Friday, March 20, 2009


MESSENGER (MErcury, Surface Space ENvironment, GEochemistry, and Ranging) is a spacecraft that had flybys of Earth, Venus, and its primary destination, Mercury. Named for Mercury being the messenger god, the main objective of MESSENGER was to observe Mercury, and also map the previously unmapped side of the planet (only 40% of the planet of Mercury was mapped in the mid 1970's by Mariner 10).

Due to the closeness of Mercury to the Sun, MESSENGER would be heading directly into the Sun's more powerful gravity and therefore would need to use a significant amount of fuel to eventually orbit Mercury. To save fuel, MESSENGER instead used gravity assists from Earth and Venus to help direct it on its path. The duration of the mission expanded because of this, but it would allow MESSENGER to practice flybys and test its equipment before reaching Mercury.

MESSENGER was launched on August 4, 2004 with a Delta II rocket from Cape Canaveral. After launch, it followed the Earth around its orbit and had a flyby of Earth on August 2, 2005, getting within 1,500 miles of Earth. Shortly after its flyby with Earth, it fired thrusters, this maneuver being called Deep Space Maneuver 1, or DSM-1. Using this thrust, MESSENGER switched orbits and fell into an elliptical orbit crossing that which is Venus's. While in this orbit, MESSENGER made two flybys of Venus. One on October 24, 2006 and one on June 5, 2007. In October 2007 MESSENGER made its second maneuver, DSM-2. This put MESSENGER directly on target for its first flyby of Mercury.

Track of MESSENGER from launch in 2004 to destination in 2011.

The space probe's first flyby of Mercury occurred on January 14, 2008. On this date, MESSENGER caught its first glimpse of the unseen side of Mercury. Below is MESSENGER's first image of this unknown surface. Two subsequent other flybys of Mercury happened on October 6, 2008 and September 29, 2009 to slow down MESSENGER's speed enough so that it could orbit Mercury.

The first image MESSENGER took during the first flyby of Mercury, showing its previously unknown side.

Finally, on March 17, 2011, MESSENGER fired the thrusters necessary to be ensnared by Mercury's gravity. It began orbiting the planet the next day, and began downloading scientific information on April 4. Many stunning images of the surface have been captured, in a higher resolution than ever before!

MESSENGER quickly completed several scientific inquires, thoroughly photographing the unknown side of Mercury, finding water in its atmosphere and possibly detecting a liquid core. Further experiments were conducted concerning the magnetosphere, as Mercury has the most volatile magnetosphere known in the Solar System.

After observing its cycle, MESSENGER determined that the magnetic field has a curious instability and asymmetry to it that is not found in another other known planetary body. The field is more concentrated to the north, resulting in very different geological formations in the north versus the south polar regions. The north polar region contains plains, with relative protection from erosion, (see below) while the south is open to a fierce bombardment of particles.

By observing the structure of craters on Mercury, MESSENGER also has indirectly determined the nature of Mercury's surface, as impacts of similar objects create different craters on different planetary bodies.

The north polar plains of Mercury in real-color (top), and false-color highlighting different rock types (bottom).

In November 2011, NASA extended the MESSENGER mission an entire year beyond its end date of March 17, 2012, in order to collect further data concerning the outer atmosphere and volcanic activity early in the planet's history. Also, this extension allowed MESSENGER to observe the effect on Mercury of the 2013 solar maximum, or a local peak in solar activity.

In addition, to facilitate more detailed observations, MESSENGER completed two orbital thrusts between April 16 and April 20, 2012 that shortened its orbital period from 11.6 hours to 8.0 hours. In its new position, the probe had more time at low altitudes, from where geologic and magnetic activity could be observed for longer periods at a time.

From its new vantage point, MESSENGER gathered enough data for scientists at NASA to make a groundbreaking announcement: there appears to be water (in the form of ice) on Mercury. Several pieces of evidence support this claim, including the fact that Mercury's near-zero axial tilt keeps many crater basins near the poles perpetually in shadow (and thus below freezing), and that chemical analyses suggested unusually high hydrogen concentrations in the same polar regions. However, perhaps the most compelling evidence is the reflectivity of these supposed ice deposits.

Areas highlighted in yellow illustrate high reflectivity, or albedo, of certain crater basins, precisely where the ice deposits would persist.

On December 30, 2012, MESSENGER captured an image of the last part of Mercury's previously unknown surface, mapping, for the first time, all 100% of the surface in daylight. This allowed the compilation of global mosaics, such as the Mercator projection below (obviously, just as with Earth maps of this type, the features near the poles are elongated).

MESSENGER's orbit continued to gradually tighten about the planet due to the influence of the Sun's gravity, and in April 2014 had its closest approach yet to the planet, dropping to an altitude of only 123.7 miles at its periapsis. In June 2014, MESSENGER made another altitude adjustment, raising its periapsis to extend the lifetime of its orbit about Mercury. Even with this adjustment, MESSENGER's orbit began to degrade over time. However, MESSENGER's decreasing altitude allowed more detailed and precise examination of Mercury than ever before. By August 3, 2014, on which date the probe had spent 10 years in space, the closest approach to Mercury had fallen below 62 miles (100 kilometers).

By September 12, the orbit had shrunk to a mere 15 miles to the surface at closest approach! This allowed high resolution images such as the one above, with only 6 meters per pixel. The smoothness of the landscape in the above image reflects past pyroclastic flow across the region. The spacecraft then performed the second of four orbital maneuvers to raise the periapsis and maintain orbit about Mercury. In October, the probe underwent another such maneuver. However, though MESSENGER's propellant was scheduled to run out in March 2015, scientists on the program's propulsion team devised a plan to use the helium which pressurized portions of the spacecraft as a makeshift fuel to extend the mission for a few additional weeks. The first application of this plan was executed successfully in January 2015, using a combination of the remaining propellant and pressurized helium to adjust MESSENGER's orbit.

Over the following months, the spacecraft took advantage of its low altitude to initiate a "hover campaign" in which the MESSENGER's magnetometer and neutron spectrometer would take observations at very low altitudes. The probe also completed its 4000th orbit of Mercury on March 27. In early April, another maneuver raised the periapsis of MESSENGER's orbit, before which it had sunk to a closest approach altitude of only 3.7 miles! After a few additional such maneuvers throughout April, the inexorable gravitational force of the Sun ultimately won out, causing the probe's anticipated crash into Mercury's surface on April 30, 2015. At this point, MESSENGER had spent well over ten years in space and four years in orbit of the Solar System's innermost planet, more than twice the original mission plan!

MESSENGER's images and data led to a enormous variety of crucial discoveries in addition to those mentioned above. For example, the probe discovered that Mercury is in fact shrinking over time (albeit very slowly) as its core cools and compresses.

The diagonal ridge in the above image was formed when Mercury's crust buckled in on itself.

The MESSENGER mission was innovative, efficient, and prolific in its results. The probe employed more inner solar system gravity assists than any prior mission, continued to operate months after its supply of propellant was exhausted, and generated many terabytes of scientific data along with over a quarter-million images as the first ever Mercury orbiter. MESSENGER revolutionized our understanding of the innermost planet.

For more info, see the MESSENGER main page.

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