Alien comet 3I/ATLAS just passed the Sun. When will it return to our skies?
This interstellar visitor should soon be 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 it look like when it passed Mars and through its closest distance to the Sun, 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
Update: 3I/ATLAS crosses perihelion!
On October 29, the alien comet passed through its closest distance to the Sun (perihelion). In case it's of interest, this was the moment that we would see any strange behaviour from it — changes in direction, dropping probes, and the like — if it was actually a ship or probe sent by aliens.
Based on imagery from the LASCO instrument on the NASA/ESA Solar and Heliospheric Observatory, two cameras on NASA's STEREO-A, and the CCOR-1 coronagraph on NOAA's GOES-19 weather satellite, 3I/ATLAS is just a comet, though.
Images of 3I/ATLAS from SOHO, STEREO-A, and GOES-19, along with a reference star from the CCOR-1 imagery, from the research paper Rapid Brightening of 3I/ATLAS Ahead of Perihelion. (NASA/ESA/NOAA/Qicheng Zhang/Karl Battams/arxiv/Cornell University)
Remarkably, the study that took a look at this imagery, which is currently available in pre-print, found that the comet was quite bright, and is "distinctly bluer than the Sun."
While the comet was much more red before, due to the amount of dust that was surrounding it, this colour shift indicates that it is now surrounded by a larger amount of gases, which were released near perihelion.
When does 3I/ATLAS return to our skies?
Unfortunately, unlike Comet Lemmon, this alien comet is far too distant and dim to see with the unaided eye.
However, 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. Note the time change starting November 2, from 6:45 a.m. local time to 5:45 a.m. local time, due to the end of Daylight Saving Time. (Stellarium/Scott Sutherland)
It will be challenging to spot it, so close to sunrise. Also, it's more difficult to observe objects (especially dim comets) that are low in the sky, since we're peering through more atmosphere at that angle, compared to when the object is higher up in the sky.
However, the comet should be found near the planet Venus, around 6:40 a.m., local time, on October 30. It will then rise earlier and earlier each morning, until it crests the horizon at around 4:40 a.m., local time, by November 7th. At least to start, Venus acts as a good guidepost to locate the comet, but the two get farther apart throughout the first week of November.
The comet doesn't reach its closest distance to Earth, at around 1.8 astronomical units or 270 million kilometres away, until December 19. While that means it will be easier to observe from Earth, being higher up in the sky at that time, it will also be farther away from the Sun than it is now, which means that it is not expected to be as bright.
Original estimates of 3I/ATLAS's maximum brightness were around a magnitude of between 14.9 to 15, during the first week of November. That would require at least a 10-inch aperture telescope, with high magnification, and would have been right at the limit of the telescope's ability to resolve. Observations submitted to the Comet Observation database (COBS) from around the world, revealed that the comet was significantly brighter, potentially reaching a magnitude of +11.2. New observations from space weather satellites (discussed above) seem to indicate that it could even be brighter!
Now, comets are notoriously unpredictable. So, we will need to wait until astronomers actually spot it to know for sure how bright it is, and thus how easy it will be seen via telescope. However, at the moment, there is reason to be optimistic about our chances, as it could be bright enough to spot with a small telescope (6" to 8" aperture). Check back for more updates!
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 image shows comet 3I/ATLAS against a backdrop of stars, taken on August 27, 2025, from the Gemini South Observatory in Chile. Credit: 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))