Alan Stern in the context of Pluto

New Horizons is an interplanetary space probe launched as a part of NASA's New Frontiers program. It was launched in 2006, becoming the first spacecraft to perform a flyby study of the Pluto system in 2015. A secondary mission contained a flyby and study of one or more other Kuiper belt objects (KBOs) in the decade to follow, where it flew past 486958 Arrokoth in 2019. It was the first space probe to ever take high-resolution photographs of Pluto in 2015.

It was engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by Alan Stern. New Horizons is the fifth space probe to achieve the escape velocity needed to leave the Solar System.

A planetary-mass moon is a planetary-mass object that is a natural satellite of another non-stellar celestial object. Because of their mass, these moons are large and ellipsoidal (sometimes spherical) in shape due to hydrostatic equilibrium caused by internal partial melting and differentiation and/or from tidal or radiogenic heating, in some cases forming a subsurface ocean.

Planetary-mass moons are sometimes called satellite planets by some planetary scientists such as Alan Stern, who are more concerned with whether a celestial body has planetary geology (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics). Thus they consider planetary-mass moons to be a subset of the planets. This conceptualization of planets as three classes of objects (classical planets, dwarf planets and satellite planets) has not been accepted by the International Astronomical Union (the IAU).