New updates released for the predicted perijove of 3I ATLAS

The predicted perijove of the interstellar object 3I/ATLAS has been updated following revisions to its non-gravitational acceleration.

Harvard Professor Avi Loeb reported that the radial acceleration component A1, initially listed as 1.6x10^{-6} au per day squared on October 30, 2025, by Davide Farnoccia on NASA’s JPL Horizons website, was reduced to 4x10^{-7} au per day squared by November 24.

Loeb noted that the minimum perijove distance during 3I/ATLAS’s encounter with Jupiter on March 16, 2026, was forecasted at 53.445 (+/- 0.06) million kilometers, which matched Jupiter’s Hill radius at that time, 53.502 million kilometers.

“Interior to that radius, Jupiter’s gravity dominates over the Sun’s tide,” Loeb stated, emphasizing the significance of the measurement.

Revisions to the non-gravitational acceleration model

Following the initial observation, the A1 value on NASA’s JPL Horizons website was revised to 6.8x10^{-8} au per day squared with a new radial dependence model.

The update applied an inverse square dependence on the heliocentric distance, 1/r², reflecting the sublimation of carbon dioxide ice within 5 au, replacing the previous water ice-based model. Loeb explained:

“This new model replaces the steeper radial dependence associated with the previous model used by NASA’s JPL Horizons — suitable for the sublimation of water (H20) ice based on work by Brian Marsden and collaborators.”

The revised perijove distance is now forecasted at 53.587 (+/- 0.045) million kilometers, slightly outside Jupiter’s Hill radius on March 16, 2026.

Loeb noted that this forecast relies on contributions from larger heliocentric distances to explain deviations of 3I/ATLAS from its gravitational trajectory.

He added that the new model of JPL Horizons is likely inadequate and that there is strong evidence that 3I/ATLAS became "brighter" near perihelion than the smooth 1/r² model would predict.

Luminosity and radial dependence

The luminosity of 3I/ATLAS, as observed by the Hubble Space Telescope on July 21, 2025, reflects mass loss in the coma.

Loeb referenced reports by Marshall Eubanks and collaborators, which indicate that the luminosity evolution suggests a steeper radial profile than the 1/r² model.

An earlier report by Qicheng Zhang and Karl Battams indicated a luminosity profile of 1/r^{7.5} inside 2 au as 3I/ATLAS approached perihelion on October 29, 2025.

“Adopting this steep radial dependence would change the expected perijove distance towards a closer agreement with Jupiter’s Hill radius,” Loeb stated.

The discrepancy between luminosity and the 1/r² model suggests that correcting the radial dependence of the non-gravitational acceleration could bring the forecasted perijove distance into agreement with the Hill radius.

Loeb clarified that since 3I/ATLAS was hidden by the Sun from terrestrial telescopes during its perihelion passage, only a tight constraint on the integrated drift of 3I/ATLAS from its gravitational path is available, but not on its radial dependence close to "perihelion."

Future observations and implications

Data from spacecraft such as Juno, Juice, or Psyche are expected to provide more precise astrometric measurements as 3I/ATLAS approaches its perijove on March 16, 2026. Loeb noted,

“We will know whether the perijove distance agrees with the Hill radius in the coming months thanks to data collected as 3I/ATLAS approaches its perijove.”

He also highlighted that the statistical likelihood of the coincidence between the perijove distance and Jupiter’s Hill radius is smaller than 0.00004.

If non-gravitational acceleration was involved in achieving this match, it could represent an anomaly in 3I/ATLAS’s trajectory.

The final verification will be posted on the JPL Horizons website.

Stay tuned for more updates.