Interstellar visitor arrives: 3I/ATLAS in our planetary backyard
The interstellar comet 3I/ATLAS has captured the attention of planetary scientists as it barrels through the inner solar system. First spotted in July 2025 by an ATLAS telescope in Chile, the object became only the third known interstellar traveler to visit our cosmic neighborhood. Its extrasolar origin is inferred from its unusual, hyperbolic trajectory and its speed—about 130,000 mph (219,000 km/h)—that does not fit a sun-centered orbit. Ground-based telescopes can still observe 3I/ATLAS in late summer and early autumn 2025, but the window to study it up close from Earth is narrow as the comet skims toward and then behind the Sun from our perspective.
To maximize scientific return, space agencies have pivoted toward a coordinated, multi-mission approach. Rather than waiting for a distant telescope to glimpse surface details, ESA and NASA are leveraging spacecraft already en route to other targets to watch the comet in real time from advantageous angles closer to the Sun’s glare and hotter vicinity.
A multi-mission watch: from Mars to the outer planets
Between 1 and 7 October, Mars Express and the ExoMars Trace Gas Orbiter (TGO) will observe 3I/ATLAS as it passes near Mars, with the closest approach planned around 3 October at about 30 million kilometers away. The Mars Reconnaissance Orbiter (MRO) and other orbiters such as Tianwen-1 and Hope provide complementary perspectives from the Martian system, helping scientists build a three-dimensional view of the comet’s path and activity from a range of altitudes and viewing angles.
Meanwhile, NASA’s Psyche mission — currently en route to the asteroid 16 Psyche between Mars and Jupiter — is slated to catch glimpses of 3I/ATLAS as it travels through the inner solar system. Psyche’s instruments, designed for high-contrast imaging and spectroscopy, will contribute data on the comet’s nucleus and coma from a vantage point distinct from Mars-bound assets, broadening the chemical and physical profile scientists can assemble.
Looking farther ahead, ESA’s Jupiter Icy Moons Explorer (JUICE) will turn its gaze toward 3I/ATLAS from November 2 through 25, beginning just after the comet’s perihelion. JUICE’s suite of remote-sensing instruments will monitor the activity peak as solar heating drives outgassing from the nucleus, creating the glowing halo and trailing tail that are characteristic of comets. This timing is deliberate: observations around perihelion capture the strongest outgassing signals and the clearest chemical fingerprint of the object’s composition.
The perihelion window: what scientists hope to learn
Perihelion is the moment when the Sun’s heat drives the comet’s ices to vaporize, releasing gases and dust into space. For 3I/ATLAS, this phase is when its nucleus becomes most active, and its halo and tail expand dramatically. By combining measurements from Mars orbiters, Psyche’s instruments, and JUICE, scientists aim to quantify the abundances of water, carbon monoxide, carbon dioxide, and more exotic ices that may hint at the comet’s formation environment beyond the Solar System.
Because 3I/ATLAS originated outside our planetary nursery, scientists expect it to carry a chemical signature that could differ from comets formed around the Sun. If its composition mirrors local solar-system comets, it would support the idea that certain building blocks are common across the galaxy. If it diverges, the findings could point to a diverse inventory of materials produced around other stars and in other protoplanetary disks.
What the data could tell us about the building blocks of planets
News from these joint observations could illuminate fundamental questions about planet formation and chemical inheritance in the galaxy. Researchers will compare 3I/ATLAS’s spectrum and dust properties with known cometary materials, seeking clues about whether organic compounds and volatiles are universal across stellar systems or if interstellar travelers bring unexpected ingredients. In addition to composition, scientists hope to infer the nucleus size, geometry, and surface activity patterns—insights that could inform future detection strategies for even more distant interstellar objects.
As 3I/ATLAS disappears from Earth-based view and continues its journey, the data gathered by Mars, Psyche, and JUICE will stand as a landmark demonstration of how international space missions can collaborate to study a visitor from outside our solar system. This rare, time-limited opportunity promises to deepen our understanding of what interstellar objects are made from and how planetary systems across the galaxy may share common building blocks — or reveal striking differences that challenge our assumptions about the cosmos.
