Click image to enlarge.
Recently achieving orbit around Mercury, NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) probe has begun a one-year imaging survey expected to send back 75,000 images of Mercury's surface, much of which has never been imaged before.
Mercury is the most difficult planet for robot probes to survey because of its nearness to the intense gravitational field of the Sun. MESSENGER required several years, and several gravitational "slingshot" flybys of Venus and Mercury, to finally position it for orbital insertion in early March 2011.
In the years since the Mariner 10 mission [launched 1973, Mercury flyby with imaging 1975], subsequent mission proposals to revisit Mercury had appeared too costly, requiring large quantities of propellant and a heavy lift launch vehicle. However, using a trajectory designed by Chen-wan Yen [Jet Propulsion Laboratory] in 1985, the study showed it was possible to seek a Discovery-class mission by using multiple, consecutive gravity assist, 'swingby' maneuvers around Venus and Mercury, in combination with minor propulsive trajectory corrections, to gradually slow the spacecraft and thereby minimize propellant needs.
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A First Look at Terrain
Near Mercury's North Pole
This WAC (Wide-Angle Camera) image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of 450 km (280 miles) above the planet during the spacecraft’s first orbit with the camera in operation. The area is covered in secondary craters made by an impact outside of the field of view. Some of the secondary craters are oriented in chain-like formations.
This image was taken during MESSENGER’s closest approach to the sunlit portion of the surface during this orbit, just before crossing over the terminator. The oblique illumination by the Sun causes the long shadows and accentuates topography. The highly elliptical orbit of MESSENGER brings the spacecraft down to a periapsis (MESSENGER’s closest approach to Mercury) altitude of 200 km (125 miles) and out to an apoapsis (MESSENGER’s farthest distance from Mercury) altitude of 15,000 km (9300 miles).
Date acquired: March 29, 2011
Johns Hopkins University Applied Physics Laboratory
Carnegie Institution of Washington (DC)