What do you get when you apply some “predictability” to the behaviour of comets but the comet was not a member of the solar system in the first place?
The third object to be proved as interstellar was 3I/ATLAS, which highlighted an unspoken fact in the current sky survey: faster discoveries have been made without a firm interpretation. The comets of interstellar are observed as moving points of light, and then hastened on to actual things, with rules supposed to govern them, how the tail ought to point, how brightness ought to develop, what a nucleus might be doing under a cloud of dust and gas. Going along that pipeline with comets which are lifted up in the long effect of the Sun. It is less dependable when an outsider comes with unknown materials, thermal background, and dust dynamics.
Orbital mechanics is not the only prediction of a comet. It is also a collection of working guesses regarding the way the sun changes ice into gas, the manner in which the gas pulls dust away, as well as the reaction of the dust to the radiation pressure. Observers with 3I/ATLAS were faced with behavior which was not the most comfortable of the visual cues the tail actually pointing away to the Sun. Bryce Bolin referred to one occasion when the dust structure seemed to “preferentially follow the direction of the sun,” indicating that large grains were taking control of the coma, and were not easily driven into the traditional anti-solar tail. A shift in dust size distribution will lead to a shift in the brightness profile, and the shortcuts commonly used to estimate nucleus size and activity lose their reliability.
The issue is further aggravated by the fact that the nucleus can hardly be seen directly. Bolin described comet nuclei as obscure, buried, within this coating of dust, and why a space resolution is important: to separate an unresolved nucleus and a resolved coma requires angular resolution which ground-based seeing blurs away. The Hubble Space Telescope is also not a camera in that context, but rather a diagnostic instrument to minimize the infinite amount of coma in the central pixels and stringent limitations to the source of what is energizing the activity. The “nucleus” in a prophecy may still be an evolved one then it may be the centre of a rapid, or of an unusually slow, flight of dust, and will reformation may be necessary each night of the coma.
There is one more twist with composition. Upon completion of a spectroscopic campaign, 3I/ATLAS was found to be spinning with a spin period of 16.16 +- 0.01 hours and dust mass-loss between 0.3 and 4.2 kg/s with an unusually high CO2-to-H2O ratio of 8:1. The reason that ratios such as that are important is that the common predictions of brightness imply that water-driven activity will take over within particular solar distances. When carbon dioxide is the cause of much of the early outgassing, the comet may brighten, fade or morphologically change on an alternate schedule than would be predicted by water-based models.
A basic constraint is also that there is no careful modeling that can eliminate: with a blistering interstellar flight, one has minimal time to study the regulations before the object disappears. The problem of the origin of the comet was summed up by David Jewitt: “No one knows where the comet came from. You can’t project that back with any accuracy to figure out where it started on its path.” You can never extrapolate that back to determine where it began in its course. He also had the observational difficulty: “It’s like glimpsing a rifle bullet for a thousandth of a second.” Those limitations imply that the forecasts need to be updated in real-time when new data is received and the modifications can be substantial.
The visitor to interstellar space tomorrow will hardly shatter comet science; it will more likely reveal comet prognostication slavery silently dependent on acquaintance with the solar system. With weaker, more rapidly receding targets, the characterization might tend to rely more on space telescopes and opportunistic spacecraft images, including a JUICE view of 3I/ATLAS, to ensure the preservation of the predictions made based on measurements, not assumptions.
