Definition of planet object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own centre of mass until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are spheroids, and several, such as the larger moons of Jupiter and Saturn and the dwarf planet , have been further distorted into ellipsoids by rapid rotation or tidal forces, but still in hydrostatic equilibrium.) However, there is no precise point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his
Gravity assist orbital inclinations from 0° to 74°. The "Rosetta" probe, launched in March 2004, used four gravity assist maneuvers (including one just 250 km from the surface of Mars) to accelerate throughout the inner Solar System - enabling it to match the velocity of the 67P/Churyumov–Gerasimenko comet at their rendezvous point in August 2014. NASA's Parker Solar Probe mission, launched in 2018, will use multiple gravity assists at Venus to remove the Earth's angular momentum from the orbit, in order to drop down to a distance of 8.5 solar radii (5.9 Gm) from the Sun. Parker Solar Probe's mission will be