Parker Solar Probe by the Helios 2 spacecraft. The "Parker Solar Probe" concept originates from a predecessor "Solar Orbiter" project conceived in the 1990s. Similar in design and objectives, the "Solar Probe" mission served as one of the centerpieces of the eponymous Outer Planet/Solar Probe (OPSP) program formulated by NASA. The first three missions of the program were planned to be: the "Solar Orbiter", the Pluto and Kuiper belt reconnaissance mission "Pluto Kuiper Express", and the "Europa Orbiter" astrobiology mission focused on Europa. The original "Solar Probe" design used a gravity assist from Jupiter to enter a polar orbit which dropped almost directly
Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in