NEPTUNE
Data from Wikipedia and NASA (C)

Neptune is the eighth and last planet of the solar system in order of distance from the Sun; it was discovered in 1846, and it came attributed the name of the roman divinity. It is the smallest between the four gaseous giants of the solar system; although its mass is greater of that one of Uranus, its dimensions is a few inferior. Visited for the first time from the probe Voyager 2 in 1989, Neptune appears like a blue disc furrowed from powerful winds and atmospheric storms of important dimensions; it is encircled from small rings and numerous natural satellites.

Historical signals

The first sure observation of Neptune was made from Galileo Galilei 27 December 1612; it designed the position of the planet on the own astronomical papers, exchanging it for one fixed star. For a fortuitous coincidence, in that period the appearing motion of Neptune was exceptionally slow, and it could not be discovered from Earth by means of the ancient instruments of Galilei. When in the 1821 Alexis Bouvard published the first study of the orbital parameters of Uranus was clear the astronomers that the motion of the planet diverged in appreciable way from the theoretical forecasts; the phenomenon could be explained only with the presence of an other body of remarkable dimensions in the external regions of the solar system. Independently between they, English mathematician John Couch Adams (in the 1843) and Urbain French the Verrier (in 1846) calculate with good approximation position and mass of this presumed new planet. While the searches of Adams came neglected from the British astronomer George Airy, which it had addressed in order to emphasize the necessity to search the new planet in the found position, those of Verrier came applied from two astronomers of the Observatory of Berlin, Johann Gottfried Galle and Heinrich d' Arrest: after less than half hour from the beginning of the searches, the 23 september 1846, they discovered the planet, less than a degree from the previewed position. To the age, independently from Adams, also Urbain the Verrier had calculated the presumed orbital parameters of the new planet. Because of its great distance, the acquaintances on Neptune remained fragmentary at least until the half of the 1900's, when Gerard Kuiper discovered its second moon, Nereide (Tritone had been discovered from William Lassell already little months after the discovery of the planet). In years seventy and eighty were accumulated indications on the probable ring presence, or arch of rings. In August 1989 the acquaintances will receive one enormous push in ahead from the visit to Neptune of the first automatic probe sended to explore the outskirtses of the planet, Voyager II. The probe discovered important details of the atmosphere of the planet, confirmed the existence of five rings and discovered new satellites beyond to those already discoveries from earth.

Observations from Earth

Neptune Earth is invisible to nacked eye from Earth; its appearing magnitudine, always comprised between 7,7 and 8,0, need at least of binoculars in order to allow the location of the planet. To the telescope Neptune appears like a small blue-green disk, similar to Uranus; the color has had to the presence of methane in the nettunian atmosphere, in reason of 2%. With an mass equal approximately to 17 times that earthling and a medium density of 1,64 times that one of the water, Neptune is smallest and denser between giant planets of the solar system. Its equatorial radius, placing the zero altimetric to the quota in which the atmospheric pressure it is 1000 hPa, is of 24 764 km. The dimensions of Neptune are inferior regarding those of Uranus (whose medium radius is equal to approximately 25 600 km), but its mass is greater. While the magnetic field of the planet wheel around the axis in 16,11 hours, the equatorial clouds complete a rotation in approximately 19,2 hours; this is a case of differentiates rotation.

Atmosphere

The neptunian atmosphere appears typically blue, but little uniform regarding that one than Urano. To the height of the Equator it is possible to observe bands parallels that cross it; to the age of the overflight from part of Voyager 2 was moreover present a extemporaneous formation, named Great Dark Spot, from the equal extension to approximately 10 000 km. In the years 1990 successive observations carried out by means of the Hubble telescope the spot was disappeared. Although the main members of the atmosphere are hydrogen and helium, it is methane (present in reason of 2%) to regulate of the meteorological phenomena. The methane molecules of the high atmosphere divide in fact in hydrocarbons, which the ethane and the acetylene, for effect of the solar radiation (900 times less than that one found on the Earth). Also the white clouds observed from the probe Voyager 2 in the 1989 are probably composed of iced methane crystals. Methane that composes the atmosphere is also responsible of the absorption of the red light, giving to the planet its characteristic coloration green-blue, so that the planet is nicknamed the "blue planet". The neptunian atmosphere is center of violent atmospheric phenomena; the winds are most powerful known in the Solar Sistem, arriving to 700 km/h. The energy necessary in order to support them cannot be supplied from the Sun, too much far away, but instead it is generated to the inside of the planet. The temperature to the topographical level of reference (defined as the quota to which the atmospheric pressure is worth 1000 hPa) is of approximately 70K (-200 °C), but it increases more is come down in depth; this implies the presence of one source of inner heat, probably responsible also of the complexity of the atmospheric formations of Neptune in comparison to those of Uranus. For against, after a short pause the atmospheric temperature resumes to grow also to increasing of the altitude, being caught up 130 K to 250 km of height regarding the level of the previous thermal maximum. The atmosphere of Neptune is divided in two main layers: the first one, situated to the -40 km regarding the quota where pressure is worth 1000 hPa, is discovered from temperatures in the order of 130 K and pressures of 3 atmospheres; it is interested from the presence of cloud of ammonia and solfuro of hydrogen. The second layer of the atmosphere, situated to the topographical level of reference, is made up mainly of methane. In the advanced part of the atmosphere they are present hydrocarbon fogs deriving from the dissociation of methane. Similarly to the atmosphere of Jupiter and Saturn, also that one of Neptune introduces great meteorological formations from the aspect similar to great spots. Most pronounced it is sure the Great Dark Spot, observed from Voyager 2 in 1989 but absent in the successive observations of the 1994 carried out by means of the telescope spaces them Hubble. Structure analogous to the hole of ozone on the earth rather than a storm. A few months after the first observations of Hubble, successive photographies of Neptune have revealed the presence of one new Dark Spot, in the boreal hemisphere of the planet. They are also present true and own atmospheric clouds, like the so-called Scooter; one thinks that they have origin from warm and relatively deep spots, that they provoke ascending currents of solfuro of hydrogen able to penetrating through methane clouds. Confronting the observations of Neptune carried out between 1996 and 2002 has been possible to characterize a discreet increase of the total Albedo of the planet (in the order of the 5-10%); the phenomenon is in truth legacy to an sensitive increase of riflettivity limited to some narrow bands, than in some cases can arrive to +100%, such variation is connected to the cycle of the seasons. The cycle ages them on Neptune is approximately 165 times slower that on the Earth, and the maximum variation in the amount of solar light incident is beyond 900 times smaller of the correspondent earth value; a simple model based on the variations ages them of energy incident is sufficient to justify the strong variations of albedo recorded experimentally. Because of the interval of time necessary because the superficial layers of the planet catch up the thermal equilibrium, Neptune catches up its maximum brightness approximately fifteen years after every solstizio; one previews that the next maximum value of Albedo will come caught up within 2025.

Inner structure

The inner structure of the planet remembers that one of Uranus. Neptune appears equipped of a superficial layer composed of hydrogen, helium and ammonia, situated under the level of clouds; more in depth, until approximately 8000 km from the visible surface of the planet, has beginning the cape, composed from ice of water, ammonia and methane. Apparently the cape is also center of the magnetic field of Neptune. Under the cape is found a envelope of molecular hydrogen and helium ; the temperature of the region catches up the 2500 K, and the pressure grazes 200 000 atmospheres. The nucleus of the planet, from the equal radius to approximately 7500 km, is finally rich of iron and other full of rocks materials; its temperature exceeds that one of the solar fotosfera, attesting itself around the 6500-7000 K. In the course of the fly-by of Neptune of the 1989 from part of the probe Voyager 2 it was possible, for the first time, to realize a model of the inner structure of the blue planet, that it remembers give that one of Urano close.

Natural satellites

Neptune possess thirteen known natural satellites, the greater one of which are Tritone; the other main satellites are Nereide, Proteo and Larissa. Tritone is the only satellite of Neptune who possesses a ellissoidale shape; it was discovered for the first time from astronomer William Lassell only 17 days after the discovery of the planet mother. Orbit in retrograde direction regarding Neptune, with the exception of all the other main satellites of the solar system; it is in sincrona rotation with Neptune and its orbit is in constant decay. The more interesting satellite, except Tritone, is Nereide, whose orbit is most eccentric. Between the July and september 1989 the American probe Voyager 2 has discovered six new satellites, like Proteo, whose dimensions would be nearly sufficient to give it a spheroidal shape; it is the second satellite of the system of Neptune, also with a mass equal to 0.25% of that one of Tritone. A new series of discoveries has been announced in 2004; they are smaller and strongly irregular satellites. Chronology of discoveries 1846 - William Lassell discovers to Tritone, seventeen days after to have discovered Neptune. 1949 - Astronomer Gerard Kuiper discovers Nereide from Earth. 1981 - In the course of a stellar occultation it comes discovered, from Earth, the third satellite of Neptune in discovery order, Larissa. 1989 – From the analysis of the images resumed from Voyager 2 are discovered Naiade, Despina, Talassa, Galatea and Proteo, carrying the total of the known nettunian satellites to eight. 2002 - Four smaller satellites of Neptune come discoveries: Neso, Alimede, Laomedea and Sao. 2003 - It comes discovered Psamate.

Nome		Diametro medio	Massa	Raggio orbitale medio	Periodo orbitale	Scoperta
Nettuno III	Naiade	58 km	~0,19×10¹⁸ kg	48 227 km	0,294 giorni	1989
Nettuno IV	Talassa	80 km	~0,37×10¹⁸ kg	50 075 km	0,311 giorni	1989
Nettuno V	Despina	148 km	~2,10×10¹⁸ kg	52 526 km	0,335 giorni	1989
Nettuno VI	Galatea	158 km	~3,70×10¹⁸ kg	61 593 km	0,429 giorni	1989
Nettuno VII	Larissa	208×178 km	~4,90×10¹⁸ kg	73 548 km	0,555 giorni	1981
Nettuno VIII	Proteo	436×416×402 km	~50×10¹⁸ kg	117 647 km	1,122 giorni	1989
Nettuno I	Tritone	2700 km	21,4×10²¹ kg	354 800 km	-5,877 giorni	1846
Nettuno II	Nereide	340 km	~31×10¹⁸ kg	5 513 400 km	0,99 anni	1949
Nettuno IX	Alimede	60 km	~0,09×10¹⁸ kg	15 728 000 km	-5,15 anni	2002
Nettuno XI	Sao	38 km	~0,09×10¹⁸ kg	22 422 000 km	7,98 anni	2002
Nettuno XII	Laomedea	38 km	~0,09×10¹⁸ kg	23 571 000 km	8,67 anni	2002
Nettuno X	Psamate	28 km	~0,015×10¹⁸ kg	46 695 000 km	-24,96 anni	2003
Nettuno XIII	Neso	60 km	~0,09×10¹⁸ kg	48 387 000 km	-25,67 anni	2002

Rings

Neptune possess planetary rings, whose composition is still unknow. Perhaps their structure seems irregular, because of the gravitational interactions with the satellites of the planet. The rings were discoveries from the probe Voyager 2 in 1989. The main ring, Adams, is constituted from three arch of ring main. The existence of similar structures has not been still totally justified; normally us one would expect uniform powder distribution and small iced bodies on the entire orbit around the planet. Some think that the gravitational attraction of Galatea can be to the base of the irregolarity observed. Voyager 2 captured the various images of other rings, between which the Verrier and the tenuous Galle ring. Observations conduct from Earth in the 2005 have carried to assume that the ring system of Neptune is extremely unstable; it appears that the Liberté ring could disappear within the end of XXI century.

Exploration of Neptune

The only probe spaces them to have visited Neptune has been Voyager 2, in 1989; with a drawn near overflight of the planet the Voyager has allowed to characterize of the main atmospheric formations, some rings and numerous satellites. On 25 August 1989 the probe has flown over the North Pole of Neptune to a quota 4950 km, in order then to head towards Tritone, the greater satellite, catching up a minimal distance of approximately 40 000 km. After the last scientific measures, lead during the phase of leaving from the gaseous giant, on 2 October 1989 all the instruments of the probe has been extinguished, leaving in function only the ultraviolet spectrometer. Voyager 2 began therefore a long march towards the interstellar space, to the speed of 470 million km to the year; the inclination of its trajectory regarding the ecliptic is of approximately 48°. One thinks that, with this speed , Voyager 2 will catch up the system of Sirio in year 358 000. In order to pick up the weakest signals of the probe was necessary to largely improve the techniques of reception of the data and to prepare new antennas quickly in order to implement the Deep Space Network; thanks to all these sagacities were possible to receive signals of Voyager 2 to the same speed exactly with which it had been possible to make it in occasion of the overflight of Urano, beyond three years and means ago.