The European Space Agency (ESA) is preparing its Mars orbiters, Mars Express and the ExoMars Trace Gas Orbiter (TGO), to observe the interstellar visitor comet 3I/ATLAS as it passes by Mars. These observations are scheduled to take place between October 1 and 7, 2025, with the closest approach to the orbiters anticipated on October 3, 2025, at a distance of approximately 30 million kilometers.
Comet 3I/ATLAS was discovered on July 1, 2025, by the ATLAS telescope in Chile. It is the third confirmed interstellar object detected in our solar system, following 1I/ʻOumuamua and 2I/Borisov. Its trajectory is distinctly hyperbolic, indicating it is not bound to our Sun and is merely passing through. The comet's composition is also of scientific interest, with early observations suggesting a high carbon dioxide to water ratio, distinguishing it from typical comets. Additionally, its tail has been observed moving in an unusual direction, leading to hypotheses about the ejection of large dust particles.
Mars Express will utilize its High Resolution Stereo Camera (HRSC) for these observations. The HRSC, originally developed for the Mars-96 mission, is capable of capturing detailed images with resolutions as fine as 2.3 meters per pixel for its Super-Resolution Channel and 10 meters per pixel for its main channels. Similarly, the ExoMars Trace Gas Orbiter will use its Colour and Stereo Surface Imaging System (CaSSIS), designed to capture high-resolution color stereo images of Mars' surface with a spatial resolution of 4.6 meters per pixel.
These instruments, primarily tasked with studying Mars, will now provide a novel perspective on comet 3I/ATLAS, aiming to reveal insights into its shape, composition, and activity. Further extending the observation window, ESA's Jupiter Icy Moons Explorer (JUICE) is scheduled to observe 3I/ATLAS from November 2 to 25, 2025. This mission will use its comprehensive suite of instruments to analyze the comet's characteristics, including its tail and coma, during a period when the comet is expected to be highly active following its perihelion passage in November 2025.