Mercury is the innermost planet and the smallest of the four terrestrial planets. It is the only terrestrial planet except Earth to have a global magnetic field. Mercury shows the most extreme surface temperature variations of all terrestrial planets. The average surface temperature is 442.5 K, with dayside temperatures up to 700 K and nightside temperatures down to 80 K. Mercury has a tenuous exosphere containing hydrogen, helium, oxygen, sodium, calcium, and potassium. This exosphere is not stable – atoms are continuously lost and replenished from a variety of sources.
So far, only two spacecrafts have visited Mercury. The NASA mission Mariner 10 made 3 flybys in 1974 and 1975. The NASA mission MESSENGER made three flybys between 2007 and 2009 and has entered orbit around the planet in March 2011. The ESA mission BepiColombo will observe the planet from orbit starting 2019. Observing Mercury from Earthis challenging. While it is bright, ranging from −2.3 to...