Despina, Moon of Neptune (APOD 3rd Sept 2009)

[jerbil]

One interesting factor about this stunning picture is that, since the Sun is so distant from Neptune and therefore has a much smaller angular diameter than our half a degree, each of the moons produces a total solar eclipse on the top of the Neptune's cloud layer.

[neufer]

http://apod.nasa.gov/apod/ap090903.html
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<<Neptune has 13 known moons. The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune and the only one massive enough to be spheroidal, is Triton, 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 probably was 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 spiraling inward because of tidal acceleration and eventually will 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 −235 °C (38 K).

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 new Neptunian moons. 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 one-quarter of one percent 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, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.>>

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<<Naiad (pronounced /ˈneɪəd/ NAY-əd, /ˈnaɪəd/ NYE-əd, or as in Greek Ναϊάδ-ες), also known as Neptune III, is the innermost satellite of Neptune named after the Naiads of Greek legend.

Naiad was discovered sometime before mid-September, 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated S/1989 N 6. Naiad is irregularly shaped and probably has not been modified by any internal geological processes after its formation. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.

Naiad orbits about 23,500 km above Neptune's cloud tops. Since this is below the synchronous orbit radius, its orbit is slowly decaying due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Naiad orbits Neptune well within its fluid Roche limit, and its density is expected to be low enough that it may be very close to its actual Roche limit already.

Since the Voyager 2 flyby, the Neptune system has been extensively studied from ground-based observatories and the Hubble Space Telescope as well. In 2002-03 Keck telescope observed the system using adaptive optics and detected easily the largest four inner satellites. Thalassa was found with some image processing, but Naiad was not located. Hubble has the ability to detect all the known satellites and possible new satellites even dimmer than Voyager 2. Still, Naiad has not been found. It is suspected that this is due to considerable errors in Naiad's ephemeris.

Orbital characteristics
Semi-major axis 48 227 ± 1 km
Eccentricity 0.0004 ± 0.0003
Orbital period 0.2943958 ± 0.0000002 d
Inclination 4.75 ± 0.03° (to Neptune equator)

Physical characteristics
Dimensions 96×60×52 km
Mean radius 33 ± 3 km
Mass ~1.9 × 10^17 kg
(based on assumed density)
Mean density ~1.2 g/cm3 (estimate)
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.07
Surface temp. ~51 K mean (estimate)
Atmosphere none>>

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<<Thalassa (pronounced /θəˈlæsə/ thə-LASS-ə, or as in Greek Θάλασσα), also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Thalassa was discovered sometime before mid-September, 1989 from the images taken by the Voyager 2 probe. Thalassa is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit. Unusually for irregular bodies, it appears to be roughly disk-shaped.

Since the Thalassian orbit is below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal decceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Relatively soon after, the spreading debris may impinge upon Despina's orbit.

Orbital characteristics
Semi-major axis 50 075 ± 1 km
Eccentricity 0.0002 ± 0.0002
Orbital period 0.31148444 ± 0.00000006 d
Inclination 0.21 ± 0.02° (to Neptune equator)

Physical characteristics
Dimensions 108×100×52 km
Mean radius 41 ± 3 km
Mass ~3.5 × 10^17 kg
(based on assumed density)
Mean density ~1.2 g/cm3 (estimate)
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.09
Surface temp. ~51 K mean (estimate)
Atmosphere none>>

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<<Despina (pronounced /dɨˈspiːnə/ dis-PEE-nə, sometimes /dɨˈspaɪnə/ dis-PYE-nə, or as in Latin Despœna, Greek Δέσποινα), also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter.

Despina as seen by Voyager 2 (smeared horizontally)


Despina was discovered in late July, 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 3. Despina is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.

Despina's orbit lies close to but outside of the orbit of Thalassa and just inside the Le Verrier ring. As it is also below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal decceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.

Orbital characteristics
Semi-major axis 52 526 ± 1 km
Eccentricity 0.0002 ± 0.0002
Orbital period 0.33465551 ± 0.00000001 d
Inclination 0.216 ± 0.014° (to Neptune equator)

Physical characteristics
Dimensions 180×148×128 km
Mean radius 75 ± 3 km
Mass ~2.1 × 10^18 kg (based on assumed density)
Mean density ~1.2 g/cm3 (estimate)
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.09
Surface temp. ~51 K mean (estimate)
Atmosphere none>>

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Galatea (pronounced /ˈɡæləˈtiːə/ GAL-ə-TEE-ə, or as in Greek Γαλάτεια), also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend.

Galatea was discovered in late July, 1989 from the images taken by the Voyager 2 probe. It is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.

Galatea's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.

Galatea appears to be a shepherd moon for the Adams ring that is 1000 km outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring. Galatea's mass has been estimated based on the radial perturbations it induces on the ring.

Orbital characteristics
Semi-major axis 61 953 ± 1 km
Eccentricity 0.00004 ± 0.00009
Orbital period 0.42874431 ± 0.00000001 d
Inclination 0.052 ± 0.011° (to Neptune equator)

Physical characteristics
Dimensions 204×184×144 km (±~10 km)
Mean radius 88 ± 4 km
Mass 2.12 ± 0.08 × 10^18 kg[6]
Mean density 0.75 ± 0.1 g/cm3
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.08
Surface temp. ~51 K mean (estimate)
Atmosphere none>>

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Explanation: Proteus is the second largest moon of Neptune behind the mysterious Triton. Proteus was discovered only in 1982 by the Voyager 2 spacecraft. This is unusual since Neptune has a smaller moon - Nereid - which was discovered 33 years earlier from Earth. The reason Proteus was not discovered sooner is that its surface is very dark and it orbits much closer to Neptune. Proteus has an odd box-like shape and were it even slightly more massive, its own gravity would cause it to reform itself into a sphere.

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<<Proteus (pronounced /ˈproʊtiəs/, or as in Greek Πρωτεύς), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. It is the largest known non-spherical moon in the solar system. It is named after Proteus, the shape-changing sea god of Greek mythology.

Map of Proteus


Proteus was discovered from the images taken by Voyager 2 probe during the Neptune flyby in 1989. Proteus is more than 400 kilometres in diameter, larger than Nereid, another moon of Neptune. However, it was not discovered by Earth-based telescopes because it is so close to the planet that it is lost in the glare of reflected sunlight.

Proteus is very cratered, showing no sign of any geological modification. It is irregularly shaped; scientists believe Proteus is about as large as a body of its density can be without being pulled into a spherical shape by its own gravity.[8] Saturn's moon Mimas has a more spherical shape due to a lower density, despite being less massive than Proteus.

Orbital characteristics
Periapsis 117,584 ± 10 km
Apoapsis 117,709 ± 10 km
Semi-major axis 117,647 ± 1 km (0.00079 AU)
Eccentricity 0.00053 ± 0.00009
Orbital period 1.12231477 ± 0.00000002 d
Average orbital speed 7.623 km/s
Inclination 0.524° (to Neptune equator)

Physical characteristics
Dimensions 440×416×404 km (± ~15 km)
Mean radius 210 ± 7 km[4]
Volume 3.4 ± 0.4 × 107km³[5]
Mass ~5.0 × 10^19 kg (estimate)
Mean density ~1.3 g/cm³ (estimate)
Equatorial surface gravity ~0.06 m/s2 (0.006 g)
Escape velocity ~0.16 km/s
Rotation period synchronous
Axial tilt zero
Albedo 0.096
Temperature ~51 K mean (estimate)
Apparent magnitude 19.7>>

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[Vivian]

http://antwrp.gsfc.nasa.gov/apod/image/ ... htened.jpg

Why there are four planets and four shadows in the image?
Vivian

[SittingDownMan]

http://antwrp.gsfc.nasa.gov/apod/image/ ... htened.jpg

Why there are four planets and four shadows in the image?
Vivian

"His composite view of Despina and its shadow is composed of four archival frames
taken on August 24, 1989, separated by nine minutes. Despina itself has been
artificially brightened to make it easier to see. "

http://antwrp.gsfc.nasa.gov/apod/ap090903.html

It's four photos, with the four images of Neptune layed on top of each other, so we see only
one Neptune. From the viewpoint of Voyager, Despina moves relative to Neptune, so each
photo shows Despina, and its shadow, in a different place. There aren't four Despina's, just
one seen four times nine minutes apart.
....................................hth SDM

[Chris Peterson]

One interesting factor about this stunning picture is that, since the Sun is so distant from Neptune and therefore has a much smaller angular diameter than our half a degree, each of the moons produces a total solar eclipse on the top of the Neptune's cloud layer.

For historical reasons, the term "eclipse" tends to get used a little sloppily here on Earth when describing what happens when the Moon passes in front of the Sun. "Occultation" is really a more accurate term, and in the case of this APOD, a viewer on the path of Despina's shadow would be observing an occultation of the Sun by that moon. Considering the length of the shadow path over just 9 minutes, it looks like the occultation would be very brief.

We see shadows like this cast by moons of Saturn and Jupiter as well.

[bystander]

One interesting factor about this stunning picture is that, since the Sun is so distant from Neptune and therefore has a much smaller angular diameter than our half a degree, each of the moons produces a total solar eclipse on the top of the Neptune's cloud layer.

Where did you get this information? I would think that all of Neptune's moons, except Triton, would be too small to produce a total eclipse. In addition, the six outer irregular moons are probably too far away from Neptune to produce total eclipses (just big rocks). The largest of the six inner regular moons, Proteus, is only 420 km in diameter. The moon pictured (just one, not four) in the APOD, Despina, is only 150 km in diameter. I'm not saying they don't eclipse the Sun (I don't know the angular sizes with respect to Neptune), but I'm rather skeptical of your claim.

[Chris Peterson]

One interesting factor about this stunning picture is that, since the Sun is so distant from Neptune and therefore has a much smaller angular diameter than our half a degree, each of the moons produces a total solar eclipse on the top of the Neptune's cloud layer.

Where did you get this information? I would think that all of Neptune's moons, except Triton, would be too small to produce a total eclipse. In addition, the six outer irregular moons are probably too far away from Neptune to produce total eclipses (just big rocks). The largest of the six inner regular moons, Proteus, is only 420 km in diameter. The moon pictured (just one, not four) in the APOD, Despina, is only 150 km in diameter. I'm not saying they don't eclipse the Sun (I don't know the angular sizes with respect to Neptune), but I'm rather skeptical of your claim.

The simple fact that you can see an umbral shadow means that there is a total eclipse in the common sense of the term. That is, if you were on the surface (i.e. at the cloud tops) and under this shadow, the Sun would be completely occulted. The size of the shadow in the images suggests that the moon is subtending a significantly greater angle than the Sun, so you wouldn't get the sort of eclipse effects that you get here on Earth, which depend on the Sun and Moon having the same angular size.

[neufer]

• ------------------------------------
  "Travels in the Morea" (1830)
  
  From Acacesium to the temple of Despoena
  there is a distance of four stades."
  
  http://en.wikipedia.org/wiki/Lycosura
  
  
  The temple of Despoena, (the Mistress,) who was supposed by the Arcadians to be the daughter of Ceres by Neptune was one of the most celebrated places of worship in the Peloponnesus. Before the entrance of the sacred inclosure of the goddess, there was a temple of Diana Hegemone, containing a brazen statue about six feet high, representing Diana with torches in her hands. After entering the inclosure, a stoa on the right contained four sculptures upon white marble in low relief inserted in the wall '. On one of these were represented the Fates, with Jupiter as their leader; on the second, Hercules carrying off the tripod of Apollo ; between them was a picture relating to the ceremony of Despoena ; on the third marble, were nymphs and Pans; the fourth contained the figure of Polybius, son of Lycortas, with an epigram attesting that Greece would not have erred if she had followed his advice, and that when she suffered for her errors, he, above all others, was useful to hen Before the temple were stars of Ceres, of Despoena, and of the Mother of the Gods. The temple contained a colossal representation, by Damophon of Messene, of Ceres, and Despoena seated on a throne. The whole composition was wrought out of a single block of marble. Ceres was holding a torch in her right hand, her left resting upon Despoena. On the knees of the latter deity there was a box, upon which her right hand rested; in her left she held a sceptre. Diana was standing by Ceres, clothed in the skin of a deer; a quiver hanging over her shoulders, a torch in one hand, and two serpents in the other; a hunting dog by her side. Anytus, one of the Titanes, in the habit of an armed man, was standing by Despoena. These two were separate statues ; below them were represented the Curetes, and on the pedestal of the Corybantes. On the right hand of the exit of the temple there was a mirror, which reflected the statues.
  --------------------------------------

[bystander]

The simple fact that you can see an umbral shadow means that there is a total eclipse in the common sense of the term. That is, if you were on the surface (i.e. at the cloud tops) and under this shadow, the Sun would be completely occulted. The size of the shadow in the images suggests that the moon is subtending a significantly greater angle than the Sun, so you wouldn't get the sort of eclipse effects that you get here on Earth, which depend on the Sun and Moon having the same angular size.

I'm not sure I agree. A total eclipse requires that the angular size of the occulting body (moon) be as large (or larger) than the occulted body (sun). A shadow does not imply an eclipse. BB's cast shadows, but no one would claim they eclipse the sun.

[Chris Peterson]

The simple fact that you can see an umbral shadow means that there is a total eclipse in the common sense of the term. That is, if you were on the surface (i.e. at the cloud tops) and under this shadow, the Sun would be completely occulted. The size of the shadow in the images suggests that the moon is subtending a significantly greater angle than the Sun, so you wouldn't get the sort of eclipse effects that you get here on Earth, which depend on the Sun and Moon having the same angular size.

I'm not sure I agree. A total eclipse requires that the angular size of the occulting body (moon) be as large (or larger) than the occulted body (sun). A shadow does not imply an eclipse. BB's cast shadows, but no one would claim they eclipse the sun.

A BB in space would not cast an umbral shadow on the ground, therefore there would be no total eclipse. But a BB a few inches above the ground does indeed cast an umbral shadow; if you were an ant under that shadow, why wouldn't you call it a total solar eclipse? In fact, the very definition of a total eclipse of the Sun is the presence of an umbral shadow.

Despina orbits about 27,700 km above Neptune's cloud tops, and is 150 km across. So it subtends 18 arcminutes. Neptune is 30 AU from the Sun, so the Sun subtends about 1 arcminute. Thus, Despina can completely occult the Sun (which could be called a total solar eclipse).

[bystander]

Despina orbits about 27,700 km above Neptune's cloud tops, and is 150 km across. So it subtends 18 arcminutes. Neptune is 30 AU from the Sun, so the Sun subtends about 1 arcminute. Thus, Despina can completely occult the Sun (which could be called a total solar eclipse).

That I can agree with. I hadn't taken the time to calculate the angles. All of Neptune's inner moons and Triton can completely occult the Sun, but none of the outer moons can.

[jerbil]

One interesting factor about this stunning picture is that, since the Sun is so distant from Neptune and therefore has a much smaller angular diameter than our half a degree, each of the moons produces a total solar eclipse on the top of the Neptune's cloud layer.

For historical reasons, the term "eclipse" tends to get used a little sloppily here on Earth when describing what happens when the Moon passes in front of the Sun. "Occultation" is really a more accurate term, and in the case of this APOD, a viewer on the path of Despina's shadow would be observing an occultation of the Sun by that moon. Considering the length of the shadow path over just 9 minutes, it looks like the occultation would be very brief.

We see shadows like this cast by moons of Saturn and Jupiter as well.

I appreciate your comment, Chris. A rarely used technical expression for the linear coincidence of three astronomical bodies is "syzygy."

[neufer]

[attachment=0]Despina.jpg[/attachment][/b]

[DavidLeodis]

The 'But looking through the Voyager 2 data 20 years later' link in the explanation has a link to the four original frames from which Ted spotted the shadows (I needed to use the enlarged version, as I could not spot the shadows in the small version). I'm not surprised they were not found before! Ted deserves great credit for spotting them, as I doubt that I ever would have done.

[bystander]

The 'But looking through the Voyager 2 data 20 years later' link in the explanation has a link to the four original frames from which Ted spotted the shadows (I needed to use the enlarged version, as I could not spot the shadows in the small version). I'm not surprised they were not found before! Ted deserves great credit for spotting them, as I doubt that I ever would have done.

I think Ted has way too much time on his hands. The moon shadows were easy enough to see. However, I knew they were there. Even knowing where to look for the moons, I'm not sure I can distinguish them. The fourth one I found, hilighted against Neptune, but the other three are hidden in the horizontal bars at the right of the pictures.

[apodman]