In a unique celestial event, ESA’s ExoMars Trace Gas Orbiter (TGO) and Mars Express successfully observed the interstellar comet 3I/Borisov, also known as 3I/ATLAS, as it passed near the Red Planet between 1 and 7 October. The two Mars orbiters provided the closest perspective of all ESA spacecraft, capturing valuable data as the comet made its closest approach on 3 October, coming within 30 million kilometres of the planet.
Each orbiter utilized its specialized camera system, originally designed for high-resolution imaging of the Martian surface from just hundreds of kilometres away. This presented a challenge, as scientists were uncertain how well these instruments would perform when pointed at a relatively dim and distant target in deep space. Despite this, the observation campaign was a success.
The ExoMars TGO used its Colour and Stereo Surface Imaging System (CaSSIS) to capture a sequence of images, compiled into a GIF, that clearly shows the comet’s motion. In these frames, the interstellar visitor appears as a faint, slightly fuzzy white dot drifting downwards near the centre of the field of view. This dot represents the comet’s nucleus—a solid body of ice and rock—surrounded by the glowing cloud of dust and gas known as the coma. While the comet’s faint tail is not visible in these particular images, the data provides crucial information about the nucleus and coma of this visitor from another star system.
This coordinated effort marks a significant scientific observation, allowing researchers to study the composition and behaviour of a confirmed interstellar object. The data from both orbiters will help astronomers understand how this comet compares to those originating within our own Solar System, offering a rare glimpse into the building blocks of planetary systems around other stars.
