Can you imagine how close Rosetta must be to comet 67P/C-G to see its own shadow? Try 19,500 feet. This photo was taken during its closest flyby yet on Valentine’s Day and reveals an exotic landscape of rock, ice and dust from an altitude of just 3.7 miles (6 km), about half the altitude of a typical jet.
Picture yourself looking out the window of a plane well before it reaches cruising altitude. While the landscape resembles the barren cliffs, canyons and hills of the southwestern U.S. it has its own unique platy cragginess. To the left, the rugged cliffs give way smoother, dust-covered terrain.
The shadow at bottom measures about 65 x 160 feet (20×50 meters), and the frame covers an area of just 750 feet square (250-m).
During the flyby, Rosetta passed through a unique observational geometry; for a short time, the Sun, spacecraft, and comet were exactly aligned. As seen from above, surface structures on the comet cast almost no shadows, allowing scientists to precisely measure the reflection properties of the materials, including determining the sizes of the mineral and ice grains.
Coming back to the shadow, notice how fuzzy it is. Even though the Sun appeared only 0.2° across (0.5° from Earth) from Rosetta’s distance of 215.6 million miles at the time, it’s still not a point source. Only point sources of light create precisely sharp shadows identical in size to the objects that produce them. Rosetta’s shadow has a fuzzy, soft-edge “penumbra” that adds an additional 65 feet (20-m) to its size.
If you look again at the first image at the start of this blog, you’ll see a faintly bright halo around Rosetta’s shadow. This is primarily caused by the “opposition effect” or shadow hiding. Grains directly below Rosetta in line with the Sun cast no shadows and so appear brighter than other grains nearby casting very short shadows.