MMX - Martian Moons eXploration
In early April 2017, the Institute of Space and Astronautical Science (ISAS), a division of the Japanese Aerospace Exploration Agency (JAXA), announced a new robotic explorer to be launched towards Mars in 2024: the Martian Moons Exploration (MMX). The goal is not to observe the red planet, but its two moons: Phobos and Deimos. These two moons (about 25 km wide) are just a fraction the size of Earth's Moon (about 3400 km wide), and their origins are still disputed. Maybe Phobos and Deimos were once asteroids, freely roaming the solar system, but which were captured into the gravitational well of Mars. An alternate hypothesis is they are ejecta: launched into space by a large impactor striking Mars billions of years ago.
In order to answer this question, ISAS/JAXA will attempt something very difficult: a sample return mission. MMX will launch towards Mars in 2024, arriving in 2025. Over three years MMX will study the moons up close, after which it will take samples and transport them back to Earth for 2029. Sample return is difficult to do; humans have had only partial success with robotic sample return missions in the past.