Mercury used to be an obscure world that was hardly more than a fuzzy blob in even the best telescopes. The dilemma is that it orbits so close to the Sun that it is always low on the horizon in bright twilight and suffers from Earth’s atmospheric turbulence. In fact legend has it that Copernicus never saw Mercury. Astronomers tried to tease out surface features and saw dusky markings. This led to false assumptions that Mercury had a thick atmosphere full of clouds. An even worse assumption was that Mercury always kept the same side facing the Sun and rotated once every 88 days, the same time it takes to orbit the Sun.
The dawn of the Space Age and advanced technology finally revealed Mercury as a true world. In 1965 the giant radio telescope at Arecibo discovered Mercury’s rotation period to be 58.65 days instead of 88 days. Therefore, Mercury did not always keep the same side pointed toward the Sun. The allusion came from Mercury’s 3:2 spin orbit resonance where Mercury’s rotational period is about two-thirds of its orbital period. This gives the false illusion of Mercury keeping the same face towards the Sun because it presents the same face at the same phase from Earth. Astronomers and scientists wanted to know what Mercury really looked like. By the 1960’s it was already known that Mercury had no clouds or dust storms and may even be a scorched version of our Moon. Mercury is a tough planet to reach with spacecraft because it orbits deep within the Sun’s powerful gravitational field. A spacecraft on a direct flight from Earth would fly past Mercury too fast for any useful observation. The spacecraft would need a lot of fuel, which would make it too large, too heavy and too expensive to even consider. It was discovered that by using Venus as a gravitational slingshot a spacecraft could be slowed down enough using minimal fuel and could be aimed to arrive at Mercury for a close and productive flyby.
On November 3, 1973 Mariner 10 blasted off and was on its way to Mercury. Mariner 10 was plagued with problems, but managed to fly within 10,000 miles of Venus on February 5, 1974 and made history on March 29, 1974 when it flew within 350 miles of Mercury for the first time revealing it to be a heavily cratered world with smooth, lightly cratered plains, long cliffs up to 2 miles high running for hundreds of miles, and had an impact basin larger than Texas. Mariner 10 also discovered an atmosphere of helium around Mercury, but it was very thin, almost a vacuum. The extreme temperature from -300º to 800º was confirmed. The biggest discovery was a magnetic field that gave support to the claim of a large iron core that contributed to Mercury’s great density, even denser than Earth. There was brief excitement when a tiny moon was thought to be discovered, but soon dismissed as a radiation burst somewhere far beyond out in the universe. Mariner 10 flew within 30,000 miles of Mercury on September 21, 1974 and only 200 miles from Mercury on March 16, 1975 before running out of fuel a week later. It succeeded in mapping the magnetic field and photographed about 45% of Mercury.
Mercury went back into obscurity for the rest of the century as no other spacecraft would fly near it for another 33 years. In spite of this, there was a renewing interest in returning, and this time to stay with an orbiting spacecraft. The possibility of water ice in the deeply shadowed craters near the poles fueled the urge to return to the scorched planet. Innovations in technology led to building lighter spacecraft that use less fuel and made it possible to put a spacecraft in orbit around Mercury. In July 1999 MESSENGER (for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was born and on August 3, 2004 it was launched on a 4.9 billion mile journey to Mercury. This sounds odd since Mercury can come as close as 50 million miles to Earth, but MESSENGER required two flybys of Earth, two flybys of Venus, and three flybys of Mercury in order to slow it down enough to equal Mercury’s orbital velocity and be captured into orbit. The Mercury flybys of January 14, 2008, October 6, 2008, and September 29, 2009 were stunning successes revealing Mercury in razor sharp detail and it photographed 80% of the planet as it flew within 125 miles of the surface each time. MESSENGER has state of the art cameras that finally revealed the true color of Mercury as an ash-colored world with a touch of tan.
MESSENGER arrived at Mercury on March 17, 2011 at 9 P.M. EDT and began a one-year tour exploring the planet in detail never thought possible. An extended mission until March 18, 2013 has been approved as the discoveries continue. MESSENGER orbits as close as 125 miles from the surface to as far as 9440 miles. This elliptical orbit is important as heat radiating from the surface of the planet would destroy the spacecraft if it constantly orbited closely. As it is, the spacecraft has a large sunshield that protects the delicate electronics and instruments from the Sun’s intense heat. MESSENGER has discovered additional impact basins and areas of unusually bright and dark soil, possibly volcanic. Some unusual craters with brighter soil around them may be vents to volcanoes that erupted not too long ago in geological times and may be interesting to explore further to determine if they are venting gases to this very day. Several of the bright areas are called hollows where volatile material, possible sulfur, vaporized in the heat creating small, irregular depressions. Mercury underwent massive compression early in its history while it was cooling, which created widespread scarps.
MESSENGER will not last much longer as the environment is harsh leaving many unanswered questions. The next chapter is already underway. The European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) plan to launch a pair of spacecraft in 2015 aboard one rocket that will both orbit Mercury in 2022. The ESA probe will concentrate on the geology of Mercury and the JAXA probe will study the magnetic field. There were even plans to deploy a small lander, but it grew too expensive and was cancelled years ago. This bold mission is called BepiColombo after Giuseppe “Bepi” Colombo, the scientist who explained Mercury’s 3:2 spin orbit resonance and suggested that Mariner 10 could fly past Mercury several times using a carefully aimed flyby of Venus. This mission will pick up where MESSENGER leaves off.
Mercury is no longer an obscure, fuzzy world, nor is it a boring version of our Moon. Future missions after BepiColombo will include a lander to study possible water ice at the poles, rovers, and finally a sample return mission. The exploration of Mercury has hardly begun and already so much has been learned.