Any dynamical evidence is every compositional hint whenever an interstellar comet accelerates in such a manner that outgassing will not be able to balance it easily. Precision tracking around perihelion left a disagreeable residual in the equations of 3I/ATLAS, the third object in the universe to be recognized to have come to us, outside its own star system, and then the comet was not moving in the correct way that a bright, noticeable plume would suggest it would have.
Astrometric solutions at perihelion distance of 1.36 astronomical units were found to converge on a recoil-like acceleration whose components were reported to be 135 kilometers per day 2 radial and 60 kilometers per day 2 transverse. In other words, the trajectory was behaving as though gasses leaving the nucleus were the propellers but the simple picture of a “rocket took” a strain on the anticipations of the amount of mass that must be lost, and the appearance of that loss in the picture.
That straining was intensified by the fact that the chemistry of the coma has not been acting like a textbook visitor to the solar-system. Optical spectroscopy detected atomic nickel vapor at 3.88 AU long before perihelion; a distance where the temperature is too low to allow metals to evaporate due to normal heating. The other abnormal signature was that not only was there presence of nickel but there was also absence of iron lines above instrumentation limits implying that the carrier of nickel might not be bulk refractory grains. One such plausible route lies in the molecular microphysics and not metallurgy: nickel contained in sunlight-weak compounds which dissociate when irradiated, releasing atoms without a hot surface.
The water, in its turn, came in as an independent constraint, that makes the simplistic accounts of the “supervolatile-only” activity more difficult. Swift demonstrated that it was able to detect hydroxyl (OH), which is the photochemical echo of water and connected it to a rate of water loss of approximately 40 kg/s at a heliocentric distance that was approximately three times more distant than that of Earth. In comets familiar to us, that is the part where water-driven activity dies, and it is suggested that the water-driven activity either takes an unusual form due to exposure, or represents an “extended source”, where small grains of ice, liberated in the nucleus, sublimate in the coma itself and supply the OH signal.
Compositional story was even narrowed by infrared spectroscopy. The coma was found to have about eight times the amounts of CO2 as water vapor by JWST measurements, which is inverted by the common hierarchy of solar-system comets. The same data correlated the brightness of the comet with drivers outside water as carbon dioxide is easily emitted at temperatures where water is slow. At this, the puzzle of acceleration can be interpreted less as a single discrepancy and more as a system in which the volatile inventories, the physics of grain, jet geometry, and the state of rotation also affect the way in which momentum departs the nucleus and how much of it can be observed as dust.
A thorough thermophysical explanation has reduced the scope of the run-of-the-mill explanations without taking away the wonder. When energy-balance calculations were combined with Monte Carlo jet geometries it was found that anisotropic outgassing with effective major fraction CO and CO2 could reproduce both the magnitude and direction of the acceleration using physically plausible active fractions, which were under percent across nucleus sizes of order 0.53 km. It does not mean that the comet is motionless, but that it may be vigorously noisy dynamically very small, as there may be vents only a few in number, and as the large grains whose mass is insensitive to radiation pressure may do much of the carrying of the dust.
Nevertheless, the actual engineering wonder might not be the interpretive, but the observational; 3I/ATLAS has been characterized in ultraviolet, optical, infrared, radio, and high-precision astrometry and has made out of a single passing body a multi-instrument laboratory of momentum transfer in porous ice. That width is important as the discovery rates are set to increase in future: the Vera C. Rubin Observatory is set to construct the first statistically significant sample and it is estimated that it should find about 5 and 50 interstellar objects during its 10-year survey. Having a population makes the field be able to cease treating every visitor as an exception and begin treating them as a dataset.
The urgency has also been converted into trajectory math by mission designers. A study by the Southwest Research Institute held that a high-speed flyby reconnaissance mission to an interstellar comet could be made possible with flight-proven technologies, stating that 3I/ATLAS alone would have been within the limits of the concept. The scientific assumption is simple: close-range imaging and in situ measurements would be able to distinguish between nucleus-scale venting and coma-scale grain sublimation, and would be able to associate exotic spectral lines, such as nickel, with a particular release environment.
In the case of 3I/ATLAS, the rest of the task does not lie in making a decision between “chemistry” and “dynamics”, but in rendering them closely connected to the point where the acceleration becomes a diagnosis, as opposed to a discussion. The loss of mass of the comet is already an observation of its own loss of mass; the spectra are a measure of what that loss is. The rift places between the two coupling.
