Monday, December 6, 2010

Mercury: MESSENGER Spacecraft in Future Headlines

Paradoxically, Mercury is the hardest planet to land on (or in) in the Solar System.  Only a single mission from Earth has flown by Mercury, the Mariner 10 spacecraft launched by NASA in 1973.  Mariner 10 used a flyby of Venus  to bend its orbital path inward to Mercury and to shorten its orbital period, eventually placing the spacecraft in an orbit around the Sun that repeatedly passed by Mercury at close range.  That orbit had a period of 176 days, exactly two Mercury years, assuring that it would fly by Mercury at close range that often.  Because Mercury is locked into a resonance with the Sun, the planet rotates exactly 3 times every 2 Mercury years—meaning that the exact same face of Mercury is presented to the Sun (and illuminated for imaging purposes) on every flyby date.  Mariner 10 carried out three brief high-speed flybys of Mercury (and one of Venus) during its operational lifetime.

Placing a spacecraft in orbit around Mercury is made very difficult by the high speed of a fly-by spacecraft.  After falling in the Sun’s gravitational field in from Earth’s orbit to Mercury’s orbit, the spacecraft is traveling so fast that it would require impractically large masses of rocket propellant to slow it down enough to be captured.  Landing is even harder, since it requires additional propellant to resist the planet’s gravitational field and decelerate to zero velocity upon arrival at the planetary surface. Instead, other deceleration methods need to be undertaken.

A second spacecraft mission, MESSENGER, uses planetary swing-bys to slow the probe. MESSENGER will soon arrive at Mercury after a 6.6 year journey and no less than six planetary flybys.  After its launch in August 2004, there were close encounters with Earth (August 2005), Venus (October 2006 and June 2007), and Mercury (January and October 2008 and September 2009), putting MESSENGER into an orbit around the Sun that will arrive at Mercury with a low relative speed on March 18, 2011.  MESSENGER will then fire its rocket engine and drop into orbit around Mercury.  The orbit will be eccentric (to permit both large-scale mapping and close-up imagery) and highly inclined (to permit coverage of the entire surface as Mercury rotates beneath the spacecraft’s orbit). 

Even with this clever trajectory design, most of MESSENGER’s launched mass (55%) had to be rocket propellant. By the end of the mission it will have been used for mid-course trajectory tweaks, to slow it down, and to bring about orbital insertion, and to maintain the desired orbit. By far the most costly of these maneuvers in terms of fuel consumption is orbital insertion. To keep posted on this exciting mission consult the website maintained by Johns Hopkins University’s Applied Physics Laboratory, the managers of the mission:  
 If you are interested in the instruments carried to Mercury to explore its atmosphere, magnetic and gravitational fields, radiation environment, and surface composition and structure, go the The Mission and click on Instruments on this same site.

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