An extrasolar visitor is currently racing through our solar system: the interstellar comet 3I/ATLAS. On October 29th, the comet passed the point in its orbit closest to the sun, which is between Earth and Mars. But because Earth is on the other side of the sun, the comet is invisible to terrestrial telescopes. However, this is different for ESA’s JUICE space probe, which is currently on its way to Jupiter. In the next few days it will fly past 3I/ATLAS at a distance of just 60 million kilometers and will use this opportunity to examine the interstellar visitor in more detail. The measuring instruments of the JUICE probe primarily focus on the gases and charged particles that emerge from the comet’s icy surface after the passage through the sun. The measurement campaign, which lasted almost the entire November, could provide valuable information about the composition of this object and perhaps also its origin.
It is only for a few years that astronomers have known that our solar system is also flown through by extrasolar asteroids and comets. The first evidence of this was provided by the strikingly elongated asteroid 1I/Oumuamua, discovered in 2017. The first interstellar comet followed in 2019, 2I/Borisov. On July 1, 2025, the robotic telescopes of the ATLAS network detected another object racing through our solar system remarkably quickly and on a hyperbolic path. The celestial body named 3I/ATLAS turned out to be a comet from interstellar space. Closer observations revealed that 3I/ATLAS is moving at more than 200,000 kilometers per hour, making it the fastest object ever observed in our solar system. According to initial size estimates, it is also the largest of the three interstellar visitors to date, with a diameter of between 440 and 5,600 meters. While approaching its closest point to the Sun just within the orbit of Mars, 3I/ATLAS developed the gas emissions typical of comets and formed the coma around its core and a tail.
Chance meeting in space
The interstellar comet passed the closest point in its trajectory to the Sun on October 29, 2025. 3I/ATLAS flew past the sun at a distance of 210 million kilometers. During this passage, the comet’s surface warmed further, causing it to emit more gas and dust. However, 3I/ATLAS cannot currently be seen from Earth because our planet is currently on the opposite side of the sun. This causes the sunlight to outshine the comet, making it invisible to terrestrial telescopes. But by chance a space probe is in a favorable observation position: the European probe JUICE (Jupiter Icy Moons Explorer). The space probe, which was launched in April 2023, is on its way to Jupiter to study the gas giant’s three largest moons – Europa, Ganymede and Callisto – in more detail. On its 778 million kilometer journey it makes several trips to the inner planets of the solar system. After a flyby of Venus in August 2025, it is currently on its way to its next Earth passage in just under a year.
The highlight, however, is that the space probe currently has a clear view of 3I/ATLAS. “The fact that JUICE and 3I/ATLAS meet on their very different paths through the solar system is an extraordinary stroke of luck. We never expected something like this,” says JUICE team member Ladislav Rezac from the Max Planck Institute for Solar System Research (MPS) in Göttingen. In the next few weeks, JUICE and the interstellar comet will be only about 60 million kilometers apart. After this happy coincidence crystallized, astronomers began to plan a spontaneous extra task for JUICE: From November 2nd to 25th, 2025, the Jupiter probe will turn on several of its measuring instruments and point them at 3I/ATLAS. However, this observation campaign is not an easy task from an operational perspective. On the one hand, the probe and the interstellar object are traveling in different directions, and the comet is also extremely fast. Therefore, the spacecraft must flexibly adjust its orientation to keep the comet in view. In addition, JUICE cannot look at the well-traveled visitor for more than 30 minutes per day. This is because it has to expose some sensitive measuring instruments to the intense solar radiation at its location.
Comet gases and water vapor
Among the measuring instruments that JUICE is now using to target the interstellar comet is one of the sensors of the Particle Environment Package (PEP) instrument package. This sensor captures high-energy uncharged atoms such as hydrogen, helium or oxygen. They are formed when charged solar wind particles hit the gas cloud surrounding 3I/ATLAS. “The measurements can tell us which gases sublimate from the surface of 3I/ATLAS and in what quantity,” explains Elias Roussos from MPS. However, the researchers expect that the yield will not be very high, so PEP will measure for twelve days in a row and hopefully capture enough particles. However, the JUICE instrument Submillimeter Wave Instrument (SWI) could provide more information, especially about the water contained in the interstellar comet. This captures radiation with wavelengths between about a quarter and half a millimeter. In this wavelength range, water molecules leave behind characteristic spectral signatures. Thanks to its high spectral resolution, the measuring instrument can, for example, provide information about whether the water molecules come from the surface of the interstellar chunk or from ice grains in its surroundings.
“We hope that SWI will enable the first high-resolution measurements of water on 3I/ATLAS,” says the scientific leader of the SWI team, Paul Hartogh from MPS. “This could help us, among other things, understand the distribution of near-surface water ice.” While 3I/ATLAS currently outgasses around two tons of water vapor per second, it could now, shortly after its passage through the sun, release much larger amounts of water in an eruption. Such behavior is also known from comets in the solar system and would make the planned measurements easier. Whether the efforts were successful will only become clear in February 2026. Only then will JUICE be close enough to Earth again to transmit the extensive measurement data to us – a kind of belated souvenir photo of a unique encounter.
Source: Max Planck Institute for Solar System Research, European Space Agency (ESA)