Astrometry with Interplanetary Spacecraft: Determination of the Non-gravitational Accelerations of the Interstellar Object 3I/ATLAS

Abstract Here, we report on the results of adding 6 observations from 2 interplanetary spacecraft to the orbit determination of 3I/ATLAS, the third known Interstellar Object. These observations, from vantage points and times impossible with terrestrial instruments, reduce the formal errors on the non-gravitational acceleration (NGA) parameters formal errors by 20%–40% compared to solutions using just the terrestrial data available from 2025 May to December. Using these data we find significant NGAs in the 3I/ATLAS trajectory, with a vector magnitude scaled to 1 au of (89.3 ± 4.6) × 10 −9 au day −2 , and a time offset (Δ T ) of −34.60 ± 2.62 days (i.e., an acceleration peaking ∼one month before perihelion). This leads to a rough mass estimate for 3I/ATLAS of 44 million tons in early 2025 August, equivalent to a CO 2 dominated nucleus radius r n ≲ 374 m.