APOD: Rosetta's Selfie (2014 Oct 16)

[APOD Robot]

Rosetta's Selfie

Explanation: This Rosetta spacecraft selfie was snapped on October 7th. At the time the spacecraft was about 472 million kilometers from planet Earth, but only 16 kilometers from the surface of comet 67P/Churyumov-Gerasimenko. Looming beyond the spacecraft near the top of the frame, dust and gas stream away from the comet's curious double-lobed nucleus and bright sunlight glints off one of Rosetta's 14 meter long solar arrays. In fact, two exposures, one short and one long, were combined to record the dramatic high contrast scene using the CIVA camera system on Rosetta's still-attached Philae lander. Its chosen primary landing site is visible on the smaller lobe of the nucleus. This is the last image anticipated from Philae's cameras before the lander separates from Rosetta on November 12. Shortly after separation Philae will take another image looking back toward the orbiter, and begin its descent to the nucleus of the comet.

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[Nitpicker]

What a beautiful image. And it builds the suspense nicely, too.

I only just noticed that (at least in this image) the dust and gas is streaming roughly towards the Sun and perhaps even vaguely in the direction of the comet's orbital motion, indicating that the stream is being expelled with a certain oomph, from pockets of sublimating ice. But I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

[Markus Schwarz]

I only just noticed that (at least in this image) the dust and gas is streaming roughly towards the Sun and perhaps even vaguely in the direction of the comet's orbital motion, indicating that the stream is being expelled with a certain oomph, from pockets of sublimating ice. But I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

According to wikipedia (German) the material loss of a comet is about 10 t/s (first pass of the sun) to 0.1 t/s. This small mass is spread over a comparatively large volume, so it is of low density (I don't have numbers, though). Thus, I don't think it will effect the path of the satellite much. Maybe the dust and gas constitute a radiation hazard.

[MargaritaMc]

http://asterisk.apod.com/viewtopic.php?p=233315#p233315
For Rosetta blog post on this image.

[MargaritaMc]

...I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

from the Rosetta blog 15 September 2014
...
A detailed operational timeline will now be prepared to determine the precise approach trajectory of Rosetta in order to deliver Philae to Site J. The landing must take place before mid-November, as the comet is predicted to grow more active as it moves closer to the Sun.

“There’s no time to lose, but now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites,” says ESA Rosetta flight director Andrea Accomazzo.

“Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that’s what makes this a risky operation.”

Margarita

[Nitpicker]

...I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

from the Rosetta blog 15 September 2014
...
A detailed operational timeline will now be prepared to determine the precise approach trajectory of Rosetta in order to deliver Philae to Site J. The landing must take place before mid-November, as the comet is predicted to grow more active as it moves closer to the Sun.

“There’s no time to lose, but now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites,” says ESA Rosetta flight director Andrea Accomazzo.

“Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that’s what makes this a risky operation.”

Margarita

A very satisfying answer, thanks Margarita. (And thanks as well, Markus.)

[Guest]

I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

I would expect that the change in mass would have an effect less than the 'margin of error' they have considered in operations.

I did wonder tho... Is this a true gravity based decent, or more like a powered rendezvous? (excluding the final braking blast, of course)

[Chris Peterson]

I only just noticed that (at least in this image) the dust and gas is streaming roughly towards the Sun and perhaps even vaguely in the direction of the comet's orbital motion, indicating that the stream is being expelled with a certain oomph, from pockets of sublimating ice. But I suppose the force and mass loss must be too small to affect the comet's path in any significant way. I can't imagine how the Rosetta team could possibly factor such random stream forces into their sums.

Dust ejection velocities from a comet are size dependent, but are probably on the order of a few tens of meters per second for the (probably) 0.1-1.0 um particles that likely form these streams. This kind of mass loss does affect the comet's orbit, but only over very long timescales (typically, multiple orbits).

The challenge with the Rosetta lander is that it could be a problem if it directly encounters a particle stream- something that isn't predictable (although they can avoid areas of observed activity).

[Kasuha]

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.

[Ann]

Alone in a fragile vessel the sailor
braves the storms of the open sea,
the canopy above him aflame with stars
the ocean below, terrifying, his grave.
Forward! So his destiny calls
and in the depths below, as in the sky above,
is God.

Sorry, I couldn't resist. This is my own quite poor translation of a previously famous (now largely forgotten) Swedish poem, På nyårsdagen 1838 (On New Year's Day, 1838) by Erik Gustaf Geijer. 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

[50bmg]

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

[Ron-Astro Pharmacist]

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.

[MargaritaMc]

...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

[MargaritaMc]

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

[ta152h0]

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

[alter-ego]

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 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.

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.

[MargaritaMc]

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

[sOnIc]

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.

[Chris Peterson]

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!

[sOnIc]

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?

[Chris Peterson]

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.

[sOnIc]

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.

[Chris Peterson]

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.

[sOnIc]

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 ...

[Chris Peterson]

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.