MESSENGER is short for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

The MLA (Mercury Laser Altimeter) laser onboard MESSENGER is originally designed to map the rocky topography of Mercury but it is also suited to study the clouds of Venus. The laser will shoot a beam with a wavelength of 1,064 nanometers (where one nanometer is equal to one-billionth of a meter), and with an electrical power rating of 20 Watts. Four receiver telescopes, which surround the MLA laser, will receive the beams as they return to the spacecraft.

Scientists are not sure whether the laser will be powerful enough to penetrate deeply enough into the atmosphere of Venus to return any useful data. However, they are hopeful that the laser will be successful. In any case, it is a good test of the laser’s ability before its real mission is carried out around Mercury in 2011.

Besides the laser experiment, scientists will also perform other experiments using high-resolution cameras, spectrometers, a particle counter, and a magnetometer aboard MESSENGER.

The European Space Agency’s (ESA's) Venus Express spacecraft, which is already orbiting Venus, will work with MESSENGER to study the planet’s atmosphere, cloud structure, and the interaction of Venus with the solar wind.

MESSENGER is part of NASA’s Discovery Program. The Applied Physics Laboratory (at Johns Hopkins University, Maryland) built and operates MESSENGER for NASA’s Science Mission Directorate (Washington, D.C.). The spacecraft was launched from a Boeing Delta II rocket at Cape Canaveral Air Force Station, Florida, at 2:15:56 a.m. EDT on August 3, 2004. In all, it will perform one flyby of Earth, two of Venus, and three flybys of Mercury before going into orbit around Mercury on March 18, 2011.

On June 5th, when it performs its second flyby of Venus, the MESSENGER spacecraft will use the gravity of the planet to slow it down enough—from 36.5 to 27.8 kilometers per second (22.7 to 17.3 miles per second)—so that it will eventually become an orbiting body around Mercury. In addition, all of its instruments will be turned on for the first time as a test before it arrives at Mercury.

Venus, the second closest planet to the Sun, is normally seen in the western sky at sunset. Venus orbits the Sun every 224.7 days. Its maximum elongation from the Sun is about 48 degrees. The planet is often referred to as the Evening Star and the Morning Star. Venus has been widely explored both on the ground and from space. The first unmanned space probe to successfully investigate Venus was the NASA Mariner 2 in 1962. It passed about 35,000 kilometers (21,700 miles) from Venus’ surface. Up through 1985, the U.S.S.R. Venera 3 through 16, U.S. Mariner 5 and 10, U.S. Pioneer Venus Orbiter and Multiprobe, and U.S.S.R. Vega were sent to Venus. Since 1986, the U.S. Magellan and ESA Venus Express have explored Venus. 

Future missions to Venus include the Japan PLANET-C, in 2010.

Mercury is the smallest planet in the solar system. It orbits the Sun in 88 days, as opposed to the Earth’s 365 days. The only spacecraft to visit Mercury was the NASA spacecraft Mariner 10. The spacecraft, launched on November 3, 1973 aboard an Atlas-Centaur rocket at Cape Canaveral, Florida, measured the atmosphere, surface, and general physical characteristics of Mercury during three flybys of the planet between 1974 and 1975. 

Another future mission to Mercury is the European Union/Japan BepiColombo, in 2019.

The JHU Applied Physics Laboratory website of MESSENGER is http://messenger.jhuapl.edu/ and the NASA website is http://www.nasa.gov/mission_pages/messenger/main/.

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