NASA's first telescope observations have shown that Comet 3I/ATLAS is unusually "active."

A Hubble image of 3I/ATLAS taken in late July. (Image credit: NASA/ESA)

One of the benefits of having so many telescopes observing large swaths of the sky is that if astronomers spot something interesting, the data archives of other satellites will likely contain images of the object taken before its official discovery that no one has bothered to examine. That’s exactly what happened with our newest interstellar visitor, 3I/ATLAS, which, although discovered in early July, was visible to other telescopes as early as May.

We previously reported on Vera Rubin's discovery of 3I/ATLAS long before its official discovery, and now a new paper says the interstellar object was discovered in TESS data as early as early May — and it appears it may have been “active” around that time.

The Transiting Exoplanet Survey Satellite (TESS) isn't designed to look for interstellar stars or anything faint. As its name suggests, it's designed to look at stars (which are bright) and the exoplanets that pass in front of them, while watching the parent star's light curve drop. But data is data, and since TESS happened to be looking at a patch of sky this year where 3I/ATLAS was expected to be, researchers Adina Feinstein and Darryl Seligman of the University of Michigan, and John Noonan of Auburn decided to search the telescope archives for any clues about it.

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It turns out they did, starting on May 7, 2025, over two separate observing periods. Because TESS takes a picture every 200 seconds, and 3I/ATLAS is moving much faster than the traditional stars TESS targets, the team had to use a technique known as “shift-stacking.” They predicted where the interstellar object (the ISO) would be in each picture, shifted the images so that the ISO was in the same place in each picture, and then stacked multiple pictures together to get a clear signal from an object that would otherwise be too faint to detect in a single picture.

3I/ATLAS began its observing period at approximately 6.35 AU and by the end of the second observing window, on June 2, had reached a distance of approximately 5.47 AU. During this time, its flux increased by a factor of 5, although the decrease in distance would have resulted in a brightness increase of only about 1.5 times.

There have been many suggestions about the causes of some of 3I/ATLAS’s most interesting features, from data collection errors to the object itself being alien technology. But the authors have a much more prosaic explanation for this seemingly bizarre occurrence: Comet ISO was likely releasing “hypervolatiles” like carbon dioxide and carbon monoxide. These have a much higher sublimation temperature than water ice and can cause a significant increase in brightness, but most comets in our solar system have no hypervolatiles left, so they don’t show such a dramatic increase in brightness at such a great distance from the Sun. For the researchers, this is yet another argument that comets from other solar systems are likely to have very different compositions than those associated with our own.

In an effort to detect even more differences, they also tried to determine the rotation period of the ISO core. However, there was not enough of a clear signal to determine whether the core was actually moving. Rather, it was caused by the coma obscuring any discernible features, making it difficult for TESS to detect any brightness changes caused by the core's rotation.