Mercury is known as an elusive, fleeting planet. It barely rises above the morning or evening horizon for a few weeks at a time and is always in the twilight. Seeing anything more than its phases is an amazing feat. It orbits too close to the Sun for the Hubble Space Telescope to safely observe it due to the risk of bright sunlight damaging it. Even spacecraft flying to Mercury have challenges because Mercury is embedded deep within the Sun's powerful gravity field. In spite of these challenges, both Mariner 10 and MESSENGER finally revealed Mercury and answered many questions, but has created others.
Mariner 10 was the first spacecraft to encounter Mercury when it flew by on March 29, 1974. It revealed Mercury as a cratered world like the Moon and yet very different from it. Mercury was found to have a huge iron core comprising almost 80% of its diameter and a magnetic field. It also has a very thin atmosphere of sodium and potassium that is more like an exosphere, but is a near-vacuum. Mercury is covered with ancient lava flows and scarps or ridges. The scarps can be over 100 miles long and several miles high. They formed when Mercury cooled and contracted, which created stress fractures and thrust faulting. Mariner 10 flew past Mercury two more times, September 21, 1974 and March 16, 1975, before it ran out of stabilizing fuel and could no longer communicate with Earth. The only next logical mission would be an orbiter, but Mercury's closeness to the Sun and its rather weak gravity posed a challenge. Technology improved over the years, but 33 years would pass before Mercury would be seen again.
MESSENGER flew past Mercury on January 14, 2008 and revealed it in higher detail than ever and in true color, but the best was yet to come when it finally entered into orbit around Mercury on March 17, 2011. MESSENGER orbited Mercury every 12 hours and as close as 125 miles during the first year allowing for complete mapping of the geology, minerals, and magnetic field. Stunning views of the giant Caloris impact basin were imaged and studied, and the gravity field refined the diameter of the iron core upwards to 85% of the planet. Parts of the core were found to be partially molten and is now theorized that Mercury has a solid silicate crust and mantle overlying a solid iron sulfide outer core layer, a deep liquid core layer, and possibly a solid inner core. Another unique feature exclusive to Mercury are the hollows that are found within several craters, along the rims, and peppering the central peaks. These are up to a mile wide, 100 feet deep and are most likely pockets where volatile material boiled away from the heat below and/or from the Sun and burst through the surface, giving it a sponge-like appearance. MESSENGER discovered a vast lava flood plain covering the northern polar region that is as large as 60% of the U.S. and up to two miles deep making it the largest feature on Mercury.
The biggest discovery came in late November 2012 when MESSENGER's neutron spectrometer confirmed evidence for the presence of both water ice and organic compounds in the permanently shadowed craters of Mercury's north pole. Mercury has an average daytime temperature of 800ºF and a nighttime temperature of -300 degrees F, but the permanently shadowed craters can remain perpetually cold, which can preserve the water ice and organic compounds delivered by impacting comets and asteroids. MESSENGER has performed beyond imagination with all of these great discoveries and more, but its time is limited; sometime during March 2015 its mission will end when it crashes into the surface of Mercury. All is not lost as a new generation of spacecraft is being built and is scheduled to launch not long after the demise of MESSENGER.
The European Space Agency (ESA) will follow up with its first mission ever to Mercury. BepiColombo will launch on July 9, 2016 and arrive at Mercury on January 1, 2024 where it will orbit for at least one year and hopefully longer. The mission comprises of two spacecraft: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). BepiColombo is a joint mission between ESA and the Japan Aerospace Exploration Agency (JAXA), overseen by ESA. BepiColombo is designed to study the composition, geophysics, atmosphere, magnetosphere, and history of Mercury. The objectives are: investigate the origin and evolution of a planet close to a star, study Mercury as a planet, its form, interior structure, geology, composition, and craters, examine the dynamics and composition of its atmosphere and magnetosphere along with the origin, investigate the composition and origin of the polar deposits, and perform a test of Einstein's theory of general relativity.
BepiColombo is named after Professor Giuseppe (Bepi) Colombo (1920-84) from the University of Padua, Italy, a mathematician and engineer of astonishing imagination. He was the first to see that an unsuspected resonance is responsible for Mercury's habit of rotating three times on its axis for every two revolutions it makes around the Sun. He also suggested to NASA how to use a gravity-assist flyby of Venus to place Mariner 10 in a solar orbit that would allow it to fly by Mercury three times in 1974-5. ESA named the mission in 1999 in honor of his achievements for a fitting tribute to a $1.3 billion cornerstone mission.
ESA's Mercury Planetary Orbiter (MPO) carries a sophisticated payload of 11 experiments and instruments comprising of cameras, spectrometers (infrared, ultraviolet, x-ray, y-ray, neutron), radiometer, laser altimeter, magnetometer, particle analyzers, Ka-band transponder, and accelerometer. JAXA's Mercury Magnetospheric Orbiter (MMO) carries a payload of five advanced scientific experiments including a magnetometer, ion spectrometer, electron energy analyzer, cold and energetic plasma detectors, plasma wave analyzer, and imager.
The launch, cruise, and arrival of BepiColombo are interesting. The spacecraft will remain as one until it arrives at Mercury and then separate into two orbiters. It uses ion and chemical propulsion along with eight gravity assists from Earth (once), Venus (twice), and Mercury (five times). BepiColombo will be launched on ESA's powerful and reliable Ariane 5 rocket and will take about 7.5 years to get to Mercury. The key dates for this mission are:
7/16/18 Earth flyby
9/22/19 First Venus flyby
5/4/20 Second Venus flyby
7/23/20 First Mercury flyby
4/14/21 Second Mercury flyby
7/6/22 Third Mercury flyby
12/29/22 Fourth Mercury flyby
2/4/23 Fifth Mercury flyby
1/1/24 Arrival at Mercury
4/1/25 End of primary mission
4/1/26 End of extended mission
BepiColombo is the new messenger to the Messenger Planet, Mercury. Actually it is two messengers where both spacecraft will continue to explore and reveal clues to the history and evolution of a world once considered elusive, obscure, and vastly unknown.