Images from the “dark side” of Mercury that have never been seen before in detail have been captured by a Nasa spacecraft that flew within 125 miles (200km) of the surface.

While the solar system’s smallest planet was photographed in detail by the Mariner 10probe in 1974 and 1975, the same hemisphere was in sunlight during each flypast, meaning that pictures of only one side were captured. Nasa’s Messengerspacecraft took 1,200 high-resolution images – of about half the previously unseen hemisphere – as it flew past on Monday. The pictures show features as small as six miles across.

The upper right part of the pictures show the giant Caloris basin, the western parts of which have never been seen before from a spacecraft.

Formed by the impact of a large asteroid or comet, Caloris is one of the largest basins in the solar system.

The new images show the complete basin interior and reveal that it is brighter than the surrounding regions and may therefore have a different composition. Darker smooth plains completely surround Caloris, and many unusual dark-rimmed craters are observed inside the basin.

Nasa scientists will publish more images over the next two weeks.

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