3I/ATLAS is not acting like a textbook comet when heated by the sun. Accurate tracking via perihelion revealed a non-gravitational shove that conventional prescriptions of outgassing should not have been capable of, making any passing interstellar visitor a sort of stress test of how engineers and dynamicists understand small bodies.
The observed aberration is not insignificant. In a perihelion where it was 1.36 AU, there were reports of a radial acceleration of 135 kilometers per day 2 and transverse acceleration of 60 kilometers per day 2, which are large enough to suggest that a simple “rocket effect” explanation would result in the mass being rapidly lost in a few weeks. It is the fundamental mismatch strong thrust usually requires conspicuous gas production but the morphology of coma prominently visible, and the behavior of dust do not necessarily scale to the suggested increase in momentum.
What complicates the problem beyond a bookkeeping exercise is the chemistry coming with the trajectory. Using IR spectroscopy proved that the coma can be carbon dioxide dominated with the CO2-to-water ratio of 7.6 +/- 0.3 measured at a time when the object was still on the side of 3AU. Practically, CO2 is a volatile which can promote activity where water is relatively slow, switching the direction of thrust on the surface and the speed at which it can change direction in response to a change in illumination. Even a seemingly water-saturated interior may exhibit CO2-led upwelling on the surface should heat conduction not be efficient or should near-surface layers be able to liberate the species that are easier to do so and release them first.
Then there is the metal. Very large telescopical optical spectroscopy observed nickel emission at large heliocentric distances and detected no iron in that same data, the combination of the two making older intuition that metals are only seen when comets are much hotter come to grief. The VLT group detected many Ni I lines and also determined an increasing nickel production with the approach of the comet to the Sun, with an actual production rate of 23.30 + 0.07 (log-scaled atoms/second) at 2.85 AU, and commented on low-temperature release pathways instead of metal sublimation. Their discussion indicates that photon-stimulated release by dust or organometallic particles, including nickel-carbonyl-like complexes, which can dissociate rapidly in sunlight are some of the mechanisms. Provided the nickel is executed with dust and released in the inner coma, there can be a redistribution of the momentum budget: the thrust can be generated by gas species that can not be easily determined based on observable appearance, and the grain size of dust grains and the directionality can cause the net acceleration vector to change over time.
The engineering lesson is increased by a short observational window. Trajectory solutions must deal with a variety of changing active fractions, rotation coupled jet geometry, and volatile mixtures that are not reflective of the solar system comets. There is no mystery in 3I/ATLAS to the extent that it cannot be measured: high-precision astrometry and compositional constraints, such as CN onset, hydroxyl detects, etc., permit models to be falsified rather than tuned.
The same mentality of measuring up is why mission planners continue to go back to interstellar flybys. A study conducted by Southwest Research Institute maintains that a high-speed, head-on flyby concept may gather definitive information on nucleus structure, activity sources, and coma physics, which are just the parameters that make an acceleration that much “too large,” merely by the chance that the wrong assumptions were introduced into the model.
The larger shift is methodological as survey capability is increased. It is anticipated that the Vera C. Rubin Observatory will encounter several tens of interstellar bodies within its lifetime of the survey, making it more difficult to treat each new object as an instance. 3I/ATLAS serves as a prototype: a situation where the dynamics, the spectroscopy and the mission design overlap around a single challenging question: what, exactly, pushes an icy body when the normal working rules no longer hold?
