Observation history

Ancient astronomers

Mercury, from Liber astronomiae, 1550

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.
[141]
 The cuneiform name used to designate Mercury on the Mul.Apin tablets is transcribed as
Udu.Idim.Gu\u4.Ud ("the jumping planet").[e][142] Babylonian records of Mercury date back to the 1st
millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their
mythology.[143]
The ancients knew Mercury by different names depending on whether it was an evening star or a
morning star. By about 350 BC, the ancient Greeks had realized the two stars were one.[144] They
knew the planet as Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion,
[145]
 a name that is retained in modern Greek (Ερμής Ermis).[146] The Romans named the planet after
the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the
Greek Hermes, because it moves across the sky faster than any other planet. [144][147] The astronomical
symbol for Mercury is a stylized version of Hermes' caduceus.[148]
The Greco-Egyptian[149] astronomer 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.[150]

Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi
couple, thus eliminating the Ptolemaic eccentrics and equant.

In ancient China, Mercury was known as "the Hour Star" (Chen-xing 辰星). It was associated with
the direction north and the phase of water in the Five Phases system of metaphysics.
  Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the
[151]

"water star" (水星), based on the Five elements.[152][153][154] Hindu mythology used the name Budha for

Mercury, and this god was thought to preside over Wednesday. [155] The god Odin (or Woden)
of Germanic paganism was associated with the planet Mercury and Wednesday. [156] The 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.[157]
In medieval Islamic astronomy, the 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 pignon,
although this insight did not influence his astronomical theory or his astronomical calculations. [158]
[159]
 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 Maragha astronomer Qotb
al-Din Shirazi in the 13th century.[160] (Note that most such medieval reports of transits were later
taken as observations of sunspots.[161])
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially
heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to
the Tychonic system later proposed by Tycho Brahe in the late 16th century.[162]

Ground-based telescopic research

Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the
center of the solar disk is a sunspot.

Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears
close to the Sun.

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 telescopic 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. [23]
A 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.[163] The next occultation of Mercury by Venus will be on December 3, 2133. [164]
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-kilometre-high (12 mi) mountains. Friedrich Bessel used Schröter's drawings to erroneously
estimate the rotation period as 24 hours and an axial tilt of 70°. [165] 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. [166] This phenomenon is known
as synchronous rotation. 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. [90] Many
of the planet's surface features, particularly the albedo features, take their names from Antoniadi's
map.[167]
In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR
Academy of Sciences, led by Vladimir Kotelnikov, became the first to bounce a radar signal off
Mercury and receive it, starting radar observations of the planet. [168][169][170] Three years later, radar
observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter Arecibo
radio telescope in Puerto Rico, showed conclusively that the planet's rotational period was about 59
days.[171][172] 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.[173]

Water ice (yellow) at Mercury's north polar region

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.[174] Data from Mariner 10 subsequently confirmed this view.
[175]
 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, because the orbital geometry meant
that these observations were made under poor viewing conditions. [165]
Ground-based optical observations did not shed much further light on Mercury, but radio
astronomers using interferometry at microwave wavelengths, a technique that enables removal of
the solar radiation, were able to discern physical and chemical characteristics of the subsurface
layers to a depth of several meters. [176][177] Not until the first space probe flew past Mercury did many of
its most fundamental morphological properties become known. Moreover, 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 Hale telescope.
They provided the first views that resolved surface features on the parts of Mercury that were not
imaged in the Mariner 10 mission.[178] Most of the planet has been mapped by the Arecibo radar
telescope, with 5 km (3.1 mi) resolution, including polar deposits in shadowed craters of what may
be water ice.[179]

Research with space probes

Main article: Exploration of Mercury

MESSENGER being prepared for launch

Mercury transiting the Sun as viewed by the Mars rover Curiosity (June 3, 2014).[180]

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much
closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over
91 million kilometres (57 million miles) into the Sun's gravitational potential well. Mercury has
an orbital speed of 47.4 km/s (29.5 mi/s), whereas Earth's orbital speed is 29.8 km/s (18.5 mi/s).
[100]
 Therefore, the spacecraft must make a large change in velocity (delta-v) to get to Mercury and
then enter orbit,[181] as compared to the delta-v required for, say, Mars planetary missions.
The potential energy liberated by moving down the Sun's potential well becomes kinetic energy,
requiring a delta-v change to do anything other than pass by Mercury. Some portion of this delta-v
budget can be provided from a gravity assist during one or more fly-bys of Venus.[182] To land safely
or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out
because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that
required to escape the Solar System completely. As a result, only two space probes have visited it
so far.[183] A proposed alternative approach would use a solar sail to attain a Mercury-synchronous
orbit around the Sun.[184]
Mariner 10
Main article: Mariner 10
Mariner 10, the first probe to visit Mercury

The first spacecraft to visit Mercury was NASA's Mariner 10 (1974–1975).[144] 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" effect and the first NASA mission to visit multiple
planets.[185] Mariner 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 that were later ascribed to the effect of the planet shrinking slightly as its iron core
cools.[186] Unfortunately, the same face of the planet was lit at each of Mariner 10's close approaches.
This made close observation of both sides of the planet impossible, [187] and resulted in the mapping of
less than 45% of the planet's surface.[188]
The spacecraft made three close approaches to Mercury, the closest of which took it to within
327 km (203 mi) of the surface.[189] 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 Earth's, which deflects the solar wind around the planet. For
many years after the Mariner 10 encounters, the origin of Mercury's magnetic field remained the
subject of several competing theories.[190][191]
On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Because
its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut
down.[192] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few
months.[193]
MESSENGER
Main article: MESSENGER

Estimated details of the impact of MESSENGER on April 30, 2015

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment,

GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of 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.[194] A first fly-by of Mercury occurred on January 14, 2008, a second on October
6, 2008,[195] and a third on September 29, 2009.[196] Most of the hemisphere not imaged by Mariner
10 was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the
planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The
probe finished a one-year mapping mission,[195] and then entered a one-year extended mission into
2013. In addition to continued observations and mapping of Mercury, MESSENGER observed the
2012 solar maximum.[197]
The mission was 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 carried imaging devices that gathered
much-higher-resolution images of much more of Mercury than Mariner 10, assorted spectrometers to
determine abundances of elements in the crust, and magnetometers and devices to measure
velocities of charged particles. Measurements of changes in the probe's orbital velocity were
expected to be used to infer details of the planet's interior structure. [198] MESSENGER's final
maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015. [199][200][201] The
spacecraft's impact with Mercury occurred near 3:26 PM EDT on April 30, 2015, leaving a crater
estimated to be 16 m (52 ft) in diameter.[202]
BepiColombo
Main article: BepiColombo
The European Space Agency and the Japanese Space Agency developed and launched a joint
mission called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the
other to study its magnetosphere.[203] Launched on October 20, 2018, BepiColombo is expected to
reach Mercury in 2025.[204] It 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 one
terrestrial year.[203] The mapper probe carries 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