For the third time in human history, a visitor from beyond the Solar System has been caught in the act of shedding its ancient secrets. In early November 2025, the European Space Agency’s Jupiter Icy Moons Explorer (Juice) spacecraft turned its instruments toward comet 3I/ATLAS, an object potentially 7 billion years old and captured a fleeting view before it continues its hyperbolic escape back into interstellar space.
3I/ATLAS, discovered on July 1, 2025 by the ATLAS survey in Chile, is no ordinary comet. Its trajectory confirms that it was formed in another star system, was ejected into the Galaxy, and has spent eons drifting through the radiation-rich void. Hurtling through the inner Solar System at roughly 130,000 mph (210,000 km/h), it has already swung past Mars and the Sun, triggering intense sublimation whereby solar heat liberated volatile ices from its surface. The resulting bright coma a halo of gas and dust appears in Juice’s image together with two distinct tails: a plasma tail of electrically charged gas shaped by the solar wind, and a fainter dust tail composed of micron-sized particles pushed by solar radiation pressure.
The observation window of Juice opened just after the comet’s closest solar approach on October 30, when activity was at its peak. On November 2, two days before its closest pass to the spacecraft at 66 million km, Juice probed the comet’s behaviour and composition using five scientific instruments: JANUS (high-resolution optical camera), MAJIS (spectrometer for visible and infrared), UVS (ultraviolet spectrometer), SWI (submillimeter wave instrument), and PEP (particle environment package). The spacecraft also snapped a frame with its Navigation Camera – a lower-resolution imager designed for guiding flybys of Jupiter’s moons. ESA engineers, eager for a preview, downloaded just a quarter of a single NavCam image despite the slow data rate imposed by Juice’s use of its high-gain antenna as a heat shield during solar approach.
This resulted in a scene with a sharply defined coma and both tails – a rare image of an interstellar comet in full post-perihelion activity. While Mars orbiters in October had been closer to the comet, Juice’s timing meant it caught 3I/ATLAS in a more vigorous state. Behind this transformation lies sublimation physics: as sunlight penetrates the comet surface, volatile species such as CO₂ and H₂O directly transition from solid to gas, dragging dust grains along with them. It is when ionized molecules are blown away by the solar wind that a plasma tail forms, while the trajectory of the dust tail is determined by radiation pressure-forces that depend on particle size and composition.
Other observatories have already hinted at unusual chemistry with spectroscopic work. The CO₂-to-H₂O ratio of roughly 8:1 in the coma, as measured by ground-based and space telescopes including the James Webb Space Telescope, is among the highest ever recorded and suggests formation near a CO₂ ice line or within a radiation-rich environment unlike our own. Reflectance spectra show slight reddening consistent with organic-rich materials, and dust mass-loss rates between 0.3 and 4.2 kg/s imply sustained activity. Large dust grains of the comet resist dispersal; they are similar to those of some distant Solar System comets with a spin period of 16.16 hours and asymmetric coma morphology.
Datasets such as these are few and far between because such interstellar captures are so rare. Most comets originate in the Solar System’s Oort Cloud or Kuiper Belt, their compositions having been sculpted by the Sun’s protoplanetary disk. 3I/ATLAS, though, carries the chemical fingerprint of a different stellar nursery altogether. Its nickel-rich plume, low water fraction, and extreme polarization-an optical property tied to dust grain structure-are anomalies that planetary scientists interpret as natural consequences of foreign formation conditions and rapid solar approach speed.
When Juice’s full dataset arrives on Earth in February 2026, scientists anticipate high-resolution JANUS imagery, detailed spectrometry from MAJIS and UVS, molecular composition from SWI, and charged particle profiles from PEP. These measurements together could disclose unprecedented detail about the building blocks of a planetary system far beyond our own. For the moment, the quarter-frame NavCam preview will have to suffice as a tantalizing reminder that in the dark space between stars, at least, ancient messengers still wander, and sometimes our spacecraft are there to welcome them.
