Data from Wikipedia and NASA (C)
Mercury is
the first planet of the solar system in order of distance from the Sun and is
the smallest in dimensions. It is a planet of modest dimensions, with an
diameter inferior to the half of that earthling; many craterized, also because
of the lack of an appreciable atmosphere that can mitigate the meteoric
impacts, and for this its aspect remembers they of the Moon close.
It is the
most close planet to the sun, not simply osservable, but however already famous
to the ancient populations, like Egizi, Chinese, Sumeri (third millenium a.C.).
The difficulties of seeing depend on the small distance from the Sun, than
disturbs always the vision of it during the twilight or little before the dawn.
The Sumeri called it Ubu-idim-gud-ud; the Babylonian, than has handed to us the
first detailed observation of planets, used the names gu-to or gu-utu. The
Greeks assigned to Mercury two names: Apollo, the star of the morning, and
Hermes, the star of the evening. The realization of the fact that was be a
matter of an only planet is attributed to Pitagora. Always because of the great
osservative difficulties, until the ' 900 it was common opinion that in truth
an other planet was most close of Mercury to the Sun still, Volcano, later on
identified with the same celestial body. More recently, in the 1631 Pierre
Gassendi was the first one to observe a transit of Mercury in front of the Sun,
second the forecasts supplied from Giovanni Keplero. In the 1639 Giovanni Zupi
Batiste, using a telescope, discovered the phases of Mercury, analogous to
those of Venus and the Moon. This supplied the definitive test that Mercury
orbits around to the Sun. Only in years Sixty of XX century, thanks to the
observations radio and radar, the period of rotation of the planet has been
estimated with precision, than before it was believed equal to that one of
revolution.
Dealing of an
inner planet regarding the Earth, Mercury appears always much neighbor to the
Sun (the its maximum elongazione is of 28,3°), so that the Earth’s telescopes
can observe it of only rarely. Its appearing magnitude oscillates between -0,4
and +5,5 according to its position regarding the Earth and to the Sun. During
the day the solar brightness prevents every observation, and the direct
observation is possible only after the sunset, on the horizon to the west, or
little before the dawn towards east. Moreover the extreme brevity of its motion
of revolution (only 88 days) allows its observation only for little consecutive
days, after that the planet becomes inosservable from Earth. Like in the case
of the Moon and Venus, also for Mercury is visible, from Earth, a cycle of phases.
The orbit
of Mercury is elliptic in first approximation; it is in fact subject to the precession
of the perihelion, effect that put in difficulty the astronomers of XIX
century. It turns out explicable at the present moment only through the theory of the general relativity,
than just on this phenomenon it has had one of its test benches. Mercury moves
on an orbit of eccentricity 0,2056, to a distance from the Sun comprised
between 46 and 69 million km, with a medium value of 58 million km (respective
0,307, 0,466 and 0,387 astronomical units). The sidereal period of Mercury is
88 days, while the sinodic period is 115,9 days. The orbital plan is tilted on
the ecliptic of 7º. The Mercury’s orbit is subject to variations, due to the
perturbations from part of other planets; the phenomenon is particularly
studied and known as far as the motion of the line of the apsidi, that it
supplies one of the tests experiences of the theory of the general relativity.
The sidereal average speed of the planet is equal to 48 km/s; it is the highest
between planets of the solar system. The mercurian motion of rotation is much
slow: it employs 58,6 days in order to complete a turn on if same, and
completes therefore three rotations every two revolutions (a clear example of
orbital resonance). This makes yes that the duration of the exposure to the
solar beams for every point of its surface is many elevated (176 days): Mercury
is the only planet of the solar system on which the duration of the day,
understanding like insolation, is greater of a period of revolution. A particular
phenomenon relative to Mercury is the so-called precession of the perihelion,
that is the perihelion of its orbit rotates progressively around the Sun in the
same direction of the planet; the entity of the movement exceeds of 43 second
of arc for century according to the Newtonian mechanics and can be justified
with discreet approximation applying the relativistiche corrections, but it is
still surveying object. The Mercury inclination regarding the plan of its
orbit, is only 2°, and renders minims the variations of insolation during the
seasons, rendering the Equator particularly warm and considerably cold the
polar regions in whichever period of the year, without appreciable differences
between the two hemispheres. In polar craters perennially protected from the
beams of the Sun it could be contained iced water. Mercury is a decidedly small
planet, being characterized from a mass equal to 0,055 earth’s masses and from
a diameter equivalent to approximately 0,38 times that earthling; its medium
density is of 2% inferior to that one of the Earth.
Analogous
to the Moon, because of its low gravitational attraction Mercury is unprovided
of atmosphere, made exception for exiles gas traces probably fruit of the
interaction of the solar wind with the surface of the planet. The atmospheric
composition has been determined like follows: potassio (31.7%), sodio (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%). The
atmospheric pressure to the ground, measured from the probe Mariner 10, is in
the order of thousandth of pascal. Because of the absence of a mechanism of
distribution of the heat received from the Sun and of its extremely slow rotation,
that the same hemisphere is exposed to the directed solar light for long
periods, the thermal excursion on Mercury is the most elevated in the entire
solar system; the illuminated hemisphere catches up the 600 K (700 K in the
equatorial zones), that one in shadow often comes down until 90 K.
Similarly
to the Moon, the mercurian ground is wide craterized because of the numerous
asteroid impacts that have marked its past and has river basins filled up from
old lavic taps, still obvious because of the nearly absolute lack of an
atmosphere. Some craters are encircle from radii. The presence of tettonic
plates is excluded on the planet. In truth, not only Mercury and the Moon have had
hit with meteorites; it is however normal that the planets in possession of an consisting
atmosphere resent in much smaller measure of the effect of the impacts, since
the incidents bodies come strongly eroded from the atmospheric friction.
Moreover the same atmosphere slowly erodes the surface of the planet,
cancelling the traces of the collision. The smaller craters of Mercury have diameter smaller of
Beyond to
craters, the more important structures of the surface are wide plainy zones,
similar to the seas of the Moon. The greater one is Planitia Caloris (from the
Latin planitia, that it means plain). The plains of Mercury, analogous to the
lunar seas but with a greater albedo, come said planitiae. With the exception
of Planitia Caloris, the most extended plainy region is Borealis Planitia, that
it occupies part of the northern hemisphere, all the other plains take the
Mercury name in the various languages. The four valley (valles) take to the
name of the radio installations on the Earth thanks to which it has been
determined the effective period of rotation of mercury. The names of slopes
(rupes) derive from the ships used for the great explorations of XV century,
with reference to the Mercury role like God of the travels and the commerce.
Two slopes take the name of dorsa and are entitle to astronomers who observed
Mercury from the Earth. The names of craters derive from the great exponents of
the arts and the umanesimo, except the crater Kuiper, that it was the first one
to be discovered being in how much visible one from far away and was entitled to
the astronomer, and the crater Hun Kal, from the Mayan term "20",
since conventionally is crossed from the twentieth meridian of longitude and is
the point of reference for the mercurian system of coordinates.
The density
of Mercury, equal to 5,43 kg/dm³, is very different much from that lunar one
and, to the contrary, is much neighbor to that earthling. This leaves to
suppose that, in spite of the likenesses with the Moon, the inner structure of
the planet is most near that one of the Earth, with a particularly massive
nucleus (until 80% of the mercurian radius) formed from heavy elements.
Searches published in 2007 on Science, lead with radar of high precision in the
Mercury is
visited for the first time in the 1974-75 from the American probe Mariner 10,
that it has televised to earth photographies recorded in the course of three
successes to you overflights. Conceived for the observation of Venus and Mercury,
Mariner 10 came launch on 3 November 1973 and caught up the planet in 1974. The
American probe was approached until to some hundreds of kilometers from the
planet, transmitting approximately 6000 photographies and mapping 40% of the
mercurian surface. NASA has launch in 2004 probe MESSENGER, whose first drawn
near Mercury passage, previewed for 14 January 2008, will be followed from
others two encounter (October 2008 and september 2009) before income in orbit around
the planet previewed for 18 March 2011. For 2009 instead is previewed the
launch, from part of the ESA, the mission spaces BepiColombo, so called in
honor of the scientist, mathematician and engineer Giuseppe Columbus (1920-1984), for the exploration of
the inner planet. The Mercury exploration has not been thought primary from the
agencies spaces for two reasons: Mercury is a planet from the excessive
temperatures, is not thought place adapted to the existence of life shapes,
neither it appears simple to take advantage of the resources, and from one
first analysis of our acquaintances was derived the idea that Mercury was the
much most similar one to the Moon of how much does not join today and that
therefore it would have been enough to study this last one for derivere
information on the planet. An other important reason that it has contributed to
reduce the number of missions towards Mercury is the high intrinsic cost
dictated from the vicinity of the Sun, that it determines: the necessity that
every probe is equipped of a heavy thermal shield , an elevated fuel
consumption in order to contrast the increment in the attractive force of the
Sun, an elevated fuel consumption in order to place and to maintain the probe
in orbit around to the planet, without the aerobraking or the parachute in order to help the landing
of a lander.