First Orbit of Mercury by MESSENGER

On 18/03/2011 at 01:10 UTC, NASA’s MESSENGER spacecraft became the first mission to enter orbit around Mercury after completing a 6.5 year journey through the inner solar system. The spacecraft executed a 15 minute braking maneuver using its main engine, slowing enough to be captured by Mercury’s gravity. The orbital insertion marked the first time a spacecraft transitioned from flybys to sustained orbital study of the innermost planet. Following capture, MESSENGER began its first full orbit at approximately 06:47 UTC on the same day. The initial orbit was highly elliptical, with a perihermion of about 200 kilometers above Mercury’s surface and an apohermion of roughly 15,000 kilometers. This 12 hour orbit was designed to balance scientific observation with thermal protection. By spending limited time close to the planet and the rest of the orbit farther away, the spacecraft reduced exposure to intense heat radiated from Mercury and the nearby Sun. The orbit also allowed repeated imaging passes and measurements of the planet’s magnetic field, surface composition, and exosphere. MESSENGER had previously conducted three flybys of Mercury between 2008 and 2009 to prepare for orbital insertion and refine navigation. Once in orbit, the spacecraft began mapping the planet globally, measuring topography, and analyzing elemental composition. Observations from the mission later revealed evidence of water ice in permanently shadowed polar craters, widespread volcanic plains, and a magnetic field offset from the planet’s center. The successful orbital insertion on 18/03/2011 enabled the first long duration exploration of Mercury. MESSENGER continued operating in orbit for more than four years before impacting Mercury’s surface on 30/04/2015 after fuel depletion.