Messenger of the Gods

MESSENGER is a NASA spacecraft whose mission is to study the planet Mercury from orbit. It should make its first flyby of the planet in 2008, and eventually enter a stable orbit in 2011. In the mean time, it has to get there, and it does so by making “gravitational assist” maneuvers near Earth and Venus. Because the spacecraft has to lose energy to go from Earth’s orbit to Mercury’s, it flies by Earth and Venus on their interior sides; that is, on the sides facing the sun. By doing this, it loses a little bit of its angular momentum to the planet, resulting in a lower energy orbit that brings it closer to the Sun, and hence closer to Mercury. (Note: spacecraft headed toward the outer solar system will do just the opposite, flying by the exterior side of Earth, Mars, or Jupiter in order to steal some of the planet’s angular momentum, and gain a higher radius, higher energy orbit. Since the ratio of spacecraft mass to planet mass is extremely small, the change in the planet’s orbit is not significant.) This clever way of navigating the solar system saves propellant, and likewise a little bit of taxpayer money. (the mission costs < $300 million, a bargain for an interplanetary mission)

Last month, while we were busying ourselves with our hectic lives involving family, friends, work, food, driving on the freeway, and television, the MESSENGER spacecraft was making just such a maneuver near the Earth. While it was leaving the vicinity of our planet, it took a series of photographs with its wide angle camera. These images have been strung together to form a little movie of the Earth as seen by the spacecraft, as it goes from near the orbit of geosynchronous satellites to past the orbit of the Moon, in a little less than 24 hours time.

Views like this are one of the reasons I love space exploration. http://messenger.jhuapl.edu/the_mission/flyby_movie.html