Alien iceberg
So, was ‘Oumuamua a comet, an asteroid, or something else? Because it was not spotted until it was well outbound, 40 days after whipping around the Sun, astronomers lacked vital information about its behavior.
Some proposed that ‘Oumuamua could be an alien spaceship or solar sail. Most astronomers discounted both of these theories since no radio signals were detected and ‘Oumuamua’s football-field length would have required a paper-thin sail in order to generate its observed acceleration.
Other researchers suggested a more natural explanation: It was venting gas along successive points of one side as they rotated into direct sunlight. This would cause it to rock back and forth like a pendulum, its surface alternately heated and cooled in searing sunlight and frigid shadow.
Most recently, some of the same authors published a paper in The Astrophysical Journal Letters last June, proposing that ‘Oumuamua is a hydrogen iceberg. Interstellar molecular hydrogen ice (H2) has long been theorized to exist, but never detected. It is a substance so mysterious that some cosmologists once believed molecular clouds might harbor vast quantities of it as the source of dark matter — the stuff that holds our galaxy together.
“It sounds farfetched because we’ve never seen hydrogen ice, but if you assume it’s correct, it explains every weird thing about ‘Oumuamua,” says Darryl Seligman, a postdoctoral fellow at the University of Chicago and a co-author of the study.
By Seligman’s reckoning, other candidates for outgassing would have been easily detectable or required a large amount of material to be ejected. Hydrogen ice explains ‘Oumuamua’s odd behavior because it is both difficult to detect and serves as a supercharged fuel.
“Molecular hydrogen is by far the best accelerant,” says Seligman. “For nitrogen [and others] to work, you would need the surface to be almost completely covered, whereas for hydrogen you could have a gunky thing with some frozen hydrogen and other stuff in there, which is a lot easier to form.”
It could also explain ‘Oumuamua’s long, shard-like shape. According to Seligman’s reconstruction, when our Sun began pelting it with photons, the surface along the object’s long axis was more exposed and melted quicker. “It’s exactly what happens to a bar of soap in the shower,” he says. “What you get is this little withered-away fragment that’s very elongated and very small.”
We will never see ‘Oumuamua again and will never know exactly what it was. But the serendipity of spotting ‘Oumuamua and Borisov in the span of just two years hints that we are missing many of their siblings. Luckily, the technology needed to spot them will soon be available. Sitting atop a mountain in Chile, the Vera C. Rubin Observatory is aiming to be fully operational in 2023. Its powerful telescope is designed to scan the entire sky every three nights with a 3,200-megapixel camera (the world’s largest) and will be capable of spotting objects in the solar system like interstellar interlopers and near-Earth asteroids, as well as more distant events like supernovae and gamma-ray bursts.