he just released a hydrogen fart

Images of ‘Oumuamua circled the globe in 2017. The wacky cigar-shaped object floating in space was the first known visitor from outside our solar system. But what was it?

‘Oumuamua didn’t have a tail of dust or a hazy cloud of gas around it like other comets. Plus, there was this weird shape that was halfway between a cigar and a pancake. The thing was also far too small to be a comet. But the craziest thing was that ‘Oumuamua was moving faster and faster as it moved away from the sun. This even prompted some scientists to dare to speak of an alien spacecraft.

hydrogen farting
Unfortunately, the reality is much less exciting. The mysterious deviation of its orbit around the sun can be explained by a simple mechanism common to icy comets: when they warm up from the sun, they release hydrogen gas.

But if it’s so normal, why didn’t the researchers think about it right away? What made ‘Oumuamua (Hawaiian for Scout) so different was its size. The comet was so small that the push from the hydrogen fart was enough to slightly alter its orbit around the sun.

Rockets
Most comets are, in fact, dirty snowballs, periodically approaching the sun from the far reaches of our solar system. As they heat up in the sun, they release water and other molecules, creating a glowing halo around the comet and often a tail of gas and dust. The gases that are released are a kind of propellants that give the comet a thrust, so that the trajectory deviates slightly from the normal elliptical orbit, which planets and asteroids travel, for example.

When ‘Oumuamua was discovered, the comet had no halo or tail. It also seemed too small and too far from the sun to release much water. This has caused so much confusion among astronomers. They wondered what the object was made of and why it was moving so fast. Was it all just a big iceberg of hydrogen? Or just a big airy snowflake propelled by sunlight? Or a light sail made by an alien civilization? Or just a spaceship with its own fuel?

Cooked by radiation
Researcher Jennifer Bergner of UC Berkeley i thought there was one much simpler explanation should be. She asked her colleague Darryl Seligman to Cornell University to test the hypothesis together. “A comet passing through the middle of our solar system is actually baked by cosmic rays, which produces hydrogen. Our thinking was, if that happens, can it stay trapped in the object so that it can expel hydrogen as it enters our solar system and heats up?” said Bergner. “Could that produce enough force to explain the acceleration?

And the answer is yes. Surprisingly, Bergner found experimental research from the 1970s, 1980s, and 1990s that already showed that ice under the influence of cosmic rays can produce a lot of hydrogen that gets trapped in the ice. Cosmic rays appeared to be able to penetrate tens of meters deep into the ice, converting at least a quarter of the water into hydrogen gas.

The little ‘Oumuamua
“With a comet several kilometers thick, it is only a very thin shell compared to the mass of the object. The release of hydrogen gas would then be barely noticeable,” explains the researcher. “But because ‘Oumuamua was so small, we believe the hydrogen provided enough force to cause the acceleration.”

The comet, which was slightly reddish in color, is estimated to be 115 meters by 11 by 19 meters, although this is not entirely certain as it was too small and too distant for telescopes to see. Thus, its size was determined based on the luminosity of the comet. Anyway, it’s a small thing. The comets known to date are all at least 1 kilometer to hundreds of kilometers in diameter.

Exactly how it should be
“What’s great about Jenny’s idea is that this is exactly what should happen with interstellar comets,” Seligman says. “We had all these stupid ideas, like hydrogen icebergs and other crazy things, and then this turns out to be the most logical explanation.”

His idea also explains the lack of a dust halo, something that had never really been seen before. “Even if there was dust in the ice, it is not necessary to release it. The ice did not evaporate, the water molecules were simply rearranged, after which hydrogen gas was released. So the dust doesn’t even come out,” says Seligman. “The main conclusion is that ‘Oumuamua is a normal comet that has just gone through a difficult process,” he concludes.