Starship Asterisk*

by **geckzilla** » Fri Oct 24, 2014 4:51 pm

50bmg wrote:
50bmg wrote:What are the spots on the solar panel? Perhaps micrometeoroid impacts?
Does anyone know anything about the spots on the solar panel? Thanks!

I know that this is the back of Rosetta's solar cells. The disorganized nature of the white dots can lead one to believe they are randomly punched holes from space debris but check out this 3d rendering of Rosetta:
http://www.esa.int/spaceinimages/Images/2013/12/Rosetta

You can see there are seemingly random white dots on the front of the cells. However, they're actually very regularly placed and clearly correspond to the white dots seen on the back of the solar panels in the APOD image. That doesn't mean there are no impact holes in Rosetta. It just means most of these spots probably are not.

by **50bmg** » Fri Oct 24, 2014 3:54 pm

50bmg wrote:What are the spots on the solar panel? Perhaps micrometeoroid impacts?

Does anyone know anything about the spots on the solar panel? Thanks!

by **sOnIc** » Mon Oct 20, 2014 5:08 pm

Thanks for the link and info alter-ago .. just gives more understanding of the scene for me .. exciting mission . . .

by **alter-ego** » Sun Oct 19, 2014 5:09 am

Chris Peterson wrote:
alter-ego wrote:I'd think that depending the brightness of the surrounding sources (array glints and comet sun-lit contributions), that eye motion would be intentional and relatively large compared to the normal (rapid) eye saccades that one experiences while attempting to fixate on a point.
Right, I'm talking about large movements. We actually only see clearly in a tiny region in the center of our gaze. But we never notice unless we really try. We scan our eyes (and even head) over a very wide field, and our brain just paints it all in, as if our eyes were taking it all in at once. Kind of a neat trick.

Absolutely. It's funny though, that process seems to have always been second nature to me. I've been conducting my personal threshold magnitudes measurements from my home. The observations range from daytime to the nighttime limit. The transition from photopic to scotopic vision is an interesting phase. It is possible that seeing that jet might fall near this transition.

by **Chris Peterson** » Sun Oct 19, 2014 4:54 am

alter-ego wrote:I'd think that depending the brightness of the surrounding sources (array glints and comet sun-lit contributions), that eye motion would be intentional and relatively large compared to the normal (rapid) eye saccades that one experiences while attempting to fixate on a point.

Right, I'm talking about large movements. We actually only see clearly in a tiny region in the center of our gaze. But we never notice unless we really try. We scan our eyes (and even head) over a very wide field, and our brain just paints it all in, as if our eyes were taking it all in at once. Kind of a neat trick.

by **alter-ego** » Sun Oct 19, 2014 4:50 am

Chris Peterson wrote:
alter-ego wrote:The full dynamic range of the composite ≈12000, or ≈13.5 f-stops. Given the eye has a static contrast ratio of about 6.5 f-stops (fixed brightness range), the image well exceeds that. However, it is well within the eye's full dynamic contrast ratio of 20 f-stops. I'd estimate the jet is near the faint range (maybe 10 to 13 f-stops), so the visibility would likely depend on blocking the brightest regions (certainly true for the fainter jet regions).
It probably would not be necessary to look away or shield the eyes. The static contrast ratio of the eyes only applies when you're fixated on something. In practice we move our eyes, and our brain paints the whole picture. So in normal operation we get better than 90dB of dynamic range (for daylight illumination levels), about three times deeper than the image.

That makes sense. I'd think that depending the brightness of the surrounding sources (array glints and comet sun-lit contributions), that eye motion would be intentional and relatively large compared to the normal (rapid) eye saccades that one experiences while attempting to fixate on a point. There is a source brightness limit, within the image FoV, beyond which the jet would not be visible regardless, but I'd say nothing in the image is near that limit.

by **Chris Peterson** » Sun Oct 19, 2014 4:24 am

alter-ego wrote:The full dynamic range of the composite ≈12000, or ≈13.5 f-stops. Given the eye has a static contrast ratio of about 6.5 f-stops (fixed brightness range), the image well exceeds that. However, it is well within the eye's full dynamic contrast ratio of 20 f-stops. I'd estimate the jet is near the faint range (maybe 10 to 13 f-stops), so the visibility would likely depend on blocking the brightest regions (certainly true for the fainter jet regions).

It probably would not be necessary to look away or shield the eyes. The static contrast ratio of the eyes only applies when you're fixated on something. In practice we move our eyes, and our brain paints the whole picture. So in normal operation we get better than 90dB of dynamic range (for daylight illumination levels), about three times deeper than the image.

by **alter-ego** » Sat Oct 18, 2014 9:30 pm

Chris Peterson wrote:
sOnIc wrote:For example if you were onboard the spacecraft you could shield you eyes and dark adapt your vision for a while; then look back at the comet and maybe see the dust; like seeing the milky way in the night sky.
Actually, this image may be doing a good job of capturing what you'd see with your naked eyes. Our visual dynamic range is many orders of magnitude greater than we can capture in any single image. That's one reason for HDR: it results in images that come closer to what we actually see. It's hard to say for certain without the exposure details of the individual images, but I wouldn't be surprised if we could comfortably see the panels and the dust jets in the same view.

My immediate thoughts were that if one were at CIVA's location, a naked eye view would comfortably show the fainter information, but that it might include shielding our eyes from the brightest sources (the saturated glints from the array panels). Interestingly, I ran across some exposure details at http://blogs.esa.int/rosetta/2014/09/10 ... -at-comet/

The full dynamic range of the composite ≈12000, or ≈13.5 f-stops. Given the eye has a static contrast ratio of about 6.5 f-stops (fixed brightness range), the image well exceeds that. However, it is well within the eye's full dynamic contrast ratio of 20 f-stops. I'd estimate the jet is near the faint range (maybe 10 to 13 f-stops), so the visibility would likely depend on blocking the brightest regions (certainly true for the fainter jet regions).

Edit: I just realized that the selfie posted on the blog site is from Sep 7 (no jet and array panels positioned differently wrt the comet, 50km away). I'm therefore assuming the exposure information also applies to the Oct 7 selfie in the APOD.

by **geckzilla** » Sat Oct 18, 2014 7:54 pm

If you were able to float out there and use either the craft or the comet itself to hide the Sun and let your eyes adjust, you could probably see all sorts of fine details. Getting on the backside of the comet would probably let you see even more detail from the jets. I'm under the impression the jets with the sunlight reflecting off them are much brighter than, say, an emission nebula like Orion until they diffuse away from the comet.

by **sOnIc** » Sat Oct 18, 2014 7:02 pm

Chris Peterson wrote:It's hard to say for certain without the exposure details of the individual images, but I wouldn't be surprised if we could comfortably see the panels and the dust jets in the same view.

Ok, how cool would that be! And that was my initial reaction .. just upon reading the caption it makes me uncertain about that point, as you are.
Thanks for your opinions and info Chris, I was just posting my thoughts ...

by **Chris Peterson** » Sat Oct 18, 2014 6:54 pm

sOnIc wrote:For example if you were onboard the spacecraft you could shield you eyes and dark adapt your vision for a while; then look back at the comet and maybe see the dust; like seeing the milky way in the night sky.

Actually, this image may be doing a good job of capturing what you'd see with your naked eyes. Our visual dynamic range is many orders of magnitude greater than we can capture in any single image. That's one reason for HDR: it results in images that come closer to what we actually see. It's hard to say for certain without the exposure details of the individual images, but I wouldn't be surprised if we could comfortably see the panels and the dust jets in the same view.

by **sOnIc** » Sat Oct 18, 2014 6:46 pm

I think I'd rather see the solar panels blown out and over exposed; that way you're looking at an accurate scene and the brain doesn't get it wrong, and I'd love to know how much dust is actually visible to the human eye.
For example if you were onboard the spacecraft you could shield you eyes and dark adapt your vision for a while; then look back at the comet and maybe see the dust; like seeing the milky way in the night sky.

I'm just moaning about the initial reaction which made me think the dust and jets were going to be visible to the eye if you were there, I was speculating about reasons for how the dust is so well lit etc .. so in my opinion it is misleading.
A great image, but ...

by **Chris Peterson** » Sat Oct 18, 2014 6:34 pm

sOnIc wrote:But my point is regarding the average person who looks at this image - they are going to assume its one image; and assume that they could see this scene with their own eyes - which they could not.

Well, they should read the caption! Showing both images is a worse solution, since the people who don't read the caption certainly aren't going to be following any links, and they'll totally miss the point of the picture. As it is, the picture is a more accurate representation of reality than anything we could see directly with our eyes- something that is true for virtually every astronomical image.

by **sOnIc** » Sat Oct 18, 2014 6:27 pm

Sorry yes you are right about HDR, I've been doing single-shot HDR lately (very effective) and commented too hastily.

But my point is regarding the average person who looks at this image - they are going to assume its one image; and assume that they could see this scene with their own eyes - which they could not.

by **Chris Peterson** » Sat Oct 18, 2014 6:14 pm

sOnIc wrote:It's artistic licence because somebody chose the exposure time to use for the dust layer. It is showing something which you would not see if you were there in person and it mislead me into thinking it was a single shot. HDR is different because the exposures are equal in length and I would much prefer that. I now don't know what this scene is showing, how long an exposure does it take to capture the dust? Would is be invisible to the naked eye? This image is misleading.

I disagree that there is anything misleading, especially as the caption is clear about how the image was made.

You are wrong about HDR. The technique uses two or more images made with different exposure times, which is exactly what was done here.

Are the individual exposures online anywhere?

Maybe, but I wouldn't count on it, or on easily finding them. Unlike NASA, ESA doesn't post many of their raw mission images.

by **sOnIc** » Sat Oct 18, 2014 6:04 pm

It's artistic licence because somebody chose the exposure time to use for the dust layer. It is showing something which you would not see if you were there in person and it mislead me into thinking it was a single shot. HDR is different because the exposures are equal in length and I would much prefer that. I now don't know what this scene is showing, how long an exposure does it take to capture the dust? Would is be invisible to the naked eye? This image is misleading.

EDIT: Are the individual exposures online anywhere?

by **Chris Peterson** » Sat Oct 18, 2014 5:59 pm

sOnIc wrote:It's not the first time I've posted regarding this kind of thing, and I know that astrophotography relies on artistic licence all the time (mixing colour channels with narrow-bands like Ha etc), but I really wish images like this were not doctored to show something which isn't there.

This isn't artistic license, nor is it showing something that isn't there. Quite the opposite, it is showing something that is there and which wouldn't otherwise be visible. It's why we design instruments like this in the first place!

by **sOnIc** » Sat Oct 18, 2014 5:54 pm

Yes what a beautiful image, I went straight to the large view and was struck by how bright and visible the dust and jet was, how amazing it would be to be there I thought.

Then I returned and read the description and was disappointed to read: "In fact, two exposures, one short and one long, were combined to record the dramatic high contrast scene..."
Because that means my initial impression of the scene was wrong, I was mislead into thinking the dust was visibly bright enough to see.

It's not the first time I've posted regarding this kind of thing, and I know that astrophotography relies on artistic licence all the time (mixing colour channels with narrow-bands like Ha etc), but I really wish images like this were not doctored to show something which isn't there .. I also know this will be a minority view. But I find images like this to be misleading and counter-educational. What a shame... I'd love to see the two 'actual' exposures separately as I will learn from that.

by **MargaritaMc** » Fri Oct 17, 2014 10:54 am

Those are links to such useful pdf documents, alter-ego. Thank you so much. I'll link your post to the Rosetta thread in Breaking Science News.

M

by **alter-ego** » Fri Oct 17, 2014 4:22 am

Kasuha wrote:
Guest wrote:I did wonder tho... Is this a true gravity based decent, or more like a powered rendezvous? (excluding the final braking blast, of course)
More like a powered rendezvous.
Check the animation of planned maneuvers on this page:

http://blogs.esa.int/rosetta/2014/10/15 ... ng-site-j/
Click to play embedded YouTube video.

Indeed the planned orbital maneuvers for Rosetta are quite involved and require many thruster firings. However, the lander (Philae) doesn't in the familiar sense. Philae has a flywheel for attitude control during decent, and the Active Descent System has different function than how we typically think about powered landings. The ADS is located on top of Philae. Its original intent was for accelerating the craft toward the comet, and upon reaching the surface it would be fired to prevent rebound. It looks like the present descent trajectory could be unpowered except for a thrust burst to make sure it sticks after contact with the surface.

The Rosetta Lander (Philae) Investigations wrote:The Active Descent System (ADS)was originally implemented to be used during
descent, to reduce the time between separation and touch down. It consists in a cold
gas system with one thruster, pointing in the Lander+z axis (“upwards”), using nitrogen
as the propellant. Since the new target comet is expected to be much larger in
mass than Wirtanen, this maneuver will most probably not be performed. However,
the ADS will still be activated at touch-down for a few seconds to avoid rebound.

From the Rosetta Lander Philae to an Asteroid Hopper: Lander Concepts for Small Bodies Missions wrote: When sufficient information on the target has been
collected and analyzed, a scenario will be worked out,
based on a separation from the main spacecraft in orbit
(it is desirable to perform this at low altitudes, i.e. 1 to
2 km), lander attitude stabilization with an internal
flywheel, the optional use of a one axis cold gas system
(propelling the lander “downwards”) and allowing
sufficient time to perform system relevant tasks (e.g.
unfolding of the landing gear) as well as the collection
of science data.
A typical descent will take 30 min to 2 hours. Mission
analysis shall provide a solution where the Lander z-axis
as well as the impact velocity vector are both
vertical to the comet surface at the landing site.
However, local slopes up to 30° can be tolerated by the
landing system (although the robustness of the landing
depends on the impact velocity).
At touch-down, the cold gas system will provide
downward-thrust and the anchoring harpoons will be
fired. The harpoons, on a tether, shall provide good
fixation to ground for a wide range of surface
parameters for the rest of the mission [19].
Additional anchoring will be provided by ice-screws
implemented in the feet of the Lander.

by **ta152h0** » Thu Oct 16, 2014 11:09 pm

Are cameras rolling as Phylae lands ? Like a " reverse rocket cam "

by **MargaritaMc** » Thu Oct 16, 2014 8:35 pm

I thought this Rosetta blog post from August might be of interest:

ESA Rosetta Blog, 18th August

Today: a quick recap of Rosetta orbital manoeuvres in the past fortnight since arrival at Comet 67P/C-G on 6 August. Today’s post is covers multiple manoeuvres, which means that the mission operations teams and flight dynamics experts at ESOC have been busy ensuring that everything is happening when it should!

First, before we go any further, a mandatory video! We say ‘mandatory’ because this animation explains in rather good detail what Rosetta has been doing and covers the current time frame up to the end of September. OK – lets watch:
Click to play embedded YouTube video.
(In this animation the comet is an artist’s impression and is not to scale with the spacecraft. The comet rotation is not representative (67P rotates once per 12.4 hours). Dates may be subject to change.)

...
• It’s important to note Rosetta has not been captured by 67P/C-G gravity, and the continuing series of thruster burns are necessary to keep the spacecraft at the comet.
• The craft will execute two of these triangular orbits, referred to by the mission team at ESOC as ‘Close Approach Trajectory’ (CAT); there will be one large, at about 100km closest pass-by distance (‘Big CAT’) and the second will be done at about 50km (‘Little CAT’). This means that the thruster burns are not only changing Rosetta’s direction on each arc, but are also lowering the pass-by distance (i.e. altitude) as well.

There's a lot more detail on the blog post and some very useful further information in the comments section.

Margarita

by **MargaritaMc** » Thu Oct 16, 2014 8:03 pm

Ann wrote:...There is something about its fateful yet beautiful sort of bluster that reminds me of Rosetta, alone in space, braving the unpredictable storms of space, including the the random outbursts of the comet on which it is supposed to land.

Ann

That's a beautiful poem, Ann, and exactly expresses the almost buccaneering quality that the Rosetta mission has. To travel six billion km around and around the solar system to reach its prize... And now to be less than 16 kilometres from the comet. And ready to get on board. Whenever I see a photo of Andrea Accomazzo,

: Click to view full size image

I picture him at the helm of a sailing vessel, brandishing a cutlass!

OK - back to science!!

Margarita

by **Ron-Astro Pharmacist** » Thu Oct 16, 2014 7:32 pm

Kasuha wrote:
Guest wrote:I did wonder tho... Is this a true gravity based decent, or more like a powered rendezvous? (excluding the final braking blast, of course)
More like a powered rendezvous.
Check the animation of planned maneuvers on this page:
http://blogs.esa.int/rosetta/2014/10/15 ... ng-site-j/

Click to play embedded YouTube video.

This is going to be one of the most anticipated events in recent history from the standpoint of new capabilities and, of course, getting some pretty cool close-ups - if all goes well. From my lay persons perspective the planned rendezvous take a pretty circuitous route. I thought they might take the "synchronize with the rotation" in approaching the comet?? That must be a more difficult accomplishment than I might expect.

by **50bmg** » Thu Oct 16, 2014 4:10 pm

What are the spots on the solar panel? Perhaps micrometeoroid impacts?

Starship Asterisk*

APOD: Rosetta's Selfie (2014 Oct 16)

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