At 6:15 GMT (2:15 EDT) this morning the Messenger spacecraft was launched on its way to Mercury, the least studied by spacecraft of the four terrestrial planets. Messenger will fly by Mercury three times, with the first flyby occuring in January of 2008, before it enters an orbit around Mercury in March of 2011. The spacecraft will map the planet and characterize the planet's structure, composition, and geological history. Along the way the spacecraft's investigators hope to solve several puzzles about the planets composition and magnetic field.
Mercury is a quarter of the distance from the sun as Earth, and its mass is 0.06 that of Earth. It orbits the Sun in 88 days, and its rotation period is two-thirds of its orbital period; Earth and Mercury come into inferior conjunction every 116 days.
A Delta 2 rocket successfully launches the Messenger spacecraft into space at 6:15:56 GMT from the Cape Canaveral Air Force Station, Florida, USA. Photo credit: Courtesy NASA.
Mercury is a sun-baked version of the Moon. Mercury's small size and nearness to the sun makes the permanent retention of an atmosphere impossible. The slight atmosphere that it has is composed of gases temporarily captured from the solar wind and volatiles release by the planet. The density of the atmosphere is so low that it is collisionless, so that the atoms and molecules of the atmosphere bounce along the planet's surface.
Mercury's rotation axis is not inclined to the planet's orbital axis, so that the floors of craters at the planet's poles are never exposed to the sun. Radar observations of the planet found the signature of ice in these craters. Where the water came from is no mystery: it came from comets, the same source for the water on Earth. The importance of ice on Mercury, beyond the peculiar notion of ice on the planet closest to the sun, is that we have one more measure of how ice formed in the outer reaches of the solar system and then propagated inward to the orbits of the terrestrial planets, where it was captured by those planets.
Mariner 10 observed a magnetic field around Mercury. The field is not well characterized, but just its presence is a surprise. The dynamo theory of magnetic field generation, which is the theory that describes Earth's magnetic field, requires the interior of a planet to be a convective liquid conductor. A planet the size of Mercury is not expected to have a liquid interior, so either some mechanism has kept the interior liquid, or the magnetic field has a non-dynamo source.
The Mariner spacecrafts's view of the planet Mercury as it flys away from the planet. Photo credit: Courtesy NASA.
Mercury is slightly denser than Earth, but this slight difference implies a radically different composition: Mercury has a much larger core of metals such as iron and nickel than either Earth or Venus. The precise measure of this composition provides information about how chemicals in the accretion disk surounding the early sun precipitatied to form the terrestrial planets.
Messenger carries two cameras, an altimeter, several spectrometers that operate in the infrared, optical ultraviolet, and x-ray wavelengths, a gamma-ray and neutron spectrometer, and a particle spectrometer that measures the characteristics of plasma particles. With these instruments Messenger will map the surface of the planet, determine the composition of Mercury's crust, and characterize the planet's magnetosphere. By measuring the doppler shift of radio signals from the probe as it orbits the planet, researhers will measure the distribution of mass within the planet.
The probe has a long journey ahead of it: it will make three passes by Mercury before finally falling into orbit about Mercury. This is dictated by the economics of rocketry. Rather than place a large amount of rocket fuel on board the spacecraft, which would require a much larger rocket booster, the flight planners decided to use passes of Earth, Venus, and Mercury as the mechanism for loosing the large amount of energy required to drop into Mercury's orbit about the Sun. With each planetary encounter, Messenger will transfer some of its energy to that planet. The amount of energy given to these planets is three time the energy required for a probe the size of Messenger to escape the solar system.
So we wish Messenger well, and we look forward to what this Rip van Winkle sends back to earth.