BY:SpaceEyeNews.
Interstellar comet 3I/ATLAS: a rare alien visitor over Mars
Every now and then, our Solar System gets a visitor that does not belong here. Interstellar comet 3I/ATLAS is one of those ultra-rare guests. Instead of forming from the same disk of gas and dust as our planets, it was born around another star, drifted for billions of years through deep space, and only recently crossed our path.
In October 2025, interstellar comet 3I/ATLAS swept past Mars. NASA took full advantage of this cosmic drive-by. The Mars Reconnaissance Orbiter (MRO), the MAVEN orbiter, and the Perseverance rover all pivoted from their usual Mars science to watch this icy traveler up close. Their coordinated campaign produced detailed images and spectra that scientists simply cannot get from Earth-based telescopes. Interesting Engineering+1
For planetary scientists, this was like watching a piece of another solar system skim past the red planet, while a full camera crew was already in orbit. The data from interstellar comet 3I/ATLAS will help reveal what it is made of, how it behaves in sunlight, and what its chemistry can tell us about the worlds that formed around its long-lost home star. And because its path is hyperbolic, 3I/ATLAS will not come back—this really is a once-in-a-lifetime encounter.
An observation of 3I/ATLAS, captured by the Hubble Space Telescope.
What makes interstellar comet 3I/ATLAS so special?
Interstellar comet 3I/ATLAS is only the third confirmed interstellar object ever seen in our Solar System, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. That tiny list shows how rare these visitors are.
Unlike ordinary comets, which orbit the Sun on long but closed paths, interstellar comet 3I/ATLAS follows a hyperbolic orbit. Its trajectory and speed show that it is not gravitationally bound to the Sun. It is simply passing through and will keep going back into interstellar space once its journey near the Sun is complete.
3I/ATLAS was first spotted on 1 July 2025 by the ATLAS survey telescope in Chile. Early tracking of its motion revealed two key clues:
- It was moving too fast to be a typical long-period comet.
- Its orbit was tilted in a way that signaled an origin outside our planetary family.
Follow-up observations with large telescopes and space missions painted a more detailed picture. The comet appears unusually rich in carbon dioxide, with a composition that suggests its surface has been processed for eons by galactic cosmic rays—high-energy particles that constantly stream through the galaxy. That processing may have altered its ices, converting simple molecules into more complex ones and building up an organic-rich crust.
Even more intriguingly, scientists estimate that interstellar comet 3I/ATLAS could be older than our Solar System by several billion years. If that estimate holds, it means we are looking at a fossil from an ancient planetary system that formed long before the Sun ever ignited.
Put simply, every photon of light that 3I/ATLAS reflects carries information not just about a comet—but about an entire alien origin story.
Closest imagery of the comet was taken by NASA’s spacecraft at Mars.
Mars orbiters zoom in on interstellar comet 3I/ATLAS
When interstellar comet 3I/ATLAS passed closest to Mars on 3 October 2025, the red planet became the perfect observation platform. Several Mars spacecraft were in just the right place at the right time, and mission teams quickly re-tasked them to track this interstellar visitor. NASA Jet Propulsion Laboratory (JPL)+1
HiRISE trades Mars landscapes for an interstellar close-up
NASA’s Mars Reconnaissance Orbiter carries HiRISE, one of the most powerful cameras ever sent to another planet. For this event, HiRISE stopped looking at Martian craters and canyons and instead turned toward the fast-moving speck of interstellar comet 3I/ATLAS.
At closest approach, the comet passed roughly 0.19 AU (about 29 million kilometers or 19 million miles) from Mars. From that vantage point, HiRISE saw a bright, slightly fuzzy point—essentially a pixelated ball of dust and ice against the black background. The image may look simple to the eye, but to scientists it contains valuable information about how big the nucleus might be and how dense the surrounding dust cloud is. Interesting Engineering+2NASA Jet Propulsion Laboratory (JPL)+2
HiRISE team members described how rare it is to watch an interstellar object from such proximity. They normally spend their time studying the Martian surface in exquisite detail. Suddenly, they had a new target that came from far beyond the Sun’s reach. This flexibility—being able to tilt the orbiter and lock onto a target rushing across the sky—turned a Mars mapper into a pop-up interstellar observatory. Interesting Engineering+1
Mars Express and ExoMars join the campaign
The European Space Agency also joined the effort. ESA’s Mars Express and ExoMars Trace Gas Orbiter performed their own observations of interstellar comet 3I/ATLAS. Their instruments focus on gases and dust in Mars’ thin atmosphere, but they can also be used to study passing objects. By combining data from multiple spacecraft, researchers hope to cross-check measurements and refine models of the comet’s activity and orbit.
Together, these Mars orbiters created a multi-angle view of 3I/ATLAS: brightness, coma size, and how its dust responded to the solar wind and sunlight. That kind of dataset simply does not exist for most comets, let alone one from another star system.
MAVEN maps the hidden chemistry of interstellar comet 3I/ATLAS
While HiRISE delivered images, MAVEN delivered chemistry. MAVEN’s primary job is to study how Mars’ atmosphere interacts with the Sun, but its instruments are perfect for sensing gas and plasma around a comet as well. NASA Jet Propulsion Laboratory (JPL)
Ultraviolet fingerprints of water and hydrogen
For about ten days, MAVEN pointed its ultraviolet spectrograph at interstellar comet 3I/ATLAS. In ultraviolet light, hydrogen glows with distinct signatures. MAVEN detected these emissions inside the comet’s coma, the fuzzy envelope of gas and dust around the nucleus. From these observations, scientists can estimate how quickly the comet is releasing water and other volatiles as sunlight heats its surface. Interesting Engineering+1
As water molecules break apart under solar radiation, they release hydrogen and hydroxyl (OH). Mapping these components around interstellar comet 3I/ATLAS helps researchers trace the flow of gas and the structure of the outgassing plume. It is like watching the comet breathe in slow motion, but across vast distances. Interesting Engineering+2NASA Jet Propulsion Laboratory (JPL)+2
Clues from the deuterium–hydrogen ratio
One of the biggest scientific prizes from MAVEN’s data may be the deuterium-to-hydrogen ratio (D/H) in the comet’s water. Deuterium is a heavier form of hydrogen with an extra neutron. The D/H ratio in water acts as a chemical “accent,” revealing where and how that water likely formed. Different planetary systems and regions of space imprint different D/H signatures.
If scientists can pin down the D/H ratio in interstellar comet 3I/ATLAS, they can compare it with comets in our own Solar System. A large difference would suggest that planetary nurseries around other stars can produce very different types of icy bodies. A surprising match, on the other hand, might hint that some aspects of planet and comet formation are universal.
Evidence for cosmic-ray processed ices
Beyond water, other telescopes and spectrographs—such as JWST and planned all-sky missions—have detected unusually high levels of carbon dioxide and carbon monoxide in interstellar comet 3I/ATLAS. These measurements support the idea that cosmic rays have reshaped its surface over vast timescales, turning simple ices into more complex chemistry.
MAVEN’s ultraviolet data fit into this larger picture, adding another layer of detail to how the comet’s outer layers behave when sunlight finally warms them after an almost unimaginably long journey.
Perseverance’s view: an interstellar smudge in the Martian sky
The Perseverance rover joined the observing campaign from the ground. Stationed in Jezero Crater, Perseverance normally explores ancient river deltas and samples Martian rocks. For interstellar comet 3I/ATLAS, the rover tried something different: a long-exposure sky image. Interesting Engineering+1
During a long exposure, the rover’s camera stays fixed while the sky slowly rotates overhead. Stars turn into streaks, tracing lines across the frame. Moving objects, like comets, stand out as small smudges. On 4 October 2025, Perseverance captured such a frame where interstellar comet 3I/ATLAS appears as a faint, compact blur against the backdrop of star trails.
This is not the kind of spectacular, wide-tail comet photo you might see from Earth. The comet remains far away and relatively faint. But scientifically, that smudge is powerful. It confirms the comet’s position from the surface of Mars, adds another data point for its brightness, and showcases how flexible modern Mars missions have become.
Together, MRO, MAVEN, Perseverance, Mars Express, and ExoMars turned Mars into a multi-platform observatory for interstellar comet 3I/ATLAS. Few other locations in the Solar System could have provided such a coordinated, high-quality dataset at just the right moment.
What interstellar comet 3I/ATLAS reveals about other star systems
Why are astronomers so excited about one distant, fuzzy object that will never come close to Earth? Because interstellar comet 3I/ATLAS carries direct samples from outside our Solar System.
Every measurement matters:
- Orbit and speed tell us how the comet entered our neighborhood and how long it has wandered through the galaxy.
- Color and polarization reveal the size and texture of its dust grains, which seem to differ from typical Solar System comets.
- Gas composition—especially the extreme enrichment in CO₂ and high CO levels—points to a surface that cosmic rays have reshaped.
- Water chemistry, including the D/H ratio, connects interstellar comet 3I/ATLAS to models of planet formation around other stars.
These clues combine into a single story. Interstellar comet 3I/ATLAS likely formed far from its original star, in a cold region rich in ices. Over billions of years, it may have been bombarded by galactic radiation, slowly modifying its outer layers while it drifted in the dark. At some point, a gravitational nudge—perhaps from a giant planet or a passing star—kicked it out into interstellar space.
Now, by watching interstellar comet 3I/ATLAS as it heats up near the Sun, we are finally seeing those processed outer layers evaporate. Instead of a pristine sample of an alien planetary system, we are sampling material that has been edited by the galaxy itself. That conclusion alone reshapes how scientists think about long-lived interstellar objects.
In a wider context, 3I/ATLAS follows on the heels of ʻOumuamua and Borisov, but with far richer data. ʻOumuamua flashed through so quickly that telescopes barely caught it, and its true nature is still debated. 2I/Borisov looked more like a “normal” comet but was observed only from afar. Interstellar comet 3I/ATLAS, by contrast, comes with a full fleet of spacecraft pointed at it, including those around Mars and others throughout the inner Solar System.
How to follow interstellar comet 3I/ATLAS from Earth
Most readers naturally ask: Can I see interstellar comet 3I/ATLAS in the night sky?
Unfortunately, the honest answer is that it will stay too faint for the naked eye. Even near its best visibility, interstellar comet 3I/ATLAS hovers around magnitude 10, meaning you need a decent telescope and dark skies to spot it.
According to the current orbit calculations, 3I/ATLAS passed closest to the Sun (perihelion) in late October 2025 and will make its nearest approach to Earth in mid-December at a distance of roughly 1.8 astronomical units—about 270 million kilometers. That is much farther away than typical “headline” comets that light up our skies.
Still, you can follow interstellar comet 3I/ATLAS in several ways:
- Online sky charts: Many observatories and astronomy websites now publish updated positions for 3I/ATLAS so that telescope owners can track it.
- NASA and ESA image releases: As new processed images and spectra come in from Mars orbiters, JWST, and solar-watching spacecraft such as SOHO and PUNCH, agencies are releasing them to the public. Live Science+1
- Science news coverage: Outlets are posting explainers, animations, and even interactive graphics that show how interstellar comet 3I/ATLAS threads its way past Mars, Venus, Earth, and Jupiter before it exits forever. Interesting Engineering+2Chron+2
Even if you never see interstellar comet 3I/ATLAS with your own eyes, you can still follow its journey in real time through these images and stories.
Interstellar comet 3I/ATLAS and the future of interstellar visitors
Interstellar comet 3I/ATLAS marks an important transition in how we study objects from other star systems. With ʻOumuamua and Borisov, astronomers struggled to gather enough data before the visitors faded away. With interstellar comet 3I/ATLAS, we have entered a new phase: coordinated, multi-mission campaigns that follow an interstellar object from discovery to departure.
Looking ahead, several ideas are already on the table:
- Dedicated “interstellar interceptors” that could launch quickly and fly out to meet future visitors.
- Automated alert systems that instantly notify space missions when a new interstellar object is discovered.
- Deep surveys—both ground-based and space-based—that increase the chances of finding these rare objects earlier and more often.
Each new object will be different. Some may look like typical comets. Others may be dark, inactive rocks. A few might show extreme chemistry like interstellar comet 3I/ATLAS. Over time, this growing sample will help answer big questions:
- How common are planet-forming disks like the one that birthed our Solar System?
- How often do planetary systems eject ice-rich bodies into interstellar space?
- Do many of these objects carry complex organic materials that could seed young worlds?
Interstellar comet 3I/ATLAS does not give all the answers, but it provides one of the clearest, best-observed case studies so far. It turns Mars into a vantage point on another cosmic neighborhood and proves that our current fleet of spacecraft can pivot fast when something extraordinary appears.
Conclusion: why interstellar comet 3I/ATLAS matters
From a distance, interstellar comet 3I/ATLAS looks like a small, faint patch of light. In reality, it is a messenger from another star system, carrying the chemical fingerprints of a world that formed long before our own. Its flyby of Mars allowed NASA, ESA, and other teams to transform the red planet into a temporary observatory, with orbiters and rovers working together to capture every photon they could. Interesting Engineering
Because of that effort, interstellar comet 3I/ATLAS is now one of the best-studied interstellar visitors in history. Scientists are using its images and spectra to probe its size, dust, gases, and water chemistry. They are testing ideas about cosmic-ray processed ices and comparing its composition with that of comets born around our own Sun.
For SpaceEyeNews readers, the message is simple: interstellar comet 3I/ATLAS is not just another comet story. It is a preview of a new era in planetary science—one where our telescopes and spacecraft can catch visitors from other stars, decode their chemistry, and fold their stories into the bigger picture of how planets and life emerge in the universe.