APOD: Despina, Moon of Neptune (2014 Jan 16)

[geckzilla]

Well walking around would be unsafe and the bigger the stone the more unsafe. Bring a milk pail on a stick and stick that out there. A robot-ish thing might do as well but a pail on a stick would be more cost effective.

Even a robot can't survive being pelted by diamonds moving at 1000 miles per hour.

[Anthony Barreiro]

Not much has happen since 1989 in the exploration of the outer solar system. Kind of sad.
(Voyager 2's closest approach to Neptune occurred on August 25, 1989.)

I wouldn't discount Cassini-Huygens, which has been intensively studying the Saturn system for the last decade, and continues to do so. And we're just over a year from New Horizons passing through the Pluto system. And both Cassini and New Horizons encountered Jupiter and returned data from there.

And Juno is on her way to spy on the hijinks Jupiter is getting up to under cover of clouds.

[Nitpicker]

Not just the "most distant gas planet"....but now days....the most distant Planet....

But this image combination shows...you can still discover something new....

:---[===] *

PLUTO LIVES!

... as a beloved Dwarf Planet, with relatively brief periods closer to the Sun than Neptune, the most distant Planet in the Solar System.

[Gr8Glaroon]

One thing puzzles me about this image. If it's a time-lapse photograph, that means it includes different pictures taken while Despina is at different points along its orbit around Neptune. Its orbit is elliptical, probably close to circular, which means that its path along its orbit has a curvature to it. So why does the time-lapse photograph show the moon Despina traveling along a straight trajectory? That trajectory should follow more-or-less the same close-to-circular path as the surface of the planet below it, since its orbit would tend to keep it at about the same distance from the surface of the planet. Yet the time-lapse photography shows the moon Despina following almost a perfect straight-line path. I don't understand.

Ron

[geckzilla]

One thing puzzles me about this image. If it's a time-lapse photograph, that means it includes different pictures taken while Despina is at different points along its orbit around Neptune. Its orbit is elliptical, probably close to circular, which means that its path along its orbit has a curvature to it. So why does the time-lapse photograph show the moon Despina traveling along a straight trajectory? That trajectory should follow more-or-less the same close-to-circular path as the surface of the planet below it, since its orbit would tend to keep it at about the same distance from the surface of the planet. Yet the time-lapse photography shows the moon Despina following almost a perfect straight-line path. I don't understand.

Ron

Optical illusion. The moon's path is not straight at all. It just looks straight compared to the curvature of Neptune's limb in the background. And, anyway, the limited perspective can really throw you off. Even if you draw a straight line to compare it to, Despina looks like it's going to crash into Neptune but we know that's not true. If you could reach into the image and rotate it around as a three dimensional projection then it would be simple to understand but all we have is a flat image.

[Nitpicker]

A bit like trying to estimate the length of a bus when it is about to run you over. It is simply the wrong vantage point to observe the elliptical orbit shape.

[Chris Peterson]

A bit like trying to estimate the length of a bus when it is about to run you over. It is simply the wrong vantage point to observe the elliptical orbit shape.

And yet, that's what orbital analysis is all about. You take the coordinates of an object observed at several locations, which typically looks very close to a line segment, and fit them to the only possible elliptical orbit.

[Nitpicker]

A bit like trying to estimate the length of a bus when it is about to run you over. It is simply the wrong vantage point to observe the elliptical orbit shape.

And yet, that's what orbital analysis is all about. You take the coordinates of an object observed at several locations, which typically looks very close to a line segment, and fit them to the only possible elliptical orbit.

Indeed, but my point was you don't typically observe the ellipse -- you derive it from your observations and calculations.