In late 2017, astronomers detected a visitor from beyond our solar system for the first time. It’s likely happened before, and will again, but this time it was not a product of science fiction, as entertaining, thought- provoking, or as banal those books and movies might be. While most of us would consider it unlikely that it is an alien spaceship, this interstellar visitor presents a useful opportunity to construct physical theories that account for the data, while also prompting us to refresh our considerations of the kinds of visits and communications that might be detected by SETI projects.

Astronomers today are engaged in the systematic tracking of asteroids, comets, and other non- planetary denizens of our solar neighborhood. For example, the Pan-STARRS telescope situated atop the Haleakala crater in Hawaii is dedicated to the detection of moving and variable celestial objects. In October of 2017, it sighted an object whose subsequent track revealed that it was moving far too quickly to remain in our solar system, or to have originated within it. It was first given a “C” classification and number representing a comet in the designation system of the International Astronomical Union. Comets are chunks of icy material originating from the outer regions of our solar system. They form a gassy envelope when nearing the sun. However, the failure to detect such a cometary coma led astronomers to switch to an “A” designation referring to an asteroid. But when its speedy hyperbolic trajectory was firmly established, a new classification of “I” for interstellar was created. 1I/2017 U1 is the first officially named interstellar vagabond. The observing team then named it Oumuamua from the Hawaiian language meaning “first messenger” or “scout”.

How surprising is such a visitor? Our solar system is littered with objects smaller than its planets and moons which are the natural products of the accretion and collision processes that formed the sun and its orbiting bodies from a primordial rotating disc of gas and dust about 5 billion years ago. We have learned in recent years from ground-based surveys and orbiting observatories such as the Kepler Satellite that planetary systems and their dusty precursors are not at all unusual, but rather accompany roughly half of the stars. Thus, the census of the real estate of our Milky Way galaxy contains not only a few hundred billion stars (plus interstellar clouds of gas and dust), but also a comparable number of planets. Smaller rocks and ice chunks like asteroids and comets are naturally expected to be part of these planetary systems. Some objects will be kicked out of their stellar systems by gravitational encounters with planets and become interstellar wanderers. Estimates of the number of these interstellar vagabonds that will visit our neighborhood vary from very few to hundreds or more per year. With n=1 observed so far, it’s hard to know.

What have we learned about our first visitor? Unfortunately, Oumuamua was already speeding away from the sun when first observed, so its entry path is a product of calculation. Its trajectory is highly inclined to the plane of planets in our solar system, and its speed well exceeds the escape velocity of our suns’ gravity. It is following a hyperbolic path which must have whipped it around the sun on September 9, 2017, closer than the orbit of Mercury, at about 200,000 miles per hour. It is now farther from the sun than Jupiter and heading out of our solar system at about 70,000 miles per hour. As it is rapidly outdistancing effective observation from even our best telescopes, we may not be able to collect more data to add to what we now have to ground our theories. The extremely large variations in its light curve indicate that it is anything but a spherical rock or ball of ice. Rather it must be a highly elongated object that has been described as “cigar shaped” or “plate shaped” (avoiding the media-favorite words “torpedo-” or “saucer-shaped”). It is tumbling at a rotation period of ~7-8 hours. Estimates of its dimensions range from 300-3000 ft x 100-500 ft x 100-500 ft. But these dimensions have been questioned, as they are based only on light measurements and assumptions of how much light its surface reflects and how smooth it might be. Its size and shape were never directly observed, since it only appears as a point source of light in the most powerful telescopes.

The plot has thickened since reports in mid 2018 that Oumuamua is not passively coasting away from the suns’ pull, but is accelerating by a slight but significant amount. Initial speculation was that Oumuamua was receiving a rocket-like push from outgassing, a jet-like evaporation that occurs in comets. But this idea ran into problems. To account for the observed acceleration it would have produced a detectable comet-like tail. But none was found. Furthermore, such outgassing is incompatible with the objects’ spin. Perhaps light provides the motive force with a push of solar radiation pressure. This “light pressure” is strong enough to force dust particles away from the heads of comets to produce their characteristic tails, but could it be strong enough to push Oumuamua? That depends on what it is made of – whether it is a lumpy rock, a cloud of dust, a cosmic leaf or a solar sail.

Unusual astronomical observations have previously led to speculations of an intelligent alien design. When rapidly repeating high energy radio signals were first detected in the 1960s, even their discoverers whimsically tossed out a suggested code-name for them as LGM, standing for little green men. We now call them pulsars and understand them to be high energy beacons emitted from fast spinning neutron stars as their beam sweeps past our line of sight. So it may not be surprising that suggestions of alien technology soon appeared for Oumuamua’s acceleration, and not just from the media. The Chair of the Harvard Astronomy Department, Abraham Loeb and Schmuel Baily published a scholarly paper making just such a suggestion in Astrophysical Journal Letters last November.

After considering the observed trajectory and light curve, and reiterating the shortcomings of the comet-like outgasing scenario, the paper details the parameters necessary for the acceleration to be produced by radiation pressure. Only a thin, sheet-like object would allow the requisite response to the light pressure at its distance from the sun. Such a “light sail” has been proposed as a means to travel between our solar planets and beyond. So the authors came right out and suggested the possibility of an artificial origin for Oumuamua. Outlandish? We are reminded of the sage advice of Sherlock Holmes: “When you have excluded the impossible, whatever remains, however improbable, must be the truth.”

Astronomers have not yet run out of natural explanations. Recently, Zdenek Sekanina proposed that Oumuamua is a “monstrously fluffy dust aggregate” that was recently ejected from a local object by an explosive event (Such a cloud would not be expected to rotate and survive in its current form if it had travelled interstellar distances.) No doubt other proposals will be forthcoming. Unfortunately, we are not likely to ever gain more data on Oumuamua as it races away from detection from our best instruments. And it is probably a forlorn hope that we would develop and launch a next generation space probe to catch up with it.

Still, the alien hypothesis was interesting enough that SETI investigators have listened for signals from Oumuamua, both with the SETI Institute’s Allen Telescope Array and with the Breakthrough Listen detector on the Green Bank Radio Telescope. Nothing was heard so far. More listening seems appropriate, as well as assiduous searches by astronomers to detect and analyze other interstellar vagabonds.

Is there value beyond stirring the press and the public imagination in placing in the scientific literature the provocative possibility of an artificial origin of Oumuamua? Perhaps. Adding this to the list of possibilities for Oumuamua and future unusual phenomena doesn’t mean one abandons the search for more-likely natural explanations. But it does serve to remind us to keep reconsidering the nature of possible encounters with, or signals from, extraterrestrial intelligence so they can best be detected and understood. And it should help to fuel the continued efforts of SETI. Even if Uamuamua is not an artificial scout, and the odds of an extraterrestrial encounter remain low, recall that the probability of winning the LOTTO is so miniscule that anyone would reasonably round off that number to zero. But not buying a ticket truly makes the odds of winning zero.

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Richard M. Lawn

Welcome to the forum, Dr. Lawn. Bearing in mind my enthusiast's hope that a new discovery will reveal the presence of extraterrestrial intelligence, I try to avoid jumping to conclusions. Having said that, though, I wonder at the the ability of a 'monstrously fluffy dust aggregate' to withstand the explosive ejection from even a nearby body. Would such a flimsy aggregate be likely to maintain a thin, elongated shape after being wrenched away from a parent body, instead of collapsing into a shape of more-or-less equal dimensions?

Then, too, we heard so much about Oumuamua being reliably determined to be of interstellar origin. Yet it's stated that, such a hugely porous object could not be expected to survive an interstellar journey.
Given these enigmas, and those others, already detailed by Dr. Loeb, I seem to glimpse a more than minute possibility that Oumuamua could be an artificial object.

Thank you and welcome

Well, I'm just a regular guy that skips flat rocks across smooth bodies of water and those rocks that do skip, tend to find a path of their own as they progress, until gravity plunges them.
If indeed there is some alien driver of this potential "vagabond vehicle" then I must say that they do quite a bad job of driving because they almost ran into our Sun on their drive-by..... !!
Welcome Dr. Lawn.....

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We are all Alien. It just depends on which planet you are standing on.

If I was an advanced race, I would turn rocks into spaceships because you could travel faster and farther. I don't think the material we use, here on earth could travel far unless they were made of "Star Stuff"! (off the mind response!)

Your final comment re LOTTO really pushes home the justification for SETI . Great article!

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Chris (craisin)

The plot has thickened since reports in mid 2018 that Oumuamua is not passively coasting away from the suns’ pull, but is accelerating by a slight but significant amount. Initial speculation was that Oumuamua was receiving a rocket-like push from outgassing, a jet-like evaporation that occurs in comets. But this idea ran into problems. To account for the observed acceleration it would have produced a detectable comet-like tail. But none was found. Furthermore, such outgassing is incompatible with the objects’ spin. Perhaps light provides the motive force with a push of solar radiation pressure. This “light pressure” is strong enough to force dust particles away from the heads of comets to produce their characteristic tails, but could it be strong enough to push Oumuamua? That depends on what it is made of – whether it is a lumpy rock, a cloud of dust, a cosmic leaf or a solar sail.

After considering the observed trajectory and light curve, and reiterating the shortcomings of the comet-like outgasing scenario, the paper details the parameters necessary for the acceleration to be produced by radiation pressure. Only a thin, sheet-like object would allow the requisite response to the light pressure at its distance from the sun. Such a “light sail” has been proposed as a means to travel between our solar planets and beyond. So the authors came right out and suggested the possibility of an artificial origin for Oumuamua. Outlandish? We are reminded of the sage advice of Sherlock Holmes: “When you have excluded the impossible, whatever remains, however improbable, must be the truth.”

What are the odds this turns out like the Pioneer Anomaly? Much ado about bad calculations and missed factors. I might just toss in space weather as a possible factor. Coronal mass ejection along the path of the object giving it a gently push. There is also the thermal explanation. As we really don't have a clue as to the size or shape of the object or its albedo or the surface variations very little is known to any precision at all. The complex light curve we say means tumbling, rests on an assumption that the surface doesn't have bright and dark patches. And we were just surprised with Ultima Thule and a sphere turned out to be a pancake. Unfortunately we didn't see it coming, just going. We don't know and can't.

However we can and should do a better job looking for possibilities so we can spot them as they approach and not just after they have waved goodbye.

Just a "galatinous" jellyfish in our galaxy swimming away from the sun. : )

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Thank you

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Dr. Michael Hutchison

Too bad Arthur C. Clarke wasn't alive when Oumuamua passed through.

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It was just a rock from interstellar space. I'm quite sure this happens often.

we don't know why everything in space is accelerating.
Maybe that unknown source has something to do with the accelaration of this thing?

If something accelerates there must be a force acting on it, otherwise it would maintain the same speed.
Tullio

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Where is the next one?

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This is very interesting Article. Imagine, the Object had 260x sonic speed when discovered , you wouldnt get hit by that ;)

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Nice write-up.

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Welcome, and many thanks for the nice article. Oumuamua is truly fascinating, and it recalls us of the classical novel by Sir Arthur C. Clarke, "Rendezvous with Rama"!

Oumuamua
Dr. Lawn, welcome to the SETI Family and thank you for an informative and entertaining article.

I gather from you writing that we had an oblate spheriod of determined size and trajectory but of undetermined density, orientation, source, intelligent contol or destination tumbling through our solar system. About the only thing we know for certain is that it came, we saw it, it is leaving and if it is a scout it is not a Boy Scout. A Boy Scout would never display that lack of truth we see here.

As for its SETI association, I don't see it's gaining speed as it leaves the area as being caused by intelligent control. Once past the sun it began to slow down in response to drag (if any) and the pull of the sun's gravity. It's rate of slowing was decreased by the propulsive force of solar radiation and maybe the solar wind. At some point the energy of gravitations attraction was overcome by radiant and wind energy. Oumaumau began to accelerate. No intelligent being intervention required (unless she was late for lunch).

Skeptics always want the last word.

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I guess if Eric can stumble grammatically so can I.

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Thanks for the post Dr. Lawn and welcome. Very interesting indeed. My own view is that unless we have something that reflects unequivocally intelligence in some way or another then we are dealing with some form of yet uncategorized natural phenomenon. If this object had stopped mid-trajectory, abruptly changed direction or reversed its trajectory then that is definitely reflective of some propulsive capacity. Likewise if we had picked up, for example, radio signatures suggesting radar mapping of our solar system then again I call that unequivocal evidence of intelligence. As things stand we can only speculate, however I personally do not consider it to be evidence of alien technology or activity. Best wishes to all fellow SETI members.