The European Space Agency’s Rosetta spacecraft was just 6 kilometers away when the picture above was snapped. You are looking at a 228 x 228 meter region taken by Rosetta’s OSIRIS narrow-angle camera on February 14. The detail is amazing, but what is that black smudge at the bottom of the picture? That would be Rosetta’s shadow.
Here’s the exact field of view of the narrow-angle camera on February 14.
And, here is the area again along with a NAVCAM image for reference.
While capturing Rosetta’s shadow is cool, the pictures “are of high scientific value,” says OSIRIS Principal Investigator Holger Sierks. When the Sun, spacecraft and comet are perfectly aligned, structures on the comet cast almost no shadows. This is key for observing the reflection properties of surface material.
“This kind of view is key for the study of grain sizes,” Sierks added.
The ESA blog post explains why Rosetta’s shadow is fuzzy.
The shadow is fuzzy and somewhat larger than Rosetta itself, measuring approximately 20 x 50 metres. If the Sun were a point source, the shadow would be sharp and almost exactly the same size as Rosetta (approximately 2 x 32 m). However, even at 347 million km from 67P/C-G on 14 February, the Sun appeared as a disc about 0.2 degrees across (about 2.3 times smaller than on Earth), resulting in a fuzzy “penumbra” around the spacecraft’s shadow on the surface. In this scenario and with Rosetta 6 km above the surface, the penumbra effect adds roughly 20 metres to the spacecraft’s dimensions, and which is cast onto the tilted surface of the comet.
Since taking a picture of its shadow, the Rosetta has backed away from the comet. The pictures below (they have been processed to bring out more detail of the comet’s activity) show activity on the comet is increasing. The comet will heading towards the sun until it reaches perihelion (closest point to sun) on August 13, 2015.