The Curiosity Rover has taken up a new hobby: astrophotography. Not content to merely photograph cool Martian rocks, the rover has captured the first image of an asteroid as seen from the surface of Mars. Nicely done, Curiosity!
Ceres is a monster of an asteroid at 950 kilometers diameter, large enough to qualify as a dwarf planet. Vesta is just barely smaller, the third-largest asteroid belt object at 563 kilometers wide. The asteroids also share a common NASA mission: the Dawn spacecraft orbited Vesta in 2011 and 2012, and is currently underway to orbit Ceres in 2015.
The photograph was taken on April 20, 2014, or Curiosity sol 606. The night-sky image captures one of the Marian moons, Deimos, in addition to the pair of asteroids Ceres and Vesta. Later that night, Curiosity as photographed the other moon, Phobos, with the planets Jupiter and Saturn. Both images are combined in one massive composite with differing exposure times, insets of images captured by other cameras, and very handy annotations:
From the NASA press release:
In the main portion of the image, Vesta, Ceres and three stars appear as short streaks due to the duration of a 12-second exposure. The background is detector noise, limiting what we can see to magnitude 6 or 7, much like normal human eyesight. The two asteroids and three stars would be visible to someone of normal eyesight standing on Mars. Specks are effects of cosmic rays striking the camera's light detector.
Three square insets at left show Phobos, Jupiter and Saturn at exposures of one-half second each. Deimos was much brighter than the visible stars and asteroids in the same part of the sky, in the main image. The circular inset covers a patch of sky the size that Earth's full moon appears to observers on Earth. At the center of that circular inset, Deimos appears at its correct location in the sky, in a one-quarter-second exposure. In the unannotated version of the 12-second-exposure image, the brightness of Deimos saturates that portion of the image, making the moon appear overly large.