Technically it’s called “outgassing,” but NASA scientists have solved a big question about a mystery object that appeared in our solar system last fall, and it has everything to do with the sudden, surprising expulsion of gas.

‘Oumuamua is a cigar-shaped hunk of rocks and metal, hurtling past Earth and through our solar system from an unknown, interstellar origin.

In a new paper published in the journal Nature, an international team of scientists confirm why ‘Oumuamua has unexpectedly picked up speed. As they outline in a new paper, they think the object, estimated to be about a half-mile long, received a speed boost through our system — to the tune of 196,000 miles per hour — from the cumulative effect of a bunch of smaller boosts created on its surface as it hurtled past inner planets and shed material. The paper’s co-author, Davide Farnocchia, Ph.D. of the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory in Pasadena, explained in a statement that ‘Oumuamua’s surprising acceleration came from within it. It’s called Outgassing.

“This additional subtle force on ‘Oumuamua likely is caused by jets of gaseous material expelled from its surface,” said Farnocchia in a statement on Wednesday. “This same kind of outgassing affects the motion of many comets in our solar system.”

Last fall, it was clear to scientists before they knew why ‘Oumuamua — named after the Hawaiian word for “scout” — was traveling so fast that it wasn’t from around here: Its trajectory and immense speed suggested it’d been whipped toward us from somewhere else and wasn’t simply coasting on our Sun’s gravity.

'Oumuamua
This artist's illustration shows 'Oumuamua racing toward the outskirts of our solar system, and is annotated with the locations of the planetary orbits. As the complex rotation of the object makes it difficult to determine the exact shape, there are many models of what it could look like.

The team’s more latest observations, made using the Hubble Space Telescope, show that ‘Oumuamua’s observed trajectory differs by “25,000 miles (40,000 kilometers) compared to where it would have been if only gravitational forces were affecting its motion,” as NASA noted Wednesday.

The trajectory of ‘Oumuamua, which now has it curving toward the outskirts of our solar system somewhere past Jupiter at a speed of 70,000 mph, has researchers thinking that ‘Oumuamua got its boost while it was taking a tour through the inner solar system, near Earth.

One of the most perplexing things about ‘Oumuamua is that it seemed to behave like a comet but didn’t look like one at first. A comet is made up of dust, rocks, and, crucially, ice, which is what creates the fuzzy halo of light and signature tail we see on established comets. Those characteristics haven’t been observed on ‘Oumuamua, leading scientists to think it was an asteroid, which is a body made up of rocks and metals.

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'Outgassing' on 'Oumuamua, shown here as little blue jets, are thought to be what gave it a small boost in speed.

Why ‘Oumuamua is a Comet, Not an Asteroid

But comets are known to pick up speed due to “outgassing,” whereas asteroids are not. In 2017, researchers publishing in Nature Astronomy suggested ‘Oumuamua might not be one or the other. As Queen’s University Belfast astronomer and lead study author Alan Fitzsimmons, Ph.D. writes in The Conversation: “[My] colleagues and I have now discovered that while it appears to be an unusually long rocky asteroid it may actually be an icy body covered in a protective crust of organic chemicals.”

Now, as the scientists behind the new paper conclude, ‘Oumuamua is in fact an icy comet — just a bit of a weird one. “It’s an unusual comet, and that’s pretty exciting,” Karen Meech, Ph.D., an astronomer at the University of Hawaii in Honolulu and co-author on the new paper, told Nature News on Wednesday.

One thing researchers are more sure about is that it’s not an alien spacecraft. Some had considered it because of its unusually aerodynamic shape, but they concluded that it just happened to be elongated because those are the types of objects that would be expected to survive a fast-paced jaunt through the universe. That said, some scientists have listened for radio signals coming from it, just in case.

In any case, at this rate, we don’t have that much time left to observe ‘Oumuamua up close, and there’s no saying how long it will be before we get another interstellar houseguest. In about four years, NASA says, our strange, cigar-shaped friend will whiz past Neptune, where it’ll zoom along its trajectory back toward the outskirts of our galaxy and we’ll wave goodbye for good.