NASA's MESSENGER spacecraft successfully completed its third and final swing by Mercury this week, gaining a critical gravity assist that will allow it to enter orbit around Mercury in 2011, and snapping images of five percent of the planet never before seen along the way.

“This third and final flyby was MESSENGER’s last opportunity to use the gravity of Mercury to meet the demands of the cruise trajectory without using the probe’s limited supply of on-board propellant,” says MESSENGER Mission Systems Engineer Eric Finnegan of the Johns Hopkins University Applied Physics Laboratory.

An unexpected signal loss prior to closest approach hampered plans for some detailed targeting of specific features uncovered in the first two flybys, but the primary purpose of the flyby – the gravity assist maneuver – was completely successful. “Furthermore, all approach observing sequences have been captured, filling in additional area of previously unexplored terrain and further exploring the exosphere of Mercury,” adds Finnegan.

Amongst the bounty of previously unseen terrain is an intriguing crater exhibiting an arc-shaped depression on its floor. This type of crater is known as a pit crater, and other examples have been seen elsewhere on Mercury. They are thought to be associated with magmatic activity – forming when shallow reserves of magma drained elsewhere and left a roof area unsupported, leading to collapse and the formation of the pit. The discovery of multiple pit-floor craters augments a growing body of evidence that volcanic activity was widespread in the geologic evolution of Mercury's crust.

A new double-ringed crater was also revealed in the MESSENGER images. The floor of the basin consists of smooth plains material with concentric troughs, formed by surface extension. Crater chains produced by the impact of material ejected from the crater in the main impact event can also be seen emanating from the basin.

Mercury's pawprint. In the top centre of the image, outlined in a white box and shown in the enlargement at upper right, is a cluster of impact craters on Mercury that appears coincidentally to resemble a giant paw print. In the “heel” are overlapping craters, made by a series of impacts occurring on top of each other over time. The four “toes” are single craters arranged in an arc northward of the "heel." The “toes” don't overlap so it isn't possible to tell their ages relative to each other. The newly identified pit-floor crater can be seen in the centre of the main image. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

MESSENGER's third flyby has afforded a close-up view at this curious bright patch seen in the last flyby. Smooth plains dominate the foreground and the rim of a newly discovered impact basin can be seen at lower left. At the centre of the bright halo is an irregular depression, which may have formed through volcanic processes. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Stay tuned for more updates, and for more information about the MESSENGER mission visit http://messenger.jhuapl.edu/index.php.