MESSENGER Mission News
July 18, 2007


Mercury's orbit is so close to the Sun that we can see it from Earth only just before sunrise or just after sunset. The best time to glimpse the planet is during one of the six or so times a year when it is at what is called "greatest elongation," referring to the angle between the Sun and a planet as seen from the Earth.

Mercury reaches its greatest western elongation Friday, and hosts of the Earth & Sky radio program will discuss this prime viewing opportunity on their show tomorrow, July 19. Mercury, the innermost planet and MESSENGER's main target, will swing to its greatest distance from the Sun Friday morning. "Even so, Mercury is rising less than 1.5 hours before the Sun now," says Earth & Sky's writer Bruce McClure. "If you're up for the challenge, these July mornings present an opportunity to catch Mercury before sunrise."

After July 20, Mercury's remaining elongation days this year are September 29 (in the eastern sky) and November 8 (in the western sky). Earth & Sky's radio program on the topic will be available online tomorrow at

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.