For those just joining the fascinating story of our interstellar visitor, on July 1, 2023, astronomers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) detected an object traveling through the Solar System at an astounding speed of around 58 kilometers per second (36 miles per second). With numerous telescopes focused on this intriguing object, it was quickly identified as a comet, specifically designated as 3I/ATLAS. This comet is not just any celestial body; it is recognized as an interstellar comet, potentially a 10-billion-year-old time capsule from an ancient era of the universe.
Astronomers are eager to delve into the properties of 3I/ATLAS, as studying this comet could provide valuable insights into the environment from which it originated and the influence of the interstellar medium on cometary objects. Remarkably, 3I/ATLAS is only the third confirmed interstellar object to be identified, following the earlier discoveries of 1I/ʻOumuamua and 2I/Borisov. However, there was a brief period during which this remarkable object was out of view from Earth-based telescopes, as it moved behind the Sun from our perspective.
During its time behind the Sun, 3I/ATLAS was imaged by the European Space Agency’s (ESA) Mars Express and ExoMars Trace Gas Orbiter (TGO) from Martian orbit. Adding to the excitement, China's Tianwen-1 orbiter, which has been in orbit around Mars since February 2021, has also returned its own stunning images of the interstellar comet. These images represent a remarkable technical achievement, as the cameras on the Tianwen-1 were primarily designed to capture well-lit surfaces of planets, rather than distant and relatively small celestial bodies like 3I/ATLAS.
This marks the first attempt to photograph such a distant and dim target—estimated to be 10,000 to 100,000 times less luminous than objects on the Martian surface. The collaborative efforts of the Tianwen-1 team involved intricate planning, which included combining the orbital characteristics and brightness features of 3I/ATLAS, as well as the geometric dimensions of its orbit. The China National Space Administration (CNSA) detailed that repeated simulations and calculations were essential in assessing the feasibility of this observation mission.
The Tianwen-1 orbiter's high-resolution camera was determined to be the ideal tool for this task, and key imaging strategies were meticulously designed to maximize its capabilities when targeting the faint comet. The resulting images reveal the comet's distinctive characteristics, including its nucleus—the icy and rocky core—and its coma, a cloud of gas and dust that forms as the comet's surface heats up and its volatile ices vaporize.
Astronomers have noted several intriguing features of 3I/ATLAS, such as the development of a rare anti-tail and an unusual chemical composition. Recently, it has been observed that the comet has slightly altered its trajectory and may have experienced a significant loss of mass. However, scientists from NASA and the Search for Extraterrestrial Intelligence (SETI) emphasize that these changes are indicative of typical cometary behavior, such as outgassing, rather than suggesting the presence of an alien mothership.
The images captured by Tianwen-1 clearly illustrate the comet’s unique features, including its nucleus and surrounding coma, which spans several thousand kilometers in diameter. Based on this observational data, researchers are conducting further in-depth studies of 3I/ATLAS, aiming to unlock the mysteries of this ancient interstellar comet and its implications for our understanding of the universe.
