Oumuamua – Non-Gravitational Acceleration Observed but Probably not Alien Tech

Oumuamua – Non-Gravitational Acceleration Observed but Probably not Alien Tech

Observation of Non-Gravitational Acceleration

A recent paper in Nature has shown (Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)), via a comprehensive analysis of multiple observations, that Oumuamua has deviated from a purely gravitational track, on its way through the solar system.  In other words, its trajectory has been influenced by some other push than the gravity of the major bodies of the solar system.  The effect is very small, so it took a lot of observational data to detect the anomalous motion.

In the graph below, Oumuamua’s non-gravitational component is shown, and can be compared to solar system bodies (it’s the black vertical line).  As you can see, it is within the range for solar system comets, in this respect, though rather far into the right tail of the distribution.  So, relative to “normal” solar system comets, its non-gravitational acceleration is actually quite large, even if it is small in an absolute sense.

Just as a reminder, Oumuamua appeared in our skies in 2017, and has been tracked with large ground-based telescopes as well as the Hubble Space Telescope.  Its velocity (26 km/second, which is more than the escape velocity from the solar system) and trajectory (highly hyperbolic eccentricity=1.3) indicate that it must have originated from outside of our solar system, probably from somewhere in the constellation Lyra.

Lyra contains the bright star Vega, which is easy to find, as it is one of the stars of the asterism The Summer Triangle.  It is also an interesting coincidence for SF fans that Vega is the star that was featured in the novel and movie “Contact”, as the home planet of the alien civilization that sent the instructions to build a wormhole device.

Some 177 ground-based and 30 space-based (Hubble) observations were analysed, to determine Oumuamua’s track through the solar system with great precision.  Various mathematical models were then produced, and compared to the actual data, using standard statistical techniques, such as Chi-square goodness-of-fit tests.

The analysis showed that the arc of the body’s path could not be explained by a solely gravitational model, even if the sun, eight planets, moon, Pluto and the 16 largest asteroid were included in the model.  Many of the observations deviated from prediction by 3 to 5 standard deviations (about 35 in all did so).  Furthermore, there appeared to be a clear trend to the offsets, so this was not likely due to random observational errors.

Below is a copy and paste graph from the paper, showing the model residuals, using a purely gravitational model (a) vs a model that also includes an extra push from some non-gravitational source (b).  The residuals are what is left over after subtracting the observational data points from the predictions of the model at the location that those observations were made (in declination and right ascension, which are sky coordinates somewhat similar to latitude and longitude).  Basically, a good model will tend to have low deviations from predictions, and the deviations will be randomly scattered throughout the dataset.  Using those criterial, one can see that model b gives a better fit with observational data.

The model with the extra push includes a term that provides a small positive impetus that varies with a 1/r**2 (or 1/r) term, with r being distance from the sun.  In other words, the push is stronger when the body is closer to the sun, and it seems to vary with something that falls off as one over R squared, as do radiation levels from the sun.

Off-Gassing Explanation of Non-Gravitational Acceleration

So, does this mean that Oumuamua is actually a spacecraft of some sort, under alien control?  Not too likely, according to the paper.  Their hypothesis is that Oumuamua is actually an extra-solar comet, rather than an extra-solar asteroid, and that the slight non-gravitational changes in the object’s track through our solar system are caused by off-gassing from the comet (primarily the sublimation of water ice, carbon dioxide ice or carbon monoxide ice).  The level of off-gassing will tend to fall off with distance from the sun, as it is the sun’s radiation that produces the off-gassing, and the strength of that falls off with distance, in a 1/R**2 manner.

Off-gassing and related dust basically creates the tail and/or halo of a comet.  By a simple application of Newton’s laws, you can see that this off-gassing will also provide a push to an object, much like a rocket does for a spacecraft (though obviously a much smaller effect).

Some nice closeup pictures of a comet off-gassing were taken by the ESA Rosetta probe, a few years ago.  Also shown, is an image of comet dust, as Rosetta made a close approach to Comet 67/P.

One problem with this hypothesis, is that earlier observations seemed to have ruled out the body being a comet.  There had been no optical signs of a coma, dust, nor spectroscopic evidence of gas emissions.  Basically, it looked like an inactive body, corresponding more to an asteroid than a comet, when compared to objects native to our solar system. 

On the other hand, a comet always seemed like a more likely explanation than an asteroid, as theory said that interstellar comets would be brighter, and therefore easier to detect than interstellar asteroids.  So there’s that.

So, why wasn’t the evidence of off-gassing and dust seen in earlier optical and spectroscopic measurements?  One possibility for the lack of dust is that Oumuamua has an atypical dust-grain size distribution, with fewer small dust particles than usual.  Solar radiation pressure would thereby not be able to create a significant dust halo, as large dust particles won’t be affected much by solar radiation pressure.  It is also possible that Oumuamua simply had a low dust to ice ratio, so there wasn’t much dust to create a dust halo.  Perhaps the long journey through interstellar space had produced this low dust-ice ratio, via a thin isolating mantle or the journey had changed the surface in some other relevant way.

Other Possible Explanations     

Some other physical models that might produce a non-gravitational acceleration were discussed.  For fun, I have included a Science Fiction possibility, which I have called SF response, where possible:

• 1.     Solar Radiation Pressure, such as that which drives a light-sail spacecraft.  This was rejected on the basis that the object would have to have an unrealistically low density, three or four orders of magnitude lower than natural solar system objects.
• SF response – what if actually was a solar sail?
•  
• 2.     Yarkovsky Effect.  This is another subtle effect, based on anisotropic thermal emission of photons from a rotating body.  As with solar radiation, this effect is far too small to produce the acceleration detected.
• 3.     Friction or Drag.  Drag from collisions with small particles would tend to have a negative acceleration (slow Oumuamua down), whereas the observed acceleration was positive.
• 4.     Single Impulsive Change in Velocity.  An example of this would be a collision with some other solar system body (or the firing of a retro rocket :)).  However, the observations seemed to indicate that the acceleration was constant and in a radial direction (away from the sun).
•  SF response – what about the slow firing of a rocket, or other impulsive force (ion thruster?), perhaps energized by solar power, which would account for the 1 over R squared form of the acceleration?
• 5.     Fragmented Object.  If the object was fragmented, with the center of mass following a different trajectory than the detected (brighter) part of the object, it could appear to be following a non-gravitational track, while its center of mass was actually following a gravitational track.  But no secondary object has been detected, and it would have to be very small to escape detection, which would mean it wouldn’t affect the apparent path enough to account for the observations.
• SF response – How about a small, reflective craft that left the mother ship?
• 6.     Odd Surface Brightness.  If one end of Oumuamua was very bright (and therefore detected) while the rest of the object was very dark, it could look as if Oumuamua was following a non-gravitational track.  But it would have to be extremely long and thin for that effect to account for the observational discrepancies.
• SF Response - Oumuamua does appear to be very long and thin, more so than almost all natural bodies in our solar system (about 10 to 1).  However, it still seems to be far too small to account for the anomalies.
• 7.     Strong Magnetic Field.  If Oumuamua had a very strong magnetic field, then interaction with the solar wind could change its motion.  But that effect would be orders of magnitude too small (about 10,000 times), even if the object was fairly highly magnetized, for a natural body.
• SF Response - Interestingly enough, a ferromagnetic body (e.g. iron) has about 100,000 times more magnetic susceptibility than even the strongest magnetized rocks (according to my text of Applied Geophysics, by Telford et al).  So, how about an inactive spacecraft, perhaps built into a natural body?

Summary

So, it appears that the enigmatic interstellar visitor is best explained as a comet that was ejected from another solar system, somewhere in the vicinity of Vega.  But, its peculiar elongated shape, and now its non-gravitational motion, render it a fascinating object for speculation, whether scientific or science fiction.

Sources:

Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua), Marco Micheli, et al, Nature 27 June 2018

Applied Geophysics, Telford et al

Now that you have read about a real interstellar interloper (natural or not), you should consider reading some Science Fiction.  How about a short story, also about interstellar interlopers.  It also features one possible scenario to explain why we haven’t met ET yet (as far as we know, anyway).  Only 99 cents on Amazon.

The Zoo Hypothesis or The News of the World: A Science Fiction Story

Summary

In the field known as Astrobiology, there is a research program called SETI, The Search for Extraterrestrial Intelligence.  At the heart of SETI, there is a mystery known as The Great Silence, or The Fermi Paradox, named after the famous physicist Enrico Fermi.  Essentially, he asked “If they exist, where are they?”.

Some quite cogent arguments maintain that if there was extraterrestrial intelligence, they should have visited the Earth by now. This story, a bit tongue in cheek, gives a fictional account of one explanation for The Great Silence, known as The Zoo Hypothesis.  Are we a protected species, in a Cosmic Zoo?  If so, how did this come about?  Read on, for one possible solution to The Fermi Paradox.

The short story is about 6300 words, or about half an hour at typical reading speeds.

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Alternatively, consider another short invasion story, this one set in the Arctic.  Also 99 cents.

The Magnetic Anomaly

Summary

An attractive woman in a blue suit handed a dossier to an older man in a blue uniform.

“Give me a quick recap”, he said.

“A geophysical crew went into the Canadian north. There were some regrettable accidents among a few ex-military who had become geophysical contractors after their service in the forces. A young man and young woman went temporarily mad from the stress of seeing that. They imagined things, terrible things. But both are known to have vivid imaginations; we have childhood records to verify that. It was all very sad. That’s the official story.”

He raised an eyebrow. “And unofficially?”

“Unofficially,” she responded, “I think we just woke something up that had been asleep for a very long time.”

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