<P> The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from . To this end, the probe carried imaging devices that gathered much - higher - resolution images of much more of Mercury than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles . Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure . MESSENGER's final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015 . The spacecraft's impact with Mercury occurred near 3: 26 PM EDT on April 30, 2015, leaving a crater estimated to be 16 m (52 ft) in diameter . </P> <P> The European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere . Once launched in 2018, BepiColombo is expected to reach Mercury in 2025 . It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit . Both probes will operate for one terrestrial year . The mapper probe will carry an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray . </P>

Which terrestrial planet has a day that is over two months long