“Before 1I/‘Oumuamua’s discovery in 2017, we had no direct evidence that objects from other star systems could reach our solar system,” explained Shokhruz Kakharov, whose research on interstellar object orbits has reshaped the way astronomers consider galactic dynamics. That context makes NASA’s recent observations of comet 3I/ATLAS more than simply another cometary research study, but a rare chance to study ancient material from outside of the Sun’s reach.
On August 6, the James Webb Space Telescope aimed its Near-Infrared Spectrograph (NIRSpec) at 3I/ATLAS, an interstellar traveler that was initially detected by the ATLAS survey telescope in Chile on July 1. The comet is over 400 million miles from the Sun and is traveling on a hyperbolic course to return it to deep space after a short solar encounter. Webb’s infrared capabilities, which maximize the detection of low-level heat signals and the penetration of cosmic dust, are specifically well-suited to the task of investigating the molecular makeup of such a far-off, icy body.
The NIRSpec instrument has the capability to break up incoming light into its component wavelengths, revealing the molecular fingerprints. For cometary science, this equates to the measurement of volatile molecules such as water vapor, carbon monoxide, and hydrogen cyanide matter that can evaporate off the comet’s nucleus as it heats up. The same methods were employed in 2019 when the Atacama Large Millimeter/submillimeter Array and the Hubble Space Telescope peered at comet 2I/Borisov and found an incredibly high CO-to-H₂O ratio of 35–155%, a far cry from the 4% solar system comet average. That made it appear that Borisov originated in an intensely frigid, carbon-rich area of its native system.
For 3I/ATLAS, Webb’s observations will be supplemented by observations from Hubble and the recently launched SPHEREx mission. Hubble, being ultraviolet-sensitive, can observe atomic and molecular emissions that infrared detectors cannot, and SPHEREx will provide an all-sky spectral survey in the near-infrared, potentially observing the comet’s changing spectrum over time. This multi-wavelength strategy enables scientists to cross-check molecular abundances and identify compounds that otherwise go undetected.
The timing of these observations is essential. As 3I/ATLAS closes in on perihelion on October 30 at 1.4 astronomical units from the Sun its outgassing will be most intense, rendering volatile signatures easier to detect. But shortly its path will take it too near the Sun to observe safely, before it returns in December for a last window of observation. Telescopes on the ground have already followed the comet’s activity since mid-June with pre-discovery images from ATLAS and the Zwicky Transient Facility, which give space-based measurements a baseline.
Kakharov and Harvard astrophysicist Abraham Loeb’s simulations of the comet’s trajectory suggest that 3I/ATLAS is most likely the oldest interstellar object ever seen, with a median age of 4.6 billion years and a birthplace in the Milky Way’s thick disk a zone inhabited by older, less-metallic stars. To arrive at that deduction, they performed 10,000 Monte Carlo simulations for each recognized interstellar object, backintegrating their trajectories for a billion years in a galactic potential model. The findings imply that, as opposed to the young 1I/’Oumuamua or the middle-aged 2I/Borisov, 3I/ATLAS was possibly ejected from its home system in the early life of the galaxy.
Observing such an object presents an window onto planetary system formation conditions distant from our own. Comets retain the chemical imprints of their formation environments, and interstellar comets bear that record across light-years. Webb’s infrared spectra may determine if 3I/ATLAS has Borisov’s CO-rich chemistry, or if it has a different volatile character suggestive of warmer or more metal-poor origins.
These same searches also support NASA’s larger mission to detect and characterize objects in the solar system, including potentially hazardous ones. Although 3I/ATLAS is no threat its closest pass by Earth will be over 150 million miles away the same detection and tracking capabilities that discovered it are needed for planetary defense. Future surveys such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time should greatly boost interstellar object detection rates, with potential to find dozens annually.
Lacking possible interstellar probe missions, these natural visitors are the only tangible samples of distant planetary systems for investigation. As Kakharov had said, “Understanding ISO origins provides a deeper context for interpreting their physical and chemical properties.” For 3I/ATLAS, that story is being inscribed these days in the infrared light gathered by Webb’s segmented gold mirrors, each photon bearing a piece of a history that originated billions of years and trillions of miles before.
