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
Planet speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth