The Wow! Signal Was Not Caused by a Comet

In 2015, a team of astronomers suggested that the Wow! Signal originated from hydrogen emissions produced by a comet. Here are two responses to this hypothesis from Robert S. Dixon and Jerry Ehman, former staff of the Big Ear telescope, which recorded the signal in 1977.

Coincidentally, the Big Ear telescope observed Comet Halley in August 1986. During this campaign, its spectral receiver—also known as the SETI receiver—was operating to search for hydrogen-line emissions. No signals associated with Comet Halley were detected.

The same conclusion applies to the Interstellar Comet 3I/Atlas, which was suggested in 2025 by astronomer Avi Loeb as a possible source of the Wow! Signal. Comets are not sufficiently strong, compact, or transient sources of neutral hydrogen to reproduce a signal with the characteristics of the Wow! event.

Moreover, in August 1977, Comet 3I/Atlas was located approximately 9 degrees away from the Big Ear detection beam. As a result, both a natural cometary origin and any associated artificial emission can be ruled out.

About the Two Comets and the Wow! Signal

(personal communication and previously unpublished)

April 20, 2016

I have read carefully the article by Professor Antonio Paris and Dr. Evan Davies regarding the two-comet-explanation for the "Wow! signal”1. Here are my comments.

The two comets were at 3.8 AU and 4.4 AU from the Earth at the time of the detection of the “Wow! signal”. The distance from the sun would be around 5 AU (similar to the orbit of Jupiter). The authors commented that ultraviolet (UV) radiation from the sun would split water vapor from each comet nucleus into hydrogen (neutral hydrogen) and oxygen. Also, the size of the hydrogen cloud could extend to over 100 million km (100 Mkm = 10^8 km) wide. The corresponding angular size as viewed from earth at 3.8 AU (or 3.8 * 150 Mkm = 570 Mkm)  would be 100/570 radian or (1/5.7 * 57.3 degrees = about 10 degrees). Note that the "Wow! signal” was from a small angular diameter source, because the correlation of its response with the beam of the Big Ear was 99.14% (nearly a perfect match). Thus, the postulated neutral hydrogen cloud had to be much much smaller than 100 Mkm wide).

If UV was able to split water vapor into neutral hydrogen, then far UV or X-rays or gamma rays would ionize at least some (maybe most) of the neutral hydrogen. In that case we would not see the neutral hydrogen emission at 21 cm (and thus the "Wow! signal” would not have occurred).

Looking at the authors' Figures 1 & 2 I deduced the following. The two comets were at 3.8 AU and 4.4 AU from the Earth with periods of 6.63 years and 6.8 years. At 6.7 years to cover 360 degrees for an orbit, we get an average speed of 360 degrees * 60 arcminutes/degree /(6.7 years*31,536,000 seconds/year) = 0.000102 arcminutes/second. This means that in the 150 seconds between the first beam response and the second, the comet would have moved only a max of 0.153 arcminute (remember that the halfpower beamwidth (HPBW) in declination was 40 arcminutes). Regardless of whether this small movement was in declination or right ascension (or a combination of both) we should have seen a second response from the other horn. I think this fact alone serves to discount the comet hypothesis. 

There is another valid reason why neither of the two comets could likely have been the source for the "Wow! signal". From the authors' Figure 1 and the table just below it I see that on August 15, 1977 (the date of the "Wow! signal”) comet P/2008 Y2 (Gibbs) was at a declination of -27 degrees 36 minutes while the other comet 266P/Christensen was at a declination of -25 degrees 58 minutes. My values for the declination of the "Wow!” source was -27 degrees 03 minutes. Thus, the magnitude of the differences are, respectively, 33 arcminutes and 65 arcminutes. The HPBW of Big Ear was 40 arcminutes, and half of that (from the peak of the beam to the half-power point on each side) was 20 arcminutes. So, the comets were, respectively, 1.65 and 3.25 of the 1/2 HPBW away. Thus, comet 266P (at 3.25) was much too far away to be detected, although comet P/2008 Y2 (at 1.65) might have been barely detectable if there were enough neutral hydrogen. But remember my previous conclusion that each comet was moving so slowly that if in (or near enough to) one beam, it should have appeared in the other beam (but didn't).

Thus, I again conclude that neither comet was the source of the "Wow! signal".

References:

1. “HYDROGEN CLOUD FROM COMETS 266/P CHRISTENSEN AND P/2008 Y2 (GIBBS) ARE CANDIDATES FOR THE SOURCE OF THE 1977 SETI “WOW” SIGNAL”; Washington Academy of Sciences Journal

Analysis by Jerry Ehman, Ph. D.

Discoverer of the Wow! Signal

Rebuttal of the Claim that the "WOW!" Signal was Caused by a Comet

(originally published on the NAAPO website)

June 6, 2017

This is a statement regarding the claim that the “WOW!” signal was caused by hydrogen emission from an unknown comet or comets. It points out inaccuracies which are fatal to the theory that the “WOW!” signal was caused by a slow-moving comet. It also points out missing details from the author’s paper.

The staff of the OSU Radio Observatory has examined the paper by Paris regarding the “WOW!” signal (Paris, Antonio. HYDROGEN LINE OBSERVATIONS OF COMETARY SPECTRA AT 1420 MHZ) and comet 266/P Christensen.

We conclude that comet 266/P Christensen is not the source of the “WOW!” signal for a number of reasons.

1. Using the ephemeris at: http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=266P%20 we see that Comet 266/P Christensen, at the time the “WOW!” signal was observed in 1977, was nowhere near the position of “WOW!”. (all positions are J2000).

“WOW!” was at: RA 19h 25m 31s  or  19h 28m 22s Dec -26 deg 57’

266/P was at: RA 18h 32m 15s Dec -27deg 22’.

The declination is near but the right ascension is nearly an hour off, placing 266P in a different part of the sky when the WOW! signal was observed.

Another comet, P/2008 Y2 was closer, at:  RA 18h 39m 39s. Dec -29 deg 38’

The declination was almost 3 degrees off and the right ascension 47 minutes off, placing P/2008 Y2 in a different part of the sky when the WOW! signal was observed.

It is important to point out that the OSU telescope beamwidth in right ascension was only about 3 minutes (of time), but the comets were 47 and 55 minutes (of time) away. The comets would have passed thru the OSU telescope beam roughly 55 and 47 minutes (of time) earlier.

The observations of the comet made by the author in 2017 near the position of the “WOW!” object are irrelevant, because that is not where the comet was in 1977 when the “WOW!” signal was observed.

2. The author does not cite any references regarding observations of hydrogen emission from comets. We have contacted a comet expert and a hydrogen expert and they are both unaware of any hydrogen emission ever having been observed from a comet.

3. The author does not cite any references of variable emissions from comets. Such variability would have to be incredibly unusual to match the “WOW!” signal. The signal would have to be exactly constant at 30 sigma for 2 minutes since it matched the OSU antenna beam exactly. It would also need to be less than 0.5 sigma for 2 minutes, at a time 3 minutes earlier or later, since it did not appear in the other OSU beam.

4. The author does not show any specific frequencies in the spectrum plots so it is impossible to compare them with the “WOW!” frequency, or to demonstrate that the comet signal bandwidth is less than the 10kHz bandwidth of the WOW! signal.

Dr. Robert S. Dixon

Director, Ohio State University SETI Program