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Mercury (planet)

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   CAPTION: Mercury Astronomical symbol of Mercury

   Mercury
          Orbital characteristics ( Epoch J2000)
   Avg. distance from Sun    57,909,176 km
                             0.387 098 93 AU
   Orbital circumference     360,000,000 km
                             (2.406 AU)
   Eccentricity              0.205 630 69
   Perihelion                46,001,272 km
                             0.307 499 51 AU
   Aphelion                  69,817,079 km
                             0.466 698 35 AU
   Orbital period            87.969 34 d
                             (0.240 846 9 a)
   Synodic period            115.8776 d
   Avg. Orbital Speed        47.36 km/ s
   Max. Orbital Speed        58.98 km/ s
   Min. Orbital Speed        38.86 km/ s
   Inclination               7.004 87 °
                             (3.38° to Sun's equator)
   Longitude of the
   ascending node            48.331 67°
   Argument of the
   perihelion                29.124 78°
   Number of satellites      0
                 Physical characteristics
   Equatorial diameter       4879.4 km
                             (0.383 Earths)
   Surface area              7.5×10^7 km²
                             (0.147 Earths)
   Volume                    6.083×10^10 km³
                             (0.056 Earths)
   Mass                      3.302×10^23 kg
                             (0.055 Earths)
   Mean density              5.427 g/cm³
   Equatorial gravity        3.701 m/s²
                             (0.377 gee)
   Escape velocity           4.435 km/s
   Rotation period           58.6462 d (58 d 15.5088 h)
   Rotation velocity         10.892 km/h (at the equator)
   Axial tilt                ~0.01°
   Right ascension
   of North pole             281.01° (18 h 44 min 2 s) ^1
   Declination               61.45°
   Albedo                    0.10-0.12
   Surface temp.
                             min  mean   max
                             90 K 440 K 700 K
   Avg. Surface temp.: Day   623 K
   Avg. Surface temp.: Night 103 K
   Adjective                 Mercurian
                Atmospheric characteristics
   Atmospheric pressure      trace
   Potassium                 31.7%
   Sodium                    24.9%
   Atomic Oxygen             9.5%
   Argon                     7.0%
   Helium                    5.9%
   Molecular Oxygen          5.6%
   Nitrogen                  5.2%
   Carbon dioxide            3.6%
   Water                     3.4%
   Hydrogen                  3.2%

   Mercury ( IPA: /ˈmɛːkjəri/) is the innermost and smallest planet in the
   solar system, orbiting the Sun once every 88 days. It ranges in
   brightness from about −2.0 to 5.5 in apparent magnitude, but is not
   easily seen — its greatest angular separation from the Sun (greatest
   elongation) is only 28.3° (it can only be seen in twilight).
   Comparatively little is known about the planet: the only spacecraft to
   approach Mercury was Mariner 10 from 1974 to 1975, which mapped only
   40%–45% of the planet's surface.

   Physically, Mercury is similar in appearance to the Moon as it is
   heavily cratered. It has no natural satellites and no substantial
   atmosphere. The planet has a large iron core which generates a magnetic
   field about 1% as strong as that of the Earth. Surface temperatures on
   Mercury range from about 90 to 700  K (-180 to 430°C) , with the
   subsolar point being the hottest and the bottoms of craters near the
   poles being the coldest.

   The Romans named the planet after the fleet-footed messenger god
   Mercury, probably for its fast apparent motion in the twilight sky. The
   astronomical symbol for Mercury, displayed at the top of the infobox,
   is a stylized version of the god's head and winged hat atop his
   caduceus, an ancient astrological symbol. Before the 5th century BC,
   Greek astronomers believed the planet to be two separate objects: one
   visible only at sunrise, the other only at sunset. In India, the planet
   was named Budha (बुध), after the son of Chandra (the Moon). The
   Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet
   as the water star, based on the Five Elements. The Hebrews named it
   Kokhav Hamah (כוכב חמה), "the star of the hot one" ("the hot one" being
   the Sun).

Structure

   Mercury is one of the four terrestrial planets, meaning that like the
   Earth it is a rocky body. It is the smallest of the four, with a
   diameter of 4879 km at its equator. Mercury consists of approximately
   70% metallic and 30% silicate material. The density of the planet is
   the second-highest in the solar system at 5.43 g/cm³, only slightly
   less than Earth's density. When corrected for gravitational
   compression, Mercury is in fact denser than Earth, with an uncompressed
   density of 5.3 g/cm³ versus Earth's 4.4 g/cm³.

Internal structure: core, mantle and crust

   Diagram showing Mercury's large core
   Enlarge
   Diagram showing Mercury's large core

   Mercury's high density can be used to infer details of its inner
   structure. While the Earth's high density results partly from
   compression at the core, Mercury is much smaller and its inner regions
   are not nearly so compressed. Therefore, for it to have such a high
   density, its core must be large and rich in iron. Geologists estimate
   that Mercury's core occupies about 42% of its volume. (Earth's core
   occupies about 17% of its volume.)

   Surrounding the core is a 600 km mantle. It is generally thought that
   early in Mercury's history, a giant impact with a body several hundred
   kilometers across stripped the planet of much of its original mantle
   material, resulting in the relatively thin mantle compared to the
   sizable core (alternative theories are discussed below).

   Mercury's crust is thought to be about 100–200 km thick. One very
   distinctive feature of Mercury's surface is numerous ridges, some
   extending over several hundred kilometers. It is believed that these
   were formed as Mercury's core and mantle cooled and contracted after
   the crust had solidified.

   Mercury has a higher iron content than any other major planet in our
   solar system. Several theories have been proposed to explain Mercury's
   high metallicity. The most widely accepted theory is that Mercury
   originally had a metal-silicate ratio similar to common chondrite
   meteors and a mass approximately 2.25 times its current mass; but that
   early in the solar system's history, Mercury was struck by a
   planetesimal of approximately 1/6 that mass. The impact would have
   stripped away much of the original crust and mantle, leaving the core
   behind. A similar theory has been proposed to explain the formation of
   Earth's Moon (see giant impact theory).

   Alternatively, Mercury may have formed from the solar nebula before the
   Sun's energy output had stabilized. The planet would initially have had
   twice its present mass. But as the protosun contracted, temperatures
   near Mercury could have been between 2500 and 3500 K, and possibly even
   as high as 10000 K. Much of Mercury's surface rock could have vaporized
   at such temperatures, forming an atmosphere of "rock vapor" which could
   have been carried away by the solar wind.

   A third theory suggests that the solar nebula caused drag on the
   particles from which Mercury was accreting, which meant that lighter
   particles were lost from the accreting material. Each of these theories
   predicts a different surface composition, and two upcoming space
   missions, MESSENGER and BepiColombo, both aim to take observations that
   will allow the theories to be tested.

Surface

   Mercury's surface is very similar in appearance to that of the Moon,
   showing extensive mare-like plains and heavy cratering, indicating that
   it has been geologically inactive for billions of years. The small
   number of unmanned missions to Mercury means that its geology is the
   least well understood of the terrestrial planets. Surface features are
   given the following names:
     * Albedo features — areas of markedly different reflectivity
     * Dorsa — ridges (see List of ridges on Mercury)
     * Montes — mountains (see List of mountains on Mercury)
     * Planitiae — plains (see List of plains on Mercury)
     * Rupes — scarps (see List of scarps on Mercury)
     * Valles — valleys (see List of valleys on Mercury)

   During and shortly following the formation of Mercury, it was heavily
   bombarded by comets and asteroids for a period that came to an end 3.8
   billion years ago. During this period of intense crater formation, the
   planet received impacts over its entire surface, facilitated by the
   lack of any atmosphere to slow impactors down. During this time the
   planet was volcanically active; basins such as the Caloris Basin were
   filled by magma from within the planet, which produced smooth plains
   similar to the maria found on the Moon.
   Mercury's Caloris Basin is one of the largest impact features in the
   Solar System.
   Enlarge
   Mercury's Caloris Basin is one of the largest impact features in the
   Solar System.

   Craters on Mercury range in diameter from a few meters to hundreds of
   kilometers across. The largest known crater is the enormous Caloris
   Basin, with a diameter of 1300 km. The impact which created the Caloris
   Basin was so powerful that it caused lava eruptions and left a
   concentric ring over 2 km tall surrounding the impact crater. At the
   antipode of the Caloris Basin is a large region of unusual, hilly
   terrain known as the "Weird Terrain". It is believed that shock waves
   from the impact traveled around the planet, and when they converged on
   the antipodal point of the impact caused extensive fracturing of the
   surface there.
   The so-called "Weird Terrain" was formed by the Caloris Basin impact at
   its antipodal point.
   Enlarge
   The so-called "Weird Terrain" was formed by the Caloris Basin impact at
   its antipodal point.

   The plains of Mercury have two distinct ages: the younger plains are
   less heavily cratered and probably formed when lava flows buried
   earlier terrain. One unusual feature of the planet's surface is the
   numerous compression folds which crisscross the plains. It is thought
   that as the planet's interior cooled, it contracted and its surface
   began to deform. The folds can be seen on top of other features, such
   as craters and smoother plains, indicating that they are more recent.
   Mercury's surface is also flexed by significant tidal bulges raised by
   the Sun—the Sun's tides on Mercury are about 17% stronger than the
   Moon's on Earth.

   Like the Moon, the surface of Mercury has likely incurred the effects
   of space weathering processes. Solar wind and micrometeorite impacts
   can darken the albedo and alter the reflectance properties of the
   surface.

   The mean surface temperature of Mercury is 452  K (353.9°F, 178.9°C),
   but it ranges from 90 K (-297.7°F, -183.2°C) to 700 K (800.3°F,
   426.9°C); by comparison, the temperature on Earth varies by only about
   150 K. The sunlight on Mercury's surface is 6.5 times as intense as it
   is on Earth, with a solar constant value of 9.13 kW/m².

   Despite the generally extremely high temperature of its surface,
   observations strongly suggest that ice exists on Mercury. The floors of
   some deep craters near the poles are never exposed to direct sunlight,
   and temperatures there remain far lower than the global average. Water
   ice strongly reflects radar, and observations reveal that there are
   patches of very high radar reflection near the poles. While ice is not
   the only possible cause of these reflective regions, astronomers
   believe it is the most likely.

   The icy regions are believed to be covered to a depth of only a few
   meters, and contain about 10^14–10^15 kg of ice. By comparison, the
   Antarctic ice sheet on Earth weighs about 4×10^18 kg, and Mars' south
   polar cap contains about 10^16 kg of water. The origin of the ice on
   Mercury is not yet known, but the two most likely sources are from
   outgassing of water from the planet's interior or deposition by impacts
   of comets.

Atmosphere

   Size comparison of terrestrial planets (left to right): Mercury, Venus,
   Earth, and Mars
   Enlarge
   Size comparison of terrestrial planets (left to right): Mercury, Venus,
   Earth, and Mars

   Mercury is too small for its gravity to retain any significant
   atmosphere over long periods of time; it has a tenuous atmosphere
   containing hydrogen, helium, oxygen, sodium, calcium and potassium. The
   atmosphere is not stable—atoms are continuously lost and replenished,
   from a variety of sources. Hydrogen and helium atoms probably come from
   the solar wind, diffusing into Mercury's magnetosphere before later
   escaping back into space. Radioactive decay of elements within
   Mercury's crust is another source of helium, as well as sodium and
   potassium. Water vapor is probably present, being brought to Mercury by
   comets impacting on its surface.

Magnetic field

   Despite its slow rotation, Mercury has a relatively strong magnetic
   field, with a magnetic field strength 1% as strong as the Earth's. It
   is possible that this magnetic field is generated in a manner similar
   to Earth's, by a dynamo of circulating liquid core material. However,
   scientists are unsure whether Mercury's core could still be liquid,
   although it could perhaps be kept liquid by tidal effects during
   periods of high orbital eccentricity. It is also possible that
   Mercury's magnetic field is a remnant of an earlier dynamo effect that
   has now ceased, with the magnetic field becoming "frozen" in solidified
   magnetic materials.

   Mercury's magnetic field is strong enough to deflect the solar wind
   around the planet, creating a magnetosphere inside which the solar wind
   does not penetrate. This is in contrast to the situation on the Moon,
   which has a magnetic field too weak to stop the solar wind impacting on
   its surface and so lacks a magnetosphere.

Orbit and rotation

   Orbit of Mercury (yellow). Orbit of Mercury as seen from the ascending
   node (bottom) and from 10° above (top).

   The orbit of Mercury is the most eccentric of the major planets, with
   the planet's distance from the Sun ranging from 46,000,000 to
   70,000,000 kilometers. It takes 88 days to complete the orbit. The
   diagram on the left illustrates the effects of the eccentricity,
   showing Mercury’s orbit with a circular orbit with the same semi-major
   axis. The higher velocity of the planet when it is near perihelion is
   clear from the greater distance it covers in each 5-day interval. The
   size of the spheres, inversely proportional to their distance from the
   Sun, illustrates the varying heliocentric distance. This varying
   distance to the Sun, combined with a unique 2:3 resonance of the
   planet's rotation around its axis, result in complex variations of the
   surface temperature.

   Mercury's orbit is inclined by 7° to the plane of Earth's orbit (the
   ecliptic), as shown in the diagram on the left. As a result, transits
   of Mercury across the face of the Sun can only occur when the planet is
   crossing the plane of the ecliptic at the time it lies between the
   Earth and the Sun. This occurs about every seven years on average.

   Mercury's axial tilt is only 0.01 degrees. This is over 300 times
   smaller than that of Jupiter, which is the second smallest axial tilt
   of all planets at 3.1 degrees. This means an observer at Mercury's
   equator during local noon would never see the sun more than 1/100 of
   one degree north or south of the zenith.

   At certain points on Mercury's surface, an observer would be able to
   see the Sun rise about halfway, then reverse and set before rising
   again, all within the same Mercurian day. This is because approximately
   four days prior to perihelion, Mercury's angular orbital velocity
   exactly equals its angular rotational velocity so that the Sun's
   apparent motion ceases; at perihelion, Mercury's angular orbital
   velocity then exceeds the angular rotational velocity. Thus, the Sun
   appears to be retrograde. Four days after perihelion, the Sun's normal
   apparent motion resumes.

Advance of perihelion

   When it was discovered, the slow precession of Mercury's orbit around
   the Sun could not be completely explained by Newtonian mechanics, and
   for many years it was hypothesized that another planet might exist in
   an orbit even closer to the Sun to account for this perturbation (other
   explanations considered included a slight oblateness of the Sun). The
   success of the search for Neptune based on its perturbations of Uranus'
   orbit led astronomers to place great faith in this explanation, and the
   hypothetical planet was even named Vulcan. However, in the early 20th
   century, Albert Einstein's General Theory of Relativity provided a full
   explanation for the observed precession. Mercury's precession showed
   the effects of mass dilation, providing a crucial observational
   confirmation of one of Einstein's theories—Mercury is slightly heavier
   at perihelion than it is at aphelion because it is moving faster, and
   so it slightly "overshoots" the perihelion position predicted by
   Newtonian gravity. The effect is very small: the Mercurian relativistic
   perihelion advance excess is just 43 arcseconds per century. The effect
   is even smaller for other planets, being 8.6 arcseconds per century for
   Venus, 3.8 for Earth, and 1.3 for Mars.

   Research indicates that the eccentricity of Mercury's orbit varies
   chaotically from 0 (circular) to a very high 0.47 over millions of
   years. This is thought to explain Mercury's 3:2 spin-orbit resonance
   (rather than the more usual 1:1), since this state is more likely to
   arise during a period of high eccentricity.

Spin-orbit resonance

   After one orbit, Mercury has rotated 1.5 times, so after two complete
   orbits the same hemisphere is again illuminated.
   Enlarge
   After one orbit, Mercury has rotated 1.5 times, so after two complete
   orbits the same hemisphere is again illuminated.

   For many years it was thought that Mercury was synchronously tidally
   locked with the Sun, rotating once for each orbit and keeping the same
   face directed towards the Sun at all times, in the same way that the
   same side of the Moon always faces the Earth. However, radar
   observations in 1965 proved that the planet has a 3:2 spin-orbit
   resonance, rotating three times for every two revolutions around the
   Sun; the eccentricity of Mercury's orbit makes this resonance stable.
   The original reason astronomers thought it was synchronously locked was
   because whenever Mercury was best placed for observation, it was always
   at the same point in its 3:2 resonance, hence showing the same face.
   Due to Mercury's 3:2 spin-orbit resonance, a solar day (the length
   between two meridian transits of the Sun) lasts about 176 Earth days. A
   sidereal day (the period of rotation) lasts about 58.7 Earth days.

Observation

   Mercury's apparent magnitude varies between about -2.0 - brighter than
   Sirius - and 5.5. Observation of Mercury is complicated by its
   proximity to the Sun, as it is lost in the Sun's glare for much of the
   time. Mercury can be observed for only a brief period during either
   morning or evening twilight. The Hubble Space Telescope cannot observe
   Mercury at all.

   Mercury exhibits moonlike phases as seen from Earth, being "new" at
   inferior conjunction and "full" at superior conjunction. The planet is
   rendered invisible on both of these occasions by virtue of its rising
   and setting in concert with the Sun in each case. The half-moon phase
   occurs at greatest elongation, when Mercury rises earliest before the
   Sun when at greatest elongation west, and setting latest after the Sun
   when at greatest elongation east (its separation from the Sun ranging
   from 18.5° if it is at perihelion at the time of the greatest
   elongation to 28.3° if it is at aphelion).

   Mercury attains inferior conjunction every 116 days on average, but
   this interval can range from 111 days to 121 days due to the planet's
   eccentric orbit. Its period of retrograde motion as seen from Earth can
   vary from 8 to 15 days on either side of inferior conjunction. This
   large range also arises from the planet's high degree of orbital
   eccentricity.
   View of Mercury from Mariner 10
   Enlarge
   View of Mercury from Mariner 10

   Mercury is more often easily visible from Earth's Southern Hemisphere
   than from its Northern Hemisphere; this is because its maximum possible
   elongations west of the Sun always occur when it is early autumn in the
   Southern Hemisphere, while its maximum possible eastern elongations
   happen when the Southern Hemisphere is having its late winter season.
   In both of these cases, the angle Mercury strikes with the ecliptic is
   maximized, allowing it to rise several hours before the Sun in the
   former instance and not set until several hours after sundown in the
   latter in countries located at South Temperate Zone latitudes, such as
   Argentina and New Zealand. By contrast, at northern temperate
   latitudes, Mercury is never above the horizon of a more-or-less fully
   dark night sky. Mercury can, like several other planets and the
   brightest stars, be seen during a total solar eclipse.

   Mercury is brightest as seen from Earth when it is at a gibbous phase,
   between half full and full. Although the planet is further away from
   Earth when it is gibbous than when it is a crescent, the greater
   illuminated area visible more than compensates for the greater
   distance. The opposite is true for Venus, which appears brightest when
   it is a thin crescent.

Studies of Mercury

Early astronomers

   Mercury has been known since at least the 3rd millennium BC, when it
   was known to the Sumerians of Mesopotamia as Ubu-idim-gud-ud, among
   other names. The Babylonians (2000–1000 BC) succeeded the Sumerians,
   and early Babylonians may have recorded observations of the planet:
   although no records have survived, late Babylonian records from the 7th
   century BC refer to much earlier records. The Babylonians called the
   planet Nabu or Nebu after the messenger to the Gods in their mythology.

   The ancient Greeks gave the planet two names: Apollo when it was
   visible in the morning sky and Hermes when visible in the evening.
   However, Greek astronomers came to understand that the two names
   referred to the same body, with Pythagoras being the first to propose
   the idea.

Ground-based telescopic research

   This Mariner 10 view from 4.3 million km is similar to the very best
   views that can be achieved telescopically from Earth
   Enlarge
   This Mariner 10 view from 4.3 million km is similar to the very best
   views that can be achieved telescopically from Earth

   The first telescopic observations of Mercury were made by Galileo in
   the early 17th century. Although he observed phases when he looked at
   Venus, his telescope was not powerful enough to see the phases of
   Mercury. In 1631 Pierre Gassendi made the first observations of the
   transit of a planet across the Sun when he saw a transit of Mercury
   predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to
   discover that the planet had orbital phases similar to Venus and the
   Moon. The observation demonstrated conclusively that Mercury orbited
   around the Sun.

   A very rare event in astronomy is the passage of one planet in front of
   another ( occultation), as seen from Earth. Mercury and Venus occult
   each other every few centuries, and the event of May 28, 1737 is the
   only one historically observed, having been seen by John Bevis at the
   Royal Greenwich Observatory. The next occultation of Mercury by Venus
   will be in 2133.

   The difficulties inherent in observing Mercury mean that it has been
   far less studied than the other planets. In 1800 Johann Schröter made
   observations of surface features, but erroneously estimated the
   planet's rotational period at about 24 hours. In the 1880s Giovanni
   Schiaparelli mapped the planet more accurately, and suggested that
   Mercury's rotational period was 88 days, the same as its orbital period
   due to tidal locking. This phenomenon is known as synchronous rotation
   and is also shown by Earth's Moon.

   The theory that Mercury's rotation was synchronous became widely held,
   and it was a significant shock to astronomers when radio observations
   made in the 1960s questioned this. If Mercury were tidally locked, its
   dark face would be extremely cold, but measurements of radio emission
   revealed that it was much hotter than expected. Astronomers were
   reluctant to drop the synchronous rotation theory and proposed
   alternative mechanisms such as powerful heat-distributing winds to
   explain the observations, but in 1965 radar observations showed
   conclusively that the planet's rotational period was about 59 days.
   Italian astronomer Giuseppe Colombo noted that this value was about
   two-thirds of Mercury's orbital period, and proposed that a different
   form of tidal locking had occurred in which the planet's orbital and
   rotational periods were locked into a 3:2 rather than a 1:1 resonance.
   Data from space probes subsequently confirmed this view.

   Ground-based observations did not shed much further light on the
   innermost planet, and it was not until space probes visited Mercury
   that many of its most fundamental properties became known. However,
   recent technological advances have led to improved ground-based
   observations: in 2000, high-resolution lucky imaging from the Mount
   Wilson Observatory 60-inch telescope provided the first detailed views
   of the parts of Mercury which were not imaged in the Mariner missions.

Research with space probes

   Reaching Mercury from Earth poses significant technical challenges,
   since the planet orbits so much closer to the Sun than does the Earth.
   A Mercury-bound spacecraft launched from Earth must travel over 91
   million kilometers into the Sun's gravitational potential well.
   Starting from the Earth's orbital speed of 30 km/s, the change in
   velocity ( delta-v) the spacecraft must make to enter into a Hohmann
   transfer orbit that passes near Mercury is large compared to other
   planetary missions.

   The potential energy liberated by moving down the Sun's potential well
   becomes kinetic energy; requiring another large delta-v to do anything
   other than rapidly pass by Mercury. In order to land safely or enter a
   stable orbit since the planet has very little atmosphere, the
   approaching spacecraft cannot use aerobraking and must rely on rocket
   motors. A trip to Mercury actually requires more rocket fuel than that
   required to escape the solar system completely. As a result, only one
   space probe has visited the planet so far.

Mariner 10

   The Mariner 10 probe, the only probe yet to visit the innermost planet
   Enlarge
   The Mariner 10 probe, the only probe yet to visit the innermost planet

   The only spacecraft to approach Mercury so far has been NASA's Mariner
   10 (1974–75). The spacecraft used the gravity of Venus to adjust its
   orbital velocity so that it could approach Mercury—the first spacecraft
   to use this gravitational "slingshot" effect. Mariner 10 provided the
   first close-up images of Mercury's surface, which immediately showed
   its heavily cratered nature, and also revealed many other types of
   geological features, such as the giant scarps which were later ascribed
   to the effect of the planet shrinking slightly early in its geological
   history. Unfortunately, the same face of the planet was lit at each of
   Mariner 10's close approaches, resulting in less than 45% of the
   planet's surface being mapped.

   The spacecraft made three close approaches to Mercury, the closest of
   which took it to within 327 km of the surface. At the first close
   approach, instruments detected a magnetic field, to the great surprise
   of planetary geologists—Mercury's rotation was expected to be much too
   slow to generate a significant dynamo effect. The second close approach
   was primarily used for imaging, but at the third approach, extensive
   magnetic data were obtained. The data revealed that the planet's
   magnetic field is much like the Earth's, which deflects the solar wind
   around the planet. The Moon's magnetic field, on the other hand, is so
   weak that the solar wind reaches the surface. However, the origin of
   Mercury's magnetic field is still the subject of several competing
   theories.

   Just a few days after its final close approach, Mariner 10 ran out of
   fuel, its orbit could no longer be accurately controlled and mission
   controllers instructed the probe to shut itself down. Mariner 10 is
   thought to be still orbiting the Sun, still passing close to Mercury
   every few months.

MESSENGER

   A second NASA mission to Mercury, named MESSENGER (MErcury Surface,
   Space ENvironment, GEochemistry, and Ranging), was launched on August
   3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing
   Delta 2 rocket. The MESSENGER spacecraft will make several close
   approaches to planets to place it onto the correct trajectory to reach
   an orbit around Mercury. It made a close approach to the Earth in
   February 2005, and to Venus in October 2006. Another Venusian encounter
   will follow in 2007, followed by three close approaches to Mercury in
   2008 and 2009, after which it will enter orbit around the planet in
   March 2011.

   The mission is designed to shed light on six key issues: Mercury's high
   density, its geological history, the nature of its magnetic field, the
   structure of its core, whether it really has ice at its poles, and
   where its tenuous atmosphere comes from. To this end, the probe is
   carrying imaging devices which will gather much higher resolution
   images of much more of the planet than Mariner 10, assorted
   spectrometers to determine abundances of elements in the crust, and
   magnetometers and devices to measure velocities of charged particles.
   Detailed measurements of tiny changes in the probe's velocity as it
   orbits will be used to infer details of the planet's interior
   structure.

BepiColombo

   Mercury as imaged by the Mariner 10 spacecraft
   Enlarge
   Mercury as imaged by the Mariner 10 spacecraft

   Japan is planning a joint mission with the European Space Agency called
   BepiColombo, which will orbit Mercury with two probes: one to map the
   planet and the other to study its magnetosphere. An original plan to
   include a lander has been shelved. Russian Soyuz rockets will launch
   the probes in 2013. As with MESSENGER, the BepiColombo probes will make
   close approaches to other planets en route to Mercury, passing the Moon
   and Venus and making several approaches to Mercury before entering
   orbit. The probes will reach Mercury in about 2019, orbiting and
   charting its surface and magnetosphere for a year.

   The probes will carry a similar array of spectrometers to those on
   MESSENGER, and will study the planet at many different wavelengths
   including infrared, ultraviolet, X-ray and gamma ray. Apart from
   intensively studying the planet itself, mission planners also hope to
   use the probe's proximity to the Sun to test the predictions of General
   Relativity theory with improved accuracy.

   The mission is named after Giuseppe (Bepi) Colombo, the scientist who
   first determined the nature of Mercury's orbital resonance with the Sun
   and who was also involved in the planning of Mariner 10's
   gravity-assisted trajectory to the planet in 1974.

   Retrieved from " http://en.wikipedia.org/wiki/Mercury_%28planet%29"
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