`Oumuamua: Listening To An Interstellar Interloper

The buzz about `Oumuamua, our first known visitor from another stellar system, seems likely to continue given yesterday’s news that the object’s axis ratio is a startling 10 to 1. Given all that, Jim Benford wondered whether there were SETI implications here. Was anyone on the case from our major SETI organizations? The answer is below, as we learn that the effort is ongoing. A frequent contributor to these pages, Jim is President of Microwave Sciences in Lafayette, California, which deals with high power microwave systems from conceptual designs to hardware. He also heads up the critical sail subcommittee for Breakthrough Starshot, the effort to send small beamed sails with miniaturized payloads to a nearby star.

By James Benford

I contacted Jill Tarter and Andrew Siemion about whether SETI researchers are conducting observations of the interstellar interloper, Oumuamua. Both say yes.

Jill said that the Allen Telescope Array has been looking at it for a while. Andrew said that Breakthrough Listen was using the Green Bank Telescope for a few hours last weekend. This was actually looking for water via hydroxyl lines using broadband 1.1-1.9 GHz data. No water was immediately evident in the coarse spectra from the standard data reduction. Breakthrough Listen is working on incorporating the appropriate windowing capabilities necessary to analyze this data, so as to use their data analysis pipeline.

Therefore there are some observations in parts of the microwave spectrum.

Image: This diagram shows the orbit of the interstellar asteroid ‘Oumuamua as it passes through the Solar System. Unlike all other asteroids and comets observed before, this body is not bound by gravity to the Sun. It has come from interstellar space and will return there after its brief encounter with our star system. Its hyperbolic orbit is highly inclined and it does not appear to have come close to any other Solar System body on its way in. Credit: ESO/K. Meech et al.

Besides astronomical observations of this unique object, there is also this remote possibility: That this interloper is an interstellar survey probe, having perhaps dropped down to interplanetary-scale velocities in order to take data during its transit of our solar system, before going on to another star.

If this is the case, then perhaps we ought to be looking rather broadly in the electromagnetic spectrum for any signal it might send to us, having easily detected leakage from Earth. That assumes it would try to respond to us using frequencies it knows we use. That would certainly include the microwave bands 1-10 GHz, where most of our radiation leakage radiation is.

I think at present the frequencies most observable coming from Earth are leakage of uplink transmissions to our satellites, of which there are now about 1200 active in orbit. Those frequencies tend to be in the upper end of the microwave where the wavelength is smaller, so we can use smaller apertures on both Earth and satellite. Downlinks, of course, would be absorbed in the Earth and not observable from afar.

Or, because they know enough about our atmosphere’s transmission windows and the Sun’s radiation spectrum, they might be signaling in the visible. Therefore our SETI optical observatories ought to be watching as well.

I would look for a pulsed beacon signal, which is more noticeable. That would be like a pulsar, but of course with no interstellar dispersion.

This matter has a very low probability of success, of course. However, it’s our first opportunity to observe at close range a truly interstellar object.

Image: This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. The large range of brightness — about a factor of ten (2.5 magnitudes) — is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object’s shape or surface albedo. Credit: ESO/K. Meech et al.