Yes, but they have not landed.
The first spacecraft to visit Mercury was Mariner 10 in 1974 -> 1975
Juan Alejandro
yes
The Mariner Ten and the Messenger
voyager
A spacecraft visited in 2011, not 2012. There is currently one spacecraft in orbit around Mercury. NASA's MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) probe.It was launched on August 3, 2004 and entered orbit on March 18, 2011. NASA has announced that it will extend the mission longer in the harsh environments of Mercury.For more information check the MESSENGER wikipedia article.
yes
Just one.
Yes.
Mariner 10 and MESSENGER flew past Mercury.
All spacecraft (from Earth) that have visited Mercury have been robotic. There were no human passengers. The chances are near zero that a stowaway microbe would have survived the long journey.
Mariner 10 and Messenger have visited Mercury. BepiColombo by the ESA is planned to be launched in 2013. Mariner 10 provider the first close up photos of the planet. Messenger is "filling" in the gaps not accomplished by Mariner.