The measurement of Interstellar comet 3I/ATLAS is underway using the instruments designed to measure Mars and the outcome is a view that is seldom given to the engineers, a view of a body that existed before the sun.
Being the third probed interstellar object to pass through the solar system, 3I/ATLAS has featured on the scale and speed: it is approximately 11 kilometers in diameter and travels at a speed exceeding 130,000 mph (210,000 km/h). Its estimated age, probably over 7 billion years, puts its raw material several billion years further into the history of the galaxy than the planets that it is passing through today. That mix would render each observation less about spectacle because of the extraction of physical constraints of a target that is not going to make it back.
The most technical exposing work has been in Martian orbit. In early October, Mars Reconnaissance Orbiter swung hi-fi in the opposite direction of its surface survey to point the HiRISE at the comet, and made observations of the comet at a distance of approximately 30 million kilometers. Those frames, the comet is represented in pixelated bright core enclosed in a coma-ejecta that make of a tiny nucleus a bigger lasting system of measurement. It is not the aesthetics, it is calibration. When a vision system that is specially constructed to detect boulders on a planet (Mars) focuses on an object moving rapidly in the deep space, it imposes limits on the brightness, dust distribution and nucleus size which would be challenging to achieve on Earth, given the same geometry. The payoff of the HiRISE is summarized by Avi Loeb in a way of viewing angle and detail: the HiRISE image would provide us with a side-view and a spatial resolution three times sharper than the Hubble Space Telescope, and even with unresolved sources, the profile of the brightness could be used to estimate the diameter.
HiRISE was not alone. During a period of approximately ten days, surrounding the encounter, ultraviolet views were captured by the MAVEN Imaging Ultraviolet Spectrograph and isolate hydrogen and other related species in the coma, which allows the calculation of water-release estimates as the sun heats up. An exposed surface captured a slight smudge using a long exposure, which is perseverance, observing that what a rover camera is capable of doing when it is requested to act as an observatory without motion is indeed noteworthy indeed, it reads as though that were a system test.
The composition, however, is where 3I/ATLAS forces the view to violate the assumptions of a visit to the interstellar by “pristine” people. JWST measurements of the NIRSpec and NASA measurements of the SPHEREx suggest that the CO2/H2O ratio is extremely high, 7.6 +- 0.3, significantly above solar system comet trends, and the CO is high as well as the spectral slope is red. When in the laboratory, galactic cosmic rays, over a long period, may transform CO into CO2 and form an organic enriched crust, such that the gas now escaping may represent an irradiated surface layer, but not an unaltered interior. Even in that frame, 3I/ATLAS serves as a record of radiation processing at gigayear timescales, not merely a sample of its place of origin.
Ground-based spectroscopy provides desirable limits. A 300-2500 nm wavelength campaign by an ESO VLT X-shooter campaign detected the comet mentioned as clearly active but only upper limits of key gas tracers, with QOH < 8.2 x 1026 s[?]1 and QCN < 5.6 x 1023 s[?]1. The identical data also gave measurements of red on the sky slopes that are associated with the very dark and outer-solar-system-like material-indications that the dust environment can be measured even in the cases when the gas bands are weak or nonexistent at greater solar distances.
Trajectory work puts that chemistry into a larger map. With the help of astrometry of Gaia DR3, researchers have been able to trace the orbit backwards over 10 million years and list dozens of stellar interactions with negligible perturbations in the recent past, which is in agreement with a steep, rapid path that is consistent with an origin in the thick disk of the Milky Way, a population that is known to contain older stars. That satellite environment contributes to that age suggested by composition: an interstellar visitor, his body overlayed with veneers of surface that have been covered over time, radiation, and low temperatures between stars.
Since the monitoring is in such an unusual state of being the closest, to date, to Earth, on Dec. 19, the engineering lesson is more than usual, namely the most fruitful “interstellar mission” in decades is being done with instruments not designed to do so, by aiming the spacecraft to the point of turning a flythrough into a dataset, which, nonetheless, can still be used to constrain size, activity, and the long-term chemistry of icy bodies across the galaxy.
