Comet ‘Oumuamua is not a spacecraft: A simple explanation rules it out

When describing it, it was determined that this object was accelerating away from the Sun in a way that astronomers could not explain. This detail, along with the fact that it did not have a bright dust coma or tail, and its strange elongated shape and small size, has led some to suggest that it was an extraterrestrial spacecraft.

The researchers now argue that the comet’s mysterious deviations from its hyperbolic trajectory around the sun can be explained by a simple physical mechanism common to many icy comets: the release of hydrogen gases as the comet heats up in sunlight, they report in Nature Magazine.

What sets ‘Oumuamua apart from any other well-studied comet in our solar system is its size: it was so small that its gravitational tilt around the sun was slightly altered by the tiny push from the explosion of hydrogen gas from the ice.

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

When it was discovered, ‘Oumuamua had no coma and no tail and was too small and too far from the sun to capture enough energy to expel much of the water, leading astronomers to speculate wildly about its formation and what made it push out.

Simpler explanation

Jennifer Bergner, assistant professor of chemistry at the University of California, Berkeley, thought there might be a simpler explanation. He raised this issue with his colleague, Daryl Seligman, now a postdoctoral fellow at the National Science Foundation at Cornell University, and they decided to work together to put it to the test and see if the comet emits hydrogen as it heats up at its entry. solar system and this would produce the force needed to explain non-gravitational acceleration.

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

The comet is believed to be about 115 x 111 x 19 meters in size, but astronomers could not be sure of its actual size because it was too small and too far away for telescopes.

“The beauty of Jenny’s idea is that it’s exactly what should happen to interstellar comets,” Seligman says. We had all these stupid ideas, like hydrogen icebergs and other crazy things, and that’s just a more general explanation. “

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

Once they realized that its inclined orbit and high speed — 87 kilometers per second — implied that it had come from outside our solar system, they named it 1I/’ Oumuamua (oh MOO-uh MOO-uh), which in Hawaiian means “messenger from afar who arrives first.” “. It was the first interstellar object, other than dust grains, seen in our solar system. The second, 2I/Borisov, was discovered in 2019, though it looked like a typical comet.

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

Since its brightness varied periodically by a factor of 12 and varied asymmetrically, it was assumed that it was very elongated and alternating from one end to the other. Astronomers have also noticed a slight acceleration away from the sun, greater than what is observed in asteroids and more characteristic of comets.

But unlike comets, astronomers haven’t spotted a coma, outgassing, or dust around ‘Oumuamua. In addition, calculations showed that the energy of the sun hitting the comet was not enough to sublimate the water or organic compounds on its surface and give it the observed non-gravity momentum. Only highly volatile gases such as H2, N2, or carbon monoxide (CO) could provide enough acceleration 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 the non-gravitational acceleration was really weird,” Seligman admits.

speculation

This led to much speculation about volatile particles that might be in the comet causing the acceleration. Astronomers have had to struggle to explain the conditions that can lead to the formation of solid bodies of hydrogen or nitrogen, which have not been observed before.

Bergner thought that the release of hydrogen gases trapped in the ice might be enough to accelerate ‘Oumuamua. Digging through previous publications, he found several experiments showing that high-energy electrons, protons, and heavier atoms can convert water ice into molecular hydrogen, and that a comet’s thin snowball structure can trap space and gas in bubbles within the ice.

Experiments have shown that when heated, for example by the heat of the sun, ice solidifies (changing from an amorphous to a crystalline structure) and forces bubbles out, releasing hydrogen gas. Bergner and Seligman calculated that the ice on the comet’s surface could emit enough gas, either in the form of a collimator 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,” says Bergner.

The idea also explains the lack of a dust coma. “Even if there is dust in the ice matrix, you don’t sublimate the ice, you just rearrange the ice and then let the H2 go. Even the dust won’t come out,” Seligman said.

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

One of these dark comets, 1998 KY26, is the next target of Japan’s 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.