3I ATLAS is expected to make its closest approach to Mars today

On October 3 2025 Mars will have a rare guest. The interstellar comet 3I ATLAS is expected to pass near the Red Planet at a distance of about 18 million miles or 29 million kilometers. That is far enough to rule out any risk of collision yet close enough that orbiting spacecraft will have a chance to study it.

This is only the third known interstellar object to visit our solar system after the enigmatic ʻOumuamua in 2017 and comet Borisov in 2019. Unlike those two which slipped past with little chance for close observation 3I ATLAS is giving scientists a longer window and a new vantage point. Its Mars flyby is not only a rare alignment but also a test of how well we can mobilize existing space assets to study a fleeting cosmic visitor.

The European Space Agency has already announced that instruments on Mars Express and the ExoMars Trace Gas Orbiter will be pointed at the comet. NASA’s James Webb Space Telescope has also taken spectra showing a coma rich in carbon dioxide. Meanwhile popular media outlets are buzzing with more sensational claims from alien probe speculation to exaggerated size comparisons. It is time to lay out the facts clearly.

This article will walk you through everything we know about 3I ATLAS its upcoming Mars encounter the science opportunities the limitations and the myths we can safely discard.

Meet 3I ATLAS

3I ATLAS is short for the third Interstellar object catalogued by astronomers. It was first detected in 2024 by the Asteroid Terrestrial-impact Last Alert System a network of survey telescopes designed to catch near Earth objects. Its unusual orbit immediately gave it away. Its hyperbolic trajectory with eccentricity well above one means it cannot be gravitationally bound to the Sun. It came from outside our solar system and will eventually leave it forever.

Initial observations showed that 3I ATLAS has a bright coma and extended tail making it more comet like than ʻOumuamua which was small faint and oddly shaped. Spectra from large telescopes and later from JWST revealed a composition dominated by carbon dioxide with significant contributions of carbon monoxide water and dust. That chemistry both overlaps with and differs from typical solar system comets. It suggests 3I ATLAS condensed in a cold outer region of its home system perhaps analogous to our Oort Cloud.

The comet’s brightness has led to headlines calling it Manhattan sized. In truth its nucleus is still poorly constrained. Estimates put it on the order of a kilometer or two across not tens of kilometers. Still that makes it substantial compared to many comets studied in the inner solar system.

The Geometry of the Mars Flyby

On October 3 2025 3I ATLAS will pass closest to Mars. At that moment its distance will be about 18 million miles or 29 million kilometers. To put that in perspective that is roughly 75 times the Earth Moon distance. This is not a grazing flyby in planetary defense terms. It is however close enough for orbital spacecraft around Mars to gather data unavailable from Earth.

The European Space Agency has confirmed that Mars Express and the ExoMars Trace Gas Orbiter will attempt to observe the comet. Because of the large distance their cameras will not resolve the nucleus. Instead they will aim to capture images and spectra of the coma and tail. The hope is to measure how the comet’s gases and dust interact with the solar wind from a different angle than Earth based observatories can provide.

Importantly 3I ATLAS’s trajectory carries it inside the orbit of Mars. That means from our perspective the comet will cross through the region of the solar system we associate with terrestrial planets. While no collision is possible this geometry is scientifically valuable. Spacecraft at Mars offer a closer and different vantage point than Earth or Earth orbit telescopes.

What We Already Know JWST and Earth Based Observations

Long before the Mars flyby the James Webb Space Telescope has been observing 3I ATLAS. In August 2025 JWST published spectra showing that the comet’s coma is dominated by carbon dioxide. Additional lines indicate carbon monoxide and water vapor as well as solid dust grains.

This mix is intriguing. Most solar system comets show water as their main volatile with CO₂ and CO playing supporting roles. A CO₂ dominated coma suggests either that the nucleus has a different composition or that CO₂ sublimates more easily under current solar heating conditions. Either way it points to chemical diversity among interstellar planetesimals.

Ground based observatories have tracked the comet’s changing brightness and tail structure. The coma has grown as the comet approached perihelion showing strong outgassing. No unusual deviations from cometary behavior have been detected despite sensational speculation.

The Science Opportunity at Mars

Why does the Mars flyby matter. First it offers a chance for parallax observation. From Earth and from Mars scientists will see the comet from different perspectives. That allows better reconstruction of the three dimensional structure of its tail and coma.

Second Mars orbiters carry instruments optimized for atmospheric studies. The ExoMars Trace Gas Orbiter has spectrometers designed to sniff out faint gases against background signals. If aligned properly these could detect subtle details of the comet’s outgassing.

Third spacecraft around Mars may catch interactions between the comet and the solar wind. Mars has no global magnetic field so its orbiters often study how charged particles from the Sun interact with its atmosphere. A comet sweeping through nearby space could add a temporary variable to that natural laboratory.

Finally there is the symbolic value. This is the first time an interstellar object will be observed by spacecraft orbiting another planet. It sets a precedent for planetary science beyond Earth.

The Limitations and Challenges

Expectations must be realistic. At 29 million kilometers distance even the best cameras at Mars cannot resolve a kilometer scale nucleus. The comet will appear as a fuzzy patch of light. Spectra may be noisy and limited by background.

The geometry of the flyby also matters. Spacecraft may only have short observing windows depending on orbital constraints and pointing restrictions. Mars Express and TGO were not built for comet interception. They will do their best but they are opportunistic observers not dedicated probes.

Moreover dust and gas distributions can be unpredictable. A sudden outburst could brighten the comet dramatically or fade quickly. Planning observations is therefore a moving target.

Clearing Up Misconceptions

Media coverage has already drifted into exaggeration. Some outlets describe the comet as heading for Mars in language that implies collision. That is false. The minimum distance is millions of miles. There is no chance of impact.

Others describe the comet as Manhattan sized or as an alien probe. The size comparison is misleading. The nucleus is likely a couple of kilometers across not tens. The alien probe hypothesis is speculation with no supporting evidence. NASA and ESA scientists have clearly stated that all data so far is consistent with a natural comet.

It is important to emphasize that 3I ATLAS poses no threat to Earth. Its trajectory does not bring it anywhere near our planet. After passing inside Mars’s orbit it will continue on a hyperbolic escape path leaving the solar system entirely.

Why Interstellar Objects Matter

Why should we care about a comet that will never return. Because interstellar objects are time capsules from other planetary systems. Each one carries information about the chemistry and physics of planet formation elsewhere in the galaxy.

ʻOumuamua gave us mystery but little data. Borisov looked more like a familiar comet but still raised questions. 3I ATLAS is the first interstellar visitor bright enough and long lasting enough to attract major observatories. Every bit of data we collect adds to a comparative sample of how common our solar system really is.

In broader context these visitors remind us that the galaxy is not empty. Planetary systems eject debris constantly. Some of that debris wanders for millions of years before passing through ours. Every detection is an interstellar handshake.

The Future of Studying Visitors

The flyby of 3I ATLAS has sparked discussion about how to prepare for the next interstellar visitor. Missions like ESA’s Comet Interceptor are designed to sit in standby orbit until a target of opportunity appears. Launched in 2029 it could in principle be retargeted for an interstellar object.

Other researchers have suggested using existing spacecraft such as NASA’s Juno which is already in deep space. One study proposed a trajectory adjustment that could allow Juno to intercept 3I ATLAS though feasibility remains debated.

Longer term astronomers advocate for a dedicated interstellar interceptor mission with high delta v capability able to chase down fast hyperbolic objects. The Mars flyby of 3I ATLAS underscores why that is needed. Opportunities arise rarely and vanish quickly.

A Visitor and a Vantage Point

On October 3 2025 Mars will be the stage for a small astronomical event with big scientific meaning. The interstellar comet 3I ATLAS will sweep past at 18 million miles away. No collision no threat. But plenty of opportunity.

Spacecraft at Mars will join Earth based and space based observatories in capturing data. Together they will sketch a fuller picture of this icy traveler from another star system. They will measure its gases its dust its interaction with sunlight. They will compare notes and sharpen models.

Meanwhile the public will watch with fascination and sometimes with confusion. Headlines will hype danger or aliens. Scientists will patiently explain distance chemistry and physics. Both groups are participating in a shared story.

3I ATLAS reminds us that the solar system is not isolated. It is part of a galactic ecosystem where debris wanders freely. Every visitor we catch is a clue about how common or uncommon our planetary history may be.

So when Mars turns its orbiters toward the faint glow of an interstellar comet remember this. You are witnessing not only a scientific campaign but also a cultural moment. A meeting of worlds in miniature. A reminder that the cosmos is larger than we imagine and sometimes it comes to visit.