View source for User:PiRSquared17/Mercury 1

==Studies==
===Ancient astronomers===
The earliest known recorded observations of Mercury are from the [[Mul.Apin]] tablets. These observations were most likely made by an [[Assyria]]n astronomer around the 14th century BC.<ref>{{cite journal | title=The Latitude and Epoch for the Origin of the Astronomical Lore in Mul.Apin | first=Bradley E. | last=Schaefer | journal=American Astronomical Society Meeting 210, #42.05 | year=2007 | volume=210 | date=May 2007 | url=http://cdsads.u-strasbg.fr/abs/2007AAS...210.4205S | publisher=American Astronomical Society| bibcode=2007AAS...210.4205S }}</ref> The [[Cuneiform script|cuneiform]] name used to designate Mercury on the Mul.Apin tablets is [[wikt:transcribe|transcribe]]d as Udu.Idim.Gu\u<sub>4</sub>.Ud ("the jumping planet").{{Ref label|B|b|none}}<ref>{{cite journal
 | first=Hermann | last=Hunger |coauthors=Pingree, David
 | title=MUL.APIN: An Astronomical Compendium in Cuneiform
 | journal=Archiv für Orientforschung | volume=24
 | publisher=Verlag Ferdinand Berger & Sohne Gesellschaft MBH
 | location=Austria | year=1989 | page=146 }}</ref> [[Babylonian astronomy|Babylonian records]] of Mercury date back to the 1st millennium BC. The [[Babylonia]]ns called the planet [[Nabu]] after the messenger to the gods in [[Babylonian mythology|their mythology]].<ref name="JHU history">{{cite web
 | year=2008 | author=Staff
 | url=http://btc.montana.edu/messenger/elusive_planet/ancient_cultures_2.php
 | title=MESSENGER: Mercury and Ancient Cultures
 | publisher=NASA JPL | accessdate=2008-04-07 }}</ref>

The [[Ancient Greece|ancient Greeks]] of [[Hesiod]]'s time called Mercury Στίλβων (''Stilbon''), meaning "the gleaming", and Ἑρμάων (''Hermaon'').<ref>{{cite book |author= H.G. Liddell and R. Scott |coauthors=''rev.'' H.S. Jones and R. McKenzie |title=Greek–English Lexicon, with a Revised Supplement |edition=9th |year=1996 |publisher=Clarendon Press |location=Oxford |isbn=0-19-864226-1 |pages=690 and 1646 }}</ref> Later Greeks called the planet [[Apollo (god)|''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 Apollo and Hermes both [[wikt:refer|refer]]red to the same body. The Romans named the planet after the fast Roman messenger god, [[Mercury (mythology)|Mercury]] (Latin ''Mercurius'') which they [[wikt:equate|equate]]d with the Greek [[Hermes]], because it moves across the sky faster than any other planet in the solar system.<ref name="Dunne" /><ref>{{cite book
 | first=Eugène Michel | last=Antoniadi
 | coauthors=Translated from French by Moore, Patrick
 | year=1974 | title=The Planet Mercury
 | publisher=Keith Reid Ltd | location=Shaldon, Devon
 | pages=9–11 }}</ref> The Roman-Egyptian astronomer [[Claudius Ptolemaeus|Ptolemy]] wrote about the possibility of planetary transits across the face of the Sun in his work ''Planetary Hypotheses''. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because the transits were too infrequent.<ref>{{citation
 | author=Goldstein, Bernard R.
 | title=The Pre-telescopic Treatment of the Phases and Apparent Size of Venus
 | journal=Journal for the History of Astronomy | page=1
 | date=February 1996 | year=1996 | volume=27
 | doi=10.1177/002182869602700101
 | bibcode=1996JHA....27....1G | s2cid=117218196
 }}</ref>

[[File:Shatir500.jpg|thumb|left|[[Ibn al-Shatir]]'s model for the appearances of [[Mercury (planet)|Mercury]], showing the multiplication of [[Deferent and epicycle|epicycle]]s using the [[Tusi-couple]], thus eliminating the Ptolemaic eccentrics and [[equant]].]]

In [[ancient China]], Mercury was known as ''Chen Xing'' (辰星), the Hour Star. It was associated with the direction north and the phase of water in the [[Wu Xing]].<ref>{{cite book
 | first=David H. | last=Kelley
 | coauthors=Milone, E. F.; Aveni, Anthony F. | year=2004
 | title=Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy
 | publisher=Birkhäuser | isbn=0387953108 }}</ref> [[Hindu mythology]] used the name [[Budha]] for Mercury, and this god was thought to preside over [[Wednesday]].<ref>{{cite book
 | first=R.M. | last=Pujari
 | coauthors=Kolhe, Pradeep; Kumar, N. R. | year=2006
 | title=Pride of India: A Glimpse Into India's Scientific Heritage
 | publisher=Samskrita Bharati | isbn=8187276274 }}</ref> The god [[Odin]] (or Woden) of [[Germanic paganism]] was associated with the planet Mercury and [[Wednesday]].<ref>{{cite book
 | first=Michael E. | last=Bakich | year=2000
 | title=The Cambridge Planetary Handbook
 | publisher=Cambridge University Press
 | isbn=0521632803 }}</ref> The [[Maya civilization|Maya]] may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the [[underworld]].<ref>{{cite book
 | first=Susan | last=Milbrath | year=1999
 | title=Star Gods of the Maya: Astronomy in Art, Folklore and Calendars
 | publisher=University of Texas Press
 | isbn=0292752261 }}</ref>

In ancient [[Indian astronomy]], the ''[[Surya Siddhanta]]'', an Indian astronomical text of the 5th century, estimates the diameter of Mercury as 3,008 miles, an error of less than 1% from the currently accepted diameter of 3,032 miles. However, this estimate was based upon an inaccurate guess of the planet's [[angular diameter]] as 3.0 [[arcminute]]s.

In [[Astronomy in medieval Islam|medieval Islamic astronomy]], the [[Al-Andalus|Andalusian]] astronomer [[Abū Ishāq Ibrāhīm al-Zarqālī]] in the 11th century described the deferent of Mercury's geocentric orbit as being oval, like an egg or a [[Pine nut|pignon]], although this insight did not influence his astronomical theory or his astronomical calculations.<ref>{{cite journal | author=Samsó, Julio; Mielgo, Honorino | url=http://articles.adsabs.harvard.edu/full/1994JHA....25..289S | title=Ibn al-Zarqālluh on Mercury | journal=Journal for the History of Astronomy | volume=25 | year=1994 | issue=4 | pages=289–96 [292] | doi=10.1177/002182869402500403 | bibcode=1994JHA....25..289S | s2cid=118108131 }}</ref><ref>{{cite journal | first=Willy | last=Hartner | title=The Mercury Horoscope of Marcantonio Michiel of Venice | journal=Vistas in Astronomy | volume=1 | year=1955 | pages=84–138 | doi=10.1016/0083-6656(55)90016-7 }} at pp. 118-122.</ref> In the 12th century, [[Ibn Bajjah]] observed "two planets as black spots on the face of the Sun," which was later suggested as the [[transit of Mercury]] and/or Venus by the [[Maragheh observatory|Maragha]] astronomer [[Qotb al-Din Shirazi]] in the 13th century.<ref>{{cite conference
 | title=History of oriental astronomy: proceedings of the joint discussion-17 at the 23rd General Assembly of the International Astronomical Union, organised by the Commission 41 (History of Astronomy), held in Kyoto, August 25-26, 1997 | first=S. M. Razaullah | last=Ansari
 | publisher=[[Springer Science+Business Media|Springer]]
 | year=2002 | isbn=1402006578 | page=137}}</ref> (Note that most such medieval reports of transits were later taken as observations of [[sunspot]]s.<ref>{{cite journal
 | last=Goldstein | first=Bernard R. | year=1969
 | title=Some Medieval Reports of Venus and Mercury Transits
 | date=December 1969
 | journal=Centaurus | volume=14 | issue=1 | pages=49–59
 | doi=10.1111/j.1600-0498.1969.tb00135.x
 | bibcode=1969Cent...14...49G }}</ref>)

In India, the [[Kerala school of astronomy and mathematics|Kerala school]] astronomer [[Nilakantha Somayaji]] in the 15th century developed a planetary model in which Mercury orbits the Sun, which in turn orbits the Earth, similar to the [[Tychonic system]] later proposed by [[Tycho Brahe]] in the late 16th century.<ref>{{cite journal
 | author=Ramasubramanian, K.; Srinivas, M. S.; Sriram, M. S.
 | title=Modification of the Earlier Indian Planetary Theory by the Kerala Astronomers (c. 1500 AD) and the Implied Heliocentric Picture of Planetary Motion
 | journal=Current Science | volume=66 | year=1994
 | pages=784–790 | url=http://www.physics.iitm.ac.in/~labs/amp/kerala-astronomy.pdf | accessdate=2010-04-23 }}</ref>

===Ground-based telescopic research===
[[File:Mercury transit 1.jpg|thumb|[[Transit of Mercury]]. Mercury is the small dot in the lower center, in front of the Sun. The dark area on the left of the solar disk is a [[sunspot]].]]

The first [[Telescope|telescopic]] observations of Mercury were made by [[Galileo]] in the early 17th century. Although he observed [[planetary phase|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 telescopic observations of the [[Astronomical transit|transit]] of a planet across the Sun when he saw a transit of Mercury predicted by [[Johannes Kepler]]. In 1639 [[Giovanni Battista Zupi|Giovanni Zupi]] used a telescope to discover that the planet had [[orbit]]al phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.<ref name=strom/>

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]].<ref>{{cite journal |last=Sinnott |first=RW |authorlink= |coauthors=Meeus, J |year=1986 |title=John Bevis and a Rare Occultation |journal=Sky and Telescope |volume=72 |issue= |pages=220 |bibcode=1986S&T....72..220S |id= |url=http://adsabs.harvard.edu/abs/1986S&T....72..220S |accessdate= |quote= }}</ref> The next occultation of Mercury by Venus will be on December 3, 2133.<ref>{{cite book
 | first=Timothy | last=Ferris | year=2003
 | title=Seeing in the Dark: How Amateur Astronomers
 | publisher=Simon and Schuster
 | isbn=0684865807 }}</ref>

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 have observed 20&nbsp;km high mountains. [[Friedrich Bessel]] used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.<ref name="sao188r">{{cite journal
 | last=Colombo | first=G. | coauthors=Shapiro, I. I.
 | title=The Rotation of the Planet Mercury
 | journal=SAO Special Report #188R
 | year=1965 | volume=188 | bibcode=1965SAOSR.188.....C | url=http://adsabs.harvard.edu/abs/1965SAOSR.188.....C
 | accessdate=2008-05-23 }}</ref> 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]].<ref>{{cite journal
 |last=Holden |first=E. S. |year=1890
 |title=Announcement of the Discovery of the Rotation Period of Mercury [by Professor Schiaparelli]
 |journal=Publications of the Astronomical Society of the Pacific |volume=2 |issue=7 |pages=79
 |url=http://adsabs.harvard.edu/abs/1890PASP....2...79H |accessdate=2008-06-03
 |doi=10.1086/120099 |bibcode=1890PASP....2...79H |s2cid=122095054 }}</ref> This phenomenon is known as [[synchronous rotation]] and is shown by Earth’s Moon. The effort to map the surface of Mercury was continued by [[Eugenios Antoniadi]], who published a book in 1934 that included both maps and his own observations.<ref name="chaikin1" /> Many of the planet's surface features, particularly the [[List of albedo features on Mercury|albedo features]], take their names from Antoniadi's map.<ref>{{cite book|url=http://history.nasa.gov/SP-423/sp423.htm|title=Atlas of Mercury|publisher=[[NASA]] Office of Space Sciences|author=Merton E. Davies, et al.|year=1978|chapter=Surface Mapping|chapter-url=http://history.nasa.gov/SP-423/surface.htm|accessdate=2008-05-28}}</ref>

In June 1962 [[Soviet Union|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.<ref>{{cite journal
 | title=Radio Echo Observations of Venus and Mercury at 23 cm Wavelength
 | year=1965 | journal=Astronomical Journal | volume=70
 | url=http://articles.adsabs.harvard.edu/abs/1965AJ.....70..486E/0000487.000.html
 | pages=487&ndash;500 | accessdate=2008-05-23
 | doi=10.1086/109772 |bibcode=1965AJ.....70..486E
 |last1=Evans
 |first1=J. V.
 |last2=Brockelman
 |first2=R. A.
 |last3=Henry
 |first3=J. C.
 |last4=Hyde
 |first4=G. M.
 |last5=Kraft
 |first5=L. G.
 |last6=Reid
 |first6=W. A.
 |last7=Smith
 |first7=W. W.
 }}</ref><ref>{{cite book
 | last=Moore | first=Patrick
 | title=The Data Book of Astronomy | page=483 | year=2000
 | publisher=CRC Press | location=New York
 | url=http://books.google.com/books?lr=&as_brr=3&q=kotelnikov+1962+mercury&btnG=Search+Books
 | isbn=0750306203}}</ref><ref>{{cite book
 | title=To See the Unseen: A History of Planetary Radar Astronomy
 | url=http://history.nasa.gov/SP-4218/sp4218.htm
 | first=Andrew J. | last=Butrica
 | publisher=[[NASA]] History Office, Washington D.C.
 | year=1996 | chapter=Chapter 5
 | chapter-url=http://history.nasa.gov/SP-4218/ch5.htm }}</ref> 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.<ref>{{cite journal
 | title=A Radar Determination of the Rotation of the Planet Mercury
 | journal=[[Nature (journal)|Nature]] | volume=206
 | issue= 1240 | pages= 451–2 | year= 1965
 |doi=10.1038/2061240a0 | last1=Pettengill | first1=G. H. | last2=Dyce | first2=R. B. | bibcode=1965Natur.206Q1240P | s2cid=31525579 }}</ref><ref>[http://scienceworld.wolfram.com/astronomy/Mercury.html Mercury] at Eric Weisstein's 'World of Astronomy'</ref> The theory that Mercury’s rotation was synchronous had become 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.<ref>{{cite book
 | first=Bruce C. | last=Murray
 | coauthors=Burgess, Eric | year=1977
 | title=Flight to Mercury
 | publisher=Columbia University Press
 | isbn=0231039964 }}</ref>

Italian astronomer [[Giuseppe Colombo]] noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.<ref>{{cite journal
 | last=Colombo | first=G. | title=Rotational Period of the Planet Mercury
 | journal=Nature | volume=208 | pages=575 | year=1965
 | issue=5010 | doi = 10.1038/208575a0 | bibcode=1965Natur.208..575C | s2cid=4213296 | accessdate=2009-05-30 | url=http://adsabs.harvard.edu/abs/1965Natur.208..575C }}</ref> Data from Mariner&nbsp;10 subsequently confirmed this view.<ref>{{cite web
 | date=October 1976 | year=1976 | author=Davies, Merton E. et al.
 | url=http://history.nasa.gov/SP-423/mariner.htm
 | title=Mariner&nbsp;10 Mission and Spacecraft
 | work=SP-423 Atlas of Mercury
 | publisher=NASA JPL | accessdate=2008-04-07 }}</ref> This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every ''second'' orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, since the orbital geometry meant that these observations were made under poor viewing conditions.<ref name="sao188r" />

Ground-based observations did not shed much further light on the innermost planet, and it was not until the first space probe flew past 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]] observations were conducted by the [[Mount Wilson Observatory]] 1.5 meter {{dp|Hale telescope}}. They provided the first views that resolved surface features on the parts of Mercury which were not imaged in the Mariner mission.<ref>{{cite journal
 | last=Dantowitz | first=R. F. | coauthors=Teare, S. W.; Kozubal, M. J.
 | title=Ground-based High-Resolution Imaging of Mercury
 | journal=Astronomical Journal | volume=119 | pages=2455&ndash;2457 | year= 2000
 | issue=5 | url=http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000AJ....119.2455D&amp;db_key=AST
 | doi = 10.1016/j.asr.2005.05.071 | bibcode=2000AJ....119.2455D }}</ref> 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]]''.<ref name=Ksa06>{{cite journal|author = L. V. Ksanfomality|title=Earth-based optical imaging of Mercury| journal= Advances in Space Research |volume= 38|pages= 594–598|year= 2006|issue=4 |url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AdSpR..38..594K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=461152a03222956|doi=10.1016/j.asr.2005.05.071|bibcode=2006AdSpR..38..594K }}</ref>
Most of the planet has been mapped by the Arecibo radar telescope, with 5&nbsp;km resolution, including polar deposits in shadowed craters of what may be water ice.<ref name=Harm06>{{cite journal|author =Harmon, J. K. et al.|title= Mercury: Radar images of the equatorial and midlatitude zones| journal= Icarus |volume= 187|pages= 374|year= 2007|issue= 2|url= http://adsabs.harvard.edu/abs/2007Icar..187..374H | doi = 10.1016/j.icarus.2006.09.026|bibcode= 2007Icar..187..374H}}</ref>

{{Clear}}

===Research with space probes===
{{Main|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 [[gravity|gravitational]] [[potential well]]. Mercury has an [[orbital speed]] of 48&nbsp;km/s, while Earth’s orbital speed is 30&nbsp;km/s. Thus the spacecraft must make a large change in [[velocity]] ([[delta-v]]) to enter a [[Hohmann transfer orbit]] that passes near Mercury, as compared to the delta-v required for other planetary missions.<ref name="DunneCh4">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapter-url=http://history.nasa.gov/SP-424/ch4.htm|publisher=NASA History Office|year=1978|chapter=Chapter Four|url=http://history.nasa.gov/SP-424/|accessdate=2008-05-28}}</ref>

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. To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. [[Aerobraking]] is ruled out because the planet has very little atmosphere. A trip to Mercury requires more rocket fuel than that required to [[escape velocity|escape]] the Solar System completely. As a result, only two space probes have visited the planet so far.<ref name="JPLprofile1">{{cite web|url=http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mercury&Display=OverviewLong|title=Mercury|publisher=[[NASA]] Jet Propulsion Laboratory|date=May 5, 2008 |accessdate=2008-05-29}}</ref> A proposed alternative approach would use a [[solar sail]] to attain a Mercury-synchronous orbit around the Sun.<ref>{{ cite journal
 | title=Mercury sun-synchronous polar orbiter with a solar sail
 | year=1996 | date=July 1996 | journal=Acta Astronautica
 | volume=39 |issue=1 | pages = 143&ndash;151 | doi=10.1016/S0094-5765(96)00131-2 | last1=Leipold
 | first1=M.
 | last2=Seboldt
 | first2=W.
 | last3=Lingner
 | first3=S.
 | last4=Borg
 | first4=E.
 | last5=Herrmann
 | first5=A.
 | last6=Pabsch
 | first6=A.
 | last7=Wagner
 | first7=O.
 | last8=Brückner
 | first8=J.
 | bibcode=1996AcAau..39..143L
 }}</ref>

====Mariner 10====
{{Main|Mariner 10}}
[[File:Mariner 10.jpg|thumb|The Mariner&nbsp;10 probe, the first probe to visit the innermost planet]]
[[File:Mercury Mariner10.jpg|thumb|View of Mercury from Mariner&nbsp;10]]

The first spacecraft to visit Mercury was [[NASA]]’s [[Mariner 10|Mariner&nbsp;10]] (1974–75).<ref name="Dunne" /> The spacecraft used the gravity of [[Venus]] to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this [[Gravitational slingshot|gravitational “slingshot”]] effect and the first NASA mission to visit multiple planets.<ref name="DunneCh4" /> Mariner&nbsp;10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and 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.<ref>{{cite web
 | date=October 1976 | year=1976 | first=Tony | last=Phillips
 | url=http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html
 | title=NASA 2006 Transit of Mercury | work=SP-423 Atlas of Mercury
 | publisher=NASA | accessdate=2008-04-07 }}</ref> Unfortunately, due to the length of Mariner&nbsp;10's orbital period, the same face of the planet was lit at each of Mariner&nbsp;10’s close approaches. This made observation of both sides of the planet impossible,<ref>{{cite web|url=http://sci.esa.int/science-e/www/category/index.cfm?fcategoryid=4586|title=BepiColumbo - Background Science|publisher=European Space Agency|accessdate=2008-05-30}}</ref> and resulted in the mapping of less than 45% of the planet’s surface.<ref name="USATMessenger">{{cite news|url=http://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm|title=MESSENGER to test theory of shrinking Mercury|publisher=USA Today|author=Tariq Malik|date=August 16, 2004 |accessdate=2008-05-23}}</ref>

On March 27, 1974, two days before its first flyby of Mercury, Mariner 10's instruments began registering large amounts of unexpected ultraviolet radiation near Mercury. This led to the tentative identification of [[Mercury's moon]]. Shortly afterward, the source of the excess UV was identified as the star 31 [[Crater (constellation)|Crater]]is, and Mercury's moon passed into astronomy's history books as a footnote.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327&nbsp;km of the surface.<ref name="AtlasM10">{{cite book|url=http://history.nasa.gov/SP-423/sp423.htm|title=Atlas of Mercury|publisher=[[NASA]] Office of Space Sciences|author=Merton E. Davies, et al.|year=1978|chapter=Mariner&nbsp;10 Mission and Spacecraft|chapter-url=http://history.nasa.gov/SP-423/mariner.htm|accessdate=2008-05-30}}</ref> 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.<ref name="Ness1">{{ cite journal | last = Ness| first = Norman F. | year = 1978| date = March 1978|title=Mercury - Magnetic field and interior| journal = Space Science Reviews | volume = 21| issue = 5 | pages = 527–553| bibcode = 1978SSRv...21..527N | url = http://adsabs.harvard.edu/full/1978SSRv...21..527N| accessdate = 2008-05-23|doi=10.1007/BF00240907| s2cid = 120025983 }}</ref>

On March 24, 1975, just eight days after its final close approach, Mariner&nbsp;10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down.<ref name="DunneCh8">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapter-url=http://history.nasa.gov/SP-424/ch8.htm|publisher=NASA History Office|year=1978|chapter=Chapter Eight|url=http://history.nasa.gov/SP-424/}}</ref> Mariner&nbsp;10 is thought to be still orbiting the Sun, passing close to Mercury every few months.<ref>{{cite web
 | date=April 2, 2008 | first=Ed | last=Grayzeck
 | url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A
 | title=Mariner&nbsp;10 | work=NSSDC Master Catalog
 | publisher=NASA | accessdate=2008-04-07 }}</ref>

====MESSENGER====
{{Main|MESSENGER}}

[[File:MESSENGER Assembly.jpg|thumb|MESSENGER being prepared for launch]]

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. It made a fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury.<ref>{{cite web|year=2005|url = http://www.spaceref.com/news/viewsr.html?pid=18956| title = MESSENGER Engine Burn Puts Spacecraft on Track for Venus|publisher = SpaceRef.com | accessdate = 2006-03-02}}</ref> A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008,<ref name="MessCountdown">{{cite web|url= http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115|title= Countdown to MESSENGER's Closest Approach with Mercury|date= January 14, 2008 | publisher= Johns Hopkins University Applied Physics Laboratory |accessdate= 2008-05-30}}</ref> and a third on September 29, 2009.<ref>{{cite web | title=MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations
| url=http://messenger.jhuapl.edu/news_room/details.php?id=136
| date=September 30, 2009 | publisher=MESSENGER Mission News
| accessdate=2009-09-30}}</ref> Most of the hemisphere not imaged by Mariner&nbsp;10 has been mapped during these fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year.<ref name="MessCountdown" />

<!--[[File:mercclose3.jpg|thumb|Mercury as imaged by the MESSENGER probe]]-->
The mission is designed to clear up six key issues: Mercury’s high density, its geological history, the nature of its [[magnetic field]], the structure of its core, whether it 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&nbsp;10, assorted [[spectrometer]]s to determine abundances of elements in the crust, and [[magnetometer]]s 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.<ref name="MSGRgrayzeck" />

====BepiColombo====
{{Main|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]].<ref name="ESAColumboGoAhead">{{cite web
 | title=ESA gives go-ahead to build BepiColombo
 | date=February 26, 2007 | publisher=[[European Space Agency]]
 | url=http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html
 | accessdate=2008-05-29 }}</ref> Once launched, the spacecraft bus is expected to reach Mercury in 2019.<ref name="Bepitelegraph1">{{cite news
 | url=http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2008/01/18/scimerc118.xml
 | title=Star Trek-style ion engine to fuel Mercury craft
 | author=Fleming, Nic | publisher=The Telegraph
 | date=January 18, 2008 | accessdate=2008-05-23 }}</ref> The bus will release a [[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.<ref name="ESAColumboGoAhead" /> 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]].<ref>{{cite web
 | title=Objectives | publisher=European Space Agency | url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350
 | date=February 21, 2006 | accessdate=2008-05-29 }}</ref>