APOD 4.8

APOD 4.8
                Ares 3 Landing Site: The Martian Revisited 

This close-up from the Mars Reconnaissance Orbiter's HiRISE camera shows weathered craters and windblown deposits in southern Acidalia Planitia. No human eyes have looked upon this terrain unless you count the eyes of astronauts in the scifi novel "The Martian" by Andy Weir (novel is about Mark Watney, an astronaut stranded at the fictional Mars mission Ares 3 landing site corresponding to the coordinates of this cropped HiRISE frame). Of course, the Ares 3 landing coordinates are only about 800 kilometers north of the (real life) Carl Sagan Memorial Station, the 1997 Pathfinder landing site.

APOD 4.7

APOD 4.7
                Gravitational Anomalies of Mercury 

The robotic MESSENGER spacecraft that had been orbiting planet Mercury for the past 4yrs (using radio waves) with precise energy. Mercury's gravity slightly changed this energy when measured on Earth (enabled the reconstruction of a gravity map of unprecedented precision). Red hues are areas of slightly higher gravity and unusually dense matter under the surface. The central area is Caloris Basin (impact feature measuring about 1,500 kms across). Last week, the MESSENGER was purposely crashed onto Mercury's surface after fulfilling its' mission.

APOD 4.6

APOD 4.6
                  Comet Churyumov Gerasimenko in Crescent

 As the 3-km wide comet (Comet 67P/Churyumov–Gerasimenkomoves) moves closer to the Sun, heat causes the nucleus to expel gas and dust. Last July, The Rosetta spacecraft arrived at the comet's double nucleus and now is co-orbiting the Sun with the comet. Recent analysis of data from the Rosetta spacecraft has shown that water being expelled from the comet is very different from the water on Earth (indicating Earth's water couldn't have originated from ancient collisions with comets like 67P). Recent data also shows that there is no detected magnetic field around the comet nucleus (magnetism might have been unimportant in the evolution of the early Solar System). In the image above, Comet 67P is shown in a crescent phase (false color) and the comet is expected to increase its evaporation rate as it nears the Sun in 2015 August.