Two robotic researchers of the European Space Agency (ESA) stationed in Mars orbit, ExoMars Trace Gas Orbiter (TGO) and Mars Express, have successfully observed a rare visitor from the depths of space. This is the interstellar comet 3I/ATLAS which flew by the Red Planet in close proximity from October 1 to 7. The two satellites had the best view of the comet to date of all European Space Agency spacecraft. At the time of its closest approach to Mars, on October 3, this interstellar traveler was located at a distance of 30 million kilometers.

Each spacecraft used its specialized camera to capture the comet's passage. These cameras are primarily intended for imaging the surface of Mars, which is only a few hundred to a few thousand kilometers away from their orbits. Because of this, scientists were unsure what results they could expect from observing a target that is extremely faint and so far away. However, the outcome is surprisingly good and provides valuable new insights.

Our galaxy is home to trillions of stars and planets, but interstellar travelers like comet 3I/ATLAS are a rarity within our Solar System. All comets studied so far, including Halley's Comet, share a common origin with our Solar System, which is 4.6 billion years old. On the other hand, interstellar comets are true strangers, carrying with them clues about the formation of worlds far beyond our own. Comet 3I/ATLAS is the third such object ever observed, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019), which makes it extremely important for science.

It was first spotted on July 1, 2025, using the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile. Its unusual trajectory, which leads it from the "thick disk" of the Milky Way - a region of old stars, led astronomers to the conclusion that it could be the oldest comet ever observed. Estimates suggest that it could be between 7.6 and 14 billion years old, which makes it potentially three or more billion years older than the Solar System itself.

Successful imaging despite extreme challenges

The ExoMars TGO spacecraft, whose primary mission is to study trace gases in the Martian atmosphere such as methane, used its Colour and Stereo Surface Imaging System (CaSSIS) to capture a series of photographs. In the published GIF, comet 3I/ATLAS is seen as a slightly blurred white dot moving downwards, near the center of the shot. This dot actually represents the comet's nucleus, composed of ice and rock, surrounded by a cloud of gas and dust known as the coma. According to Nick Thomas, the principal investigator for the CaSSIS camera, this was an extremely challenging observation because the comet was 10,000 to 100,000 times fainter than usual targets, such as the surface of Mars.

Although CaSSIS could not distinguish the nucleus from the coma due to the great distance - imaging a nucleus only a few kilometers wide would be as impossible as observing a mobile phone on the Moon from Earth - the coma, which extends for several thousand kilometers, is clearly visible. The coma forms as comet 3I/ATLAS approaches the Sun. The heat and radiation of the star heat the comet, causing the release of gases and dust that create a glowing halo around the nucleus. Interestingly, although a tail was also expected from the comet, it was not visible in the images. This does not mean that the tail does not exist, but that it is too faint to be registered. The tail and coma are expected to increase and intensify further as the comet approaches the Sun.

Data analysis is in progress

Although ExoMars TGO managed to image the comet, the situation with Mars Express is different. Comet 3I/ATLAS is still not visible in the Mars Express images, partly due to the shorter exposure time, which was only 0.5 seconds (which is the maximum limitation for Mars Express), compared to five seconds for ExoMars TGO. Scientists are still optimistic and will continue to analyze the collected data, including combining multiple images from Mars Express to try to detect this dimly lit object.

In addition to imaging, the teams also tried to measure the comet's light spectrum using spectrometers on both spacecraft. On Mars Express, these are OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité), which maps the mineral composition of the surface and atmosphere, and SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars), which analyzes the composition of the atmosphere. ExoMars TGO used its NOMAD spectrometer for similar measurements. It is currently unknown whether the coma was bright enough for a spectral analysis of the chemical composition. In the coming weeks and months, scientists will continue to analyze to discover what comet 3I/ATLAS is made of and how it behaves as it approaches the Sun.

Colin Wilson, a scientist on the Mars Express and ExoMars projects at ESA, stated that it is always exciting when spacecraft have to deal with unexpected situations like this. Their primary goal is the scientific exploration of Mars, but observing external objects like this comet provides a unique opportunity to expand our knowledge. It is expected that further data analysis will reveal more about the mysterious visitor.

What awaits us?

The journey of comet 3I/ATLAS through the inner part of the Solar System will continue, and its next major stop will be a close encounter with the Sun at the end of October. In November, another ESA mission, Jupiter Icy Moons Explorer (Juice), will also point its instruments at the comet. Although Juice will be further from the comet than the Mars orbiters were, it will observe it after its closest approach to the Sun, when it will be in a much more active state. Although the data from the Juice mission will arrive only in February 2026, expectations are high because it will be an opportunity to observe the comet in its full glory.

Such icy wanderers outside the Solar System represent a rare, tangible connection to the rest of the galaxy. The possibility of a physical visit to one of them represents an opportunity for humanity to establish a connection with our universe on a completely new level. It is with this goal that ESA is developing the Comet Interceptor mission.

This unique mission is planned to launch in 2029 into a parking orbit at the stable point L2 (Lagrange point), located approximately 1.5 million kilometers from Earth in the direction opposite to the Sun. There it will wait for a suitable target - a primitive comet from the distant Oort Cloud or, what is even more challenging and attractive, an interstellar object like comet 3I/ATLAS. The mission is designed to meet an object that has not even been discovered yet, which is a completely new approach to space exploration. If a suitable target appears, the Comet Interceptor will set off in pursuit. A few weeks before the encounter, the main spacecraft will release two smaller probes that will independently collect data and create a 3D profile of the comet and its interaction with the solar wind.

Michael Kueppers, a scientist on the Comet Interceptor project, points out that at the time the mission was selected in 2019, only one interstellar object was known, 1I/ʻOumuamua. After that, two more were discovered, and each of them shows great diversity in appearance. Visiting such an object could lead to a breakthrough in understanding their nature. Although it remains unlikely that an interstellar object will be found that is reachable for the Comet Interceptor mission, this spacecraft will serve as a pioneer for future rapid-reaction missions, proving the concept of waiting in space for the right target. It will be a key step towards shedding light on the secrets of these mysterious visitors from the unknown depths of space.