3I/ATLAS Is Fastest Interstellar Comet Ever Recorded, Clocking 130,000 MPH

Comet 3I/ATLAS has been observed by the Hubble Space Telescope, which delivered new insights into this third interstellar visitor. The orbital observatory has refined the estimates about the size of the cometary nucleus and confirmed that the comet is the fastest ever to come from beyond the Solar System.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

The comet was discovered on July 1 by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS), and it is the third interstellar object after ’Oumuamua, discovered in 2017, and Comet 2I/Borisov, discovered in 2019. From the get-go, Comet 3I/ATLAS looked different. It was twice as fast as the previous two, and it looked a lot bigger too.

In terms of speed, the comet is still holding that record. The interstellar object has a larger velocity than some of the Solar System comets, including Halley’s Comet at closest approach to the Sun, which reaches 54 kilometers (33 miles) per second. The Great Comet of 1680 and the Great Comet of 1843 were likely 10 times as fast since they got much closer to the Sun. And the Sun-grazing comets that often end up destroyed by our star are faster still.

That said, Comet 3I/ATLAS is speedy because it reaches that speed hundreds of millions of kilometers from the Sun.

“This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene that will gradually emerge,” said David Jewitt of the University of California, Los Angeles, science team leader for the Hubble observations, in a statement. “This is now possible because we have powerful sky survey capabilities that we didn't have before. We've crossed a threshold."

Hubble was able to measure how much it is outgassing, which is perfectly consistent for a comet that far from the Sun. The outgassing, though, made it difficult to estimate the size of its nucleus.

‘Oumuamua was shaped like a cigar, with its longest axis between 100 and 1,000 meters (328 and 3,280 feet). Comet Borisov had a nucleus of less than half a kilometer (0.3 miles). 3I/ATLAS was initially estimated to be 20 kilometers (12 miles) days after discovery, and the value has been reduced since. The Hubble estimations place it between 5.6 kilometers to 320 meters (3.5 miles to 1,000 feet).

“No one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second. You can't project that back with any accuracy to figure out where it started on its path,” said Jewitt.

We do not know the exact star system, but based on its speed and orbit, researchers have an idea of the region it comes from: the thick disk of the Milky Way, a region that is above and below the plane of the galaxy, where the spiral arms and the Sun are located. While that is not yet confirmed, the team has a testable hypothesis: the amount of water the comet has.

Continuous observations, especially upcoming ones with JWST, TESS (Transiting Exoplanet Survey Satellite), and Swift in space, as well as ground-based observatories, will provide more insight into the object.

Comet 3I/ATLAS will get as close as 210 million kilometers (130 million miles) from the Sun, far beyond the orbit of Earth, on October 30, 2025. The interstellar interloper will be visible from Earth until September, then it will be behind the Sun from our point of view. It should then be back being visible in December. 

A paper with the Hubble results will be published in The Astrophysical Journal Letters and is available on arXiv.