NASA and ESA spacecraft are taking advantage of the proximity to Mars to study the comet's chemical composition; perihelion is expected on October 30 and will lead to a peak in activity
on-October 3 2025 Interstellar comet 3I/ATLAS passed close to the orbit of Mars, about 30 million kilometers from the Red Planet. This is only the third time in history that an object originating from outside the Solar System has been recorded crossing its inner space. This event has aroused enormous interest in the scientific community, as it has opened a unique observation window that provides an opportunity to better understand the composition of materials originating from other stars.
In the window of days October 1–7ESA is directing Mars Express and ExoMars TGO to observe the comet; the close approach on 3 October (≈30 million km) will allow imaging and spectroscopic measurements of the gas and dust released from the nucleus. ESA's JUICE spacecraft is then intended to follow 3I/ATLAS near perihelion and in November, when its activity is at its peak. NASA's Psyche mission, en route to asteroid 16 Psyche, will also attempt to capture observations. esa.int+1
Spacecraft and large ground-based images have already revealed a halo and tail, and early measurements indicate water ice and CO₂ similar to "local" comets, but with an estimated exceptionally large mass. For scientists, this is a natural laboratory for testing the "recipe" of the raw materials of planetary systems beyond the Sun: if the composition matches that of solar system comets, it is a hint at the universality of the building blocks; if it deviates, we may be exposed to "exotic" materials from another system. Chron
When and what will be visible from Earth? During perihelion, the comet is angularly close to the Sun, so viewing it with the naked eye is unlikely and even dangerous. After it passes behind the Sun, it will reappear in the December sky at a great distance, faintly visible to telescopes and tripods only—most of the data will come from spacecraft.
Discovery and unique characteristics
The comet was first discovered on July 1, 2025, by the ATLAS telescope in Chile, as part of a NASA-funded monitoring project. Analysis of its trajectory clearly showed that it was moving on a hyperbolic trajectory—that is, it was not bound by the Sun’s gravity and was coming from outside. Its incredible speed, about 219 km/h (130 mph), solidified its identification as an interstellar object.
The nucleus of 3I/ATLAS is relatively small, estimated to be less than one kilometer in diameter. However, it is surrounded by a bright halo of gases and dust—the comet—that are emitted as the ice within it heats up. Observations from space indicate that the comet is rich in water ice, carbon dioxide, and carbon monoxide, similar to local comets. However, its mass is estimated to be in the tens of billions of tons, a figure of particular interest because it may distinguish it from the comets we know.
A close crossing to Mars – a natural laboratory
Mars' proximity has allowed several research spacecraft to point sensors at it. The European Space Agency (ESA) has operated Mars Express and the ExoMars Trace Gas Orbiter to collect data on its structure, while NASA has planned observations with the Psyche spacecraft on its way to asteroid 16 Psyche.
Scientists are especially looking forward to information coming later this year: around October 30The comet will reach perihelion — the closest point to the Sun, just inside the orbit of Mars. During this time, its nucleus is expected to release large amounts of gas and dust, which will allow its chemical composition to be measured with great precision.
Observations from JUICE and other spacecraft
After perihelion, ESA is also directing the JUICE (Jupiter Icy Moons Explorer) mission to follow 3I/ATLAS, between 2 and 25 November. JUICE is equipped with instruments that can measure light, heat and gas emissions with high precision. The coordination of observations between JUICE and spacecraft around Mars, and subsequently with renewed ground-based observations in December, creates a unique set of global collaboration.
Other spacecraft, including NASA's Mars Reconnaissance Orbiter, China's Tianwen-1, and the Emirati spacecraft "Hope," could add valuable information. Even the Parker Solar Probe is expected to provide data on the plasma environment when the comet is close to the sun.
Comparison to previous guests
3I/ATLAS joins a very short list of interstellar guests. The first was 1I/ʻOumuamua, discovered in 2017 and its elongated shape and mysterious acceleration sparked debate about whether it might be an artificial object. The second was 2I/Borisov in 2019, which looked like a classic comet with a bright tail. Now, 3I/ATLAS offers a combination of familiar features with an unusual mass, providing another sample for comparison between solar systems.
Visibility from Earth
Despite the excitement, the comet will not be easily visible to the general public. Its angular proximity to the Sun at perihelion makes direct viewing hazardous. It will be visible again in large telescopes in December 2025, when it will approach Earth at a distance of about 1.8 astronomical units (about 270 million km). This distance ensures that there is no threat to Earth, but also prevents observation with the naked eye.
Broad scientific significance
The 3I/ATLAS study could provide answers to fundamental questions: Are the building blocks of stellar systems similar across the galaxy? Or are they unique to each system? If the composition is similar to that of comets in our solar system, this would suggest a chemical uniformity in the universe. If "exotic" materials are found, science would get a rare glimpse into a completely different environment.
The Mars orbit crossing on October 3 marked the beginning of a series of coordinated observations that will continue until at least March 2026, when 3I/ATLAS will pass near Jupiter on its way out of the solar system. This is a rare cosmic visitor, making our solar system an open laboratory for understanding processes occurring in other planetary systems throughout the galaxy.
To track the comet on the NASA website
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