Comet 3I/ATLAS, the third interstellar comet ever seen, has passed close to Mars, with NASA’s ExoMars and Mars Express orbiters capturing its first-ever observations. The two spacecraft watched as the comet, originating from outside our Solar System, approached the Sun.
On October 3, the comet was 30 million km away from the Mars orbiters, providing the closest view of an interstellar comet for any ESA spacecraft. However, the images captured by ExoMars’ Colour and Stereo Surface Imaging System (CaSSIS) show a fuzzy white dot moving downwards near the centre of the image.
The CaSSIS camera struggled to distinguish the comet’s nucleus from its surrounding coma due to its distance. The coma is created as the comet approaches the Sun, releasing gas and dust that collects into a halo around the nucleus. While the full size of the coma cannot be measured in the images, it is clear that the comet is creating a visible feature.
The comet’s tail, which can grow up to millions of kilometers long, may not be visible in the current images but could become more visible as the comet continues to heat up and release more ice. Scientists are unsure if the comet’s coma and tail were bright enough for a spectral characterisation.
Comet 3I/ATLAS has been observed by several space telescopes since its discovery on July 2025, with astronomers suspecting it may be the oldest comet ever observed, potentially three billion years older than the Solar System. Next month, the Jupiter Icy Moons Explorer (Juice) will observe the comet from a further distance.
The European Space Agency is preparing the Comet Interceptor mission, which aims to visit an interstellar object like 3I/ATLAS or another pristine comet from the Oort Cloud in 2029. This mission would provide a breakthrough in understanding the nature of these mysterious visitors and their potential for revealing secrets about the formation of worlds beyond our Solar System.
The observations made by ExoMars and Mars Express orbiters offer a rare, tangible connection to the broader galaxy, connecting humankind with the Universe on a greater scale.
Source: https://www.esa.int/Science_Exploration/Space_Science/ESA_s_ExoMars_and_Mars_Express_observe_comet_3I_ATLAS