The Phys.org article 'Mystery' signal from space is solved. It's not aliens cites a summary of the conclusion "by Abel Mendez, director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo in a blog post Friday, revealing the true nature of the signals."

After further fueling speculation by summoning the world experts in the hunt for life elsewhere in the universe—The SETI Berkeley Research Center at the University of California—the team issued its conclusion.

"We are now confident about the source of the Weird! Signal," Mendez wrote.

"The best explanation is that the signals are transmissions from one or more geostationary satellites."

The signals only appeared around Ross 128 because it is located "close to the celestial equator where many geostationary satellites are placed," Mendez added.

The article also goes on to say:

Astronomers detected strange signals that seemed to be coming from a dwarf star about 11 light-years away, but have now determiend that the signals are interference from a distant geostationary satellite

Mendez is further quoted the Space.com article Weird Radio Signals Detected from Nearby Red Dwarf Star:

Each of these hypotheses has its issues, he said. For example, solar flares of the type that could be responsible generally occur at lower frequencies. In addition, Mendez wrote, there aren't a lot of other objects in the Ross 128 field of view, "and we have never seen satellites emit bursts like that."

and in the Space.com article Not Aliens: Weird Radio Signal from Star Likely Has Duller Explanation:

"This explains why the signals were within the satellite’s frequencies and only appeared and persisted in Ross 128; the star is close to the celestial equator, where many geostationary satellites are placed," Mendez added. "This fact, though, does not yet explain the strong dispersion-like features of the signals (diagonal lines in the figure); however, it is possible that multiple reflections caused these distortions, but we will need more time to explore this and other possibilities."

Wikipedia lists the star Ross 128 at at declination of about +0° 48', and the latitude of the Arecibo telescope to be about +18° 21'. Satellites in geostationary orbits are in a circular ring approximately coincident with the equator at a distance from the center of the Earth of only about 42,000 km. From the Northern hemisphere, geostationary satellites will appear several degrees south in declination. (See this interesting answer.)

For Arecibo, I calculate about -3.2° declination.

QUESTION: How could a radio telescope have picked up satellite signals from a satellite at about -3.2° declination while observing a star at +0.8° declination? And why wasn't this immediately ruled out - surely radio telescopists know about satellites by now, especially the crowded ring of geostationary satellites! Aren't there standard procedures to rule these out before making announcements?

Could the half-angle acceptance really have been 4 degrees? Since the brown red dwarf is extremely close to the solar system, could proper motion put it 4 degrees away from the value listed in Wikipedia?

enter image description here

enter image description here

above: "The signal that seemed to emanate from the red dwarf star Ross 128, as detected by the Arecibo Observatory in May 2017 (enclosed in the red frame)." Credit: PHL @ UPR Arecibo From here.

  • 1
    $\begingroup$ You sometimes use "brown dwarf" in your question, Ross 128 is a red dwarf. Its proper motion is a little over 1 arcsecond south per year, so it would take over 10000 years for it to move 4 degrees south. $\endgroup$
    – James K
    Jul 22, 2017 at 9:36
  • $\begingroup$ @JamesK Thank you for catching my mistake - I've fixed it. Also thank you for looking into Ross 128's proper motion. $\endgroup$
    – uhoh
    Jul 22, 2017 at 10:29
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    $\begingroup$ @SQB please do not try to edit the grammar of text that is clearly quoted from another source. Anything in quotation marks, or in a block-quote (indented, shaded areas) are directly quoted from the other sources indicated. Also, the gas-giants tag is totally irrelevant. Thanks! $\endgroup$
    – uhoh
    Jul 22, 2017 at 18:04

1 Answer 1


The original team suggested three main possibilities: "(1) unusual stellar activity, (2) emissions from other background objects, or (3) interference from satellite communications" Clearly radio astronomers know about satellites.

They noted "in the absence of solid information about the signal, most astronomers would think that [radio interference or instrumental failures] would probably be the most likely explanation."

The satellite hypothesis has some problems it "does not yet explain the strong dispersion-like features of the signals (diagonal lines in the [weird] figure [in the question])" But on a balance of probablities this is the most likely explanation.

There are satellites in the region of Ross 128:

enter image description here

Location of Geostationary satellites operating between 4-8 GHz in the same region of the sky as Ross 128 (yellow dot).
Credit: Enriquez et al. (SETI Berkeley), http://seti.berkeley.edu/ross128.pdf

Your analysis of the location of a satellite, based on the geometry of satellite orbiting over the equator is not supported by this image.

The observers did not claim to have received a SETI signal. Indeed if you look at their original report they note that "aliens" is at the bottom of their possible explanations. The signal looks different from typical satellite bursts. They hoped the signal was astronomical in nature so there may have been a little wishful thinking.

So the most likely explanation is that we are observing multiple reflections from geostationary satellites transmitting on the 4-8 GHz band.

All quotations from the press release from the planetary habitability observatory.

  • $\begingroup$ So my analysis is certainly right as far as it goes. There are approximations of the order of 0.1° that I've made to keep the geometry simple. If it's not supported by the image, then something else is going on here. Part of the problem may be the difference between geosynchronous and geostationary. Satellites that have run out of propellant for station keeping will start to "drift" from their station and inclination starts increasing steadily. Another problem is that during 10 minutes, those satellites will move by about 2.5 degrees across the celestial sphere, not shown in this image! $\endgroup$
    – uhoh
    Jul 22, 2017 at 10:37
  • $\begingroup$ Now that I have those satellite numbers I can use Skyfield to try to recalculate that plot correctly. There is no epoch shown for that plot, so it's pretty much useless without a reference to the time of observation. I'll start with the time quoted in the waterfall plot of the radio signal in my question. Remember geostationary satellites appear fixed in the sky and so cycle through 360° in RA every sidereal day! $\endgroup$
    – uhoh
    Jul 22, 2017 at 10:39
  • $\begingroup$ All of those listed are pretty old. Newest is 2002, but they go back to 1980's and 1970's. Very doubtful if any of these are transmitting. Two are moving much faster than anything near GEO. My plot looks different also, I can't account for the difference yet though. i.stack.imgur.com/MOoZE.png I don't understand the science behind multiple reflections of satellite signals explaining the (huge!) dispersion. By the way, if you are quoting, you should use block quotes (begin line with the"> greater-than symbol) and quote exactly rather than paraphrase. $\endgroup$
    – uhoh
    Jul 22, 2017 at 18:01
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    $\begingroup$ The quotes are copy paste from the listed source, with editoral changes shown in brackets. Quote marks are a common method of indicating quotations in English. I do know how to do block quotes. (check my rep, I've been here a while)! but thanks for the reminder. Its clear that the scientists also don't fully understand the dispersion, but satellite transmission was the best explanation. Who knows what old satellites do : dord.horse $\endgroup$
    – James K
    Jul 22, 2017 at 18:10
  • $\begingroup$ OK I see now, the multiple reflections from satellites sentence near the end is your own summary of the explanation, and not at all intended as a direct quote of it. Got it! The editorial markings earlier are fine with me. I can't imagine SE life without block quotes, but seems you're enjoying it quite nicely :) $\endgroup$
    – uhoh
    Jul 22, 2017 at 18:18

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