On October 4, 2025, at precisely 21:33:39 mean local solar time on Mars, NASA’s Perseverance rover captured a remarkable image using its Right Navigation Camera (Navcam). This image features the interstellar object 3I/ATLAS, which was approximately 38 million kilometers away from Mars at that moment. The photograph reveals an elongated stripe, which is about four times longer than its width, leading to intriguing discussions on social media regarding the nature of 3I/ATLAS. Many speculated whether it could be a large cylindrical object.
In response to inquiries from Rep. Anna Paulina Luna, I conducted an analysis of the image. The Navcam on the Perseverance rover is designed to be sensitive to visible light, with an angular resolution of 0.33 milli-radian (equivalent to 68 arcseconds) per pixel. This specification translates to a spatial scale of roughly 12,500 kilometers at the distance of 3I/ATLAS when the image was taken. This scale is crucial in determining the width of the elongated stripe observed in the Navcam image.
The projected length of the stripe in the image was calculated to be around 50,000 kilometers. Additionally, the upper limit on the diameter of 3I/ATLAS, as derived from the SPHEREx space observatory, is reported to be 46 kilometers based on an albedo of 4%. The Navcam’s elongated stripe is significantly longer—by about a factor of a thousand—than this upper limit, indicating that it is likely an optical artifact created by the long integration time as the object traversed the Martian sky.
If 3I/ATLAS were a cylinder measuring 50,000 kilometers in length, it would present an angular size of 23 arcseconds in the Hubble Space Telescope image captured on July 21, 2025, when it was positioned three times the distance between the Earth and the Sun from the Hubble camera. However, in reality, 3I/ATLAS appeared considerably smaller, by at least an order of magnitude, in the Hubble image. This discrepancy suggests that the elongation seen in the Navcam image was a result of the integration time used in creating the composite image, during which 3I/ATLAS moved across the Martian sky.
Mars travels around the Sun at a speed of 24 kilometers per second, while 3I/ATLAS follows a retrograde orbit in the opposite direction at approximately 67 kilometers per second relative to the Sun. Consequently, the speed of 3I/ATLAS in relation to Mars translates to a path length of around 50,000 kilometers for an integration time of about 10 minutes.
In summary, the stripe observed in the Navcam image was likely produced by stacking hundreds of images over a 10-minute interval. In a single snapshot, which has a maximum exposure time of 3.28 seconds for the Navcam, 3I/ATLAS would have appeared as a circular spot. The motion of 3I/ATLAS would cause a smear of merely 300 kilometers in an individual frame—approximately 3% of the much larger smearing effect of 12,500 kilometers caused by the Navcam's limited angular resolution.
The stacking of numerous images enhanced the apparent brightness of 3I/ATLAS in the final composite image. One way to visualize this stripe is as a series of smeared snapshots, reminiscent of a pod of green beans. As 3I/ATLAS traversed the Martian sky, it shared the spotlight like a movie star, with numerous cameras—including seven Mars orbiters and a ground-based rover—capturing its journey.
The highest resolution image of 3I/ATLAS was taken by the HiRISE camera onboard the Mars Reconnaissance Orbiter. HiRISE boasts an angular resolution that equates to 30 kilometers per pixel as of October 3, 2025. The brightest pixel in the HiRISE image will likely provide the best constraints on the area of 3I/ATLAS. There is considerable anticipation within the scientific community for the HiRISE team to release their images promptly, as they promise to shed more light on this fascinating interstellar object.
