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Simple explanation rules out that the comet ‘Oumuamua is a ship

'Oumuamua


‘Oumuamua – NASA, ESA/ J. OLMSTED/ FRANK SUMMERS STSCI

March 22 () –

A pair of astronomers from Berkeley and Cornell Universities have found a surprisingly simple explanation for the strange orbit of comet ‘Oumuamua, detected in 2017.

Upon being described, it was determined that this object it was accelerating away from the Sun in a way that astronomers could not explain. This detail, together with the fact that it did not have a bright comma or dust tail, its peculiar elongated shape and its small size, led some to suggest that it was an alien spacecraft.

The researchers now argue that the comet’s mysterious deviations from its hyperbolic path around the Sun can be explained by a simple physical mechanism common to many icy comets: the outgassing of hydrogen as the comet warms up in sunlight, as published in the journal ‘Nature’.

What set ‘Oumuamua apart from any other well-studied comet in our solar system was its size: It was so small that its gravitational deflection around the Sun was slightly altered by the small push it created. when hydrogen gas burst out of the ice.

Most comets are essentially dirty snowballs that periodically zoom in on the sun from the far reaches of our solar system. When heated by sunlight, a comet ejects water and other molecules, producing a bright halo or coma around it and often gas and dust tails, They act like the thrusters of a spacecraft.

When discovered, ‘Oumuamua had neither a coma nor a tail and was too small and too far from the Sun to capture enough energy to expel much water, which led astronomers to speculate wildly about its composition and what was pushing it outward.

Jennifer Bergner, an associate professor of chemistry at UC Berkeley, thought there might be a simpler explanation. She raised the issue with a colleague, Darryl Seligman, now a National Science Foundation Postdoctoral Fellow at Cornell University, and they decided to work together to put it to the test and see if the comet might be emitting hydrogen as it heated up when it entered the system. solar and that would produce the force that is needed to explain the non-gravitational acceleration.

He found that experimental research published since the 1970s showed that when ice is struck by high-energy particles similar to cosmic rays, abundant molecular hydrogen (H2) is produced and trapped in the ice. “Because ‘Oumuamua was so small, we think it produced enough force to drive this acceleration,” reveals.

The comet is believed to have been about 115 by 111 by 19 meters in size, but astronomers could not be sure of its actual size because it was too small and distant for telescopes to resolve.

“The beauty of Jenny’s idea is that it’s exactly what should happen with interstellar comets,” Seligman says. “We had all these stupid ideas, like hydrogen icebergs and other crazy stuff, and it’s just the most generic explanation.”

On October 19, 2017, on the island of Maui, astronomers using the Pan-STARRS1 telescope, operated by the Institute for Astronomy at the University of Hawaii at Manoa, observed for the first time what they thought was a comet or a asteroid.

Once they realized that its inclined orbit and high speed — 87 kilometers per second — implied that it came from outside our solar system, they gave it the name 1I/’Oumuamua (oh MOO-uh MOO-uh listen)), which is Hawaiian for “a messenger from afar who arrives first”. It was the first interstellar object, other than dust grains, ever seen in our solar system.. A second, 2I/Borisov, was discovered in 2019, although it looked and behaved more like a typical comet.

As more telescopes focused on ‘Oumuamua, astronomers were able to trace its orbit and determine that it had already circled the sun and was heading out of the solar system.

Since its brightness changed periodically by a factor of 12 and varied asymmetrically, it was assumed that it was very elongated and rotated from end to end. The astronomers also observed a slight acceleration away from the Sun, greater than that observed in asteroids and more characteristic of comets.

But unlike comets, the astronomers detected no coma, outgassing molecules, or dust around ‘Oumuamua. In addition, the calculations showed that the solar energy impinging on the comet was insufficient to sublimate the water or organic compounds on its surface and give it the observed non-gravitational momentum. Only hypervolatile gases such as H2, N2 or carbon monoxide (CO) could provide an acceleration sufficient to match the observations, given the incoming solar energy.

“We’ve never seen a comet in the solar system that didn’t have a dust coma. So non-gravitational acceleration was really weird.Seligman admits.

This led to much speculation about what volatile molecules might be in the comet to cause the acceleration. Astronomers had to struggle to explain what conditions could lead to the formation of solid bodies of hydrogen or nitrogen, which had never been observed before.

Bergner thought that outgassing of ice-trapped hydrogen could be enough to speed up ‘Oumuamua. Searching through previous publications, he found many experiments showing that high-energy electrons, protons, and heavier atoms could turn water ice into molecular hydrogen, and that a comet’s fluffy, snowball structure could trap space. gas in bubbles inside the ice.

Experiments showed that when heated, for example by the heat of the sun, ice anneals (changes from an amorphous to a crystalline structure) and forces the bubbles out, releasing hydrogen gas. Bergner and Seligman calculated that the ice on the surface of a comet could emit enough gas, either in the form of a collimated beam or a fan spray, to affect the orbit of a small comet like ‘Oumuamua.

“The main conclusion is that ‘Oumuamua is a standard interstellar comet that has just undergone heavy processing,” Bergner says. “The models we have run are consistent with what we see in the solar system in comets and asteroids. So, Basically, you could start with something that looks like a comet and have this scenario work.”

The idea also explains the absence of dust coma. “Even if there was dust in the ice matrix, you’re not sublimating the ice, you’re just rearranging the ice and then letting H2 release. So the dust won’t even come outSeligman said.

Seligman points out that his conclusion about the source of ‘Oumuamua’s acceleration should settle the comet question. Since 2017, he, Bergner, and their colleagues have identified six other small comets with no observable coma, but with small non-gravitational accelerations, suggesting that such “dark” comets are common.

One of these dark comets, 1998 KY26, is the next target of the Japanese Hayabusa2 mission, which recently collected samples from the Ryugu asteroid. 1998 KY26 was thought to be an asteroid until it was identified as a dark comet in December.

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Written by Editor TLN

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