The pair of bright star-like features in the upper panel are not stars at all, but the Earth and Moon! MESSENGER was at a distance of 98 million kilometers (61 million miles) from Earth when this picture was taken. The computer-generated image in the lower left shows how the Earth appeared from Mercury at the time. Much of the Americas, all of Europe and Africa, the Middle East, and much of Asia were visible.
MESSENGER took this image as part of a campaign to search for natural satellites of Mercury. Mercury has no moons that we know of. If any exist, they must be small (less than a few kilometers), or we would have seen them by now. The strategy for the satellite search involves taking multiple images of locations at predetermined distances from Mercury, from 2.5 to 25 times the planet radius. Pictures of these points in space are captured at intervals ranging from seconds to nearly an hour, depending on their distances from Mercury. A moving satellite will appear at different positions in images of the same region of space taken at different times.
The Earth and Moon appear very large in this picture because they are overexposed. When looking for potentially dim satellites, long exposures are required to capture as much light as possible. Consequently, bright objects in the field of view become saturated and appear artificially large. In fact, the Earth and Moon are each less than a pixel in size, and no details on either can be seen. The "details" pointing downward from the Earth and Moon are artifacts caused by the image saturation. These can be seen clearly in the zoomed image in the center lower panel.
This image was taken on the same day that images with Earth in the scene were acquired by the Cassini spacecraft, as part of a mosaic of the backlit Saturn system ( http://saturn.jpl.nasa.gov/news/waveatsaturn ).
One day later in the search for satellites of Mercury, MESSENGER again took similar images of the Earth and Moon. The date of those images, July 20, was the 44th anniversary of the Apollo 11 landing on the Moon. All six Apollo landing sites are illuminated in the MESSENGER images, although they cannot be resolved. Their locations are marked on the simulated image in the lower right panel.
The background for this image is the region between the constellations Sagittarius (the archer) and Scutum (the shield). The right ascension and declination of Earth are 18h 55m 44s and -18° 25' 31", respectively. Pluto is also in the field of view but far too dim to be seen.
July 19, 2013, 11:54:41 UTC
Image Mission Elapsed Time (MET): 1016558881
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
WAC Filter: 2 (clear filter)
Field of View: The WAC has a 10.5° field of view
|Target||Earth||Mercury, Moon, Pluto, Saturn|
|System||Earth||Mercury, Pluto, Saturn|
|Target Type||Earth||Dwarf Planet, Planet, Satellite|
|Instrument Host||MESSENGER||Cassini Orbiter|
|Instrument||Mercury Dual Imaging System (MDIS)||Imaging Science Subsystem (ISS)|
|Detector||Wide Angle Camera|
|Date in Caption||2013-07-19|
|Image Credit||NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington|