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

[geckzilla]

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.

That reminds me of the time I asked my high school science teacher if atoms could be orbiting around various objects in the room. I think the answer is no because the atmosphere is constantly swishing atoms around and there are other atoms and other forces many times more effective at moving atoms than gravity in a room but at the time she just gave me a weird look and said she didn't know.

[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.

That reminds me of the time I asked my high school science teacher if atoms could be orbiting around various objects in the room. I think the answer is no because the atmosphere is constantly swishing atoms around and there are other atoms and other forces many times more effective at moving atoms than gravity in a room but at the time she just gave me a weird look and said she didn't know.

RIght. A good question it was. In practice, of course, two bodies could become too small to orbit each other due to gravity. Not because the rules of gravity change, but because other forces dominate. For debris in the Solar System, below a few centimeters you get effects from solar radiation. Even smaller and you get effects from solar wind. Smaller yet and electrostatic forces come into play.

But for some honking big comet massing 10 trillion kilograms, there are no other forces which come close to unseating gravity.

[guestII]

Hello,
I wonder obout the gravitational field of Comet Churyumov-Gerasimenko: whether there is a single center point of mass (as here on sphere like Earth) or what effects the 'non-homogenous mass' could have on the landing path of lander 'Philae'. Take the view of today's APOD: if Philae was to land on that rock wall (which for myself I call the 'C-P's Eiger North Face ') the view is directly apointing to, the gravitational field certainly is not normal to that surface, maybe even turned to the opposite direction(?) at least with some distance to that wall, since this face belongs to the smaler part (the head) of 67P/C-G. As I read this rock wall is not part of the set of the possible landing-sites - but somehow the control-crew here on earth has to cope with the effects of that duck-shaped body. Did this non-homogenous mass-allocation in some way influence the selection of possible landing-sites - and how?

[Beyond]

But for some honking big comet...

I like the way scientific minded people are using more and more easy to spell and pronounce words, that have more meaning to the average person than a lot of the scientific jargon does.

[Evenstar]

Amazing picture. I wish if they describe the comet as coal-like in color they would have used those colors instead of some browns and tans. You don't have to make the comet look like 'home' you know. Also, where is this comet heading--swinging in toward the Sun? What is its orbit? It will be a fantastic ride if the probe works holding the spacecraft to the comet's surface while it passes around the Sun and on out! What powers the spacecraft--solar or reactor?

[Chris Peterson]

I wonder obout the gravitational field of Comet Churyumov-Gerasimenko: whether there is a single center point of mass (as here on sphere like Earth) or what effects the 'non-homogenous mass' could have on the landing path of lander 'Philae'.

Well, no matter how oddly shaped a mass is, no matter what its density distribution, it still has a single center of mass. Whether it can be treated as a point when considering orbits is simply a matter of how far away the orbiting body is. Satellites, even in low Earth orbit, largely see the Earth as a point mass. Low orbits around the Moon are not stable, however, because the mass concentrations are greater. If you are in orbit around 67P many tens of kilometers away, it's going to essentially act like a point mass. From just a few kilometers, though, the mass variation will have a significant impact.

I doubt that the lack of homogeneity played much of a role in determining the landing site. Its effects should be very manageable for a controlled landing (as opposed to a sustained orbit).

[Chris Peterson]

I like the way scientific minded people are using more and more easy to spell and pronounce words, that have more meaning to the average person than a lot of the scientific jargon does.

Boy howdy!

[Chris Peterson]

Amazing picture. I wish if they describe the comet as coal-like in color they would have used those colors instead of some browns and tans.

Coal is not black. If you light it up bright enough, or take an image and stretch the contrast, you'll see all sorts of color- browns and tans like in today's image.

Also, where is this comet heading--swinging in toward the Sun? What is its orbit?

It has a six-and-a-half year period that takes it as close to the Sun as 1.2 AU, and as far as 5.7 AU. That is, from a little outside Earth's orbit to a little outside Jupiter's. It is heading inwards, about a year from perihelion.

What powers the spacecraft--solar or reactor?

Photovoltaic panels.

[Ron-Astro Pharmacist]

The full moon seems bright to me so I can't really imagine if, per chance, it had an albedo near that of Enceladus. Makes one wonder if those seas where seas our biology's might be a bit sundry. Two suns for the price of one. At least light-wise.

[Beyond]

I like the way scientific minded people are using more and more easy to spell and pronounce words, that have more meaning to the average person than a lot of the scientific jargon does.

Boy howdy!

Well a big YEE HAA to you, pardner, who sits tall in the saddle as we all share the ride round the nearest star, on this unique rock.

[Ron Davis]

I think the light areas are patches of dust (snow?) with level surfaces. If so, one can see the various directions in which gravity pulls on different parts of the surface. The directions are qualitatively consistent, by visual estimate, with the gravity field of adjacent spheres.

[alter-ego]

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.

Thanks for the clarification! I was puzzled why you asked.

This image was taken with the Narrow Angle Camera, and both Wheel 1 and 2 have "clear" filters with 600nm bandwidth, centered on 600nm. If these (or any other) filters were used, then, as you wondered, the color reconstruction would have been made on educated, best-guess basis. I'm thinking that the lack of filter information may indicate the clear filters were used. Maybe the answer is coded into an image filename somewhere.

[stargene1]

Amazing and beautiful. And in addition to fascinating, rather forbidding, if they hope to land something. I'm thinking
that even harpoons and explosive bolts (other than chewing gum) could even cause an impulse-back-reaction which
might just kick the little lander back into space! BTW, has anybody else noticed the large number of roughly parallel
grooves, which almost look like strata? I understand that some asteroid collision modelling has suggested that these
are essentially 'shock' lamellae writ large, due to occasional big impacts (either current or during the formative
age of the solar system. However, is it possible that some grooves are actual stratigraphy, scaled down to the
level of planetesimal formation? Just a thought.

[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.

Thanks for the clarification! I was puzzled why you asked.

This image was taken with the Narrow Angle Camera, and both Wheel 1 and 2 have "clear" filters with 600nm bandwidth, centered on 600nm. If these (or any other) filters were used, then, as you wondered, the color reconstruction would have been made on educated, best-guess basis. I'm thinking that the lack of filter information may indicate the clear filters were used. Maybe the answer is coded into an image filename somewhere.

The creator responded on Flickr with this:

Hi all, no raw data.
No OSIRIS image has been released with different filters so can get an RGB image as result.
We started with a single image flic.kr/p/p6kuZs working on the information that we all know (low albedo, dusty surface, and so on), obtaining three virtual layer. Processing, as long as even our eyes were pleased and believed what they was looking at.
In a way, we pushed to limit a technique that we use for a long time to make color native b/w shots to increase the visual perception.

In short, colors are best guess.

ESA is still not releasing color data or even raw data (we only get JPEGs) and also that makes me (and a lot of other people) sad.

[alter-ego]

The creator responded on Flickr with this:

Hi all, no raw data.
No OSIRIS image has been released with different filters so can get an RGB image as result.
We started with a single image flic.kr/p/p6kuZs working on the information that we all know (low albedo, dusty surface, and so on), obtaining three virtual layer. Processing, as long as even our eyes were pleased and believed what they was looking at.
In a way, we pushed to limit a technique that we use for a long time to make color native b/w shots to increase the visual perception.

In short, colors are best guess.

ESA is still not releasing color data or even raw data (we only get JPEGs) and also that makes me (and a lot of other people) sad.

Yup, makes sense. Thanks for getting to the bottom of this.

[ta152h0]

Neil Armstrong RIP would be doing a lot of drifting here, right and left

[Ron-Astro Pharmacist]

Looks like this will be touch and go. Well - at least touch I hope. 50/50 odds don't seem so good.

http://www.nature.com/news/lander-to-ai ... E-20140918