New images from the OSIRIS camera on European Space Agency’s Rosetta probe taken during the close flyby on February 14, 2015, reveal the surface of the comet in unprecedented detail and show the shadow of the spacecraft.
Close view of a 228 x 228 m region on 67P/Churyumov-Gerasimenko was taken 6 km above the surface. Rosetta’s fuzzy shadow, measuring about 20 x 50 m, is seen on the right of the image. Image credit: ESA / Rosetta / MPS for OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
ESA’s Rosetta is not the first spacecraft to capture its own shadow in this way. In 2005, the Japanese Hayabusa space probe captured its shadow on asteroid Itokawa.
However, because Hayabusa was only a few tens of meters above the surface, the penumbral effect was much less, resulting in a sharper and darker shadow of the spacecraft.
The new image from Rosetta’s OSIRIS camera shows an area near the edge of the comet’s belly close to the Imhotep-Ash regional boundary, where a mesh of steep slopes separates smooth-looking terrains from a craggier area.
It was taken from a distance of 6 km from the comet’s surface and covers an area of 228 x 228 m.
During the 14 February flyby, the spacecraft not only passed closer by the comet than ever before, but also passed through a unique observational geometry: for a short time the Sun, spacecraft, and comet were exactly aligned.
In this geometry, surface structures cast almost no shadows, and therefore the reflection properties of the surface material can be discerned.
The OSIRIS camera image from the close flyby, bottom left, shown here in context with NavCam images. Image credit: ESA / Rosetta / NAVCAM – CC BY-SA IGO 3.0 / MPS for OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
“Images taken from this viewpoint are of high scientific value. This kind of view is key for the study of grain sizes,” said Dr Holger Sierks of the Max Planck Institute for Solar System Research in Germany, who is the principal investigator of the OSIRIS.
As a side effect of this exceptional observational geometry, space probe’s shadow can be seen cast on the surface of the comet as a fuzzy rectangular-shaped dark spot surrounded by a bright halo-like region.
The shadow is fuzzy and somewhat larger than Rosetta itself, measuring 20 x 50 m. 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 20 m longer and wider than Rosetta’s dimensions.
