank Telescope (whereas the ATA, like Arecibo, can be used continuously for SETI, the Green Bank Telescope must share its time amongst many astronomers all with different research goals), it does have the advantage of being able to search over an 800 MHz bandwidth (by comparison, the ATA has an instantaneous bandwidth of 50–100 MHz, and in total can survey a much wider slice between 0.5 to 10 GHz, if taken in steps). This bandwidth is then divided up into eight billion channels, each less than a hertz, and each channel is carefully analysed for narrowband signals. Find a narrowband signal that is not terrestrial interference and you may be onto something – all known natural radio sources in the Universe are broadband, rather than narrowband.
The Green Bank radio telescope.
“We had been working closely with Kepler scientists for a long time before the Kepler candidates were announced in February,” says Dr Dan Werthimer, who is leading the survey. Indeed, Werthimer is also chief scientist on the world’s longest running SETI project, SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations), which for 30 years has been operating on the Arecibo radio telescope in Puerto Rico, as well as SETI@home.
The Kepler candidates chosen for the survey have been picked by what some people see as slightly contentious reasoning. “There were two things that we were interested in,” says Werthimer. “One is that they were small so that they have a good chance of being rocky, and the other criteria was to have a planet at the right distance from its sun where you have a good temperature for liquid water.”
This is the standard description for the habitable zone, but for some critics the definition is limited. In recent months alone there have been scientific papers relating to how dark matter trapped inside the cores of planets, or worlds with huge swathes of hydrogen wrapped around them, could keep a planet warm and toasty for liquid water much farther from a star than the traditional habitable zone. Furthermore, in our own Solar System, we see how gravitational tidal forces from the giant planets Jupiter and Saturn can heat moons such as Europa, Io and Enceladus. By narrowing our search to such just the conventional habitable zone, are we missing other abodes for life?
Werthimer agrees. “There are other ways to make heat besides being at just the right distance from a star,” he acknowledges. “The original habitable zone was a narrow region at just the right distance, but what we’re learning now is that there are lots of habitable zones.”
As Kepler confirms or rules out its candidate planets, and as new and more interesting candidates are found, and as more time can be booked on telescopes like Green Bank, the search list will change and widen, pushing the envelope of where we think life could exist. “This is just the start,” promises Werthimer. “We’re going to go back and re-observe if possible once a year.”
SETI is beginning to target its search towards exoplanetary systems discovered by Kepler. Image: NASA/JPL–Caltech.
“One possibility is to try a different approach all together,” he says. “I like to sit around and think, if I were ET, what sort of strategy would I have to get in touch? We could try things like looking for infrared excess in stars from Dyson swarms, or use radar and see if we can find any [alien probes] hiding in Earth’s Lagrangian points. Or let’s do something with optical SETI, because that doesn’t require a huge investment in antennas.”
Optical SETI (see our previous story) searches for brief flashes of laser light rather than radio transmissions, and utilises normal astronomical telescopes rather than giant radio dishes. The most powerful laser on Earth is the National Ignition Facility at the Lawrence Livermore National Laboratory in California, which can attain energies of petawatts (a quadrillion, or 1015, watts) for a tiny fraction of a second. If shone into space the laser would outshine the Sun for this amount of time – imperceptible to the naked eye but sensitive photomultipliers, which essentially count photons, could detect a similar laser pulse coming from the stars were it aimed at us.
With the right equipment – a 250mm aperture amateur telescope equipped with the requisite detectors will suffice – such a pulse would be easy to spot, for a torrent of photons detected in any given nanosecond would give it away. It’s also cost effective for ET; beaming a continuous, high power radio message in all directions would gobble up energy and require a huge transmitter.
“The radio searches look for a steady tone that is on all the time, and if the aliens are deliberately pinging Earth they might make something like that,” says Shostak. However, because ET are likely to be so far away they are unlikely to know we are here, although they may see that Earth has an oxygen atmosphere and a biosphere – a planet with promise, one of many that ET could be investigating. “In such a case they can’t have an always-on signal being broadcast to the entire Galaxy all the time, at least not one at a level we could detect because that requires on the order of 1017 watts if you want it to be strong enough to be picked up by our SETI instruments. That’s a big power bill!”
To overcome this, it has been suggested there may be two signals to find. “One will be a beacon to get our attention, perhaps a pulsed signal, and then there will be some weaker message that will be harder for us to detect and decode,” says Werthimer, echoing an idea first developed by Shostak.
Optical SETI would detect powerful flashes of laser light from alien civilisations. Image: ESO/H H Heyer.
These ‘flashes’ may cycle around hundreds or thousand of worlds before returning to us a week, a month, a year later, and the idea is reminiscent of ‘Benford beacons’ (see our article last year) which work on the premise of cost optimisation. Once they’ve got our attention, we would train all our telescopes on that part of the sky, straining to hear a continuous low power omnidirectional message. A laser pulse would work for the initial flash, and they are also much more efficient than radio. “If you want to send really high bandwidth messages, like all of Wikipedia in a few seconds, you might be better off with fine beams like a laser,” says Werthimer.
Although there have been some optical SETI experiments, including those led by Paul Horowitz at Harvard, by Dr Ragbir Bhathal of the University of Western Sydney and Werthimer himself at Berkeley, if lasers hold so much promise and are less expensive to search for, why are we not looking for them en masse?
Creating an amateur community intent on searching for SETI signals is not new. It was initially pioneered in the mid-1990s as Project Argus, led by Dr H Paul Shuch at the SETI League. The idea was to create 5,000 amateur radio stations using old, secondhand three- to five-metre C-band dishes that would be located all around the world, giving continuous coverage of the sky. However, because of various factors – the technical know-how to set up an Argus station, the cost involved (a few hundred to a few thousand dollars), the absence of high-profile pro-SETI movies like Contact and the lack of user-friendly off-the-shelf radio equipment, Argus has floundered at just 144 stations in 27 countries. And while Shuch explains that meaningful science can still be done with that many stations, he’s less optimistic about amateur optical SETI.
“[What Seth Shostak faced] exactly parallels the problem I had when I approached the largest electronics retailer in the US, a decade and a half ago, about producing and marketing an amateur radio telescope product line,” he says. “They weren’t even willing to talk to me unless I could realistically project a market of a million systems per year. The industry thinks in terms of amortizing development costs over huge production volumes, and that’s no more likely to exist in the optical realm than it does in the radio. So I’m afraid optical SETI will also remain the province of the DIY experimenter, rather than the masses. But I’d love to be proven wrong!”
Shuch is a little more optimistic about Project Argus. “The challenge is to sustain the enthusiasm of that small group of participants, as years of a null result will naturally lead to a loss of interest. We’ve probably tapped out the bulk of those experimenters with the interest to lash together their own stations – in the Internet age the masses would rather crunch SETI@home data than build equipment.”
It seems there are plenty of plan B’s, but if plan A is struggling at the moment, what hope do the likes of optical SETI have of getting off the ground? Is there a lack of interest? Yes and no, says Shostak. He mentions how his public talks are always packed out, and how young undergraduates and PhD students are always asking him how they can get into his line of work, and yet governments, space agencies and private individuals are reluctant to put up the money. The Allen Telescope Array, named after Microsoft’s Paul Allen who has donated over 30 million dollars to the project, relies on private donations (interested readers can make a donation here and ten dollars buys a forty million channel inspection of a Kepler candidate planet). Shostak also points out what may be a cultural issue: “SETI has been primarily an American experiment for the last 20 years; the Italians are the only ones outside of the US that are routinely doing SETI, while there have also been very good experiments in South Korea and Australia. But the UK doesn’t do it. The Netherlands don’t do it. Germany doesn’t do it. I don’t get why that is.”
There is new hope in the future. With the ATA, as well as the recent WISE infrared survey mission from NASA, the VISTA infrared survey telescope at the European Southern Observatory in Chile, the planned building of the giant ALMA and Square Kilometre Array radio observatories, the proposed Large Synoptic Survey Telescope, the upcoming Gaia astrometry mission, the James Webb Space Telescope and a plethora of other next-generation observational instruments, we’re headed into a new era of research based on large scale astronomical surveys. Many of these survey telescopes will have the ability to piggyback many experiments on them, just as happens now with Arecibo, and future SETI researchers could simply mine the reams of data that will come from these surveys.
“You could map the sky in just a few days – the ATA could do that,” says Shostak. “Then you would come back and map it again and then essentially use a blink comparator like in the old days, and compare the two maps and see what has changed. It could be the way to go simply on the basis that aliens may not be relentlessly targeting us with a high powered signal because that is expensive.”
Having a plan B really increases SETI’s chances of success, for none of us can really be sure how, or even if, ET is transmitting to us. Werthimer champions the multi-pronged strategy, citing twenty current ongoing SETI projects besides the ATA. SETI, as a field, is still hanging in there, and as computing power increases, and giant survey telescopes come online, there’s every chance that Shostak will be right in his prediction that we’ll detect a signal by the year 2025. And until then, as Werthimer succinctly states, “SETI continues.”