`Oumuamua: Red, Tumbling, and Silent

Astronomers' recent observations of our first-known interstellar visitor reveal that it is very strange indeed.

Portrayal of 'Oumuamua (1I/2017 U1)

This portrayal of 'Oumuamua (1I/2017 U1), by the same artist as the one above, suggests a less elongated shape. Some asteroid specialists have questioned how extreme this body's length-to-width ratio might be.
ESO / M. Kornmesser

Back in late October, in the days following the discovery of what proved to be an object from interstellar space, astronomers scrapped the observing plans at observatories worldwide to scrutinize this unique visitor before it zipped out of the inner solar system, never to return. Right now ‘Oumuamua (or 1I/2017 U1), as it's now known, is some 350 million km (2.3 astronomical units) from Earth, has dimmed to only 27th magnitude, and is receding at another 5½ million km (15 Earth-Moon distances) each day.

But the flurry of observations made weeks ago, when it was closer and brighter, have fueled a second flurry of activity — writing scientific papers — to detail what we've learned. Here are three highlights that have come to light since S&T's previous update.

It's Reddish

Some of the first observations of 1I/2017 U1 showed that it had a slightly reddish color, not unlike a class of red-tinged bodies (called D types) found in the outer asteroid belt. Careful spectral measurements made by Alan Fitzsimmons (Queens University Belfast) and others using the 4.2-m William Herschel Telescope and the 8.2-m Very Large Telescope bear out this initial assessment.

Portrayal of 'Oumuamua (1I/2017 U1)

This portrayal of 'Oumuamua (1I/2017 U1), by the same artist as the one above, suggests a less elongated shape. Some asteroid specialists have questioned how extreme this body's length-to-width ratio might be.
ESO / M. Kornmesser

Writing in December 18th's Nature Astronomy, the observers note that they don't see any spectral evidence for outcrops of rocky minerals. This suggests that all of `Oumuamua might be covered with a veneer of icy organic compounds that have turned red after prolonged exposure to space radiation. This conversion doesn't take very long on cosmic time-scales — roughly 10 million years — and for all we know this body had been drifting through interstellar space for billions of years before showing up on the Sun's doorstep.

In fact, the Fitzsimmons team contends, the dark, reddish exterior of ‘Oumuamua might be masking an icy interior deeper down. The researchers calculate that the surface temperature soared to roughly 600 Kelvin during its brief but close encounter with the Sun (just 0.25 astronomical unit, well inside Mercury's orbit). But they also find that a topside layer of irradiated, carbon-rich goo would only need to be a half meter thick to keep the Sun's heat from reaching deeply buried water ice and even frozen carbon dioxide. They'd remain as ices, instead of sublimating and escaping in comet-like fashion.

"We recognize one obvious problem with this model," the authors admit. Comets in our own Oort Cloud should likewise have built up thick mantles due to 4½ billion years of exposure to cosmic rays, yet most are clearly give off lots of gas as they near the Sun. One possible out: Maybe ‘Oumuamua spent enough time near its host star that become completely devolatilized before heading our way.

It's Tumbling

It didn't take observers long to realize that ‘Oumuamua displayed wild swings in brightness of up to 2½ magnitudes — a factor of 10! — that defied easy explanation. Surely it must be highly elongated, with a length 5 to 10 times its width (depending on whom you ask). Some of the initial reports suggesting a spin period of 7.5 hours proved premature.

Wesley C. Fraser (Queen's University Belfast) and others combined brightness measurements from multiple telescopes to test for a rotation period. They conclude that "no single rotation period can explain the exhibited brightness variations." Instead, the object appears to be tumbling in an "excited rotational state."

Likewise, a team led by Michal Drahus and Piotr Guzik (Jagiellonian University, Poland) used the Gemini North telescope to bag hundreds of images on October 27th and 28th. "While the light curve of 1I/‘Oumuamua is clearly periodic, it does not repeat exactly from one rotation cycle to another," they write in an online preprint. "Furthermore, the light curve does not appear to have a single, unique periodicity because the rotation periods reported by other studies differ from one another and are inconsistent with our data."

The object could only be spinning this erratically if it were tumbling, a characteristic that's rare among our solar system's asteroids and comets. (Comet 1P/Halley is a tumbler, for example, driven by assorted gas jets from its nucleus.) But ‘Oumuamua shows no hint of outgassing, and the chance that it struck something in our solar system en route to its discovery seems impossibly remote. Most likely, they conclude, it had a rough time while escaping from the system of its origin.

It's Not Broadcasting!

Green Bank Telescope

The world's largest fully steerable radio telescope is the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The dish measures 100 by 110 meters.

One of the most unexpected studies occurred earlier this month when Breakthrough Listen — the initiative to find signs of intelligent life in the universe — funded an effort to eavesdrop on any radio transmissions that ‘Oumuamua might be broadcasting. On December 13th astronomers used the huge Robert C. Byrd Green Bank Telescope in West Virginia to listen to billions of radio frequences in four bands spanning 1 to 12 gigahertz.

The Green Bank search didn't turn up any radio "beacons" or other transmissions in narrow frequency bands, though the analysis of three of the bands isn't complete. Honestly, the Breakthrough Listen team didn't expect to pick up alien broadcasts from this interloper — but, hey, why not try, right?

One final set of observations of 1I/2017 U1 is under way. Karen Meech and others are attempting to record images with the Hubble Space Telescope. Their aim is simply to locate its position against background stars as accurately as possible. Those measurements, in turn, will fine-tune calculations of the object's in-bound trajectory and help us identify where this strange object came from and perhaps shed some light on how it got here.

24 thoughts on “`Oumuamua: Red, Tumbling, and Silent

  1. rocksnstarsrocksnstars

    Why not try? Maybe because telescope time shouldn’t be wasted? True, this was a one time thing, so not a big deal, but scopes dedicated to alien searches are another story. Fermi’s Paradox tells us, at lease some of us, that we ARE alone in the Milky Way. If anyone reads Rare Earth: Why Complex Life Is Rare in the Universe, by Ward and Brownlee, they might agree also. Obviously my opinion, and the two authors among others, but I would be really surprised if we ever make contact. It would be interesting, although likely not a good thing – ask Hawking, but at 70 years old, it probably won’t happen in my lifetime. So good luck to all you youngsters who are counting on it. If it ever happens, maybe you can let me know.

    1. David-S

      Even the great Fermi was wrong in his life. His paradox was another example. We are visited hourly but this will not become public until the public demands the governments to release it. Conspiratorial? One person’s conspiracy theory is another person’s strategy. They haven’t “made contact” public within my lifetime of 71 years, but I learned the truth way back in ’65. As they say in the sci fi movies, “Keep watching the skies.”

    2. Frank-ReedNavigation.comFrank-ReedNavigation.com

      The “Rare Earth” solution to the Fermi Paradox, advocated with such energy by Ward and Brownlee, is merely one end of the spectrum of hypothetical solutions to the issue, and a great many of the specifics in the Rare Earth hypothesis have been grossly exaggerated in recent decades. For example, rare earthers often claim that life on Earth would be impossible with Jupiter to protect us… or that it would be impossible without a large Moon. But these claims are pseudoscience –they resemble science.

      For anyone reading along curious about this Rare Earth hypothesis, the links from the Wikipedia article on the topic (https://en.wikipedia.org/wiki/Rare_Earth_hypothesis) provide lots of interesting reading, both pro and con.

    3. millerf1

      Keep looking up, there is still a chance depending upon your life expectancy. Everything is in the assumptions. If they use entangled particles to communicate it would be instantaneous at local galactic distances; there would be no RF, too primitive. If it was a flyby, a com package could arrive in as little as ten years or so because it takes time to speed up and slow a significant mass.

  2. goodricke1

    The Rare Earth book is no different from any other work on this subject – speculation and guesswork for which we have no proof one way or the other. There are as many answers to Fermi’s ‘paradox’ as there are questions raised by it.

    This latest attempt to explain Oumuamua is rather unsatisfactory; as the authors themselves admit, our own Oort cloud comets should have thick mantles and if Oumuamua has already been devolatilized then the need for a veneer to explain its lack of cometary activity becomes self-evidently redundant.

  3. db

    In Rare Earth, Peter and I pointed out arguments that et life is likely to be dominated by microbial life rather than advanced lifeforms. Microbial life might be easy to form when the conditions are right but it is very tough to extinct once it “infects” a planet. Most of Earth history has been dominated by microscopic life and this will be even more so when the Sun gets just a little brighter. If Oumuamua was made by advanced organisms, it may have been made billions of years ago. The crew may have been so bored living so long in such a small object that they neglected to notice that they eventually did pass close a star. With no power for such a long time what could they do except to freeze? They certainly were not traveling at warp speed so it must have taken a very long time to get here even if the were on a directed trajectory from a nearby star.

  4. Jim-BaughmanJim-Baughman

    I don’t see how any object could keep “tumbling” for billions of years. Pluto’s minor moons tumble because they are in the presence of disrupters. It seems that eventually the irregularities in Oumuamua, by exerting slightly different gravitational tugs at different points in a chaotic tumble, would eventually smooth the chaos into a simple rotation.

    1. Jim DeCamp

      Of course it will “tumble” for billions of years. It’s know as conservation of angular momentum. If a body has an axis of symmetry and is reasonably well balanced around that axis, and is “spun up” around that axis it will be perceived to be spinning, like the earth, moon, or planets. Bodies that are not symmetrical, or not well balanced, appear to tumble, regardless of initial conditions. Had it not been tumbling, that would have been more surprising. Spacecraft and satellites are carefully designed to control rotation, random pieces of space debris, not so much.

      1. Frank-ReedNavigation.comFrank-ReedNavigation.com

        Uh, the problem here is that many “tumbling motions” die out rather quickly. Of course they don’t violate conservation of angular momentum by doing this, but the rotation of something that’s been in space for a billion years will have had plenty of time to lose rotational anomalies through internal “relaxation”.

        For example, suppose you build a cylindrical spaceship and set it spinning about its long axis. That is, the rotation axis of the spaceship is coincident with the long axis of the cylinder. What will happen to it over time if it’s “adrift”? Internal flexing of any body will be slightly “non-rigid” and this will lead to a slight precession of the rotation around the initial axis –in effect, a “wobble”. And over time, the angle of precession will steadily grow through that dissipative flexing of the structure. After some period of time, the entire rotation will shift to a new stable state (and slower) about an axis of rotation perpendicular to the long axis of the cylinder –rolling end over end, like the rotation initially described for Oumuamua. This process happens without violating any of the conservation laws. So, yes, tumbling motions –of a certain type– do tend to die out, even in a perfect vacuum. The problem so far is that we don’t have any idea what sort of tumbling motion Oumuamua is exhibiting, and indeed it sounds as if the principal investigators on this issue are not sure from their light curve data. It’s a complicated problem!

        By the way, for a real world example of the rotational transition I described above, look up what happened to Explorer 1.

  5. StanR

    Couldn’t tumbling have been introduced as the object sped by the sun? The sun would most likely have “pulled” on the very-not-round object from different angles relative to the (incoming) rotation axis of the object.

    1. Frank-ReedNavigation.comFrank-ReedNavigation.com

      Tumbling on the scale they’re reporting could not have been induced by tidal interaction (the “pulling” on a “very-not-round” object as you’ve described it) –it’s not up to the task by a wide margin. But it is conceivable that there was a brief cometary outburst that went undetected when Oumuamua was near perihelion. If we accept the model of an encrusted comet (which is described in the article, but also is admittedly in conflict with the behavior our own Oort cloud comets), then it’s possible a single crack might have opened up, venting gas from a small section, creating a simple rocket-like jet which could have created the unusual rotation.

  6. Frank-ReedNavigation.comFrank-ReedNavigation.com

    Even if we set aside all thought of technological origin, Oumuamua is an astonishing and extraordinary discovery worth every possible observation in every part of the electromagnetic spectrum. The light curve astronomers have seen simply makes no sense. The very first object seen on a definitively extra-solar hyperbolic orbit displays a light curve unlike anything ever seen in our own Solar System and with features that are still not understood. This is one weird piece of debris!

    Now picking up those technological thoughts that we set aside… just to keep an open mind! Here’s one: maybe it is literally debris –space junk. Nature likes spheroids, but technology likes elongated structures. Perhaps it’s just a great girder from some destroyed structure built by a civilization five billion years ago… And here’s another: if it’s targeting our Solar System, maybe it has merely found its way home after a long absence. If we could catch up to it and get inside, perhaps we would find, not aliens from some distant star system, but the bones of Venusians who left their planet in an ancient epoch, perhaps three billion years ago, when Venus was habitable and might have hosted its own thriving civilizations. Ya never know unless you look… 🙂

      1. Jim DeCamp

        Prior probabilities would seem to exclude an artifact from an alien civilization, especially given its huge size. For every kilogram of material launched into interstellar space by advanced civilizations, a couple of trillion tons of rocks are ejected by natural processes. Would seem to be a non-starter, but fun, if true.

        1. Frank-ReedNavigation.comFrank-ReedNavigation.com

          You wrote: “For every kilogram of material launched into interstellar space by advanced civilizations, a couple of trillion tons of rocks are ejected by natural processes.”

          And you know this… how? Since the only example we have of an advanced civilization launching material into space is our own, it’s impossible to put any numbers on this in the general case. How much stuff does a Kardashev “Type II” civilization launch into interstellar space? Or a Type III? We’re accustomed to imagining civilizations a few centuries beyond our own capabilities… that’s the playground of most science fiction after all. But what does a civilization do if it’s been around for ten million years? Can you really put numbers on the amount of interstellar “junk” it creates?? How many demolished Dyson Spheres are littering the Milky Way?? 🙂

  7. Mining EngineerMining Engineer

    All the tumbling theories depend solely on the light curve, which in turn assumes that the outer surface is homogenous. What if `Oumuamua’s surface is non homogenous, like that of Iapetus. Such a variegated surface could be natural like on Iapetus, or could be a device to control internal temperature in an artificial device.
    `Oumuamua’s close approach to the sun is also a little surprising. Assuming that a similar object would have been detected if it had approached within 1 AU of the sun, its actual approach to 0.25 AU has only a 6.25% probability.

    1. Kelly BeattyKelly Beatty Post author

      that’s a good point. however, it seems that the spectrum of `Oumuamua didn’t change as it rotated — something you’d expect if bright and dark patches were creating the light curve. as best we can tell, the surface is homogeneous in albedo and spectrum.

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