Mercury is the closest planet to the Sun and, with a diameter of 4,880 kilometres (40% smaller than Earth and 40% greater
than the Moon), is the second smallest - larger only than Pluto, which is the most distant planet in our solar system. It
has a highly eccentric orbit that takes the planet from 47 million kilometres from the Sun at perihelion to 70 million
kilometres from the Sun at aphelion. Mercury completes an orbit around the Sun every 88 days at a pace of nearly 50
kilometres per second – faster than any other planet.
Mercury has no known satellites and a very thin atmosphere composed mainly of helium and sodium.
Until the early 1960s it was thought that Mercury`s day was the same length as its year, always keeping the same face to
the Sun (as the Moon does to the Earth). It is now known that Mercury rotates on its axis three times during two of its
years. The combination of a slow rotation (59 Earth days) and a rapid revolution around the Sun (88 Earth days) means that
one Mercury solar day takes 176 Earth days or two Mercury years.
The consequence of Mercury`s unusual orbit and rotation combined would cause very strange effects for observers on the
planet`s surface. At some longitudes observers would see the Sun rise, gradually increase in apparent size moving slowly
toward the zenith where it would stop, briefly reverse course, and stop again before resuming its path toward the horizon
and decreasing in apparent size. Observers at other points on Mercury's surface would see different, but equally odd,
motions. The Sun would look very large (nearly three times as large as from Earth) and the sky would be black due to lack of
atmosphere to cause scattering of light. Earth would be seen as a blue-coloured `star`.
At its closest to the Sun, temperatures on the surface can reach a sizzling 467ºC on the sunlit side but, because Mercury
has very little atmosphere to keep it warm, temperatures can drop to a freezing –183ºC on the dark side. This range in
surface temperature is the largest for any body in the solar system. The temperature on Venus is slightly hotter but more
Mercury is visible to the naked eye only during the intervals when it is far enough away from the Sun in the sky. These
intervals vary from a week to about two weeks and there are usually six of them in each year. Each one is centered around
the time of greatest elongation. At a maximum elongation of 28º from the Sun as seen from Earth, Mercury is difficult to
see except during twilight, just after sunset or before dawn. During daylight the solar glare makes observation difficult
and during twilight the planet is so low on the horizon that its light must pass through ten times the amount of Earth`s
atmosphere than if the planet were directly overhead. Even through powerful telescopes, observations from Earth lack surface
detail but its phases, similar to the Moon and Venus, might be seen. The best times to observe Mercury are when it is an
evening star in the spring and a morning star in the autumn. In midsummer the lighter skies make visibility difficult close
to the horizon.
Changing Phase & Apparent Size of Mercury
We say that a planet is in conjunction with another planet or object when the two are in line as viewed from Earth. The inner planets of Venus and Mercury (so called because they revolve around the sun inside of Earth's orbit), can line up at superior conjunction when they lie behind the sun or at inferior conjunction when they lie between the sun and the earth. The outer planets can only line up at superior conjunction as viewed from Earth.
The relative positions of Mercury and the earth are indicated on the diagram, which has been simplified by keeping the earth's position fixed. As Mercury revolves around the sun, its sunlit hemisphere is presented to the earth in varying amounts showing phases resembling those of the moon. They can only be seen through a telescope and were first observed by Galileo in 1610. The full phase is shown at superior conjunction, the quarter phases at greatest elongations and the new phase at inferior conjunction. Usually a thin crescent remains at the new phase because the planet crosses a little above or below the sun.
As Mercury passes from superior to inferior conjunction, its distance from the earth decreases, causing its apparent diameter to increase. However, this is more than offset by the change in phase. The magnitude varies from -2.0 near superior conjunction to 3.0 when approaching inferior conjunction.
After passing greatest eastern elongation, Mercury appears to move westward relative to the sun and passes between the earth and the sun at inferior conjunction. It thus changes from an evening star to a morning star.
As Mercury's orbit is notably elliptical the planet is further from the sun at some elongations than at others, varying from 18-28 degrees. It is this varying brightness that makes favourable apparitions rather infrequent. The best apparitions occur between February and April in the evening and September and November in the morning. In midsummer the lighter skies make visibility difficult near the horizon.
In contrast to the 60+ multi-national missions to Mars, so far only one spacecraft has ever
visited Mercury. This was NASA`s Mariner 10 launched in November 1973. Mariner 10 was the 7th successful launch in the
Mariner series and the first spacecraft to visit two planets and use the gravitational pull of one planet (Venus) to reach
another (Mercury). After a journey of nearly five months, including a flyby of Venus, the spacecraft passed within 703
kilometres of the solar system's innermost planet on March 29 1974. The photographs Mariner 10 radioed back to Earth
revealed an ancient, heavily cratered surface, closely resembling our Moon. The pictures also showed huge cliffs as high
as 3 kilometres and as long as 500 kilometres criss-crossing the planet. Mariner 10's discovery that Mercury has a very
weak magnetic field, similar to but weaker than Earth's, was a major surprise.
After the initial Mercury encounter, Mariner 10 made two further flybys in September 1974 and March 1975 before control
gas used to orient the spacecraft was exhausted and the mission was concluded. Each flyby took place at the same local
Mercury time when the identical half of the planet was illuminated and, as a result, only 45% of the planet's surface was
In 1991, astronomers using radar observations showed that Mercury may have ice at its north and south poles. The ice
appears inside deep craters, the floors of which remain in perpetual shadow, not melted by the Sun.
NASA`s Messenger, launched in August 2004, has embarked upon a route via an Earth flyby (August 2005) and two Venus flybys
(October 2006 and June 2007). There will then be three Mercury flybys in January 2008, October 2008 and September 2009 each
at an altitude of 200 kilometres.
After more than six and a half years and five deep space manoeuvres (course corrections using gravity assist), Messenger
will finally go into orbit around Mercury in March 2011 until the end of the primary mission in March 2012. The goal is to
study the characteristics and environment of Mercury from orbit; specifically the scientific objectives of the mission are
to study the surface composition, geologic history, core and mantle, magnetic field, and tenuous atmosphere of Mercury, and
to search for water ice and other frozen volatiles at the poles.
BepiColumbo is a European Space Agency mission planned to launch in 2012 sending two orbiters to Mercury. The orbiters
will be launched either together via an Ariane rocket or separately in two Soyuz-Fregats. The spacecraft will take an
interplanetary cruise to Mercury lasting more than 4 years using solar-electric propulsion and gravity assists from the
Moon, Venus, and Mercury before going into polar orbit around Mercury. The scientific objectives for the mission are to
study Mercury's form, interior structure, geology, composition, craters, origin, structure, dynamics of its magnetic field,
composition and dynamics of the vestigial atmosphere, test Einstein's theory of general relativity, search for asteroids
sunward of Earth, and to generally study the origin and evolution of a planet close to a parent star. Collaboration on the
mission with the Japanese Space Agency ISAS is being discussed. ISAS may contribute in whole or part to one of the orbiters.
In Roman mythology, Mercury (Hermes in Greek) was the messenger of the gods and the product of a
secret affair between Zeus and the nymph Maia. He was born in a cave on Mount Cyllene. Although best known as a messenger
he undertook other errands for the gods, and the ancient Greeks also regarded him as the god of commerce and trading and
the god of thieves. At other times he was known as the god of travellers, shepherds, cunning, and athletics.
In terms of astrology, Mercury is the planet of intellect, communication, perception and reason.
In its positive form, Mercury endows a person with cleverness, articulate speech, a good memory, public speaking ability and
even a talent for acting and singing. In its negative form, Mercury's influence results in sarcasm, deception, fraud and
argumentativeness. As Mercury is so close to the sun, it always lies within a person's sun sign or a sign immediately next