 |
| Urbain Le Verrier. |
History
Discovery
The first possible sighting of Neptune is thought to be
by Galileo as his drawings showed Neptune near Jupiter.
But Galileo was not credited for the discovery since he
thought Neptune was a "fixed star" instead of a planet.
Because of Neptune's slow movement across the sky,
Galileo's small telescope was not strong enough to
detect Neptune as a planet.
In 1821, Alexis Bouvard published the astronomical
tables of the orbit of Uranus. Later observations showed
that Uranus was moving in an irregular way in its orbit,
making some astronomers think of another large body
being the cause of the irregular motions of Uranus. In
1843, John Couch Adams calculated the orbit of an eighth
planet that would possibly be influencing the orbit of
Uranus. He sent his calculations to Sir George Airy, the
Astronomer Royal, who asked Adams for an explanation.
Adams began to make a copy of the reply, but never sent
it.
In 1846, Urbain Le Verrier, who was not working with
Adams, made his own calculations but also failed to get
much attention from French astronomers. However, in the
same year, John Herschel began to support the
mathematical method and encouraged James Challis to
search for the planet. After much delay, Challis began
his unwilling search in July 1846. Meanwhile, Le Verrier
had convinced Johann Gottfried Galle to search for the
planet.
Although Heinrich d'Arrest was still a student at the
Berlin Observatory, he suggested that a newly drawn map
of the sky, in the region of Le Verrier's predicted
area, could be compared with the current sky to look for
the displacement characteristic of a planet, as compared
to a fixed star. Neptune was then discovered that very
night on September 23, 1846, within 1° of where Le
Verrier had predicted it to be, and about 10° from
Adams' prediction. Challis later found out that he had
seen the planet twice in August, failing to recognize it
owing to his careless approach to the work.
After the news of the discovery of Neptune spread, there
was also a lot of arguing between the French and the
British about who was to be deserved credit for the
discovery. Later, an international agreement decided
that both Le Verrier and Adams together deserved credit.
However, historians are now reviewing the topic after
the rediscovery in 1998 of the "Neptune papers"
(historical documents from the Royal Greenwich
Observatory), which had seemingly been stolen by
astronomer Olin Eggen for nearly three decades and were
only rediscovered (in his ownership) right after his
death. After reviewing the documents, some historians
now think that Adams does not deserve equal credit with
Le Verrier.
Naming
Shortly after its discovery, Neptune was temporarily
called "the planet exterior to Uranus" or "Le Verrier's
planet". The first suggestion for a name came from Galle.
He proposed the name Janus. In England, Challis
suggested the name Oceanus. In France, Arago suggested
that the new planet be called Leverrier, a suggestion
which was met with a lot of opposition outside France.
French almanacs promptly reintroduced the name Herschel
for Uranus and Leverrier for the new planet.
Meanwhile, on separate and different reason, Adams
suggested changing the name Georgian to Uranus, while
Leverrier (through the Board of Longitude) suggested
Neptune for the new planet. Struve gave support of that
name on December 29, 1846, to the Saint Petersburg
Academy of Sciences. Soon Neptune was internationally
agreed among many people and was then the official name
for the new planet. In Roman mythology, Neptune was the
god of the sea, identified with the Greek god, Poseidon. |
|
 |
| The internal
structure of Neptune: |
- 1. Upper atmosphere, top clouds
- 2. Atmosphere consisting of hydrogen,
helium and methane gas
- 3. Mantle consisting of water, ammonia
and methane ices
- 4. Core consisting of rock (silicates
and nickel–iron)
|
Structure
Mass and composition
At 10.243×1025 kg, Neptune's mass puts the planet
between Earth and the largest gas giants; Neptune has
seventeen Earth masses but just 1/18th the mass of
Jupiter. Neptune and Uranus are often considered to be
part of a sub-class of gas giant known as "ice giants",
given their smaller size and big differences in
composition compared to Jupiter and Saturn. In the
search for extrasolar planets, Neptune has been used as
a reference to determine the size and structure of the
discovered planet. Some discovered planets that have
similar masses like Neptune are often called "Neptunes".
just as astronomers refer to various extra-solar "Jupiters."
The atmosphere of Neptune is made up mostly of hydrogen,
with a smaller amount of helium. A tiny amount of
methane is also detected in the atmosphere. Important
absorption bands of methane happen at wavelengths above
600 nm, in the red and infrared portion of the spectrum.
This absorption of red light by the atmospheric methane
gives Neptune its blue hue.
Because Neptune orbits so far from the Sun, it gets very
little heat with the uppermost regions of the atmosphere
at −218 °C (55 K). Deeper inside the layers of gas,
however, the temperature rises slowly. Like Uranus, the
source of this heating is unknown, but the differences
are larger: Neptune is the farthest planet from the Sun,
yet its internal energy is strong enough to create the
fastest winds seen in the Solar System. Several possible
explanations have been suggested, including radiogenic
heating from the planet's core, the continued radiation
into space of leftover heat made by infalling matter
during the planet's birth, and gravity waves breaking
above the tropopause.
The structure of the inside of Neptune is thought to be
very similar to the structure of the inside of Uranus.
There is likely to be a core, thought to be about 15
Earth masses, made up of molten rock and metal
surrounded by a mixture of rock, water, ammonia, and
methane. The heavy pressures keep the icy part of this
surrounding mixture as solids, in spite of the large
temperatures near the core. The atmosphere, extending
about 10 to 20% of the way towards the center, is mostly
hydrogen and helium at high altitudes. More mixtures of
methane, ammonia, and water are found in the lower areas
of the atmosphere. Very slowly this darker and hotter
area blends into the superheated liquid interior. The
pressure at the center of Neptune is millions of times
more than that on the surface of Earth. Comparing its
rotational speed to its degree of oblateness shows that
it has its mass less concentrated towards the center
unlike Uranus. |
 |
| The Great Dark Spot
(top), Scooter (middle white cloud), and the
Small Dark Spot (bottom), with contrast
exaggerated. |
Weather and magnetic field
One difference between Neptune and Uranus is the level
of meteorological activity that has been observed (seen
or measured). When the Voyager spacecraft flew by Uranus
in 1986, that winds on that planet were observed to be
mild. When Voyager flew by Neptune in 1989, powerful
weather events were observed. The weather of Neptune has
extremely active storm systems. Its atmosphere has the
highest wind speeds in the solar system, thought to be
powered by the flow of internal heat. Regular winds in
the equatorial region have speeds of around 1,200 km/h
(750 mph), while winds in storm systems can reach up to
2,100 km/h, near-supersonic speeds.
In 1989, the Great Dark Spot, a cyclonic storm system
the size of Eurasia, was discovered by NASA's Voyager 2
spacecraft. The storm resembled the Great Red Spot of
Jupiter. However, on November 2, 1994, the Hubble Space
Telescope did not see the Great Dark Spot on the planet.
Instead, a new storm similar to the Great Dark Spot was
found in the planet's northern hemisphere. The reason
why the Great Dark Spot has disappeared is unknown. One
possible theory is that heat transfer from the planet's
core disrupted the atmospheric balance and existing
circulation patterns. The Scooter is another storm, a
white cloud group farther south than the Great Dark
Spot. Its nickname was given when it was first noticed
in the months leading up to the Voyager encounter in
1989: it moved faster than the Great Dark Spot. Later
images showed clouds that moved even faster than
Scooter. The Wizard's eye/Dark Spot 2 is another
southern cyclonic storm, the second most strongest storm
seen during the 1989 encounter. It originally was
completely dark, but as Voyager came closer to the
planet, a bright core developed and is seen in most of
the highest resolution images.
Unlike other gas giants, Neptune's atmosphere shows the
presence of high clouds making shadows on a thick cloud
deck below. Though Neptune's atmosphere is much more
active than that of Uranus, both planets are made up of
the same gases and ices. Uranus and Neptune are not
exactly the same type of gas giants like to Jupiter and
Saturn, but are rather ice giants, meaning they have a
larger solid core and are also made of ices. Neptune is
very cold, with temperatures as low as −224 °C (−372 °F
or 49 K) recorded at the cloud tops in 1989.
Neptune also has similarities with Uranus in its
magnetosphere, with a magnetic field strongly tilted
comparative to its rotational axis at 47° and offset at
least 0.55 radii (about 13,500 kilometres) from the
planet's physical center. Comparing the magnetic fields
of the two planets, scientists think the extreme course
may be characteristic of flows in the interior of the
planet and not the result of Uranus' sideways rotational
movement. |
 |
| Four images taken a
few hours apart with the NASA/ESA Hubble Space
Telescope's Wide Field Camera 3. |
Neptune's Rings
Very small blue colored rings have been discovered
around the blue planet, but they are not as well known
as the rings of Saturn. When these rings were discovered
by a team led by Edward Guinan, originally they thought
that the rings may not be complete rings. However, this
was proven wrong by Voyager 2. Neptune's planetary rings
have a weird "clumpy" arrangement. Although the cause is
currently unknown but some scientists think that it may
be because of the gravitational contact with small moons
that orbit near them.
Proof that the rings are incomplete first began in the
mid-1980s, when stellar occultation were found to rarely
show an extra "blink" just before or after the planet
occulted the star. Pictures from Voyager 2 in 1989
solved the problem, when the ring system was found to
have several faint rings. The farthest ring, Adams, has
three famous arcs now named Liberté, Egalité, and
Fraternité (Liberty, Equality, and Fraternity).
The existence of arcs is very hard to understand because
the laws of motion would predict that arcs spread out
into a single ring in a very short time. The
gravitational effects of Galatea, a moon just inward
from the ring, are now thought to have created the arcs.
Several other rings were discovered by the Voyager
cameras. Also with the thin Adams Ring about 63,000 km
from the center of Neptune, the Leverrier Ring is at
53,000 km and the wider, smaller Galle Ring is at 42,000
km. A very small outward expansion to the Leverrier Ring
has been named Lassell; it is surrounded at its outer
edge by the Arago Ring at 57,000 km.
New Earth-based observations published in 2005 appeared
to show that Neptune's rings are a lot more unstable
than thought before. To be exact, it looks like that the
Liberté ring might disappear maybe quickly in less than
100 years. The new observations seems to puzzle our
understanding of Neptune's rings into a lot of
confusion.
Moons
Neptune has 14 known moons. Triton is the largest
Neptunian moon, comprising more than 99.5% of the mass
in orbit around Neptune, and it is the only one massive
enough to be spheroidal. Triton was discovered by
William Lassell just 17 days after the discovery of
Neptune itself. Unlike all other large planetary moons
in the Solar System, Triton has a retrograde orbit,
indicating that it was captured rather than forming in
place; it was probably once a dwarf planet in the Kuiper
belt. It is close enough to Neptune to be locked into a
synchronous rotation, and it is slowly spiralling inward
because of tidal acceleration. It will eventually be
torn apart, in about 3.6 billion years, when it reaches
the Roche limit. In 1989, Triton was the coldest object
that had yet been measured in the Solar System, with
estimated temperatures of 38 K (−235 °C).
Neptune's second known satellite (by order of
discovery), the irregular moon Nereid, has one of the
most eccentric orbits of any satellite in the Solar
System. The eccentricity of 0.7512 gives it an apoapsis
that is seven times its periapsis distance from Neptune.
From July to September 1989, Voyager 2 discovered six
moons of Neptune. Of these, the irregularly shaped
Proteus is notable for being as large as a body of its
density can be without being pulled into a spherical
shape by its own gravity. Although the
second-most-massive Neptunian moon, it is only 0.25% the
mass of Triton. Neptune's innermost four moons—Naiad,
Thalassa, Despina and Galatea—orbit close enough to be
within Neptune's rings. The next-farthest out, Larissa,
was originally discovered in 1981 when it had occulted a
star. This occultation had been attributed to ring arcs,
but when Voyager 2 observed Neptune in 1989, Larissa was
found to have caused it. Five new irregular moons
discovered between 2002 and 2003 were announced in 2004.
A new moon and the smallest yet, Neptune XIV, was found
in 2013. Because Neptune was the Roman god of the sea,
Neptune's moons have been named after lesser sea gods. |
|
 |
| A size comparison of
Neptune and Earth. |
Observation
Neptune cannot be seen with naked eye alone, since
Neptune's normal brightness are between magnitudes +7.7
and +8.0, which can be out-shined by Jupiter's Galilean
moons, the dwarf planet Ceres, and the asteroids 4 Vesta,
2 Pallas, 7 Iris, 3 Juno and 6 Hebe. A telescope or
strong binoculars will show Neptune as a small blue dot,
similar in appearance to Uranus. The blue color comes
from the methane in its atmosphere. Its small obvious
size has made it difficult to study visually; most
telescopic data was quite limited until the arrival of
the Hubble Space Telescope and large ground-based
telescopes with adaptive optics.
Exploration
Voyager 2 is the only spacecraft that has visited
Neptune. The spacecraft's closest approach to the planet
occurred on 25 August 1989. Because this was the last
major planet the spacecraft could visit, it was decided
to make a close flyby of the moon Triton, regardless of
the consequences to the trajectory, similarly to what
was done for Voyager 1's encounter with Saturn and its
moon Titan. The images relayed back to Earth from
Voyager 2 became the basis of a 1989 PBS all-night
program, Neptune All Night.
During the encounter, signals from the spacecraft
required 246 minutes to reach Earth. Hence, for the most
part, Voyager 2's mission relied on preloaded commands
for the Neptune encounter. The spacecraft performed a
near-encounter with the moon Nereid before it came
within 4,400 km of Neptune's atmosphere on 25 August,
then passed close to the planet's largest moon Triton
later the same day.
The spacecraft verified the existence of a magnetic
field surrounding the planet and discovered that the
field was offset from the centre and tilted in a manner
similar to the field around Uranus. Neptune's rotation
period was determined using measurements of radio
emissions and Voyager 2 also showed that Neptune had a
surprisingly active weather system. Six new moons were
discovered, and the planet was shown to have more than
one ring.
The flyby also provided the first accurate measurement
of Neptune's mass which was found to be 0.5 percent less
than previously calculated. The new figure disproved the
hypothesis that an undiscovered Planet X acted upon the
orbits of Neptune and Uranus.
After the Voyager 2 flyby mission, the next step in
scientific exploration of the Neptunian system, is
considered to be a Flagship orbital mission. Such a
hypothetical mission is envisioned to be possible in the
late 2020s or early 2030s. However, there have been
discussions to launch Neptune missions sooner. In 2003,
there was a proposal in NASA's "Vision Missions Studies"
for a "Neptune Orbiter with Probes" mission that does
Cassini-level science. Another, more recent proposal was
for Argo, a flyby spacecraft to be launched in 2019,
that would visit Jupiter, Saturn, Neptune, and a Kuiper
belt object. The focus would be on Neptune and its
largest moon Triton to be investigated around 2029. The
proposed New Horizons 2 mission (which was later
scrapped) might also have done a close flyby of the
Neptunian system. |
|
 Kiddle:
Neptune
Wikipedia: Neptune |
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