Alien comet 3I/ATLAS just swept past Mars. When will it return to our skies?
We're just weeks away from this interstellar visitor reaching its brightest in Earth's night skies. Will we see it?
There's been a lot of hype and rumours about alien comet 3I/ATLAS lately, so where is it now, what did the spacecraft around Mars see when it passed by, when will it reappear in our skies, and is it really anything we need to worry about? Here's the details.
Since its discovery back in early July, our latest visitor from interstellar space — 3I/2025 N1 (ATLAS), or 3I/ATLAS for short — has been speeding through the inner solar system at over 200,000 kilometres per hour, taking only three months to travel from just inside the orbit of Jupiter to sweeping past Mars at the beginning of October. For comparison, the fastest spacecraft ever launched from Earth, New Horizons, took more than 10 months — from April 7, 2006 to February 28, 2007 — to travel a similar distance.
The positions of comet 3I/ATLAS on July 1, when it was discovered, and October 3, when it made its closest pass by Mars. (NASA/JPL-Caltech/Scott Sutherland)
During the past three months, telescopes both on Earth and in space have been snapping images and collecting data about this galactic wanderer. After all, this object presents us with a unique opportunity, and for a limited time only.
We can't journey to other planetary systems to explore them, but 3I/ATLAS was likely ejected from around some distant star. So, by studying its composition, we can get an idea of what the environment was like there, at least when the star and its planets were first forming. And, we only have until sometime next year to learn as much as we can from it, as it will pass outside of Jupiter's orbit in March 2026, heading back towards interstellar space, after only a brief visit.
3I/ATLAS as seen by the Gemini South observatory on August 27, 2025. (International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab))
Unfortunately, our observations of 3I/ATLAS currently have a bit of an interruption. In late September and through most of October, it is passing behind the Sun from our perspective. So, it's not visible to any telescopes on Earth's surface, and it's actually dangerous to aim any of our space telescopes at it, as the intense light from the Sun can damage their optics.
3I/ATLAS at Mars
As luck would have it, though, on October 3 the comet passed just 30 million kilometres away from the only planet in the solar system, other than Earth, with multiple spacecraft orbiting around it, as well as more than one mission on the ground. And, as that happened, the space agencies that handle the majority of those missions — NASA and the ESA — were ready for it.
Both the ESA's Mars Express and ExoMars Trace Gas Orbiter (TGO) spacecraft turned their cameras towards 3I/ATLAS at its closest point to the Red Planet.
"3I/ATLAS has not yet revealed itself in the Mars Express images, partly because these were taken with an exposure time of just 0.5 seconds (the maximum limit for Mars Express)," the ESA said in a press release from October 7.
ExoMars TGO's Colour and Stereo Surface Imaging System (CaSSIS) managed to capture the alien comet, though, thanks to the camera's longer, 30-second exposure time.
Comet 3I/ATLAS (upper right of centre) is seen here as captured by ExoMars TGO's CaSSIS instrument on October 3, 2025. (ESA/TGO/CaSSIS)
During ExoMars TGO's normal mission operations, CaSSIS is used to image the surface of Mars, at a range of hundreds to thousands of kilometres away.
"Despite not being designed to capture something so far away, ExoMars TGO revealed the coma of gas and dust surrounding the icy-rocky nucleus," the ESA explained.
NASA orbiters, such as Mars Odyssey and the Mars Reconnaissance Orbiter, plus the Perseverance and Curiosity rovers on the surface, were likely also instructed to aim their own cameras at the comet. However, there is no official word yet from these missions.
The latest updates to the image catalogs for MRO's HiRISE camera and Odyssey's THEMIS instrument were on October 1, two days before the comet reached its closest approach. Curiosity's raw image catalog shows several images taken at night by the rover's Navigation cameras, but none show any indication that they captured the comet. There are hints that Perseverance may have succeeded, though!
These two images may show comet 3I/ATLAS, as captured by the Right Navigation camera on the Perseverance rover on Mars, using two different filters. If this is the comet, it is elongated into a streak in both views due to the long exposure time used to capture the images. (NASA/JPL-Caltech)
With the current U.S. government shutdown in effect, we will need to wait until the mission teams return to work to receive any official analyses of the results.
READ MORE: An alien comet is now passing through our solar system. Here's what we know
When does 3I/ATLAS return to our skies?
This comet is far too distant and dim to see with the unaided eye. However, anyone with a decent backyard telescope — something with an 8-inch to 10-inch aperture — may want to get it ready for the end of October.
Once 3I/ATLAS clears the Sun and reappears in our predawn sky, it is expected to reach its brightest between October 30 and November 7.
The location of 3I/ATLAS in the morning sky from the end of October through the first week of November, 2025. (Stellarium/Scott Sutherland)
Go out in the predawn hours and point your telescope towards the east. Look for the planet Venus shining brightly above the horizon as a good guidepost to locate the comet.
According to estimates from the International Astronomical Union's Minor Planet Center, 3I/ATLAS will reach between magnitude 14.9 to 15 in the first week of November. To see something that faint will require at least a 10-inch aperture telescope, with high magnification, and that will be right at the limit of that telescope's ability to resolve. Long exposure astrophotography through such a telescope might gather enough light to see it more clearly.
Amateur sources such as Gideon van Buitenen and Seiichi Yoshida are much more optimistic. Both estimate that 3I/ATLAS could be even brighter when it reappears, possibly up to magnitude 11.5. (In astronomy, the lower an object’s magnitude, the brighter it is.)
If these optimistic estimates are correct, observers under clear, dark skies may only need a smaller telescope (with a 6- to 8-inch aperture) to see the comet at the end of October and during the first week of November.
DON'T MISS: Hubble reveals new details about alien comet 3I/ATLAS
Is 3I/ATLAS a threat to Earth?
The TLDR answer to this question is: No.
Even at its closest approach to Earth, 3I/ATLAS will be well over 200 million km away, and on the other side of the Sun from us.
By now, months after its discovery, it would be glaringly obvious, both from professional and amateur sources, if this comet was on an impact trajectory with Earth. There would be no way to hide this information from the public.
The simple truth is that 3I/ATLAS comes nowhere near our planet, which is unfortunate for those of us who would like to see this rare visitor shining in our sky, and it will soon be headed back out of our solar system, never to return.
This diagram shows the path of Comet 3I/ATLAS through the inner solar system. Inset is an image of the comet taken by the European Southern Observatory, shortly after its discovery. (NASA/JPL/ESO/O. Hainautiu)
As for the speculation that 3I/ATLAS could be some kind of probe sent by hostile aliens: regardless of the hype about this subject making the rounds on the news and the internet, all indications are that it is a completely natural object.
It is certainly a strange comet:
travelling faster than any other interstellar object we've seen so far,
on a path through the solar system almost directly along the ecliptic (the plane the planets orbit in),
with far more carbon dioxide than water in its ices, compared to comets from our own solar system,
but at the same time, while at a distance to the Sun where most comets are fairly inactive, still spewing around 40 litres of water into space every second, which is roughly equal the rate of a mid-sized firehose on full blast.
However, despite all that strangeness, it still displays all the characteristics of a comet, with a coma of gases and dust around its nucleus, and a tail extending away from it. Nothing about its composition or behaviour gives any solid evidence of it being anything but natural.
Instead, it is a rare celestial curiosity for us to observe from afar while it zips through our region of space.
We may uncover stronger clues to its origin, possibly learning more about star systems deeper in the Milky Way. Also, this encounter may help us to locate other interstellar objects in the future, and might even inspire us to put missions into space to intercept those future visitors, to get a closer look at them as they pass by.
(Thumbnail combines a view of Mars and 3I/ATLAS using NASA's Eyes on the Solar System with an image of the comet captured by the ESA's ExoMars Trace Gas Orbiter (TGO))