We already knew that interstellar comet 3I/ATLAS was different from the other two interstellar objects known to have traversed the Solar System, but a slew of new observations suggest it may be even weirder than the weirder that it was.
NASA and ESA instruments Hubble, SPHEREx, JWST, and TESS have all captured the object as it makes its way towards the Sun. The results show that not only was 3I/ATLAS actively outgassing long before we spotted it, but its atmosphere (or coma) has a higher proportion of carbon dioxide than scientists usually see in comets, interstellar or otherwise.
This could tell us something about the environment in which 3I/ATLAS formed, the space conditions through which it has traveled, or even the internal composition of the comet.
Related: Fuzzy, Large, And Very Old: Everything We Know About Interstellar Comet 3I/ATLAS
The comet first came to our attention on 1 July 2025, and astronomers have been avidly goggling at it ever since – not least because they have a very limited window in which to do so. Its closest approach to the Sun, or perihelion, will take place on October 29; but, because it's on the other side of the Sun from Earth, it will be hidden behind the star's blazing glow by this time.
This means that the best time for observing the comet before perihelion is rapidly slipping by, so scientists are making the best of it, turning some of our most powerful instruments to the task – or, in the case of TESS, scouring pre-discovery data for glimpses of the object.
It's for this reason that we now know the first known glimpse of 3I/ATLAS took place back in May, nearly two full months before the official discovery. The comet was moving a lot faster than the targets TESS is designed to study, so researchers had to use some image-stacking techniques to reveal it.
This is where it gets interesting. The TESS data suggest that the comet was already active at that time, at a distance of around 6 astronomical units (AU) from the Sun – out past the orbit of Jupiter. That's a much greater distance than expected: most comets begin showing activity no closer than 5 AU from the Sun.
When we call a comet active, it means that it has warmed up enough for the ices on and just under its surface to sublime – transition directly from a frozen to a gaseous state. This produces a coma and, eventually, if the comet comes close enough to the Sun to be affected by radiation pressure, cometary tails.
In their preprint paper, the researchers who made the TESS discovery posit that the comet's early awakening may have had something to do with its composition. Some ices sublime more readily than others – and carbon dioxide is one of those ices.
This was confirmed by two separate, independent measurements using two different instruments. In mid-August 2025, NASA's brand-spanking-new space telescope SPHEREx took multi-spectral observations of the comet at distances between 3.3 and 3.1 AU from the Sun, clearly resolving a coma rich in carbon dioxide, as well as water.
No tails or jets were observed at this time, and the coma was measured out to a radius of 23 kilometers, suggesting that production rates were pretty high. (The comet itself, according to Hubble measurements, has a radius of about 2.8 kilometers.)
This is supported by measurements from JWST, which observed the comet at a distance of 3.32 astronomical units from the Sun in early August. Its data suggests that carbon dioxide and water exist in the coma at a ratio of 8 to 1 – among the highest proportion of carbon dioxide ever seen in a comet.
There could be several reasons for this.
"Our observations are compatible with an intrinsically CO2-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk," the researchers explain in their preprint paper.
"A low coma H2O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H2O sublimation rate relative to CO2 and CO."
We're not really going to know more without more information on the comet, for which we may have to wait. As you can see in the animation above, its trajectory is going to take it behind the Sun relative to Earth, but at perihelion it may be close enough to Mars for Mars orbiters to catch a glimpse.
After perihelion is when things will get even more exciting. At this time, the comet will make its closest approach to Earth as it continues to zoom right on out of the Solar System. In theory, Juno could intercept it when it whizzes past Jupiter in March next year.
This is such a fascinating object, we can't wait to see what else our intrepid astronomers are able to find.
The Hubble, TESS, SPHEREx, and JWST findings, which have not yet been peer-reviewed, are all available in separate listings on the preprint server arXiv.