[en] 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.
