NASA, ESA, David Jewitt (UCLA); Image Processing: Joseph DePasquale (STScI)
The strange behavior of interstellar object 3I/ATLAS has scientists scratching their heads again. Astronomers observing the comet say it has changed color for the second time and is showing signs of moving in a way that cannot be explained by gravity alone.
Discovered on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 3I/ATLAS is only the third confirmed interstellar visitor after ?Oumuamua and Borisov. It is moving through our Solar System on a hyperbolic orbit with an eccentricity of around 6.1, meaning it is not bound to the Sun and will eventually head back into deep space.
As it approached its closest point to the Sun, or perihelion, on October 29, astronomers tracked it using NASA’s STEREO, SOHO, and GOES-19 satellites. They noticed that the comet had brightened dramatically to about magnitude 9, making it visible with small telescopes, and that it appeared to have turned bluer than before. Researchers noted this brightening is unusually fast compared to most comets from the Oort cloud.
Some have suggested far-fetched theories. Harvard astronomer Avi Loeb speculated that an alien spacecraft could theoretically use perihelion to disguise a maneuver, echoing his earlier claims about ?Oumuamua. But most scientists point to a simpler explanation: outgassing. As the comet’s icy surface heats up, it releases gas jets that act like tiny thrusters, altering its speed and direction slightly.
NASA’s Davide Farnocchia at the Jet Propulsion Laboratory reported that 3I/ATLAS shows signs of what’s called non-gravitational acceleration, meaning its motion isn’t due to gravity alone. Measurements taken at about 1.36 astronomical units from the Sun, or roughly 203 million kilometers, showed small but measurable shifts consistent with gas jets pushing it as it neared the Sun.
Loeb calculated that if the comet’s material evaporates at several hundred meters per second, it could lose about ten percent of its total mass over six months. That would create a large cloud of gas and dust around it, possibly detectable by the European Space Agency’s Juice spacecraft in early November.
Scientists also suspect the comet’s unusual color and brightness changes are due to its chemistry. Earlier in its journey, carbon dioxide sublimation may have kept its water ice from vaporizing. Once it got close enough for water to sublimate, the result was a sudden burst of outgassing and a visible color shift toward blue.
The comet is currently hidden behind the Sun from Earth’s view, but once it reappears later this month, astronomers will be watching to see whether it fades, stabilizes, or brightens even more.
As the research team notes in their paper on arXiv, the cause of 3I/ATLAS’s rapid changes remains unclear. Whether it settles down or keeps surprising us, this visitor from beyond the Solar System is already rewriting what we know about interstellar comets.