Mercury (planet)

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Mercury  

MESSENGER false color image of Mercury
Orbital characteristics[1]
Epoch J2000
Aphelion	69,816,900 km 0.466697 AU
Perihelion	46,001,200 km 0.307499 AU
Semi-major axis	57,909,100 km 0.387098 AU
Eccentricity	0.205630
Orbital period	87.9691 d (0.240846 a)
Synodic period	115.88 d[2]
Average orbital speed	47.87 km/s[2]
Mean anomaly	174.796°
Inclination	7.005° 3.38° to Sun’s equator
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Satellites	None
Physical characteristics
Mean radius	2,439.7 ± 1.0 km[3][4] 0.3829 Earths
Flattening	< 0.0006[4]
Surface area	7.48×107 km² 0.108 Earths[3]
Volume	6.083×1010 km³ 0.054 Earths[3]
Mass	3.3022×1023 kg 0.055 Earths[3]
Mean density	5.427 g/cm³[3]
Equatorial surface gravity	3.7 m/s² 0.38 g[3]
Escape velocity	4.25 km/s[3]
Sidereal rotation period	58.646 day (58 d 15.5 h)[3]
Equatorial rotation velocity	10.892 km/h
Axial tilt	0.027°[5]
North pole right ascension	18 h 44 min 2 s 281.01°[2]
North pole declination	61.45°[2]
Albedo	0.119 (bond) 0.106 (geom.)[2]
Surface temp.    0°N, 0°W    85°N, 0°W	min mean max 100 K 340 K 700 K 80 K 200 K 380 K
Apparent magnitude	up to -1.9[2]
Angular diameter	4.5" — 13"[2]
Adjectives	Mercurian
Atmosphere
Surface pressure	trace
Composition	[citation needed] 31.7% Potassium 24.9% Sodium 9.5% Atomic Oxygen 7.0% Argon 5.9% Helium 5.6% Molecular Oxygen 5.2% Nitrogen 3.6% Carbon dioxide 3.4% Water 3.2% Hydrogen

Mercury (pronounced [ˈmɝkjʊəri] (help·info)) is the innermost and smallest planet in the solar system (since Pluto was re-labelled as a dwarf planet), orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface.^[6] The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008.^[6] MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.

Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.^[7] It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (−180 to 430 °C),^[8] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.^[9] The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.^[10]

Contents 1 Internal structure 2 Surface geology 3 Atmosphere 4 Magnetic field and magnetosphere 5 Orbit and rotation 5.1 Advance of perihelion 5.2 Spin–orbit resonance 6 Observation 7 Studies of Mercury 7.1 Ancient astronomers 7.2 Ground-based telescopic research 7.3 Research with space probes 7.3.1 Mariner 10 7.3.2 MESSENGER 7.3.3 BepiColombo 8 See also 9 Notes and references 10 External links

Internal structure

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Mercury is one of the four terrestrial planets that is a rocky body like the Earth. It is the smallest of the four, with a diameter of 4879 km at its equator. Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. 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 (water is 1.00 g/cm³), only slightly less than Earth’s density. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.^[11]

Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.^[12] Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.^[13][14]

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.^[15]

Mercury’s crust is believed to be 100–200 km thick. One distinctive feature of Mercury’s surface are numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.^[16]

Mercury has a higher iron content in the core than any other major planet in our solar system, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of average solar system rocky matter, and a mass approximately 2.25 times its current mass. However, early in the solar system’s history, Mercury may have been 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 as a relatively major component.^[15] A similar process 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 2,500 and 3,500 K (about 4,000 to 5,800 °F), and possibly even as high as 10,000 K (about 17,500 °F).^[17] Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.^[17]

A third theory proposes 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.^[18] 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.^[19][20]

Surface geology

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Main article: Geology of Mercury

Mercury’s surface is overall 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. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets.^[14] As data from the recent MESSENGER flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling "the spider."^[21]

The currently known surface features are given the following names in planetary geology:^[22]

Feature name	Type of terrain	List of features
Albedo features[23]	—	List of albedo features on Mercury
Dorsa	ridges	List of ridges on Mercury
Montes	mountains	List of mountains on Mercury
Planitiae	plains	List of plains on Mercury
Rupes	scarps	List of scarps on Mercury
Valles	valleys	List of valleys on Mercury

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago.^{[citation needed]} 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.^{[citation needed]} 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.^[24][25]

Craters on Mercury range in diameter from a few meters to hundreds of kilometers across. The largest known craters are the enormous Caloris Basin, with a diameter of 1550 km,^[26] and the Skinakas Basin with a diameter of 1,600 km, but known only from low resolution Earth-based imaging of the non-Mariner-imaged hemisphere. The impact that 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". One hypothesis for the origin of this geomorphological unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin’s antipode (180 degrees away) the high stresses were capable of fracturing the surface.^[27] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.^[28]

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.^[29] Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.^[30]

The mean surface temperature of Mercury is 452 K (179 °C/354 °F),^{[citation needed]} but it ranges from 90 K (−183.1 °C/−297.7 °F) to 700 K (427 °C/800 °F),^{[citation needed]} due to the absence of an atmosphere. The mean noon-time temperature is 620 K (347 °C/656 °F),^{[citation needed]} while the dark-side temperature averages 110 K (−163 °C/−262 °F).^[31] 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. 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.^{[citation needed]}

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 by the 70m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.^[32] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.^[33]

The icy regions are believed to be covered to a depth of only a few meters, and contain about 10¹⁴–10¹⁵ kg of ice. By comparison, the Antarctic ice sheet on Earth has a mass of about 4×10¹⁸ kg, and Mars’ south polar cap contains about 10¹⁶ 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.^{[dead link]}

Atmosphere

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Main article: Atmosphere of Mercury

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. This 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 striking its surface.^[34]

Magnetic field and magnetosphere

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Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s.^[35][36] Mercury's magnetic field is dipolar and nearly aligned with the planet's spin axis.^[37] It is likely that this magnetic field is generated in a manner similar to Earth’s, by a dynamo of circulating liquid core material^[38] or a partially solid core.^[39] A mechanism that has been suggested for keeping it liquid are particularly strong tidal effects during periods of high orbital eccentricity.^[40]

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. It is also strong enough to trap solar wind plasma within the magnetosphere, which contributes to the space weathering of the planet's surface.^[37]

Orbit and rotation

The orbit of Mercury is the most eccentric of the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having 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, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.^[41]

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.^[42]

Functionally, Mercury’s axial tilt is nonexistent,^[43][44] with measurements as low as 0.027°.^[5] This is many times smaller than that of Jupiter, which boasts 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. Conversely, at the poles the Sun never rises more than 0.01° above the horizon.^{[citation needed]}

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 move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.^[41]

Advance of perihelion

Main articles: Tests of general relativity#Perihelion_precession_of_Mercury and Perihelion precession of Mercury

During the 19th century, French mathematician Le Verrier noticed that that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He proposed 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 the orbit of Uranus led astronomers to place great faith in this explanation, and the hypothetical planet was even named Vulcan. However, no such planet was ever found.^[45]

In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller effects, operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.^[46][47]

Spin–orbit resonance

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—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.^[48]

The original reason astronomers thought it was synchronously locked was that 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.^[41] A sidereal day (the period of rotation) lasts about 58.7 Earth days.^[41]

Orbital simulations indicate that the eccentricity of Mercury’s orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years.^[41] 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.^[49]

Observation

Mercury’s apparent magnitude varies between about −2.0—brighter than Sirius—and 5.5.^[50] 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, due to safety procedures which prevent its pointing too close to the Sun.^[51]

Like the Moon, Mercury exhibits 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 first and last quarter phases occur at greatest elongation east and west, respectively, when Mercury's separation from the Sun ranges anywhere from 18.5° at perihelion to 28.3° at aphelion. At greatest elongation west, Mercury rises earliest before the Sun, and at greatest elongation east, it sets latest after the Sun.^{[citation needed]}

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. At the nearest approach, Mercury currently comes as close as 82 Gm to Earth. 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 orbital eccentricity.^{[citation needed]}

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.^{[citation needed]} 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 southern temperate zone latitudes, such as Argentina and New Zealand.^{[citation needed]} By contrast, at northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can also, like several other planets and the brightest stars, be seen during a total solar eclipse.^[52]

Mercury is brightest as seen from Earth when it is at a gibbous phase, between either quarter phase 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.^[50] The opposite is true for Venus, which appears brightest when it is a thin crescent, because it is much closer to Earth than when gibbous.^[53]

Studies of Mercury

Ancient astronomers

The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC.^[54] The cuniform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU₄.UD ("the jumping planet").^[55][56] Babylonian records of Mercury date back to the 1st millenium BC. The Babylonians called the planet Nabu after the messenger to the Gods in their mythology.^[57]

The ancient Greeks of Hesiod's time knew the planet as Στίλβων Stilbon ("the gleaming") and Hermaon. Later Greeks called the planet Apollo when it was visible in the morning sky and Hermes when visible in the evening. Around the 4th century BC, however, Greek astronomers came to understand that the two names referred to the same body. The Romans named the planet after the Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes.^[9][58]

Ground-based telescopic research

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.^[41]

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.^[59] The next occultation of Mercury by Venus will be on December 3, 2133.^[60]

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, claiming to observed 20 km high mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.^[61] 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.^[62] This phenomenon is known as synchronous rotation and is also shown by Earth’s Moon.

In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences lead by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet.^[63][64][65] Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days.^[66][67] The theory that Mercury’s rotation was synchronous became widely held, and it was a surprise to astronomers when these radio observations were announced. 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.^[68]

Italian astronomer Giuseppe Colombo noted that the rotation 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.^[69] Data from Mariner 10 subsequently confirmed this view.^[70]

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 1500 mm telescope provided the first views that resolved some surface features on the parts of Mercury which were not imaged in the Mariner missions.^[71] Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.^[72] Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice.^[73] Ground-based telescopes have detected the bright rays around some radar-mapped craters.^{[citation needed]}

Research with space probes

Main article: Exploration of Mercury

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.^{[citation needed]}

The potential energy liberated by moving down the Sun’s potential well becomes kinetic energy; requiring another large delta-v change to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit the spacecraft must rely entirely on rocket motors since aerobraking is ruled out because the planet has very little atmosphere. A trip to Mercury actually requires more rocket fuel than that required to escape the solar system completely. As a result, only two space probes have visited the planet so far.^{[citation needed]}

Mariner 10

Main article: Mariner 10

The first spacecraft to approach Mercury was NASA’s Mariner 10 (1974–75).^[9] 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.^{[citation needed]} 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 as its iron core cools.^[74] Unfortunately, because Mariner 10's orbital period was almost exactly 3 sidereal Mercury days, the same face of the planet was lit at each of Mariner 10’s close approaches,^{[citation needed]} resulting in less than 45% of the planet’s surface being mapped.^[75]

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. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.^[76]

Just a few days after its final close approach, Mariner 10 ran out of fuel; since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.^[77]

MESSENGER

Main article: 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 fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007.^[78] The first fly-by of Mercury occurred on January 14, 2008.^[79] Two more fly-bys of Mercury are scheduled, in October 2008 and September 2009. Most of the hemisphere not imaged by Mariner 10 will be mapped during the fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year. Orbital periapsis will be over the northern hemisphere. Communications with Earth occur near apoapsis, every 12 hours.

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.^[19]

BepiColombo

Main article: BepiColombo

The European Space Agency is planning a joint mission with Japan 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. A Russian Soyuz rocket will launch the bus carrying the two probes in 2013, from ESA's Guiana Space Center to take advantage of its equatorial location.^{[citation needed]} As with MESSENGER, the BepiColombo bus will make close approaches to other planets en route to Mercury for orbit-changing gravitational assists, passing the Moon and Venus and making several approaches to Mercury before entering orbit.^{[citation needed]} A combination of chemical and ion engines will be used, the latter thrusting continuously for long intervals. The spacecraft bus will reach Mercury in 2019.^{[citation needed]} The bus will release the magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for a terrestrial year.^{[citation needed]}

The mapper probe will carry an array of spectrometers similar 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.^{[citation needed]}

The mission is named after Giuseppe (Bepi) Colombo, the scientist who first determined the nature of Mercury’s spin-orbit resonance and who was also involved in the planning of Mariner 10’s gravity-assisted trajectory to the planet in 1974.^[20]

See also

• Colonization of Mercury
• Mercury in astrology
• Mercury in fiction

Notes and references

External links

Find more about Mercury on Wikipedia's sister projects:
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• Mercury — About Space
• Atlas of Mercury — NASA
• Nine Planets Information
• NASA’s Mercury fact sheet
• Mercury Profile by NASA's Solar System Exploration
• ‘BepiColombo’, ESA’s Mercury Mission
• Merkur(dt.)
• ‘Messenger’, NASA’s Mercury Mission
• SolarViews.com — Mercury
• Planets — Mercury A kid’s guide to Mercury.
• Mercury World Book Online Reference Center
• Astronomy Cast: Mercury
• Geody Mercury World’s search engine that supports NASA World Wind, Celestia, and other applications.
• A Day On Mercury flash animation

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