In early October, the third interstellar object (ISO) to visit our Solar System (3I/ATLAS) made its closest flyby to Mars, coming within 30 million km (18.6 million mi) of the Red Planet. This placed it within view of several missions currently operating there, which are operated by three space agencies: NASA, the European Space Agency (ESA), and the China National Space Agency (CNSA). While the ESA released images taken by the Mars Express* and *ExoMars Trace Gas Orbiter (TGO), and China released images taken by the Tianwen-1 orbiter, NASA was unable to release any data due to the government shutdown.
But with the government back up and running, NASA has finally released images acquired by three missions that captured 3I/ATLAS as it flew past Mars. This included the *Mars Reconnaissance Orbiter* (MRO), the *Mars Atmosphere and Volatile EvolutioN* (MAVEN) orbiter, and the Perseverance rover. Whereas the two orbiters captured images of 3I/ATLAS in optical and ultraviolet light (respectively), the rover also captured a faint image of the ISO.
The MRO took its visible light images of 3I/ATLAS (shown at top) on Oct. 2nd using its High-Resolution Imaging Science Experiment (HiRISE) camera, when the interstellar comet was about 0.2 astronomical units (AUs) distant. These images were among the closest views that any NASA spacecraft or Earth-based telescopes are expected to obtain of 3I/ATLAS and will allow scientists to get a better estimate of the comet's size. Ordinarily, this camera observes well-lit features on the Martian surface, but the orbiter rotated to obtain a better view of the celestial object.
This same technique was used in 2014 when the MRO and MAVEN both adjusted their orientation to observe the comet Siding Spring. The comet looks like soft, glowing white ball in the image, which captures the cloud of dust and gas (coma) created by outgassing as the ISO continued to approach the Sun. The annotated version of the image indicates that the comet and its gaseous envelop measure approximately 1500 km (932 mi) in diameter.
Further study of the HiRISE images could help scientists estimate the size of the comet’s nucleus, and its central core of ice and dust, as well as the color of particles in its core. "Observations of interstellar objects are still rare enough that we learn something new on every occasion. We’re fortunate that 3I/ATLAS passed this close to Mars," said Shane Byrne, HiRISE principal investigator at the University of Arizona.
"One of MRO’s biggest contributions to NASA’s work on Mars has been watching surface phenomena that only HiRISE can see,” added MRO project scientist Leslie Tamppari of NASA’s Jet Propulsion Laboratory. “This is one of those occasions where we get to study a passing space object as well."
Meanwhile, MAVEN captured images of 3I/ATLAS using its Imaging Ultraviolet Spectrograph (IUVS) over the course of ten days (Sept. 27th to Oct. 7th) in multiple wavelengths. In the image shown above, the comet appears as a bright, pixelated dot, white at the center, blue at the edges, and darker purple towards the outer coma. Similarly, high-resolution UV images by the IUVS provided the most detailed breakdown of 3I/ATLAS' chemical composition and established upper limits on the hydrogen-to-deuterium ratio and other chemical data, which could also provide insight into its place of origin.
An annotated composite image was also provided that identifies the three hydrogen sources detected by MAVEN's IUVS camera, including atmospheric hydrogen (the brightest streak on the right), interplanetary hydrogen (center), and hydrogen released as part of 3I/ATLAS outgassing water (left), “The images MAVEN captured truly are incredible,” said Shannon Curry, MAVEN’s principal investigator at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado Boulder. “The detections we are seeing are significant, and we have only scraped the surface of our analysis.”
Lastly, NASA’s Perseverance rover caught sight of 3I/ATLAS from its position inside the Jezero Crater on Oct. 4th using its Mastcam-Z camera. Since the object was so faint as seen from the surface, the exposure times had to be extremely long to acquire the resulting images shown below. This led to the stars appearing as steaks in the sky while the comet (indicated by the crosshairs) is barely visible. 3I/ATLAS has since reemerged from behind the Sun and is exhibiting new behavior, including a dramatic increase in brightness, new ultraviolet and X-ray activity, and a more pronounced color.
It will make its closest approach to Earth on Friday, Dec. 19th, 2025, when it will be almost twice the distance between the Earth and the Sun (2 AUs), thus posing no threat to Earth.
Further Reading: NASA