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Return to ‘Wow!’

The famous Wow! signal, picked up on August 15, 1977 at the Big Ear radio telescope (Ohio State University) is back in the news, with a new theory suggesting a source for the signal right here in the Solar System. Antonio Paris (St. Petersburg College, FL) asks us to consider a cometary origin for the signal, generated as two comets released hydrogen as they passed near the Big Ear’s search field. The now-dismantled telescope had a fixed field of view, so a bright signature at 21 centimeters — the hydrogen line — would have appeared short-lived.

Specifically, 21-cm refers to the line in the spectrum of neutral hydrogen atoms, a wavelength corresponding to 1420 megahertz associated with the most common element in the universe. It was back in 1959 that both Philip Morrison and Frank Drake fixed on the hydrogen line as a rational place to look for interstellar beacons, the assumption being that any civilization trying to reach another would choose a wavelength associated with some sort of universal constant.

Project Ozma grew out of this as Drake studied the nearby stars Tau Ceti and Epsilon Eridani at this wavelength, while Morrison, working with Giuseppe Cocconi, wrote the most famous paper in the history of SETI, ”Searching for Interstellar Communications,” which appeared in Nature in 1959 and is a fascinating read to this day (available online). Hence the interest of Jerry Ehman at Ohio State’s Big Ear, and the enthusiasm with which he wrote “Wow!” on the printout of the signal detected that day in 1977. Had we found an interstellar beacon?


The Cometary Hypothesis

We do know that the ‘Wow!’ signal’s intensity rose and fell over the same 72-second interval that the Big Ear itself could keep an object in its view — with a fixed field of view, the Earth’s rotation governed this. Hence Ehman could assume the signal had an origin in space, and Antonio Paris makes the same assumption. Scheduled to appear in the Journal of the Washington Academy of Sciences, the paper notes that the size of a comet’s hydrogen cloud is determined by the size of the comet, extending for as much as 100 million kilometers in width. The cloud increases significantly as the comet approaches the Sun. From the paper:

Since the rate of hydrogen production from the comet’s nucleus and coma has been calculated at 5 x 1029 atoms of hydrogen every second, the hydrogen cloud is the largest part of the comet. Moreover, due to two closely spaced energy levels in the ground state of the hydrogen atom, the neutral hydrogen cloud enveloping the comet will release photons and emit electromagnetic radiation at a frequency along the hydrogen line (1420.40575177 MHz).

Two comets are of interest. Looking back to 1977, Paris found that from July 27 to August 15, the Jupiter-family comets 266P/Christensen and P/2008 Y2 (Gibbs) were transiting near the Chi Sagittarii star group, placing them close to the source of the “Wow!” signal. Back to the paper:

Of significance to this investigation, the purported source of the “Wow” signal was fixed between the right ascension and declination values… of comets 266P/Christensen and P/2008 Y2 (Gibbs). On their orbital plane, moreover, 266P/Christensen was 3.8055 AU from Earth and moving at a radial velocity of +13.379 km/s; and P/2008 Y2 (Gibbs) was 4.406 AU from Earth and moving at a radial velocity of +19.641 km/s…

If the cometary hypothesis is correct, this would explain why subsequent searches using the Very Large Array and the Ohio State University Radio Observatory between 1995 and 1999 found nothing, for neither comet would then have been near the right ascension and declination values of the original signal. Paris suggests that the period of 266P/Christensen (6.63 years) and P/2008 Y2 (Gibbs) (6.8 years) can explain why the signal was never again detected.

The idea that the Wow! signal was produced from clouds of neutral hydrogen emanating from the two comets seems quite a stretch, but usefully, Paris offers a way to falsify the hypothesis. We learn that comet 266P/Christensen will again pass through the neighborhood of the “Wow!” signal on January 25, 2017, while comet P/2008 Y2 (Gibbs) will be in the area on January 7 of 2018. So we will have the opportunity to test the notion and analyze the hydrogen spectra of the two comets. Shouldn’t the Big Ear have picked up the same cometary signature 24 hours later? We can’t be sure, but scanning the hydrogen signal from each comet sounds like a good idea.

The paper is Paris and Davies, “Hydrogen Clouds from Comets 266/P Christensen and P/2008 Y2 (Gibbs) are Candidates for the Source of the 1977 “WOW” Signal,” accepted at the Journal of the Washington Academy of Sciences (abstract).


Comments on this entry are closed.

  • Keith Cooper January 12, 2016, 13:02

    “Shouldn’t the Big Ear have picked up the same cometary signature 24 hours later?”

    I think (and someone may correct me) that this is incorrect, and it becomes an even bigger problem for the comet hypothesis. The Big Ear telescope, like Arecibo, would just look up and let the sky move above it and objects drift in and out of the field of view of its receivers. The Big Ear was designed in such a way that it had a two-horn feed system, and it would switch between the two horns, or receivers, 79 times per second, and the signal strength was measured by subtracting the signal in one horn from the other (I guess this was to help identify terrestrial interference – a signal detected by both horns at the same time would be close by and not on the celestial sphere). The two horns were separated by about five feet, and a celestial object would appear in one of the horns for 72 seconds, then the sky would drift around and the object would appear in the second horn just five minutes later. However, the Wow signal did not appear in the second horn and one of the common SETI explanations for this is that the signal was switched off in the five minutes between horns. The problem was, the computer hadn’t been programmed to tell which was the positive horn and which was the negative horn, so no one knows which horn detected the signal. The declination on the sky was the same for both horns, but depending on which horn detected the signal, the RA would be different.

    If it has been a comet, it would have been detected in both horns within the space of five minutes, and Paris even acknowledges this in the conclusions to his paper. Based on this I’m not sure the comet hypothesis can stand up to scrutiny?

  • Keith Cooper January 12, 2016, 14:26

    I feel I should follow up my earlier comment. I contacted Professor Paris and asked him about this five minute delay between the horns. He suggested that the high angular velocity of the comets could have taken them out of the field of view of the second horn. I haven’t done the maths to determine whether that could be the case, and I’m not fully convinced by the theory yet, but I am beginning to warm to it!

  • Ron S January 12, 2016, 14:33

    Sort of like a chart with one data point and then fitting a curve to it. Almost any will do. Similarly for the Wow signal. Maybe this hypothesis works out, or maybe not. The science of comets is interesting enough without positing it as a solution to the Wow issue.

  • H. Floyd January 12, 2016, 15:08

    Comets are quickly earning the David Drumlin Award for biggest SETI buzzkill.

  • TLDR January 12, 2016, 16:09

    So do we get “Wow”-like signals off of known comets experimentally?

  • Mark Zambelli January 13, 2016, 7:51

    I’m also wondering what kind of activity the sun was undergoing around this time period… any CME or localized magnetic disturbance would take some short time to get out to around 6 a.u. Could an energetic re-connection provide the means of ionizing the neutral H2 to provide a (brief?) burst? If cometary comas are normally quiescent (why aren’t we picking up more ‘wow’s from comets?) then maybe a rare circumstance must come about to make comets sing as suggested.

  • Harry R Ray January 13, 2016, 10:37

    Correct me if I am wrong, but I have always been led to believe tha the way to distinguish between a NATURAL signal and a NON-NATURAL one is the NARROWNESS of the bandwidth! Non-natural transmitters transmit their signal at a bandwidth of roughly 1 Hz. A NATURAL source signal will be SPREAD OUT over a RANGE of bandwidths(doe to dopplar shifts caused by various components of the source MOVING IN DIFFERENT DIRECTIONS) in the form of a POINT-SPREAD FUNCTION! The NARROWNESS of the WOW signal should rule OUT ANY NATURAL SOURCE(by the way, this is how pulsars were determined to come from neutron stars)! The explanation for the WOW signal has always been EARTHLY* INTERFERENCE, with the asterisk indicating that it DID come from space(i.e. NOT on the ground or in the air), but from a top secret military spy satalite whose co-ordinates were NEVER made public.

  • ljk January 13, 2016, 11:09

    I have posted this multiple times here and elsewhere in the past and it has yet to be seriously addressed. Instead everyone keeps replaying the same old discussions and debates about the 1977 Wow! Signal, as if it were the only suspicious signal we have ever encountered.

    Gerry Harp of The SETI Institute has some definite views of the Wow! Signal here:


    To quote:

    Harp: The “Wow!” signal was almost certainly radio frequency interference. The signal failed to pass even the simplest tests to exclude interfering signals from that observation campaign. From another perspective, at the Allen Telescope Array (ATA), we see dozens of signals comparable to “Wow!” every day. This is simply because we have much more computational power than they did back when “Wow!” was seen. If the “Wow!” signal were seen today, it would be a yawn. However, there is a silver lining to the “Wow!” signal. “Wow!” has inspired a lot of public interest in SETI. Despite being a not very scientific result, public awareness of “Wow”! has been beneficial to SETI. So I generally think of “Wow!” as being a good thing from that perspective.


    Harp: It isn’t at all special or different from signals that we observe every day at the ATA. I hope you’re not disappointed that I’m not so impressed by the “Wow!” signal. I think you will find that many professional scientists in the field do not find the “Wow!” signal very convincing. But that doesn’t mean that SETI isn’t a good thing to do. There is still a 50% chance, by my estimates, that our first discovery of life off of our planet will be a discovery of a transmitting civilization.

    Earlier comments by Harp on the subject here:


    So in conclusion, had the Wow! Signal been detected with today’s technology, it would have been ignored, due to our far improved computer technology. How many people realize that signal from 1977 was not monitored by someone in real time but that an engineer saw it hours later in a printout? The Planetary Society is doing something rather similar with their Optical SETI program, meaning if they do detect something by the time they get back to it the signal may be gone.

    How many real, serious SETI programs where the sky is being constantly monitored 24/7 and alerts are addressed right away are actually happening right now?

  • Ashley Baldwin January 13, 2016, 11:31

    This got me thinking Kardashev I civilisations and Dyson Spheres. I wondered that any civilisation able to utilise all the energy of its parent star would have a more sophisticated approach to such an undertaking than building a solid shell. How clunky is that ? Wouldn’t it be better to directly tap a proportion of the stars energy to create some sort of energy based shell , I hate to say “force field ” as that is just so Star Wars/ Doctor Who passé . In which case one wouldn’t see the light of a star gradually diminish as the shell was completed piecemeal but suddenly disappear altogether when the field was operated to be replaced witha weak infrared spectrum alone. Watch out for any sudden stellar disappearances !

  • Michael January 13, 2016, 16:48

    The thing with Dyson spheres is that you need all the available materials in the system to make it but when you have made it you will have very little material left to throw to other star systems to start the cycle again. It will more likely be a sphere with large holes in it making a strange infrared skewed spectrum.

  • Marshall Eubanks January 13, 2016, 20:34

    TLDR January 12, 2016 at 16:09

    So do we get “Wow”-like signals off of known comets experimentally?

    No. Comets have been detected in the radio, including (weakly) the 1667 MHz OH line, but no comet has been observed to emit any 21 cm radiation, much less the strong emissions that would be required to explain the “Wow” signal.

    Also, the beam of the “Big Ear” was eight minutes of arc in right ascension (and 40 in declination). To move 8 minutes of arc (2 milliradians) in 5 minutes would mean an angular rate of (at least) 7 microradians per second. At 3 AU, that would require a transverse velocity of 3300 km/sec (and that, remember, is a lower bound). That is roughly 100 times the actual transverse velocities of these objects.

    There is, in short, no way that this cometary hypothesis can explain the Wow signal.

  • NS January 13, 2016, 21:20

    Re the discussion linked by ljk:

    Vakoch: The “Wow!” signal had all the characteristics of a real signal from ET with one critical exception – it never repeated. Without this one essential requirement of science – that observations can be replicated – we have no basis for thinking it was really from an extraterrestrial civilization.

    Aside from the fact that Vakoch is a more favorably disposed to the “Wow!” signal than Harp is, the former also raises the issue of repeatability, which to me is a real problem. If ET signals are right on the edge of what we’re able to detect, and not aimed at us to begin with, aren’t very intermittent detections what we would expect? We shouldn’t just dismiss signals that don’t repeat if they meet other criteria for being ET. I wouldn’t announce that we’d discovered ET based on signals like that either, but to the extent we can we ought to follow up. Brief, not-repeating signals may be all we get for a very long time.

  • xcalibur January 14, 2016, 1:54

    TLDR – that’s what I was going to ask. if there’s data to support a mundane explanation for the wow signal, I’ll accept it. but do comets give off signals of that magnitude in hydrogen spectra? are they capable of doing so? if not, could there be an alternate explanation, like a quaking pulsar or gamma ray burst?

    the wow signal is interesting, even more so if we can rule out natural causes.

  • Mike Fidler January 14, 2016, 4:02

    A little off topic – but- some food for thought!
    I’ve been reading Andre Fuzfa paper on – How Current Loops and Solenoids Curve Space-time – http://arxiv.org/pdf/1504.00333v3.pdf and wondering about FTL communications could be possible?

    Then should this be what we are looking for in SETI?

    The one problem is the huge magnetic fields required, but if we were to only use it like a pinhole, could it work like a camera and be able to see great distances in real time – project a image of Alpha Centauri from a million miles away!

  • Keith Cooper January 14, 2016, 7:44

    @Ashley Baldwin

    What you describe, with the star disappearing and everything, is exactly what happens at the start of Peter F Hamilton’s 2004 novel Pandora’s Star.

    @Marshall Eubanks

    Thanks for the calculations. The paper quotes the comet’s relative velocities as 13.4 and 19.6 km/s, so… yeah. According to Wikipedia Comet Hyakutake in 1996 (which I was lucky enough to have seen, it rivalled Hale-Bopp in my opinion) covered half a degree every 30 minutes during its closest approach to Earth, so an arcminute per minute, which is the kind of angular velocity comets Gibbs and Christensen would have needed to move out of the way of the Big Ear’s second horn. Hyakutake though was just 0.1AU when it made its closest approach, plus it was a long period comet and they move much faster than short-period Jupiter-family comets like Gibbs and Christensen.

  • Scott Gordon January 14, 2016, 8:15

    In response to Harry’s comments :

    The ‘narrow wavelength is artificial’ rule applies to thermal, synchrotron etc. emission, not to a spectral line (otherwise there would be no such thing as a natural spectral line in nature!).

    Some might argue that this shows a big flaw in the idea of using natural spectral lines as METI beacons – it looks like a wierd natural signal !

    The 21cm hydrogen line is an spectral line made by cold (hence un-ionised) hydrogen atoms. There are two almost identical energy levels in the atom depending on whether the proton and electron spin the same way or opposite ways; occasionally the atom flips to the lower energy of the two states and the energy difference is given off as a photon at a very consistent known wavelength (about ~21cm, which is radio).

    Hydrogen produced by a comet would be travelling within +/- a few km/second of the comet’s velocity, hence in a narrow velocity range, and a narrow wavelength/frequency range due to the Doppler effect. So no it would not be spread out. In fact the fact that is not spread out is one of the most valuable observational tools in radio astronomy.

    Due to the Doppler Effect, the frequency range divided by the line frequency is roughly the velocity range divided by the speed of light (eg. 10 km/s velocity range –> frequency range 10/300000*1420 MHz = only 47 kHz; or 0.0007 cm if you want wavelength range – extremely narrow).

    In response to Mark’s comments :

    The 21cm line is produced by neutral hydrogen atoms so anything which might ionise the hydrogren stops the line occurring (not causes it). It’s not like the H-alpha line which is emitted by atoms cooling down.

  • Mark Zambelli January 14, 2016, 8:45

    Thanks for posting those links again… I read through them before, after you’d posted them last time the ‘Wow!’ signal was discussed here, so it was good to get a refresher. It helps to put some perspective on the matter and I know it was a surprise for me to learn that astronomers regularly pick up ‘signals’ such as these.

    But then again, the ‘Wow!’ candidate was an early, strong, perhaps the most famous good

  • Mark Zambelli January 14, 2016, 8:50

    (Continued, doh!)…example and for this it’s a good thing it gets so much media coverage. It’ll always be with us as one of those wonderful and enigmatic mysteries, even if it seems overwhelmingly more likely to be something more mundane/terrestrial, with a small chance it might less mundane but non-ETI in origin.

  • ljk January 14, 2016, 12:06

    More attention needs to be paid to TYC 1220-91-1, which has been suggested has the hallmarks of a Benford Beacon:


    It is very frustrating that most of the information about this SETI find is relatively scant and most of the attention paid to it is via UFO fringe groups.

    Some straightforward astronomical data on the star:


  • ljk January 14, 2016, 16:37

    Adam Crowl just made me aware of this other Benford Beacon candidate:


    They had the signal for almost one hour! Yet they could not get another dish on it in time.

    I had heard the rumor in the early 1990s but now I have a stellar name to the story. Again, why isn’t SETI focusing on this one too?

  • Cynthia Russ January 15, 2016, 6:18


    Have you contacted the SETI Institute about either candidate? I suspect they will tell you those are on their observing lists.

  • Ashley Baldwin January 15, 2016, 9:04

    Well spotted Keith C . I have to hold my hands up to being a big fan of that very book and the “Dyson Pair” . There is an eerie resemblance which is why I made the subtle reference . I hope no more though as we haven’t yet developed the ability to construct wormholes , gravispatial ultra drives and Nova bombs !

  • Eniac January 19, 2016, 12:36

    If the signal was picked up by only one horn, it really means that it did not pass an important test for coming from a celestial source. This is why there are two horns, to distinguish true sources from artifacts. That, in my view, is the reason that most reasonable astronomers are not overly excited by the Wow signal. Not some sort of conspiracy or conservatist fear of the extraordinary, as some appear to believe.

  • ljk January 21, 2016, 11:29

    Jill Tarter says Stephen Hawking is wrong about alien behavior:


    I have written in several articles here on Centauri Dreams why ETI would likely not attack humanity, or if they did it would be swift and over before we knew what hit us. However, it is possible that while aliens may not have deliberate malevolent intentions, they could see us as too different/ unsophisticated to warrant regard and protection. We could subsequently be “run over” or left off the cosmic warning alert list if, say, a relatively supernova were about to sweep our region of the Milky Way galaxy.

  • ljk January 21, 2016, 11:45

    Oh wait, they’re all dead – never mind…


  • ljk January 22, 2016, 10:15
  • ljk February 8, 2016, 13:07