Interstellar Comet 3I/Atlas Looks Back-To-Front, With Dust Shooting Toward The Sun

Comet 3I/ATLAS, the third known and fastest interstellar object to fly across the Solar System, is at it again. The latest oddity shown by this comet is how it is spewing dust. As they get closer to the Sun, comets typically release a tail of gas and dust and a second tail of ions. Comet 3I’s tail is still pretty weak – understandable since it’s still pretty far away – but it is spewing a lot of dust. However, this ejecta is not being released behind it as it goes, but in the direction of the Sun.

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Comets are icy, made up of frozen gas, rock, and dust, hence their nickname "dirty snowballs". As they get closer to the Sun, their surface begins to sublimate – going from solid to gas – and form the coma, the fuzzy atmosphere around the comet nucleus. A nucleus as small as a neighborhood can make a coma larger than Earth. It is the solar wind that shapes the tail, with dust and gas usually becoming the primary tail, the curved one, and the straighter one is the ion tail, formed by glowing ionized gas. A sunward tail or anti-tail is rare, though not unheard of, and is a layer of debris left behind by the comet that appears pointed at the Sun.

What’s happening to Comet 3I/ATLAS has more to do with its true nature than any possible anti-tail or sunward tail shenanigans. In a yet-to-be-peer-reviewed paper, astronomers using Hubble reveal the comet seems to be experiencing a lot of dusty sublimation on the Sun side, but this disappears on the night side. This is creating a distinct release of stuff on the sunny side.

There are two leading explanations for this. It could be possible that the comet is spinning with a pole pointing at the Sun, so the night side is never getting any light. The alternative is that its surface has been dramatically weathered. As we covered in an exclusive feature, the comet is likely significantly older than the Solar System, which means it has spent several billion years being bombarded with cosmic rays. Over time, this process has stripped the comet of surface hydrogen, making it more difficult to create a long, beautiful tail, as we have seen from Solar System comets.

The comet was discovered on July 1 by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS), and is just the third interstellar object ever observed 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, reaching  58 kilometers (35 miles) per second. It might be a bit bigger, too.

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 to finish the year. 

The study is available on the arXiv.