Mercury is the smallest and densest of the four inner planets, the one with the oldest surface and the biggest extremes in daily temperature. It is also the least explored; the only spacecraft to have visited Mercury was Mariner 10 in 1974. New exploration of Mercury will begin now that the Messenger spacecraft, launched in August 2004, has reached the innermost planet of our solar system. The spacecraft took a most extraordinary route in getting there! Messenger started out at the orbit of the Earth, and had to change its speed considerably in order to match that of Mercury, which is much closer to the Sun and moving much more quickly. Messenger followed a great looping orbit around the Sun, ending up at the spot where Mercury was four years later. To do this, it passed closely by Venus twice on its way inwards; the gravitational force of Venus changed Messenger's orbit enough to send it further inwards towards Mercury. Once it reached Mercury, Messenger used that planet's gravity to loop past the planet (three times in 2008 and 2009), mapping the entire planet in full colour and measuring its magnetic field and atmosphere. A course correction will place it in orbit around Mercury in March 2011. Messenger's 12-month orbit will take two Mercury 'days'; one Mercury day, from sunrise to sunrise, is about 176 Earth days long. (Mercury rotates very slowly). From its orbit above Mercury, Messenger will continue to send data and images from that planet back to Earth for many years. Instrumentation on the spacecraft includes the following: Imaging Systems to take colour and black&white photographs Gamma Ray, Neutron & X-Ray Spectrometers to analyze surface and crust composition Magnetometer to measure the planet's magnetic field Laser Altimeter to measure the height of surface features Atmosphere and Surface Spectrometers to identify atmospheric composition and search for minerals Doppler Radar to determine Mercury's mass distribution As it was leaving Earth in August 2005, the Messenger spacecraft took a series of photographs of Earth with a wide-angle lens; these images, comprising one full rotation of Earth, were taken as the spacecraft moved from 65,000 km above the planet to more than 435,000 km away (farther than the moon). The images were made into a movie, which you can see at the right; a larger and more detailed version is available at the NASA site (see the link below). A spectacular view of Mercury's hidden side is shown below. This is the first of many images that will be received from the Messenger spacecraft. It shows the side of Mercury that we have never seen before. Mercury is difficult to view from Earth, since it's so close to the sun. When the Mariner 10 probe visited Mercury in 1974-1975, only one side of the body was facing sunlight. Mercury rotates three times during every two orbits, so the same side of the planet is illuminated every other time it is nearest to the sun. We've never before been able to see this side. Visit the NASA Messenger site at  http://messenger.jhuapl.edu/  for more information.

Mercury

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