In 2020 the extra-solar object, tagged Oumuamua, screamed through the solar system. Weirdly shaped, oddly colored, it made some very odd changes in velocity without the usual cloud of dust characteristic of comets. It was so odd that the “alien spaceship” hypothesis was not only alive, but near the top of the stack.
That got our nerdy hearts a thumpin’, that’s for sure!
Since that time, excited nerds scientists have been proposing hypotheses to explain Oumuamua. In particular, how did it change velocity, and why did we not see evidence of out gassing during those events? This was very odd, and has provoked some pretty exotic hypotheses, including, of course, that it is an alien space probe maneuvering with a light sail.
Our nerdy hearts are still thumpin’, and the hypotheses are still coming in.
This spring, researchers from Berkeley and Cornell discuss a new hypothesis, arguing that Oumuamua is a water rich comet that had accumulated pockets of dissociated Hydrogen [1]. When it neared our sun the Hydrogen out gassed, but was not detected by our fleeting observations.
The key observation is that cosmic ray bombardment can dissociate water molecules, forming molecular Hydrogen, H2. Over the long trip to us, Oumuamua was continuously “baked” by this high energy radiation, producing considerable amounts of H2. This gas would accumulate in bubbles in the ice.
Their analysis indicates that the accumulated H2 would explode out of the ice when heated, potentially producing enough thrust to account for the observed behavior.
The researchers note that the amount of H2 would be small enough to not be easily detected, and the out gassing would not have water to detect. The acceleration would be pretty small, but Oumuamua is a small enough object that it would move it. But larger comets generally seen would have very little effect from such H2. Also, the heating and out gassing is a surface effect, so larger objects would have relatively little effect for their mass.
This hypothesis is certainly interesting (and, for some, “compelling” [3]), and raises interest in examining other small dark comets in our solar system—if we can find them. Oumuamua was noticed because it was so weird, but objects that size and that dim are mostly not seen. So no one really knows very much about them. This kind of odd behavior might be common for these objects, but just hasn’t been seen.
Of course, deep in our nerdy hearts, we definitely want it to be an alien space probe! With a light sail!
But it probably was a wandering comet.
- Jennifer B. Bergner and Darryl Z. Seligman, Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice. Nature, 615 (7953):610-613, 2023/03/01 2023. https://doi.org/10.1038/s41586-022-05687-w
- Nell Greenfieldboyce, Scientists think they know why interstellar object ‘Oumuamua moved so strangely, in NPR News – Science, March 22, 2023. https://www.npr.org/2023/03/22/1164814086/scientists-think-they-know-why-interstellar-object-oumuamua-moved-so-strangely
- Marco Micheli, A compelling explanation for the enigmatic small object ‘Oumuamua. Nature, 615 (7953):591-592, 2023. https://www.nature.com/articles/d41586-023-00797-5
- Dennis Overbye, Oumuamua Was a Comet After All, a Study Suggests, in New York Times. 2023: New York. https://www.nytimes.com/2023/03/22/science/astronomy-oumuamua-comet.html
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