Starship Asterisk*

Chris Peterson wrote:

JeffLawrence wrote:
How do astronomers calculate the density of extraterrestrial objects? I think I understand calculation of speed (red shift) and perhaps distance, but density has me puzzled.

Your question is very broad, since it covers a wide range of different objects. But in this case we are talking about a small body in orbit around a much more massive body. Since the mass of Saturn is known, and the orbital period of Hyperion is known, the mass of the Hyperion can be calculated. We've observed this moon closely enough to directly measure its size and shape, and therefore we can calculate its volume. Density is just the mass divided by the volume.

JeffLawrence wrote:How do astronomers calculate the density of extraterrestrial objects? I think I understand calculation of speed (red shift) and perhaps distance, but density has me puzzled.

alphachap wrote:

garry wrote:
Great picture of Hyperion!
How can you have an impact crater so large on a moon so small and not have the moon fracture?
It's not like you can have a soft impact!

Actually you can have a soft impact.

garry wrote:Great picture of Hyperion! How can you have an impact crater so large on a moon so small and not have the moon fracture? It's not like you can have a soft impact!

NoelC wrote:

neufer wrote:
Molecules are copiously formed on the surface of dust grains in the protoplanetary disk (or proplyd).

In our environment, the elements must be brought to the temperature of combustion.
Is this formation you're speaking of just burning then, or some kind of catalytic reaction?

neufer wrote:Molecules are copiously formed on the surface of dust grains in the protoplanetary disk (or proplyd).

NoelC wrote:
I finally figured out what this satellite reminds me of: A piece of broken coral, such as one might find at a Florida beach.
Space coral, perhaps, with animals popping out sweeping the surroundings for complex organics?

NoelC wrote:
A serious question, not related to that frivolous observation, but on the origin of the water: Given that we see so much water (ice) out there... Are there mechanisms where hydrogen and oxygen combine out in otherwise calm space? I know temperatures can sometimes be extreme... Or is it likely that the compound was made in some past violent event?

neufer wrote:Some Hyperion features do suggest a kinship to comet Wild 2 which, itself, has never been closer than 1.59 AU to the sun.

Chris Peterson wrote:

WatchPuppy wrote:
I second the opinion of other posters that Hyperion is a captured comet. The closeup photos of comets provided by the APOD site show outgasing that would create the crater like features.

This body is far larger than any comet nucleus. If you assume that the features are vents of some sort, you'd also need to assume the body was once in an orbit that took it much closer to the Sun. But dynamically, it would be very difficult to construct a scenario where Saturn could capture a body in such an orbit. Given the shape and extremely high porosity of Hyperion, it is much more likely to be the product of an ancient collision. The odd appearing craters are probably what you get when a rubble pile is impacted, rather than a truly solid body.

http://en.wikipedia.org/wiki/81P/Wild wrote:

Wild 2 (Vilt) in 3D

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<<Comet 81P/Wild, also known as Wild 2 (Vilt), is a comet named after Swiss astronomer Paul Wild, who discovered it in 1978 using a 40-cm Schmidt telescope at Zimmerwald. For most of its 4.5 billion-year lifetime, Wild 2 probably had a more distant and circular orbit. In September 1974, it passed within one million kilometers of the planet Jupiter, whose strong gravitational pull perturbed the comet's orbit and brought it into the inner Solar System. Its orbital period changed from 43 years to about 6 years, and its perihelion is now about 1.59 AU (astronomical unit).

NASA's Stardust Mission launched a spacecraft, named Stardust, on February 7, 1999. It flew by Wild 2 on January 2, 2004, and collected particle samples from the comet's coma, which were returned to Earth along with interstellar dust it collected during the journey. 72 close-up shots were taken of Wild 2 by Stardust. They revealed a surface riddled with flat-bottomed depressions, with sheer walls and other features that range from very small to up to 2 kilometres across. These features are believed to be caused by impact craters or gas vents. During Stardust's flyby, at least 10 gas vents were active. The comet itself has a diameter of 5 kilometres.

Stardust's "sample return canister," was reported to be in excellent condition when it landed in Utah, on January 15, 2006. A NASA team analyzed the particle capture cells and removed individual grains of comet and interstellar dust, then sent them to about 150 scientists around the globe. As of 2006, the composition of the dust has contained a wide range of organic compounds, including two that contain biologically usable nitrogen. Indigenous aliphatic hydrocarbons were found with longer chain lengths than those observed in the diffuse interstellar medium. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust. Very few pure carbon (CHON) particles were found in the samples returned. A substantial amount of crystalline silicates such as olivine, anorthite and diopside were found, materials only formed at high temperature. This is consistent with previous observations of crystalline silicates both in cometary tails and in circumstellar disks at large distances from the star. Possible explanations for this high temperature material at large distances from Sun were summarised before the Stardust sample return mission by van Boekel et al.

Both in the Solar System and in circumstellar disks crystalline silicates are found at large distances from the star. The origin of these silicates is a matter of debate. Although in the hot inner-disk regions crystalline silicates can be produced by means of gas-phase condensation or thermal annealing, the typical grain temperatures in the outer-disk (2−20 au) regions are far below the glass temperature of silicates of approx 1,000 K. The crystals in these regions may have been transported outward through the disk or in an outward-flowing wind. An alternative source of crystalline silicates in the outer disk regions is in situ annealing, for example by shocks or lightning. A third way to produce crystalline silicates is the collisional destruction of large parent bodies in which secondary processing has taken place. We can use the mineralogy of the dust to derive information about the nature of the primary and/or secondary processes the small-grain population has undergone.

Results from a study reported in the September 19, 2008 issue of the journal Science has revealed an oxygen isotope signature in the dust that suggests an unexpected mingling of rocky material between the center and edges of the solar system. Despite the comet’s birth in the icy reaches of outer space beyond Pluto, tiny crystals collected from its halo appear to have been forged in the hotter interior, much closer to the Sun.>>

WatchPuppy wrote:I second the opinion of other posters that Hyperion is a captured comet. The closeup photos of comets provided by the APOD site show outgasing that would create the crater like features.

jeffduncan wrote:
Those do not look like craters on Hyperion... they look like rocket orifices. I suspect that Hyperion is a captured comet, and that escaping gas blasted those vents in the surface and left the interior honeycombed during its passes near the sun.

orin stepanek wrote:
It does look like a sponge! I think it is a captured comet. It would be an interesting study for some future space probe.

WatchPuppy wrote:
I second the opinion of other posters that Hyperion is a captured comet. The closeup photos of comets provided by the APOD site show outgasing that would create the crater like features.

http://en.wikipedia.org/wiki/Hyperion_%28moon%29 wrote:
<<Hyperion is one of the largest bodies known to be highly irregular (non-spherical) in the solar system. The only moon larger than it that is known to be irregular in shape is Neptune's moon Proteus. Hyperion has about 15% of the mass of Mimas, the least massive spherical body. The largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for the irregular shape is that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past. A proto-Hyperion could have been from 350 to 1000 km in diameter. Over about 1,000 years, ejecta from a presumed Hyperion breakup would have impacted Titan at low speeds building up volatiles in the atmosphere of Titan.

Like most of Saturn's moons, Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition. However, unlike most of Saturn's moons, Hyperion has a low albedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.>>

http://en.wikipedia.org/wiki/Proteus_%28moon%29 wrote:

Neptune's Proteus

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<<Proteus (Πρωτεύς), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus circles Neptune in a near equatorial orbit at the distance of about 4.75 equatorial radii of the planet.

Despite being more than 400 km in diameter Proteus has a somewhat irregular shape with several slightly concave facets and relief as high as 20 km. Its surface is dark, neutral in color and heavily cratered. The largest crater is more than 200 km in diameter. There are also a number of scarps, grooves and valleys related to large craters.

Proteus is probably not an original body that formed with Neptune; it may have accreted later from the debris created when the largest Neptunian satellite Triton was captured.

Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity and is inclined by about 0.5° to the planet's equator. Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously with the orbital motion, which means that one face always points to the planet.

Physical characteristics

Proteus is the second largest moon of Neptune. It is about 420 kilometres in diameter, larger than Nereid, the second to be discovered. 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. The surface of Proteus is dark—its geometrical albedo is about 10%. The surface's color is neutral as the reflectivity does not change appreciably with the wavelength from violet to green. In the near-infrared part of the spectrum the surface becomes less reflective around 2 μm pointing to a possible presence of complex organic compounds such as hydrocarbons or cyanides. These compounds may be responsible for the low albedo of the inner Neptunian moons. While Proteus is usually thought to contain significant amounts of water ice, it has not been detected spectroscopically on the surface.

The shape of Proteus is close to a sphere with the radius of about 210 km, although deviations from the spherical shape are large—up to 20 km; scientists believe it is about as large as a body of its density can be without being pulled into a perfect spherical shape by its own gravity.

Proteus is heavily cratered, showing no sign of any geological modification. The largest crater, Pharos, has a diameter from 230 to 260 km. Its depth is about 10–15 km. The crater has a central dome on its floor a few kilometers high. Pharos is the only named surface feature on this moon: the name is Greek and refers to the island where Proteus reigned. In addition to Pharos there are several craters 50–100 km in diameter and many more with diameters less than 50 km.

Proteus, like the other inner satellites of Neptune, is unlikely to be an original body that formed with it, more probably having accreted from the wreak rubble that remained after Triton's capture. Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites.>>

• The Grand Imperial Master of Hyperion