The next series of posts will look at potential missions to some of the orphans in NASA's planetary exploration program: The inner planets ex Mars but including the moon.
NASA's last mission to Venus was the Magellan radar mapping mission that mapped that world from 1990 to 1994. The last science mission to the moon was in the Apollo missions. (The currently orbiting Lunar Reconnaissance Orbiter was funded as a pathfinder to future manned missions and not as a science mission, even though some scientific questions will be addressed.)
The exception to the orphan status is Mercury, which is the target of the MESSENGER mission, which has accomplished three highly productive flybys and will enter orbit in about 18 months.
Other space agencies have recent, current or planned missions for these worlds. Russia will send its Venera-D mission to orbit, balloon, and land at that world mid decade. ESA and JAXA have a combined mission with two orbiters planned for Mercury. ESA has an active orbiter at Venus, and JAXA has a Venus orbiter planned to study its atmosphere for the early part of the coming decade. And a slew of science oriented lunar orbiters have just completed their missions.
All of this activity puts U.S. scientists in something of a bind. To build upon recent and planned missions by other nations, they must propose sophisticated new missions. Sophisticated tends to equal expensive, and NASA's planetary budget is likely to be largely devoted to Mars and Jupiter-Europa for the coming decade.
The next few posts will look at proposed strategies and missions to advance exploration of the inner planets in light of these constraints.