Understanding the Mysterious Stripe in the Latest Mars Image
During a routine exploration, the Mars Rover captured an intriguing image of the interstellar object 3I/ATLAS against the Martian sky. This object was approximately 38 million kilometers from Mars at that specific time. The image, which has sparked curiosity and speculation online, showcases an elongated stripe that is about four times longer than it is wide.
Is It a Giant Cylinder?
Given the unusual shape, many have questioned whether 3I/ATLAS could be a colossal cylindrical object. We can shed some light on this mystery by looking at the numbers. The Rover's camera, capable of capturing visible light, has an angular resolution of 0.33 milli-radian per pixel. This translates to a spatial scale of roughly 12,500 kilometers at the distance of 3I/ATLAS from Mars when the image was snapped. It's this scale that sets the width of the elongated stripe in the image. Thus, the projected length of the stripe is about 50,000 kilometers.
How Big is 3I/ATLAS Actually?
The upper limit of the diameter of 3I/ATLAS, as determined by a space observatory, is 46 kilometers for an albedo of 4%. This figure is a thousand times smaller than the length of the stripe in the image. Therefore, the stripe must be a result of a long integration time, seeing as the source object moves across the Martian sky. If 3I/ATLAS were a 50,000-kilometer long cylinder, it would have appeared far larger in a Hubble Space Telescope image taken when 3I/ATLAS was at a distance of three times the Earth-Sun separation from the Hubble camera. However, 3I/ATLAS appears much smaller in the actual Hubble image. This discrepancy suggests that the elongation of the stripe was created by the integration time used to make the composite Rover image, during which 3I/ATLAS moved across the Martian sky.
Motion Across the Sky
Mars orbits the Sun at a speed of 24 kilometers per second, while 3I/ATLAS travels in the opposite direction at a speed of about 67 kilometers per second relative to the Sun. This speed translates to a path length of roughly 50,000 kilometers over an integration time of about 10 minutes.
The Stripe Explained
This stripe in the Rover image is likely the result of stacking hundreds of images over a total time span of about 10 minutes. In an individual snapshot, 3I/ATLAS would have appeared as a circular spot, with a maximum exposure time per shot of 3.28 seconds. In a single frame, the motion of 3I/ATLAS across the Martian sky would have smeared its image by merely 300 kilometers. This figure is only about 3% of the larger smear associated with the limited angular resolution of the Rover's camera. Stacking hundreds of images enhanced the apparent brightness of 3I/ATLAS in the final image.
Assumptions and Possibilities
These conclusions are based on the assumption that the images captured by the Rover are indeed of 3I/ATLAS. If the images represent single snapshots with an exposure time below 3.28 seconds, then the brightness and angular length of the observed source suggest that it is not 3I/ATLAS. In such a case, the source might be much closer to the camera and capable of being smeared into an elongated image with a smaller spatial size and a slower speed.
A Movie Star in the Martian Sky
If the source is indeed 3I/ATLAS, we can visualize the stripe as a sequence of smeared snapshots, similar to a pod of green beans. 3I/ATLAS has had the experience of a movie star, with several Mars orbiters and a ground-based rover taking snapshots as it passed across the Martian sky. The highest resolution image was captured by a camera aboard a Mars orbiter, which has an angular resolution that translated to 30 kilometers per pixel at a specific time. The brightest pixel in this high-resolution image will offer the best constraint yet on the area of 3I/ATLAS. Everyone is keenly waiting for the team to release these images as soon as possible.