A team of astronomers observing the interstellar object 3I/ATLAS has discovered something surprising. Our third confirmed interstellar visitor appears to have changed color for a second time. Meanwhile, another team found that the comet began to show signs of non-gravitational acceleration as it approached perihelion.
For those of you who haven't been following the story of our third interstellar visitor, on July 1, astronomers at the Asteroid Last Alert System (ATLAS) spotted an object hurtling through our solar system on an escape path. Considering his speed and his eccentricity between 6.1 and 6.2it was confirmed be an interstellar object, the third one we've seen since 1I/'Oumuamua And 2I/Borisov.
Astronomers have been closely monitoring the object, which is now known as a comet due to its characteristic outgassing behavior. They found a row unusual features object, which may well be A 10 Billion Year Time Capsule from the Early Age of the Universe. Studying it could tell us about the exotic environment in which it grew up, although astronomers are still trying to pinpoint exactly which part of the galaxy it came from.
Unfortunately, as the object approached perihelion, or the point in its path where it was closest to the Sun, from Earth's perspective it was behind our parent star. Harvard astronomer Avi Loeb made a big fuss about this, suggesting that an alien spacecraft trying to see our solar system or destroy Earth might decide to move out of our sight at perihelion to disguise the maneuver by redirecting it towards Earth.
This hypothesis is rejected by almost all astronomers, given the behavior of the comet, including astronomers Search for extraterrestrial intelligence (NETWORKS) and NASA. However, when the comet approached the Sun, it was The Key to Testing This Very Unlikely Hypothesis (aliens have been proposed as an explanation for cosmological mysteries many times, but we haven't found one so far).
So what did we learn as the object approached perihelion? In a new study that has not yet been peer-reviewed, astronomers report that the comet appears to have changed color (for the second time).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 brightened significantly, to about magnitude 9making it brighter than many of the Herschel 400 galaxies, and can be seen with a small telescope or decent binoculars. They add that “color photometry shows that the comet is clearly bluer than the Sun.” However, there is still no satisfactory explanation for its rapid brightening, which far exceeds the brightening rate observed in the more frequently observed Oort cloud comets.
Meanwhile, sightings reported Davide Farnoccia, a navigation engineer at NASA's Jet Propulsion Laboratory, points out that the comet appears to be showing signs of “non-gravitational acceleration” or acceleration. caused not only by gravity.
“Non-gravitational acceleration was measured at a perihelion distance equal to 1.36 times the distance between the Earth and the Sun (defined as an astronomical unit or “au”), equivalent to 203 million kilometers,” Loeb explains in his paper. blog postadding that it showed a “radial acceleration from the Sun of 135 kilometers (=9×10^{-7}AU) per day squared” and “a transverse acceleration from the direction of the Sun of 60 kilometers (=4×10^{-7}AU) per day squared.”
Before you scream”ATTACK OF THE DARK FORESTIn fact, this may be further evidence of its cometary behavior: its acceleration changed as it approached the Sun and began to release gas, losing mass in the process.
“At a thermal release rate of several hundred meters per second, the evaporation half-life of 3I/ATLAS is 6 months,” Loeb writes. “This means that in the month it takes 3I/ATLAS to cross the spatial scale of the order of perihelion separation from the Sun, 3I/ATLAS will lose about a tenth of its mass. This massive mass loss should be detected as a large plume of gas surrounding 3I/ATLAS in the coming months of November and December 2025.”
The European Space Agency's (ESA) Juice spacecraft could detect such mass loss. early November. But according to a preliminary paper using solar observatories, significant outgassing was likely occurring, further indicating the comet's interesting, if unusual, behavior.
“Our quick analysis of these data suggests that the comet will likely emerge from the conjunction significantly brighter than when it entered, with an extrapolated geocentric magnitude of V ~9 at perihelion, possibly driven by a significant visible gaseous outburst,” the team explains, although they add that there are several mysteries to be cleared up (after all, this is only our third confirmed interstellar object).
“The reason for the rapid increase in brightness of 3I, which far exceeds the rate of increase in brightness of most Oort cloud comets with similar r (CE Holt et al. 2024), remains unclear,” the researchers add. “It is possible that its H2O sublimation was constrained by earlier cooling by CO2 sublimation, which remained unusually dominant at r ~ 3 AU, possibly due to its rapid approach to the Sun compared to other comets.”
Now it's really a waiting game to see what happens when we see it again.
“Without an established physical explanation, the prospects for 3I's behavior beyond perihelion remain uncertain, and a brightness plateau—or even a brief continuation of its brightening into the preperihelion—seems as plausible as a rapid decay beyond perihelion,” the team concludes. “Continued observations may help provide a more accurate explanation of the comet's behavior.”
The study is posted on a preprint server. arXiv.
