Double planet the four, small, outer satellites of Pluto. A now-abandoned hypothesis for the origin of the Moon was actually called the "double-planet hypothesis"; the idea was that the Earth and the Moon formed in the same region of the solar system's proto-planetary disk, forming a system under gravitational interaction. This idea, too, is a problematic condition for defining two bodies as "double planets" because planets can "capture" moons through gravitational interaction. For example, the moons of Mars (Phobos and Deimos) are thought to be asteroids captured long ago by Mars. Such a weak definition would also deem Neptune-Triton a double planet,
Mars Exploration Program used toward a mission that is part of the Mars Sample Return program. As a result of the smaller budget for the MEP, NASA was forced to cancel the Mars Science Orbiter planned for 2016 that would have studied the climate of the planet. NASA does not have any missions for the future of the MEP, as a plan is still being developed that will encompass the next few decades. Mars Exploration Program The Mars Exploration Program (MEP) is a long-term effort to explore the planet Mars, funded and led by NASA. Formed in 1993, MEP has made use of