Astronomers captured rare images of the interstellar comet 3I/ATLAS as it passed near Mars on October 3, using ESA’s ExoMars and Mars Express orbiters.

The comet, one of only three interstellar objects observed entering our solar system, was about 18.6 million miles from the orbiters and extremely faint compared to usual targets.

First spotted in July by the Asteroid Terrestrial-impact Last Alert System in Chile, 3I/ATLAS is estimated to be 3 billion years older than the solar system and weighs over 33 billion tons, with a nucleus of carbon dioxide gas and water ice.

Solar heat is activating the comet, creating its coma of gas and dust, ABC News has reported.

ESA astronomers plan further observations with the Jupiter Icy Moons Explorer in November and will continue analyzing orbiter data to better understand the comet’s composition.