The first pictures taken by the Messenger probe as it passed Mercury on Monday have started to arrive at Earth.
They include images that show parts of the surface missed by the Mariner 10 spacecraft when it flew by the planet in the 1970s.
Like the previously mapped portions of Mercury, the new sections appear heavily cratered.
Messenger needs to perform another two fly-bys and a number of engine firings to get itself into orbit in 2011.
Monday's manoeuvre took the probe to within just 200km (125 miles) of the planet at closest approach.
Messenger was programmed to collect more than 1,300 images and make other observations during the encounter.
The data began transmission to Earth on Tuesday.
This image was taken just 21 minutes after closest approach. Messenger's Narrow Angle Camera spies a variety of surface features, including craters as small as about 300m across.
Nasa says such detailed close-ups will be used by planetary geologists to study the processes that have shaped Mercury's surface over the past four billion years.
The picture shows part of a giant impact crater to the bottom-right.
This Narrow Angle Camera image was obtained about 37 minutes after closest approach to the planet.
It shows a previously unseen crater with distinctive bright rays of ejected material extending radially outward from the crater's centre.
Resolution in the picture is about 360m per pixel, and the width of the image is about 370km.
Monday's flyby has produced a sequence of pictures that will allow scientists to build a high-resolution mosaic of the northeast quarter of the region not seen by Mariner 10.
It is to be expected that after 30 years of technological development, the images returned by Messenger are of a superior quality to those sent back by Mariner 10.
The large, shadow-filled, double ringed crater in this picture was glimpsed by the 1970s probe and named Vivaldi, after the Italian composer.
Its outer ring has a diameter of about 200km. Messenger sees much more detail, including the broad ancient depression overlapped by the lower-left part of the Vivaldi crater.
Messenger captured this image as it closed in on the planet.
It was taken when the probe was about 18,000km from the surface of Mercury, and about 55 minutes before closest approach.
The image shows smooth plains, many impact craters (some with central peaks), and rough material that appears to have been ejected from the large crater to the lower right.
This 200km-wide crater was also seen in less detail by Mariner 10.