By the date of its perihelion, 3I/ATLAS displayed the first evidence of a non-gravitational acceleration. The report (accessible here) was filed by Davide Farnoccia, a navigation engineer at NASA’s Jet Propulsion Laboratory in Pasadena who received his PhD in Mathematics from the Galileo Galilei School of Graduate studies at the University of Pisa, Italy.
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. It had two components in the orbital plane of 3I/ATLAS:
· A radial acceleration away from the Sun of 135 kilometers (=9×10^{-7}au) per day squared.
· A transverse acceleration relative to the Sun’s direction of 60 kilometers (=4×10^{-7}au) per day squared.
If 3I/ATLAS is propelled by the rocket effect of ejected gas, then momentum conservation implies that the object would lose half its mass over a characteristic timescale equal to the ejection speed divided by the measured non-gravitational acceleration. For a thermal ejection speed of a few hundred meters per second, the evaporation half-life of 3I/ATLAS is 6 months. This implies that over the month it takes 3I/ATLAS to cross a spatial scale of order 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 upcoming months of November and December 2025.
Hellbound and Down 