A tiny planet Mercury before the huge sun

A tiny black spot - this is how Mercury looks in front of the solar disk, which it passed on Monday. It took the planet more than seven hours to travel the entire length.

An astronomer took those pictures with an 800mm telescope from Kempten in the Allgäu region of Bavaria. The sun is shining with such intensity that the planet is reduced to a small shadow. Even this picture could only be taken because the telescope has been darkened with a special sunray-filter.

This picture is from 2006. Italy's National Institute of Astrophysics took it during a previous Mercury flyby. At least here it is easy to distinguish between the planet and frequently ocurring sunspots, or coronal mass ejections, which also appear as dark spots on the surface of the sun. The planet, however, is clearly round with sharp edges before the background of solar flares.

Only a spacecraft will do. NASA's spacecraft Messenger took the best pictures of Mercury so far. The probe circled the planet from 2011 to 2015 until it finally crashed there.

Messenger found a planet with a surface which does not look too differently from our Earth-Moon - at first glance: rock, sand, gravel and lots of craters left behind by meteorite impacts. But there are big differences, too: temperatures on Mercury are much more extreme. Because there is practically no atmosphere, it can get icy cold. On the other hand, the sun can make it extremely hot.

This picture is a composite from thousands of pictures taken by different spectrometers in the course of countless orbits around the planet. For the first time, there is a really high-resolution image of Mercury.

This area shows the eventual crash site where Messenger came down in 2015. Clearly visible: meteorite impact craters of all sizes.

The astronomers who have watched Monday's Mercury flyby did not see pictures like this. The photo taken by Messenger shows an area of plains formed by volcanic activity. The different colors indicate different rock materials.