Mercury and Venus: Significant Results from MESSENGER and Venus Express Missions

Understanding how the solar system evolved has been one of the driving reasons for exploring the solar system until recently. Now the focus includes planets around other stars and particularly terrestrial planets and habitability. Mercury and Venus are two extreme members in our solar system but are poorly understood. Our closest planetary neighbors, with their proximity to the sun made it challenging to learn much about them from telescopes, as they are accessible for only a short time before sunrise or after sunset for large telescopes (due to scattered light and sensitive detectors). Venus with it global cloud cover made it impossible to learn about its surface, until new advances in radar and microwave techniques. Only partly surveyed by Mariner 10 from three fly-bys during 1974–1976, Mercury remained enigmatic until MESSENGER. By contrast, Venus has been explored from fly-by spacecraft, orbiters, entry probes and landers and even balloons, yet the major science questions have only become sharper. MESSENGER and Venus Express, the two current spacecraft visitors from Earth to the innermost, entered the final phase of their mission lives in summer 2014 as the fuel required for orbit maintenance was depleted. Orbiting Venus since 15 April 2006, Venus Express conducted an aerobraking experiment in June 2014. It collected its last observations on 27 November 2014 when the fuel was exhausted during orbit raise maneuvers, and the spacecraft entered the atmosphere on 18 January 2015. Over more than eight years of observing Venus from its 24 h, polar elliptic orbit, it collected a large amount of data from its operating instruments which have provided new insights into the atmosphere of Venus and to a limited extent, its surface. The MESSENGER spacecraft also observed Venus on its third fly-by of Venus which also yielded some new results.