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Planetary News: SETI (2002)Still Searching for the Elusive "Wow!"By Amir Alexander, with a personal account by Robert Gray21 October, 2002
A quarter century after a startled Jerry Ehman made his famous mark on the margins of a computer printout, the search for the "Wow!" signal continues. In the October 20, 2002 issue of The Astrophysical Journal, veteran "Wow!" signal-hunter Robert Gray and collaborator Simon Ellingsen of the University of Tasmania report on the latest efforts to find that elusive signal, the most promising one ever detected by a SETI program. This time Gray and Ellingsen made use of the 26 meter radio telescope in Mount Pleasant at Hobart, Tasmania. Here, for the first time, the researchers had the luxury of pointing the telescope towards the source of the "Wow!" signal for long hours at a time. Even so, the signal, so strong and clear when detected twenty five years ago, remained elusive. The "Wow!" signal was originally detected on the night of August 15, 1977 at the Ohio State University Big Ear Observatory. As on every other night, as Big Ear was searching the skies for an alien signal, its observations were being recorded on a printout sheet. An exceptional string of characters and numbers struck Jerry Ehman, a professor at Franklin University in Columbus, when he was going over the printout that night: 6EQUJ5. He circled the code and added a single comment in the margin: "Wow!" The signal represented by the string "6EQUJ5" was indeed remarkable. Not only was it extremely strong, but it also almost certainly came from outside the Earth. This was because the signal increased and then decreased over a time span of 37 seconds - exactly the time it takes one of Big Ear's two beams to scan a point in the sky. Furthermore, because the signal was detected by only one of Big Ear's two beams and not by the other, which scanned the same location within a few minutes, it was clear that the signal was not continuous. A strong and intermittent signal from outer space is a rare thing indeed. Could it be that an alien civilization is sending a beacon our way? This was an astounding possibility, but unless the signal was detected again, there was no way to know. Numerous attempts to relocate the "Wow!" signal have been made in the past twenty five years. In the months following the discovery, the Big Ear team tried repeatedly to reproduce the signal, but to no avail. In 1987 and again in 1989 Robert Gray led "Wow!" searches using the 84 foot radio telescope of the Planetary Society-funded META array at the Oak Ridge Observatory in Massachusetts, but found nothing. In 1995 and 1996 Gray managed to secure the services of the entire Very Large Array in New Mexico, composed of twenty seven 25-meter dishes. Taking advantage of the remarkable sensitivity of the array, Gray and his collaborator, Kevin Marvel, searched the "Wow!" section of the sky for signals as much as 1000 times weaker than the original. Again, they found nothing.
An intriguing possibility remained: perhaps the signal was not merely intermittent, but periodic, repeating at intervals of hours or days? The signal might illuminate us like a lighthouse, Gray speculated, continuously rotating and regularly but rarely pointing in our direction, perhaps once every extraterrestrial planetary 'day.' Listening for that type of signal would require pointing a radio telescope continuously towards the source of the "Wow!" signal for a long stretch of time, so as not to miss the brief signal. This kind of study, however, could not be conducted from the Northern Hemisphere, where the celestial coordinates of the "Wow!" rise above the horizon for only a few short hours a day. Gray turned south, and in collaboration with Ellingsen secured observing time on the 26 meter radio telescope of the University of Tasmania, off the Southeastern corner of the Australian continent. What happened then is best described in Robert Gray's own words: "Simon Ellingsen and I searched for the elusive 'Wow!' signal from the Mount Pleasant Radio Observatory in Tasmania, observing constantly for 14 hours at a time in the hope of catching periodic lighthouse-like radio signals--or something else of astronomical interest that would explain the Ohio State event. The Tasmanian site was used because the 'Wow!' locale is above the horizon for only 4 to 6 hours a day viewed from the U.S., while it's high in the sky in the Southern hemisphere and can be tracked much longer."
"Our search showed that there is no strong signal flashing 'on' more often than every 14 hours, and we had a good chance of spotting signals repeating as rarely as every 20 hours. One might argue that we didn't look long enough-that maybe the 'Wow!' has a longer schedule. That could be, but the statistics on Ohio State's sky survey procedure say that they would have been quite unlikely to catch anything repeating as rarely as every 12 or 18 hours. So I think we looked long enough to test a periodic or 'lighthouse' scenario over a plausible range of times." "Should the 'Wow!' now be dismissed as having been interference? I don't think so, although it's proven awfully elusive The possibility that it was interference now seems stronger, but the signs of a celestial origin were pretty strong too. All observations to date add up to less than a day of listening at any one position, over a fairly narrow range of frequency. It is still possible that a signal sweeps across us once every day or longer, or changes frequency, and that Ohio State just happened to be looking at the right time." "I think it's probably worth a more systematic follow up-over a longer time and wider range of frequency. But I'll leave that venture for someone else, with the caution that there are no easy-pickings here." To learn more about Robert Gray's search for the "Wow!" signal, see his article in the January/February 2001 issue of The Planetary Report. |
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