MESSENGER Space Probe at Mercury

March 15, 2011

The Roman god, Mercury, was known as the messenger of the gods. The planet, Mercury, has the most rapid motion of all the planets. Since the god was known for his swiftness, the planet is named after him. The orbit of Mercury around the Sun is only 88 days. The chemical element, Mercury, known as "quicksilver," is also named after him.

The Roman Mercury is identified with the Greek god, Hermes, from whom we get the word, "hermetic." Hermetic means different things to different people. Technologists think of an hermetic seal, while it has a mystical connotation to others.

Illustration of Hermes by John Flaxman (1755-1826) for Homer's Odyssey (detail), via Wikimedia Commons Note the winged feet and the caduceus, the god's symbol. The caduceus is often used (mistakenly) as a symbol for medicine. The proper classical symbol for medicine is the Rod of Asclepius, which has a single snake.

The idea of the god, Mercury, being a messenger is the reason behind the name of the MESSENGER spacecraft. The name, which is in all capitals, stands for Mercury Surface, Space Environment, Geochemistry and Ranging. MESSENGER will attempt to establish an orbit about the planet Mercury on March 17.

This is not the first time this particular spacecraft has been to Mercury, but it's only the second spacecraft to visit. Our first close view of Mercury happened on March 29, 1974, when the Mariner 10 spacecraft passed the shadowed side of Mercury at less than 450 miles range. This was several weeks after its flyby of Venus that occurred on February 5, 1974.

Mariner 10 was designed to orbit the Sun and make multiple encounters with Mercury at every second orbit. Mariner 10 reached Mercury again on September 21, 1974, viewing its southern hemisphere; and once again on March 16, 1975, viewing its northern hemisphere. This last flyby was at a range of about 200 miles, after which the spacecraft's fuel supplies were depleted, and the mission was over.

US ten cent stamp commemorating Mariner 10. An image of Mercury is on the left, and an image of Venus is shown on the right. The sun is in the background. (Via Wikimedia Commons).

One scientific controversy solved by Mariner 10 was Mercury's rotation period. It had always been believed that Mercury was tidally locked to the sun, so that it would always have one face pointed sunwards, and the other always shadowed. This would be nice for manned exploration, were that ever to occur. Modern observations showed that the "darkside" of Mercury was too hot for that to be the case; and it was confirmed by Mariner 10 that Mercury makes three full rotations for every two obits, so its tidal locking is 3:2 instead of 1:1.

MESSENGER flew by Mercury first on January 14, 2008, and then again on October 6, 2008, and September 29, 2009. During a planned encounter, just a few days away on March 17, MESSENGER will enter orbit around Mercury for a planned year of observation. The spacecraft will make a fourteen minute engine burn at 8:45 PM (EDT). High resolution mapping of Mercury's surface will begin around April fourth.

The first flyby detected large amounts of water in Mercury's albeit tenuous atmosphere, which was an unexpected finding. Imaging showed recent volcanism, as well, so Mercury should have a molten core. Another probe to Mercury, BepiColombo, is in preparation for a 2014 launch. This joint European Space Agency and Japan Aerospace Exploration Agency mission was to include a lander, but the lander was canceled for money reasons.

Image of Mercury taken by Messenger on September 29, 2009, during its third flyby of the planet. This image shows craters whose rims are just visible after being filled with lava. (Via Wikimedia Commons).

Although Mercury is very close to the Sun, the shadowed portion of the planet can go as low in temperature as 90 K (-185 °C). The hot side can get as hot as 700 K (425 °C). However, just as for the Moon, there are craters at the poles that have been shaded since they were first made, and there are indications that they may harbor water ice. Maybe there's a chance for some human presence on Mercury, after all, but probably not in this century. We can't even make it back to our own moon, a scant quarter of a million miles away.

References:

1. Mission page for MESSENGER, NASA.
2. Clara Moskowitz, "NASA's Messenger Spacecraft Gears Up to Orbit Mercury," Space.com, March 11, 2011.
3. Mercury Exploration page on Wikipedia.