First Mercury orbiter

The first spacecraft to orbit the planet Mercury was NASA's MESSENGER probe, which was successfully inserted into the planet’s orbit at 01:10 on 18 March 2011 (UTC; 21:10, 17 March EDT).

Placing MESSENGER into orbit around Mercury was an incredibly complex task. First, a powerful launch vehicle was required to shed as much of Earth’s orbital energy as possible, allowing the spacecraft to be pulled towards the sun. This manoeuvre on its own would have brought MESSENGER close to Mercury, but would also have accelerated it to an extremely high speed, making orbital insertion impractical. To control the spacecraft’s approach, the MESSENGER mission planners came up with an intricate series of gravity-assist fly-bys of Earth, Venus and Mercury, where the spacecraft used each planet’s gravity to slow itself down.

MESSENGER’s trip to Mercury ended up taking 7 years 227 days. It launched on 3 August 2004 and covered 7.9 billion km (4.9 billion miles) – orbiting the sun 15 times – before finally reaching its destination.

The spacecraft’s primary mission ran from orbital insertion to 30 April 2015, when it was de-orbited and deliberately crashed into the planet’s surface – becoming the first man-made object to reach Mercury’s surface. During its four years in orbit, MESSENGER revolutionized scientific knowledge of the closest planet to the sun. It carried out a full topographical survey of the planet, revealing in the process that Mercury was still tectonically active, with fault canyons and active volcanos. It also measured the strength of the magnetic field, discovered water ice on the surface, and took readings that revealed Mercury had a liquid-iron core.

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. It was only the second probe to reach the innermost planet since Mariner 10's first fly-by in 1974. The third mission to Mercury (the ESA/JAXA BepiColumbo mission) is currently (as of 2019) en route to the planet, expected to arrive in 2025.