MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) is a probe designed to orbit and study Mercury, the closest planet to the sun. Planned to launch May 11, 2004, it is the eighth mission of NASA's Discovery Program.

Intrigue of Mercury

Mercury is a particularly difficult planet to study due in part to its elusive placement in our sky. Because of its proximity to the sun, Mercury is rarely present in the Earth's night sky. Using a telescope to observe Mercury can be quite risky, since catching the sun through a telescope can be damaging on the eye. The planet is only visible during 30 to 40 nights of the year. Moreover, Mercury's elliptical orbit made its trajectory through our sky seemingly unpredictable to astronomers of yesteryear, and so it has been one of the last planets for science to understand.

In 1976, NASA's Mariner 10 probe made a major discovery on its flyby of Mercury. Data suggest the probe encountered a magnetic field much larger than the Earth's. Although a small planet, only about a third the size of the Earth, Mercury's iron core is actually relatively much bigger than in the other inner planets. The foremost objective of the Messenger mission is to begin to uncover the secret of Mercury's large iron core. Proposed theories suggest that perhaps Mercury was once a larger planet whose outer crust was once stripped away from either violent solar activity or a collision with another world. Another theory states that the center of the early solar system, then just a disc-like solar nebula, contained drastically different material or formed differently due to gravitational constraints. A clearer understanding of Mercury's evolution will enhance our understanding of how other planetary systems form in the universe. The Messenger mission might yield more mysteries than it solves, although this seems to be the norm for most space missions of discovery.

Another objective of the probe is to confirm the theory that water exists in the puzzling polar regions of Mercury.  Because of Mercury's slow rotation (58 Earth days) and fast orbit around the sun (87 Earth days), an apparent day on Mercury lasts 176 Earth days.  That is, if standing on Mercury, an apparent day would last months, as the sun slowly rises to high noon over the course of several months then seems to lazily meander around the sky before finally heading towards sunset months later.  This means that day-time temperatures on Mercury reach 427°C, while the areas left dark for many Earth months fall to an astonishing -173°C.  The polar extremities of the planet never see the light of day, and recent data suggests ice may exist in these parts.

Difficulty of the mission

Inserting an orbiter around Mercury is not an easy task. Huge amounts of fuel would be needed to slow down a probe to keep it from succumbing to the sun's powerful draw. Instead, NASA decided to send Messenger on a five year journey around around the sun, moving slowly inward until finally inserting itself in an orbit around Mercury in 2009.  Messenger will take advantage of the gravitational stopping power of three Venus flybys en route to its final destination.

Solar activity, such as solar flare and radiation, is a problem for space activity around Earth, and the problem is magnified for any mission that intends to navigate as far inward as Mercury.  For this reason, the Messenger probe's hardware must be more protected than on any other NASA craft to date.  Engineers chose to use a carbon composite frame for the spacecraft in order to maximize protection from heat.  At this proximity to the sun, the typical aluminum body used in other missions would simply melt.

The Messenger probe plays host to a packed house of instrumentation. The probe features a gamma ray and neutron spectrometer, an x-ray spectrometer, magnetometer, laser altimeter, atmospheric and surface composition spectrometer, and energetic particle and plasma spectrometer.  Messenger will also take advantage of the Doppler effect with a radio emitter and receiver for navigational purposes.

Critical mission dates

  • May 11, 2004 - Launch

  • November 2, 2004 - First Venus flyby

  • August 28, 2005 - Second Venus flyby

  • October 26, 2006 - Third Venus flyby, followed by first Deep Space Maneuver

  • October 16, 2007 - First Mercury flyby, followed by second Deep Space Maneuver

  • July, 2, 2009 - Orbital insertion

  • July 2, 2010 - End of primary mission

Other NASA Discovery Program Missions
·Mars Pathfinder·
·Lunar Prospector·
·Deep Impact·

Mes"sen*ger (?), n. [OE. messager, OF. messagier, F. messager. See Message.]


One who bears a message; the bearer of a verbal or written communication, notice, or invitation, from one person to another, or to a public body; specifically, an office servant who bears messages.


One who, or that which, foreshows, or foretells.

Yon gray lines That fret the clouds are messengers of day. Shak.

3. Naut.

A hawser passed round the capstan, and having its two ends lashed together to form an endless rope or chain; -- formerly used for heaving in the cable.

4. Law

A person appointed to perform certain ministerial duties under bankrupt and insolvent laws, such as to take charge og the estate of the bankrupt or insolvent.

Bouvier. Tomlins.

Syn. -- Carrier; intelligencer; courier; harbinger; forerunner; precursor; herald.

Messenger bird, the secretary bird, from its swiftness.


© Webster 1913.

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