APOD: 62 Kilometers above Comet... (2014 Sep 15)

[APOD Robot]

62 Kilometers above Comet Churyumov-Gerasimenko

Explanation: Spacecraft Rosetta continues to approach, circle, and map Comet Churyumov-Gerasimenko. Crossing the inner Solar System for ten years to reach the vicinity of the comet last month, the robotic spacecraft continues to image the unusual double-lobed comet nucleus. The reconstructed-color image featured, taken about 10 days ago, indicates how dark this comet nucleus is. On the average, the comet's surface reflects only about four percent of impinging visible light, making it as dark as coal. Comet 67P/Churyumov-Gerasimenko spans about four kilometers in length and has a surface gravity so low that an astronaut could jump off of it. In about two months, Rosetta is scheduled to release the first probe ever to attempt a controlled landing on a comet's nucleus.

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[geckzilla]

Does anyone have any idea what "reconstructed-color" means? Does that mean it was essentially guessed at based on the texture and known color properties of other objects?

[Nitpicker]

Fantastic APOD. And the "dark as coal" image of LA is excellent (and a bit Hollywood.)

[Boomer12k]

Now THAT is just an awesome picture. Ummmm....where is the ICE????? I see what looks like sand....I see dark rock....

:---[===] *

[Guest]

Does anyone have any idea what "reconstructed-color" means? Does that mean it was essentially guessed at based on the texture and known color properties of other objects?

I'd also like to know how they arrived at the colors used.

[alter-ego]

Does anyone have any idea what "reconstructed-color" means? Does that mean it was essentially guessed at based on the texture and known color properties of other objects?

I'd also like to know how they arrived at the colors used.

A detailed description of the cameras and filters can be found here. Since I don't think Rosetta has a built-in subject palette to compare colors, a set of images using a suitable set of filters must be used to create RGB reconstruction. Varying degrees of true color reproduction are affected by filter characteristics(wavelength, bandwidth), and how they are combined. I didn't see a specific press release for this image that describes the filters and image combination algorithm.

Regarding the cameras, the article states:
There are two overlapping filter wheels on each camera. For true-color NAC images, look for RGB combinations made with images ending with 12/13/14 or 82/83/84.
And specifically for narrow band filter use:
The combination of narrowband filters that is closest to the RGB combinations in the narrow-angle camera is OI/NH2/CN (17/15/14), but proceed with caution.

Edit:
As a separate example of Mars Viking Lander color reproduction:
The Viking cameras have six spectrally narrow band detectors, three in the visible and three in the near infrared. The use of all six channels has been shown (Huck et al., 1977) to provide the most accurate color rendition. Because many of the images in our study had not been taken in six channels, three component color reconstruction was used. The three components correspond approximately to Blue, Green, and Red. The color reconstruction of these images was performed in a "radiometric" sense, meaning that the components were each linearly amplified to effect an equal average sensitivity over the spectral bandpass. Therefore, the reconstructed triplet, while possessing the same general color characteristics, is not intended to be an exact photometric reproduction of the actual sense as perceived by a human observer.

[Ann]

Those bright patches sure look like snow.

Ann

[geckzilla]

alter-ego: I know how color images are created from multiple images taken with different filters. However, as far as I know, all of Rosetta's release images have all been black and white with no filter data included.

[madtom1999]

Is there a coma and or tail yet and if so is it just so tenuous close up its not visible?

[APODFORIST]

Fantastic.

It looks like a piece of the alps floating in space.

[CURRAHEE CHRIS]

Really an exciting picture. Such a historic bit of work being done. Very exciting times.

[Lanulos]

Archive page is not loading.

[Guest]

An interesting comment in the text of the photo description. Specifically, "... the unusual double-lobed comet nucleus.". I'm wondering what the author was referring to, given that we have a miniscule & negligible sample size with which to compare and evaluate. Perhaps it is just an assumption that all comet cores are round(ed) objects, making this one unusual??? Assumptions make for questionable science. Notwithstanding that, it is an amazing photo; and impressive that mankind could get a camera that close. Waiting for the landing attempt, and hope the lander does not bounce off. I hope the landing position and orbit selected can provide a scenic view as the comet dives toward the sun, tho science must take priority here (or out there...).

[PeterNord]

When I first saw the picture I thought another photo of: http://www.atoptics.co.uk/fz1051.htm
Same stuff.

[MadCat-75]

Fantastic.

It looks like a piece of the alps floating in space.

this was my first guess, too!

[Psnarf]

I'd keep the lander away from the snow. An undisturbed bit of charcoal may be safe from geysers. Can't tell if the holes are sleeping geysers or collisions. Depending upon where the geysers appear, this comet could start tumbling. Hope the harpoon is strong enough to account for the changes in acceleration from centripetal forces.

[Chris Peterson]

An interesting comment in the text of the photo description. Specifically, "... the unusual double-lobed comet nucleus.". I'm wondering what the author was referring to, given that we have a miniscule & negligible sample size with which to compare and evaluate.

Well, dozens of comets have been mapped with radar, of which I believe two are almost certainly contact binaries, and two have elongated shapes that may or may not actually be described as "double-lobed". So while it would be a stretch to call this rare, I don't think "unusual" is an unreasonable assessment.

[LocalColor]

Mountains in space! Go Rosetta!

[HunterofPhotons]

Can Rosetta orbit the comet utilizing just the low gravity of the comet or does it have to use its thrusters to maintain the orbit?

dan k.

[BMAONE23]

Is there a coma and or tail yet and if so is it just so tenuous close up its not visible?

This thread has a post with an image of the tail in August

[Chris Peterson]

Can Rosetta orbit the comet utilizing just the low gravity of the comet or does it have to use its thrusters to maintain the orbit?

Since August the spacecraft has been in orbit. Initially, it was in a series of hyperbolic (open) orbits, and the thrusters were used to define subsequent orbits. For a few days now it's been in a closed elliptical orbit. The thrusters are not required to maintain this orbit, outside of minor corrections to compensate for tidal effects, non-homogenous mass, and other secondary effects.

[HunterofPhotons]

.... The thrusters are not required to maintain this orbit....

Thanks for the information, Chris.
I wouldn't have thought that a body that size would have enough 'pull' to hold Rosetta in orbit.

dan k.

[madtom1999]

Is there a coma and or tail yet and if so is it just so tenuous close up its not visible?

This thread has a post with an image of the tail in August

Thanks DMAONE23
Well Cool!

[Yoduh99]

has a surface gravity so low that an astronaut could jump off of it

that sounds pretty scary when you think about how gentle you'll need the rover to land to not accidentally bounce off back into space!

[Chris Peterson]

I wouldn't have thought that a body that size would have enough 'pull' to hold Rosetta in orbit.

No body is too small to be unable to hold another body in orbit. For any pair of bodies, there is a simple equation relating orbital period, radius, and the mass of the bodies (simplest when the central body is much more massive than the orbiting body, as in the case of Rosetta). An orbital speed on the order of a meter per second at a distance of a few tens of kilometers works out just fine.