Key points: ESA used images from the ExoMars Trace Gas Orbiter during 3I/ATLAS’s Oct. 3 Mars flyby to reduce trajectory uncertainty by roughly tenfold. The comet — the third confirmed interstellar visitor — was first seen in July traveling at over 130,000 mph and likely originated billions of years ago. Perihelion on Oct. 29 produced a brightening and temporary tail changes. Additional observations from NASA and ESA orbiters may reveal more before its closest Earth approach on Dec. 19 (~168 million miles / 270 million km).
Mars Orbiter Sharpens 3I/ATLAS Exit Path — Trajectory Uncertainty Cut Tenfold
Key points: ESA used images from the ExoMars Trace Gas Orbiter during 3I/ATLAS’s Oct. 3 Mars flyby to reduce trajectory uncertainty by roughly tenfold. The comet — the third confirmed interstellar visitor — was first seen in July traveling at over 130,000 mph and likely originated billions of years ago. Perihelion on Oct. 29 produced a brightening and temporary tail changes. Additional observations from NASA and ESA orbiters may reveal more before its closest Earth approach on Dec. 19 (~168 million miles / 270 million km).
02:40 AM, 11/18/2025Science
Mars orbiter sharpens the path of interstellar comet 3I/ATLAS, cutting uncertainty tenfold
Scientists at the European Space Agency (ESA) have dramatically improved predictions for the outbound path of the interstellar comet 3I/ATLAS after analyzing images taken during the object’s close pass by Mars. Data from ESA’s ExoMars Trace Gas Orbiter (TGO) reduced the uncertainty in the comet’s trajectory by roughly tenfold, allowing astronomers to aim telescopes and instruments with greater confidence as the comet leaves the solar system.
3I/ATLAS — the third confirmed interstellar object detected — was first spotted in early July traveling sunward at more than 130,000 mph (210,000 km/h). Researchers believe it may have been ejected from a star in the Milky Way’s outer regions as long as seven billion years ago. Observers have reported unusual features on the comet, such as a heavily irradiated surface and a puzzling anti-tail; these oddities led to unsupported speculation about artificial origins, a claim most astronomers reject.
The comet reached perihelion (closest approach to the Sun) on Oct. 29, an event accompanied by a sudden brightening, a temporary color change and a brief disappearance of its tail. Earlier, on Oct. 3, 3I/ATLAS passed about 18.6 million miles (30 million kilometers) from Mars, giving TGO a unique vantage point to photograph the object.
By adding observations from Mars to the usual ground- and Earth-orbit-based tracking, ESA scientists effectively gained an extra baseline for triangulation, shrinking positional uncertainty significantly. ESA said the improvement was far greater than expected: what teams anticipated as a modest gain became a roughly tenfold leap in accuracy.
Why this matters: tighter trajectory predictions let observatories schedule targeted observations and increase the chance of capturing high-quality imagery and spectra while the object is still accessible.
Processing the data was challenging. TGO’s Colour and Stereo Surface Imaging System (CaSSIS) is optimized for imaging Mars’ surface, not thousands of miles-distant, fast-moving small bodies. Incorporating the orbiter’s precise motion around Mars into the comet’s astrometric calculations required careful calibration. The result marks a milestone: this is the first time measurements from a planetary orbiter have been accepted into the International Astronomical Union’s Minor Planet Center database, the central repository for small-body tracking.
ESA plans to apply the same technique with other missions. The Jupiter Icy Moons Explorer (JUICE) is expected to observe 3I/ATLAS later this month, and teams have noted that spacecraft such as Hera and NASA’s Europa Clipper could potentially intersect or sample material from the comet’s tail as it departs the inner solar system.
NASA’s Mars Reconnaissance Orbiter (MRO) also imaged the comet during the flyby and may have captured one of the clearest views yet. Those images were delayed from public release by a recent government shutdown but are expected to appear soon, according to reports.
3I/ATLAS will make its nearest approach to Earth on Dec. 19, closing to about 168 million miles (270 million kilometers). Although still distant, that pass represents the best viewing opportunity from Earth before the object continues on its one-way path out of the solar system.