Several days after Rosetta’s close flyby of Comet 67P on February 14, 2015, images taken on this day by OSIRIS, the scientific imaging system on board, have now been downlinked to Earth. With a resolution of 4 inches (11 centimeters) per pixel, these data from OSIRIS’ Narrow Angle Camera reveal highly detailed structures on the comet’s surface. Since at closest approach, Sun, spacecraft, and comet were almost perfectly aligned, few shadows are visible in the images, with one exception: As a side-effect of this exceptional observational geometry, Rosetta’s shadow on the surface can be seen surrounded by a bright halo-like region.
The image shows an area near the edge of the comet’s belly at the boundary of the Imhotep region covering 748 feet by 748 feet (228 meters by 228 meters) on the comet’s surface. A mesh of steep slopes separates smooth-looking terrain from a more craggy area. The image was taken from a distance of 4 miles (6 kilometers) from the comet’s surface, thus making structures with a pixel scale of only 4 inches (11 centimeters) visible. In resolution, it is only surpassed by images taken by Philae’s camera ROLIS on November 12, 2015, during descent.
During the flyby, Rosetta not only passed closer by a comet than ever before, but also engaged in a unique observational geometry: For a short time during the maneuver, the Sun, spacecraft, and comet were exactly aligned. “Images taken from this viewpoint are of high scientific value,” said Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. Since the surface structures cast almost no shadows, the surface’s reflection properties can be discerned. “This kind of view is key for the study of grain sizes,” he added.
At the bottom of the image, Rosetta’s shadow can be seen as a fuzzy rectangular-shaped dark spot measuring approximately 65 by 165 feet (20 by 50 meters). These dimensions are determined by the spacecraft’s penumbra. Such penumbras occur when an object is illuminated by more than one light source — or an extended one like the Sun. In both cases, light reaches the object from different directions, leading to a dark core shadow where the object blocks the entire light source and an adjacent penumbra where only part of the light source is concealed. Considering the distance between Rosetta and the comet’s surface, the penumbra effect leads to a shadow both 66 feet (20m) longer and wider than Rosetta’s dimensions of approximately 7 by 105 feet (2 by 32 meters), which is cast on the tilted surface of the comet.
In addition, the region surrounding the shadow appears brighter than the rest of the comet’s surface seen in the image. Scientists refer to this effect as an opposition surge. It is known, for example, from photographs showing astronauts on the Moon and occurs typically on highly structured, regolith surfaces when light incidents from the same direction into which it is reflected. In this situation, shadows normally cast by small surface grains disappear leading to a pronounced increase in brightness. This is further enhanced by the backscattering of light by the small particles on the surface of the comet.