New evidence for the nature of interstellar object ‘Oumuamua is in, making it far more likely that the unusual interloper is a comet rather than an asteroid. The data come from an array of instrumentation — the Hubble Space Telescope, the Canada-France-Hawaii Telescope, ESO’s Very Large Telescope and the Gemini South Telescope — and show that `Oumuamua is slowing down slightly less than expected. We are talking about a tiny force, about 1/1000 as strong as the pull of the Sun’s gravity, according to this overview of new work in Nature.

The science paper on this work, which also appears in Nature, looks at a variety of possible explanations for the velocity change. The one the authors think most likely is that `Oumuamua (pronounced “oh-MOO-ah-MOO-ah”), now moving at some 114,000 kilometers per hour, has vented material during its pass through our system, behaving the way many comets do. Marco Micheli (ESA), lead author of the paper, puts it this way: “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”

Co-author Karen Meech (University of Hawaii) concurs. It was Meech who led the initial discovery team characterisation in 2017. Although the scientists found no visual evidence for outgassing, it remained true that the composition of its surface resembled a cometary nucleus. Meech adds: “We think that ‘Oumuamua may vent unusually large, coarse dust grains.” If passage through interstellar space had eroded smaller dust grains on the surface of the object, a cloud of larger particles would not have been bright enough for Hubble to detect.

Image: This artist’s impression shows the first interstellar object discovered in the Solar System, ?Oumuamua. Observations made with the NASA/ESA Hubble Space Telescope, CFHT, and others, show that the object is moving faster than predicted while leaving the Solar System. Researchers assume that venting material from its surface due to solar heating is responsible for this behavior. This outgassing can be seen in this artist’s impression as a subtle cloud being ejected from the side of the object facing the Sun. Because outgassing is a behavior typical for comets, the team thinks that ?Oumuamua’s previous classification as an interstellar asteroid should be changed to a comet. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser.

For my part, I like the description offered by Michele Bannister (Queen’s University Belfast) in the article cited above. Bannister compares the object to a “‘baked Alaska’ dessert, with a frozen heart and warm exterior.” An object like this, approaching the Sun, would begin outgassing, though the rate here is tiny. Let me quote from the Nature article:

The outgassing rate is small compared to what typical comets experience, says Jessica Agarwal, an astronomer at the Max Planck Institute for Solar System Research in Göttingen, Germany. ‘Oumuamua also emits relatively little debris, perhaps because its dust particles are too large and heavy for the weak outgassing to carry aloft. That could explain why ‘Oumuamua never developed a visually stunning, comet-like tail.

Comets can be affected by non-gravitational accelerations, however, as ‘Oumuamua now apparently shows. From the paper:

After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.

The paper also considers solar radiation pressure, the Yarkovsky effect (in which thermal variations on the surface of a rotating object like an asteroid can lead to asymmetric forces), and the possibility of a collision with another object, none of which fit the bill. The unlikely idea that `Oumuamua is an alien spacecraft is rejected because the object is tumbling on all three axes. Our short-term interstellar guest, in any case, has been nudged a bit faster than expected.

Assuming we’re dealing with a comet, the outgassing may make our attempts to trace its home star that much more difficult. The new observations were carried out to help make that call, but team member Olivier Hainaut (European Southern Observatory, Germany) now wonders whether we will ever know its true home. As to its apparently cometary nature, he adds:

“It was extremely surprising that ?Oumuamua first appeared as an asteroid, given that we expect interstellar comets should be far more abundant, so we have at least solved that particular puzzle. It is still a tiny and weird object that is not behaving like a typical comet, but our results certainly lean towards it being a comet and not an asteroid after all.”

The paper is “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (`Oumuamua)”, Nature 27 June 2018 (abstract).