Mercury symbol

Mercury is the closest planet to the Sun, and is a very difficult object to observe, as it never leaves the twilight zone, either in morning or evening. Very little is known about this world. It has only been visited by one spacecraft, Mariner 10, which imaged approximately 40% of its surface in 1974 and 1975. The image below is a mosaic of some of those images.

Mercury, imaged by Mariner 10 in 1974

[ NASA / JPL / Caltech ]


Axial Tilt
Axial Period
(d, h, m)
87.97 d 0.31 0.47 7.0 0.0 58, 15, 30.5
Diameter (km)
Oblateness Mass
(Earth = 1)
(water = 1)
No. of
4, 878 0.000 0.06 5.4 0.11 0

Missions to Mercury

Only one spacecraft has ever been sent to study Mercury. Mariner 10 executed 3 fly-bys of the small planet on 29th March 1974 (at 705 km), 21st September 1974, (at 47,000 km), and on 16th March 1975 (at 327 km). The spacecraft discovered that Mercury has an extremely tenuous helium atmosphere, and is unusually dense for such a small body. This means that Mercury must posess a very large iron core.

A future mission called MESSENGER (or MErcury Surface, Space ENvironment, GEochemistry and Ranging mission), is due for launch in 2004. It will enter orbit around Mercury on 30th September 2009, and complete its mission one year later in September 2010.