Mercury is the smallest and innermost planet in the solar system, orbiting the Sun at 43.5 million miles (70 million kilometres), or roughly half the distance from the Earth to the Sun. Although a genuine planet, visually it most resembles our own Moon: a scarred, cratered, dead world. Unlike Venus or Mars, the other planets in the inner solar system, Mercury has not been extensively explored by human spacecraft. Only one probe, Mariner 10, has ever flown past the planet, although a second probe named Messenger is scheduled to orbit Mercury and make a lengthy study of the planet beginning in 2011.
Although it is generally accepted that the planet has no chance of having ever supported life (the current priority which is driving exploration of Mars and of several moons around gas giants Jupiter and Saturn), there are a number of interesting features of Mercury. The planet has an exceptionally large iron core, which has led to suggestions that some ancient collision resulted in the loss of much of the planet’s crust. It also has virtually no axial tilt, and therefore (unlike Earth) no seasons. The craters at Mercury’s north and south pole get no sunlight at all, and may be among the coldest places in the solar system. It has an extremely thin atmosphere of hydrogen, helium, and oxygen, which is constantly being stripped away by the solar wind and replenished by dust escaping the planet’s surface.
Reaching Venus, however, is much more difficult than travelling to nearby Mars or even to distant Jupiter. A space probe travelling into the inner solar system must accelerate to escape Earth, spiral inwards towards the Sun, and then accelerate again, fighting against both the Sun’s gravity and its own momentum, to enter orbit around Mercury. Once in Mercury’s orbit, moreover, the nearby Sun’s gravity is so strong that continual course corrections will be required to maintain position. This is the same reason, incidentally, that Mercury probably could not sustain moons of its own.
For these reasons, only one space probe has so far ventured as far into the inner system as Mercury. In 1974 and 1975, Mariner 10 made two approaches towards Mercury, building a composite map of about one-tenth of the planet’s surface. Mariner 10 was on a one-way trip to both Venus and Mercury, and ran out of fuel during its second approach to Mercury. Contact has now been lost with the spacecraft for decades, but it is probably still in solar orbit, and may even have made several subsequent approaches to within sight of Mercury.
A second, more advanced NASA probe, named Messenger, is due to be inserted into Mercury orbit in 2011. Unlike Mariner 10, Messenger is expected to spend at least one year in Mercury orbit, conducting an extensive research mission. It launched in 2004 from Cape Canaveral and is meandering inwards on a long, spiralling course intended to maximize fuel economy. This spiralling path has already brought Messenger vast Venus twice and past Mercury three times. On its fourth approach to Mercury, in 2011, it will enter the planet’s orbit. Messenger has already made headlines – in 2008, during its first flyby, it discovered water in Mercury’s thin atmosphere.
After Messenger, a third probe may eventually reach Mercury, this one built by the European and Japanese space agencies. BepiColombo is still on the drawing board, but is tentatively scheduled for a 2014 launch, which would put it at Mercury in about 2020. The original mission plan called for three research components – a main orbiter, a specialized component to study the planet’s weak magnetic field, and a lander. However, the lander has now been cancelled to keep costs down. Had the lander been kept in the project, it would have made Mercury only the third planet (and fifth planet or moon) upon which a spacecraft had been landed.