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| Image credit: NASA, ESA, David Jewitt (UCLA); Image Processing: Joseph DePasquale (STScI) |
A group of scientists keeping an eye on the interstellar object 3I/ATLAS discovered something somewhat unexpected. For the second time, our third verified interstellar visitor seems to have changed hue. As the comet approaches perihelion, a different team discovers that it has started to exhibit non-gravitational acceleration.
In case you haven't been following the news about our third interstellar visitor, astronomers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) discovered an object speeding through our Solar System on an escape track on July 1. It was determined to be an interstellar object, the third we have observed after 1I/'Oumuamua and 2I/Borisov, based on its velocity and eccentricity between 6.1 and 6.2.
The object, which is now recognized as a comet because of its unusual outgassing characteristics, has been closely monitored by astronomers. The object, which may very well be a 10-billion-year-old time capsule from an earlier era of the universe, has several peculiar characteristics. Although astronomers are still trying to determine the precise region of the galaxy from whence it originated, studying it could reveal the unique environment in which it grew up.
Unfortunately, from Earth's perspective, the object was behind our host star as it approached perihelion, the point in its path where it was closest to the Sun. Avi Loeb, a Harvard astronomer, has made a huge deal out of this fact, speculating that an extraterrestrial spacecraft trying to observe our Solar System or destroy Earth would decide to avoid our view around perihelion in order to conceal a movement that would reroute it toward Earth.
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Given the cometary behavior, almost all astronomers—including those at NASA and the Search for Extraterrestrial Intelligence (SETI)—reject this theory. However, the comet's approach to the Sun was a crucial time to test this extremely improbable theory (aliens have been proposed as a solution to cosmological problems numerous times, and we have discovered zero).
As the object got closer to perihelion, what did we learn? Astronomers claim that the comet appears to have changed color (for the second time) in a recent paper that has not yet undergone peer review. On October 29, the team observed 3I/ATLAS as it neared perihelion using Solar Terrestrial Relations Observatory (STEREO), SOlar and Heliospheric Observatory (SOHO), and the weather satellite GOES-19.
The object is now visible with a modest telescope or a good pair of binoculars, and the scientists discovered that it has brightened dramatically to about magnitude 9, making it brighter than several Herschel 400 galaxies. They further state that "color photometry shows the comet to be distinctly bluer than the Sun" . Its quick brightness, which is significantly faster than the rate of brightening observed in Oort cloud comets that are seen more regularly, has not yet been adequately explained.
Meanwhile, the comet seems to show indications of "non-gravitational acceleration," or acceleration not only due to gravity, according to observations made by Davide Farnoccia, a navigation engineer at NASA's Jet Propulsion Laboratory.
"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 writes in a blog post. He adds that the demonstrated a "radial acceleration away from the Sun of 135 kilometers (=9x10^{-7}au) per day squared" along with a "transverse acceleration relative to the Sun’s direction of 60 kilometers (=4x10^{-7}au) per day squared" .
Non-gravitational acceleration does not imply that the item moved independently, so stop yelling "DARK FOREST ATTACK" and constructing a bunker world around Jupiter. Its acceleration changed as it got closer to the Sun and started outgassing, losing mass in the process, which could actually be more proof of its cometary habit.
"For a thermal ejection speed of a few hundred meters per second, the evaporation half-life of 3I/ATLAS is 6 months," adds Loeb. This suggests that 3I/ATLAS would lose roughly a tenth of its mass during the month it takes it to cross a spatial scale of order its perihelion separation from the Sun. In November and December of 2025, a gigantic gas plume around 3I/ATLAS should be visible as a result of this massive mass loss.
Early in November, such mass loss might be detected by the European Space Agency's (ESA) Juice probe. However, substantial outgassing is probably likely to have occurred, indicating once more intriguing, albeit peculiar, cometary behavior, according to the pre-print report utilizing solar observatories.
The team says, "Our cursory analysis of this data indicates the comet will likely emerge from conjunction considerably brighter than when it entered, with an extrapolated geocentric V magnitude of ∼9 at perihelion, perhaps driven by prominent, visible gas emission," but notes that there are still a number of unanswered questions (after all, this is only our third confirmed interstellar object).
"The reason for 3I’s rapid brightening, which far exceeds the brightening rate of most Oort cloud comets at similar r (C. E. Holt et al. 2024), remains unclear," the researchers state. "It is possible that its H2O sublimation had been held down earlier by cooling from its CO2 sublimation, which remained unusually dominant at r ∼ 3 au, perhaps related to its rapid approach toward the Sun compared to other comets."
Now that we get another glimpse of it, it truly is a waiting game to see what transpires.
"Without an established physical explanation, the outlook for 3I’s postperihelion behavior remains uncertain, and a plateau in brightness—or even a brief continuation of its preperihelion brightening—appears as plausible as rapid fading past perihelion," the group concluded. "Continued observations may help provide a more definitive explanation for the comet’s behavior."
The study is posted to pre-print server arXiv.
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