Everyone’s favorite interstellar comet 3I/ATLAS isn’t hiding near perihelion this week, as amateur astronomers reveal.
Don’t believe all the breathless ballyhoo that you’re currently reading around the web about interstellar Comet 3I/ATLAS. In a clockwork Universe, comets are the big wildcard, and interstellar comets are doubly so. This comet is scientifically interesting enough in its own right, no alien interlopers needed.
There has been no shortage of hype around this comet. Despite the seemingly endless stream of conspiracy-minded videos, NASA isn’t hiding anything. The U.S. space agency has simply fallen silent due to the ongoing government shutdown. This means the task of scouring images for the faint comet has often fallen on the shoulders of dedicated amateur astronomers.
Here’s what we actually know about the comet thus far. 3I/ATLAS was discovered this past summer on July 2nd by the Asteroid Terrestrial-impact Last Alert System (ATLAS) sky survey. Its high orbital eccentricity of more than 6, which is well above a value of 1, immediately caught astronomers’ attention. This pegged it as an object from beyond our solar system. So far, 3I/ATLAS has all of the observed hallmarks of a run-of-the-mill comet: a coma, a dust tail, etc. It is a fascinating object to be sure, a rare visitor from outside our solar system.
“Initial detections of the comet showed that it had a condensed coma, with an offset pointing towards the Sun,” Bryce Bolin (Research Scientist, Eureka Scientific) told *Universe Today*. “However, starting in August-early September, the comet started to evolve an extended anti-solar tail. It has been suggested that the development of the tail is due to ~1mm scale dust particles slowly moving in the anti-solar direction due to solar radiation pressure.”
The emergent ‘anti-tail of Comet 3I/ATLAS. Credit: Gemini South.
Again, anti-tails are very comet-like feature that we’ve seen before. “The comet appears to have started to turn green due cometary gases,” says Bolin. “This is due to the emission of diatomic carbon (cyanogen gas) which give comets their green color. This has been seen in recent spectroscopy taken at Gemini South.”
What we do know about Comet 3I/ATLAS is that seems hail from the thick disk of the Milky Way, and is probably a very ancient object far older than our solar system. It’s also really whizzing through, moving at a blazing 58 kilometers per second relative to the Sun. That’s too fast for us to chase down, though one day soon, ESA’s proposed Comet Interceptor mission could be ready to do just that.
Comet 3I/ATLAS at perihelion. Credit: NASA/JPL.
3I/ATLAS reaches perihelion this week on Wednesday, October 29th around 11:47 Universal Time (UT) at 1.36 Astronomical Units (AU) from the Sun. It’s also nearly exactly opposite to Earth at this point, and won’t become generally visible from the Earth until November, emerging low in the dawn.
Now, lots of hay has been made about the comet’s trajectory. Its low inclination versus the ecliptic moving in a retrograde path inclined at just five degrees is interesting, but in and of itself not an exceptional fact. Note that asteroid and comet hunting surveys like ATLAS and PanSTARRS favor scouring the ecliptic region looking for new objects. Also, the fact that the comet is making a poor apparition versus the Earth isn’t really all that strange, as most solar system comets tend to do the same. It would be more exceptional to see a bright interstellar comet headed directly for our vicinity.
Comet 3I/ATLAS from September 16th. Credit: Efrain Morales.
Occam’s Razor is applicable here. Yes, 3I/ATLAS could be an artificial object, the vanguard of an alien invasion fleet, or a large bowl of petunias, brought into existence by an Infinite Improbability Drive straight out of *Hitchhiker’s Guide to the Galaxy*. But there’s no evidence of this. What there is evidence for in terms of activity and dust production is a pretty pedestrian comet, albeit one with a pretty fascinating back story from beyond the solar system.
The observed brightness of 3I/ATLAS, versus various observation platforms. Credit: Marshall Eubanks.
3I/ATLAS passed just 0.19 AU from Mars on October 3rd. Assets on and around the Red Planet were at the ready, including NASA’s rovers on the planet, and ESA’s missions in Martian orbit. There was lots of flap that the comet looked like a cylinder in Percy’s view (remember Arthur C. Clarke’s *Rendezvous with Rama*?) But even novice astrophotographers know what a basic star trail looks like (!)
Do you see it? The ghostly +8th magnitude glow of comet 3I/ATLAS, as seen from the Perseverance Rover’s Mastcam-Z imager. Credit: NASA/Simeon Schmaub.
Meanwhile, the European Space Agency isn’t shut down, and has more plans to observe the comet. The Jupiter Icy Moons Orbiter (JUICE) cruising to Jupiter is up next, and will start monitoring the comet from about 0.4 AU distant starting in early November.
ESA missions versus Comet 3I/ATLAS. Credit: ESA.
Check this out: when interstellar comet 3I/ATLAS was 2.35 AU from Earth last week on the farside of the Sun, it was actually visible in the new CCOR-1 (Compact Coronagraph) aboard NOAA’s GOES-19 satellite. Worachate Boonplod found the +11th magnitude comet in recent images, and traced out its path through CCOR-1’s imager:
3I/ATLAS flies through the field of view of NOAA’s CCOR-1 imager. Credit: Worachate Boonlplod/NOAA.
The Polarimeter to Unify the Corona Heliosphere (PUNCH) and the Solar Heliospheric Observatory (SOHO’s) LASCO C2 coronagraphs have also followed 3I/ATLAS this past week as it nears perihelion. This is tricky, as the comet is faint at +11th magnitude. This is right at the grim edge of what image stacking and processing can tease out.
faint Comet 3I/ATLAS (bright smudge in the left image) crossing through SOHO’s LASCO C3 field of view. Credit: NASA/ESA/SOHO/Qicheng Zhang.
November and December present the best prospects for seeing the comet for yourself as it reemerges from behind the Sun, low in the dawn. Though it never gets much brighter than +11th magnitude, it’s in range of large backyard telescopes as it moves through Virgo and Leo. 2026 sees the comet exit the solar system in the direction of the constellation Gemini towards the star Zeta Geminorum, resuming its long journey through deep interstellar space.
Here’s my final thought on where the science of interstellar objects is headed. As the name states, 3I/ATLAS was only the third interstellar comet to be seen visiting the inner solar system, after 1I/Oumuamua in 2017 and 2I/Borisov in 2019. Note that unlike 1I/Oumuamua, which was only seen on its way out of the solar system, 3I/ATLAS was discovered while it was still 4.5 AU out from the Sun. We’re getting better at this. With systems such as the Vera C. Rubin telescope just now coming online in 2025 and scouring the sky down to ever fainter magnitudes, I would not be surprised if we start seeing lots more smaller, fainter interstellar objects slipping through our solar system, very soon.
Certainly, there’s enough amazing facts to consider about 3I/ATLAS, without venturing into the realm of wild conjecture and fantasy. To once again evoke Douglas Adams: “A garden is amazing enough, without having to believe that there are fairies in it, too.”