Behind The Sun, Interstellar Object 3I/ATLAS Shows Signs Of Non-Gravitational Acceleration And Is Now Blue

For those of you who haven’t been following the story of our third interstellar visitor, on July 1, astronomers with the Asteroid Earth Impact Warning System (ATLAS) spotted an object passing through our solar system on an escape trajectory. Given its speed and eccentricity between 6.1 and 6.2, it was confirmed to be an interstellar object, the third we have seen after 1I/’Oumuamua and 2I/Borisov.

Astronomers have kept a close eye on the object, now known to be a comet due to its distinctive outgassing behavior. They discovered a number of unusual features of the object, which could well be a 10 billion year old time capsule from an earlier era in the universe. Studying it could tell us the exotic environment it grew up in, although astronomers are still trying to determine exactly what part of the galaxy it came from.

Unfortunately, as the object approached perihelion, or the point in its trajectory where it was closest to the Sun, it was behind our host star from Earth’s perspective. Harvard astronomer Avi Loeb has made quite an outcry about this, suggesting that an alien spacecraft attempting to observe our solar system or destroy Earth might choose to be out of our field of view at perihelion in order to conceal a maneuver and redirect it toward Earth.

This is a hypothesis that almost all astronomers reject, given the behavior of comets, including astronomers at the Search for Extraterrestrial Intelligence (SETI) and NASA. Nevertheless, as the comet approached the Sun, it was a key moment to test this highly unlikely hypothesis (aliens have been suggested several times as an explanation for cosmological mysteries, and so far we have found none).

So what did we learn as the object approached perihelion? In a new study, which has not yet been peer-reviewed, astronomers report that the comet appears to have changed color (for the second time). Using the Solar Terrestrial Relations Observatory (STEREO), the Solar and Heliospheric Observatory (SOHO), and the GOES-19 weather satellite, the team was able to observe 3I/ATLAS as it approached perihelion on October 29.

The team found that the object had brightened significantly, to around magnitude 9, making it brighter than many Herschel 400 galaxies and visible with a small telescope or a good pair of binoculars. They add that “color photometry shows that the comet is significantly bluer than the Sun.” However, there is not yet a satisfactory explanation for its rapid brightening, which far exceeds the rate of brightening seen in the more frequently observed Oort cloud comets.

In the meantime, Observations reported by Davide Farnoccia, a navigation engineer at NASA’s Jet Propulsion Laboratory, indicate that the comet appears to exhibit signs of “non-gravitational acceleration” or acceleration. not caused by gravity alone.

“The non-gravitational acceleration was measured at the perihelion distance of 1.36 times the Earth-Sun separation (defined as an astronomical unit or ‘au’), equivalent to 203 million kilometers,” Loeb explains in a blog post, adding that it showed a “radial acceleration relative to the Sun of 135 kilometers (=9×10^{-7}au) per square day” and a “transverse acceleration relative to the direction of the Sun of 60″. kilometers (=4×10^{-7}au) per square day”.

Before you shout “DARK FOREST ATTACK” and start building a bunker world around Jupiter, non-gravitational acceleration does not mean the object moved on its own. In fact, this could be further evidence of its cometary behavior; its acceleration changed as it approached the Sun and began to outgas, losing mass.

“For a thermal ejection velocity of a few hundred meters per second, the evaporation half-life of 3I/ATLAS is 6 months,” Loeb writes. “This implies that in the month it takes for 3I/ATLAS to traverse a spatial scale on the order of its perihelion separation from the Sun, 3I/ATLAS would lose about a tenth of its mass. Such a massive mass loss should be detectable in the form of a large plume of gas surrounding 3I/ATLAS during the coming months of November and December 2025.”

The European Space Agency’s (ESA) Juice spacecraft may be able to detect such a mass loss in early November. But according to the pre-printed paper using solar observatories, significant outgassing likely occurred, once again indicating interesting, if unusual, cometary behavior.

“Our rapid analysis of these data indicates that the comet will likely emerge from the conjunction much brighter than upon entry, with an extrapolated geocentric V magnitude of ∼9 at perihelion, perhaps due to significant visible gas emission,” the team explains, while adding that there are several mysteries to unravel (this is only our third confirmed interstellar object, after all).

“The reason for the rapid brightening of 3I, which far exceeds the brightening rate of most Oort Cloud comets at similar r (CE Holt et al. 2024), remains unclear,” the team adds. “It is possible that its H2O sublimation was dampened earlier by the cooling of its CO2 sublimation, which remained unusually dominant at r ∼ 3 au, perhaps related to its rapid approach to the Sun relative to other comets.”

Now it’s really a waiting game to see what happens as we get a view of it again.

“Without an established physical explanation, the prospects for 3I’s post-perihelion behavior remain uncertain, and a plateau in brightness – or even a brief continuation of its pre-perihelion brightening – seems as plausible as a rapid disappearance after perihelion,” the team concludes. “Continued observations could help provide a more definitive explanation of the comet’s behavior.”

The study is published on the arXiv preprint server.