“Every interstellar comet so far has been a surprise,” said Dr. Zexi Xing of Auburn University, looking back on the wide variety of objects that have wandered into the solar system from far-off star systems. That surprise grew deeper in early October when the European Space Agency’s Mars orbiters, Mars Express and ExoMars Trace Gas Orbiter, took highly unusual photographs of comet 3I/ATLAS as it flew by Mars even though it was 10,000 to 100,000 times fainter than they normally capture.
The achievement was a technical feat. Both of the orbiters are designed to take pictures of Mars’ surface from a few hundred miles up, not follow a very swift-moving object some 18.6 million miles away. Still, with precise calibrations and long exposures, the cameras captured the comet’s icy-rocky core and its coma, the faint halo of gas and dust pushed off by solar heat. The readings were captured as 3I/ATLAS hurtled through space at about 130,000 mph, its path verifying an origin well outside the Sun’s gravitational grasp.
First spotted on July 1, 2025, by the Chilean ATLAS survey, 3I/ATLAS is just the third confirmed interstellar visitor to the solar system, and probably the largest between 5 and 11 kilometers in diameter, with a mass of over 33 billion tons. Astronomers estimate it may be three billion years older than the solar system, its journey through the galaxy preserving a record of ancient planetary chemistry. NASA’s SPHEREx mission observations found a water ice-rich nucleus and a carbon dioxide gas-dominated coma, but surprisingly low in carbon monoxide a hallmark indicating the comet was “well baked and boiled before being ejected from its parent solar system,” according to Carey Lisse of Johns Hopkins University.
The makeup has created interesting comparisons to indigenous solar system comets. In all but one, water, carbon dioxide, and carbon monoxide are the major volatile ices, their relative composition determined by the formation temperature and radiation environment. The extremely high CO₂-to-H₂O ratio of about 8:1 in 3I/ATLAS’ coma is one of the highest ever measured, and it suggests an origin close to a CO₂ ice line or within a radiation-dense zone in its native system. Ground-based telescopic reflectance spectra have evidenced slight reddening, in agreement with organic-rich substances like those for outer solar system comets.
The detection of such faint interstellar travelers demands dedicated imaging techniques. Methods such as spectral deconvolution, applied in exoplanet and faint companion research, capitalize on the principle that instrumental artifacts vary linearly with wavelength whereas the target location is constant. By removing wavelength-dependent noise, astronomers can mask faint objects from the light of brighter sources. Although ESA’s Mars orbiters did not use this specific technique, their ability to image 3I/ATLAS was made possible by similar noise-removal principles and precise motion following across frames.
More spacecraft have become involved. NASA’s Neil Gehrels Swift Observatory detected hydroxyl a chemical signature of water from 3I/ATLAS when it was almost three times further away from the Sun than Earth and when most comets are inert. The recorded water-loss rate of around 40 kilograms per second at such a distance indicates sublimation from small ice grains that are levitated off the nucleus, a process observed in only a few far-off comets. “Detecting water is a major breakthrough for understanding how interstellar comets evolve,” commented Auburn University’s Dennis Bodewits. “Finding the same signal in an interstellar object means that, for the first time, we can begin to place 3I/ATLAS on the same scale used to study native solar-system comets.”
ESA’s Jupiter Icy Moons Explorer (Juice) will then study the comet in November, shortly after perihelion its closest to the Sun when solar heating will power maximum activity. By this time, coma and tail are supposed to swell tremendously, providing a more distinct chemical signature through gases and dust emitted. For planetary scientists, this moment is pivotal: it is a case of matching or comparing 3I/ATLAS’ chemistry with solar system comets to determine if the assembly materials of planets are universal in the galaxy or influenced by idiosyncratic local circumstances.
Although no tail was observable in the Mars-orbiter photos, geometry and low dust generation most probably concealed it. Sublimation of its water ice should create a more visible dust coma and tail as 3I/ATLAS moves further inward, making it observable when it crosses behind the Sun during December. By March 2026, it will pass by Jupiter before turning back into interstellar space, taking the secrets of a planet system that was formed billions of years ago compared to our own.
