“What can a comet from another star system teach us about our own?” That’s the question at the heart of a rare astronomical moment unfolding far from Earth’s view. On October 30, the interstellar comet 3I/ATLAS reached perihelion-its closest approach to the Sun-while hidden behind our star from terrestrial observers. Yet, across the solar system, a network of spacecraft is positioned to watch this ancient traveler ignite in the sunlight.
Perihelion is the time when a comet reaches its most dramatic level of activity. Solar heating causes volatile ices to sublimate, expelling gas and dust into an outer coma and often creating two different tails: one of dust shaped by the pressure of sunlight and one of ions swept away by the solar wind. For 3I/ATLAS, this moment has particular value, since it is not gravitationally bound to the Sun; it is a visitor from far beyond, following a hyperbolic trajectory that will carry it back to interstellar space. At perihelion, it lay 1.35 astronomical units from the Sun, or about 202 million kilometers, a distance where the balance of heat and chemical interactions can reveal its origins.
Earth-based and near-Earth telescopes are missing the show due to solar conjunction, but Mars-based missions have already provided early glimpses. ESA’s ExoMars Trace Gas Orbiter (TGO) has captured the comet as a faint, fuzzy dot during its closest approach to Mars on October 3 at 0.19 AU distance using its Colour and Stereo Surface Imaging System (CaSSIS). Several-thousand-kilometre-wide coma was visible, though the nucleus remained unresolved. Mars Express and TGO also attempted spectral measurements with OMEGA, SPICAM, and NOMAD instruments, hoping to detect volatile signatures in ultraviolet and infrared wavelengths.
Beyond Mars, NASA’s Psyche spacecraft and the Lucy mission to Jupiter’s Trojan asteroids are in positions to observe the comet’s perihelion activity. ESA’s Jupiter Icy Moons Explorer (JUICE), fresh from a Venus flyby, will have one of the best vantage points, though its use of its high-gain antenna as a sunshield means data will not reach Earth until February. Coordinated campaigns may extend to Europa Clipper and even solar observatories such as Parker Solar Probe and STEREO-A, which can image regions near the Sun where 3I/ATLAS now resides.
Chemistry is the central scientific draw. Observations from the Very Large Telescope in Chile have revealed unusually high nickel vapor emissions at distances far too cold for metals to sublimate, suggesting that nickel may be bound in volatile compounds that disintegrate under sunlight. The James Webb Space Telescope measured a coma rich in carbon dioxide-far more than in typical solar system comets-along with carbon monoxide and water ice particles. That composition suggests a molecular cloud environment markedly different from the one that birthed our own planets.
NASA’s Neil Gehrels Swift Observatory added another layer of intrigue when it detected emissions from hydroxyl (OH), a photodissociation product of water, when the comet was nearly three times farther from the Sun as Earth. The measured water-loss rate of about 40 kilograms per second at such a distance is in conflict with the models based on solar system comets and suggests that small icy grains may be releasing water vapor well before their nucleus warms up enough for surface sublimation.
Other scientists are also investigating various anomalies in both the trajectory and behaviour of the comet. Avi Loeb at Harvard University mentioned eight peculiarities that put 3I/ATLAS at a rank of 4 out of 10 on the Loeb scale of a possible technological origin, although he stresses it is most likely natural. These include being aligned within 5 degrees of the ecliptic, a sunward jet seen in mid-2025, and a path timed just so as to bring it close to Mars, Venus and Jupiter while hidden from Earth during perihelion.
Other spacecraft may yet contribute opportunistic measurements. Hera and Europa Clipper could pass through regions of the comet’s ion tail in coming weeks, with Europa Clipper’s plasma instruments and magnetometer capable of detecting charged particles and magnetic field draping caused by the coma’s interaction with the solar wind. Models such as “Tailcatcher” predict ion distributions millions of kilometers wide, potentially within reach of these missions despite large miss distances from the tail’s central axis.
To planetary scientists, each dataset from 3I/ATLAS represents a fragment in a large puzzle: whether the building blocks of planets-volatiles, organics, and metals-are consistent across the galaxy or highly variable between stellar systems. Given its estimated age of more than seven billion years, this comet dates back to long before our Sun formed and, unaltered, carries material from its natal system. The coordinated observations now underway will define just how much of that story we can read before 3I/ATLAS disappears into the dark between stars.
