Dawn taking chem makeup data at 385 km

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Better than expected, speed rel. Ceres under 64 m/s @ 46 kkm

Postby Marshall on February 28th, 2015, 12:35 pm 

As of 8am pacific on 28 Feb, Dawn was 46.0 kkm from Ceres going 63.9 m/s. Speed is now the critical factor. To achieve capture the plan was to get slowed down to 46 m/s by 6 March, when the distance will have increased to 61 kkm. Judging from current status figures. the craft is doing better than expected.
Based on Rayman's November 2014 projections (see the table a couple of posts back) we were looking at:
...45.46 .... 52.68 .... 64.25 for Feb 28.
It's only 8am and speed is already under the 64 m/s figure that could be taken as the goal for the day.

I just checked again, a little before 4pm pacific and the speed was 63.0 m/s, well below the target for the day.
Marshall
 


Re: Better than expected, speed rel. Ceres under 64 m/s @ 46

Postby Darby on February 28th, 2015, 11:32 pm 

Darby
 


Re: Better than expected, speed rel. Ceres under 64 m/s @ 46

Postby Marshall on March 1st, 2015, 1:48 am 

Emily Lakdawalla. She impresses me---good a combining geological smarts with keen photo interpretation. Suspected you might approve of some of what she has to say.

since we turned a page let's bring forward the current status view to have it handy for reference
Image
At 9:30 pm pacific 28 Feb (already 5:30 UT 1 March), speed is now 62 m/s
the target speed for 1 March, in the table, is roughly 61 m/s, more precisely 60.73. So it looks fair to make it.
Code: Select all
date      X          Y          Z        distance  v_esc  v_probe
F25     5.62894    25.0851    -29.7158    39.29    56.67    71.36
F26     11.407    26.4613    -29.1488    40.98    55.48      69.67
F27     17.2899    27.6663    -28.1919    43.11    54.10     68.44
F28     22.8583    28.5286    -27.0313    45.46    52.68    64.25
M1      27.9985    29.1842    -25.6846    47.90    51.32     60.73
M2      32.8862    29.7513    -24.1873    50.51    49.98     58.67
M3      37.6439    30.1647    -22.7166    53.31    48.65     55.28
M4      41.9734    30.4246    -21.3167    56.05    47.44     50.18
M5      45.8274    30.5605    -19.8726    58.55    46.42     46.96
M6      49.5028    30.6491    -18.2955    61.02    45.47     44.35
M7      52.8252    30.4896    -16.7451    63.24    44.66     40.49
M8      55.7681    30.3242    -15.1946    65.27    43.97     37.71

X Y Z are coordinates relative to Ceres, which is (0,0,0), measured in kkm---thousands of km.
X is directed out from sun, in Ceres orbit plane
Y is directed perpendicularly up off the orbit plane, approximately in Ceres' north pole direction
Z is directed forwards in Ceres orbit plane, the direction Ceres is moving, a negative shows the probe trailing behind.
distance from Ceres continues increasing for a while because the probe has some excess momentum
vescape is the escape velocity at that given distance
vprobe is the predicted velocity the probe will actually have that day. It must fall below vesc to achieve capture.The table starts 17 Feb and it shows the probe overshooting Ceres in the X direction (it has not had time to slow its X motion and will need Ceres gravity to pull it back in line)
likewise it shows the probe overshooting in the upwards Y direction.
Ceres' orbit inclination is 10 degrees and it just recently passed its descending node, so it is sloping down relative to the ecliptic and to Dawn's prior orbit.
This gives Dawn some unwanted speed in the "up" or Y direction. Again Ceres gravity will help draw the probe in.
Marshall
 


Re: Better than expected, speed rel. Ceres under 64 m/s @ 46

Postby Darby on March 1st, 2015, 4:10 am 

Yes, my inner polymath wannabe seems to gravitate to people with a flair for cross disciplinary connections.

If I wasnt already married, and if she wasnt a ginger (im genetically programmed to only fixate on brunettes) i'd probably hit on her.:-)
Darby
 


Re: Better than expected, speed rel. Ceres under 64 m/s @ 46

Postby Marshall on March 1st, 2015, 4:30 am 

If she knew your inner polymath cared she could always dye her hair

lak.jpg
Marshall
 


Re: Better than expected, speed rel. Ceres under 64 m/s @ 46

Postby Darby on March 1st, 2015, 10:25 am 

But then the curtains wouldn't match the drapes.

Ok, back on topic. {faux innocent whistle}

BTW, if you want to kick off a hot women of science thread over in the lounge, I'm game.
Darby
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Marshall on March 1st, 2015, 12:33 pm 

I'll think about it. It would have to be respectful.
BTW Dawn already as of 8 pm pacific has made its speed reduction target for the day
According to that table three-or-so posts back, to stay on track for capture by 6 March it has to slow down to 60.73 m/s on 1 March.
And according to current status as of 8 pm pacific, or 16h UT the speed was 60.35.

this is just a rough check based on different people's estimates but at least it is not way off which would worry me.
Marshall
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Darby on March 1st, 2015, 1:28 pm 

Firmly agreed on respectful.
{on a related note, feel free to redact my slightly too off-color jest above ... the self-edit timer already lapsed}
Darby
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby TheVat on March 1st, 2015, 3:05 pm 

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Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Marshall on March 1st, 2015, 3:47 pm 

Personally I find these comments to be entirely appropriate and anything less would fail to adequately address the exquisite reality. Given that our comments (including Braininvat's helpful pun) are *sufficient* acknowledgement of the facts of the matter, let us move on.

http://dawn.jpl.nasa.gov/multimedia/Cer ... allery.asp

The "Ceres science gallery" so far has only pictures from shoots up thru 19Feb. Those from 25Feb are not posted. So we look for those tomorrow. I'm hoping they will be higher resolution and reveal additional detail.
There could be some more intelligent comment online.

Personally I'm still in some suspense about the approach trajectory. It's complicated, with overshoots in both the "up" and "out" direction, as well as the lag in keeping up with Ceres. This comes on top of a shortage of hydrazine for attitude control
Marshall
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Watson on March 1st, 2015, 4:18 pm 

Image

This image was taken by NASA’s Dawn spacecraft of dwarf planet Ceres on Feb. 19 from a distance of nearly 29,000 miles (46,000 km). It shows that the brightest spot on the dwarf planet has a dimmer companion which lies in the same crater. Note also the “cracks” or faults in its crust at bottom right. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
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Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Marshall on March 1st, 2015, 4:33 pm 

Watson, thanks for posting that enigmatic shot that shows TWO white spots. If it were just some exposed icy material why would it be divided into two?
What does "volcanism" mean in this case?
Say that radioactive decay melts material at some depth. And a chilly liquid trickles up to the surface through some cracks. Does that constitute volcanism under Ceres circumstances?

BTW there is an OpNav picture shoot scheduled for today:
Code: Select all
...
Feb 12   (83,000)   122   (7.8)  3.8   98%   RC1
Feb 19   (46,000)   222   (4.3)  7.0   87%   RC2
Feb 25   (40,000)   255   (3.7)  8.0   44%   OpNav 4
Mar 1    (49,000)   207   (4.6)  6.5   23%   OpNav 5
Apr 10   (33,000)   306   (3.1)  9.6   17%   OpNav 6
Apr 14   (22,000)   453   (2.1)  14   49%   OpNav 7

so we should be seeing the attitude (orientation) of the craft flip around in the simulated current status view.
It has to aim its camera at Ceres which means its retro-thrust has to be interrupted. Thruster turned off.
The resolution will not be as good as that of 25 Feb. And only a 23% thin sliver of the planit will be sunlit.

They need optical navigation shots of Ceres with the known stars in the background for navigation purposes because they have very little to go on otherwise. Only the SLIGHT DOPPLER SHIFT in the signal that tells of Dawn's speed relative to Earth---towards or away---radially away in this case after allowing for the Earth's own 30 km/s orbital motion.
And yet they have to tell the probe which way to point the thruster when it gets turned back on! It is like a blind person finding their way in unmarked terrain.

They can't afford to make mistakes given the shortage of hydrazine (and the loss of reaction wheels which could otherwise help with attitude control). They can't "wander around" until they find the right orbit. They have to go right to the desired orbit with the least number of photoshoots (which are costly in terms of attitude control) in the most direct way possible.

And the most direct way possible turned out (after several days lost thrust in September) to be a complex trajectory utilizing a maximum of gravity assistance from Ceres.
Om.jpg
Marshall
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Watson on March 1st, 2015, 5:07 pm 

Glad to share. It does seem a bit bright to be a reflection and what is it continuously reflecting. There is the obvious lite left side and the dark right side, but the spots are right between at an apparent 45 degrees to the light source.

Are there other potential bright spots that face away from us at an angle, like towards the lower edge at about 8 o'clock.
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Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Marshall on March 1st, 2015, 7:11 pm 

In her blog entry Emily Lakdawalla has discussion of the enhanced contrast. She says you cannot take it literally to be what you would see. True the difference in reflectivity (albedo) is stark, but they have numerically scaled everything up.
The albedo of Ceres is only 10%, quite low. Emily compares it with other bodies like moons of jupiter and saturn.
Ceres crust is quite dark. So the white spots are only white by comparison. They might really be navy gray or butterscotch brown. Just guessing, but you get the idea.
Marshall
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Watson on March 1st, 2015, 11:04 pm 

There is a news conference tomorrow morning.

http://www.ustream.tv/NASAJPL2
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Re: Better than expected, speed rel. Ceres 58.6 m/s @ 49.9 k

Postby Marshall on March 1st, 2015, 11:22 pm 

Watson, thanks for the Ustream link!
http://www.ustream.tv/NASAJPL2
I gather the press conference will also be watchable at:
http://www.nasa.gov/multimedia/nasatv/i ... PPXsUI-DVo

When a 25 Feb photo is posted, one place to find it will be at the Dawn website
click on: multimedia > photos/images > Dawn science at Ceres
and you get: http://dawn.jpl.nasa.gov/multimedia/Cer ... allery.asp



The probe has been "retro" thrusting in an effort to slow itself down to around 46 m/s by 6 March
Current status view shows the ion thruster beam pointed straight out ahead in the direction the probe is going.
The phase of Ceres indicates the direction to the sun
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Current status now gives the speed relative to Ceres as 58.6 m/s (as of 8pm pacific, 1 March) and a distance of 49.9 kkm.
that is better than what we have listed in the table for 1 March.
It's rather more like what the table gives for 2 March.
Maybe the date labeling each row of the table means "4am in the morning, UT, on that date"?
Anyway, according to current status the slow-down is proceeding well
Marshall
 


Re: Better than expected, speed rel. Ceres 60 m/s @ 48.6 kkm

Postby Darby on March 2nd, 2015, 12:28 pm 

Watson, thanks for posting that enigmatic shot that shows TWO white spots. If it were just some exposed icy material why would it be divided into two?

Say that radioactive decay melts material at some depth. And a chilly liquid trickles up to the surface through some cracks. Does that constitute volcanism under Ceres circumstances?


I'll take a stab at the two above:

a) I suspect the division of the two spots will likely turn out to be icy deposits in two smaller subcraters located in the basin of the larger and more clearly defined crater. Deep craters probably help slow the solar radiation driven sublimation process, and the smaller and deeper the crater, the more likely the deeper the ice deposits are likely to be, and the longer it will then take for said deposits to sublime away.

b) As evidenced by the plastic deformation and pressure-driven melting of glacial undersides in long-term heavily subzero environments here on Earth, I suspect that sheer mass alone can account for most or all of the migration of icy substances from the core to further out in the mantle. I mean come on, if it has enough mass to crush itself into a sphere under its own gravity, I think it's highly likely that only solid matter is left in the core, and anything non-solid (such as ice and/or liquid) would have long since been squeezed much further away from the core. Perhaps not all the way to the surface, but definitely out of the deep core. And no, such migration doesnt constitute volcanism. Cryovolcanism manifests in different structures, and seems to require recurring periodic gravimetric tidal or radiative forces (or a combination thereof) that cause rhythmic changes of state across a critical state change boundary that then result in quasi permanent plumes or volcano like structures that have some sort of internal plumbing that has developed for said gases/liquids. I really dont think that's the case here.
Last edited by Darby on March 2nd, 2015, 12:34 pm, edited 2 times in total.
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Re:Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Marshall on March 2nd, 2015, 12:30 pm 

Good analysis! Makes sense. Maybe we'll hear various possible explanations from JPL people.

Picture taking!
speed 57.2 m/s (as of 8am pacific) at a distance of 51.3 kkm
talking to Goldstone antenna

Goldstone is talking with Dawn (as of 8 am pacific)
https://eyes.nasa.gov/dsn/dsn.html
And current status shows the spacecraft flipped around in photo+communication mode, thruster off.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
It looks like the navigation photo-shoot that was earlier scheduled for 1 March was postponed until 2 March, otherwise everything seems normal.

The current speed is nice and slow 57.2 m/s (as of 8am pacific) at a distance of 51.3 kkm
Marshall
 


Re:Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Darby on March 2nd, 2015, 12:33 pm 

Darby
 


Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Darby on March 2nd, 2015, 1:23 pm 

My assertion in the first part of item B above seems to loosely parallel what is being said in the interview currently underway, because they're calling the large impact basin we've been jawing about a "relaxed" basin (meaning a certain degree of "plastic" rebound/flattening has occurred due to the presence of pressure-deformable ice(s) in the mantle).

They also took a question just now on cryovolcanism, and pretty much gave a similar version of what I already posted ... it's not at the top of their list of explanations ATM. IOW, yes to the possibility of subsurface ice(s), but no to self-constructing volcano like structure for the bright spot under discussion.

Edit: they also made a comment in passing that the bright spots could also correspond to a salt-like deposit, but they also seemed pretty tenuous on that idea ... but they went on record with the possibility. I'm still leaning towards CO2 and/or water ice.
Last edited by Darby on March 2nd, 2015, 1:55 pm, edited 1 time in total.
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Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Darby on March 2nd, 2015, 1:52 pm 

And the conference is over ... my impression of the the overall feel of the answers given is that they've only just arrived on scene, and they havent got a lot of data yet, and wont have more definitive theories in place until they swing back around to the light side in a lower orbit. They spent most of their time bringing people up to speed on what's already been said on their own blogs. In other words, nothing much new to report yet.

There was one question of interest for me though, concerning whether or not they might attempt a lower final orbit than initially planned, based on the current reserves of fuel left from the transit from vesta, and their reply was that they want to wait until they finish principal science for a while, and then they'll take stock of their remaining hydrazine and consider attempting a lower orbit as the project gets closer to wrapping up.
Darby
 


Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Marshall on March 2nd, 2015, 2:02 pm 

Image

I like the salt hypothesis for the white spots.

Impact breaks the surface and heats subsurface, some very salty water percolates up, the water evaporates or sublimes, leaving a reflective salt deposit. Makes sense. Have to see.
Marshall
 


Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Darby on March 2nd, 2015, 2:10 pm 

It's a possibility. We'll just have to wait until mid April to find out.

One factor I can see in favor of the reflective salts is the presence of some signs of reflectivity outside of just the bright craters. I can picture an impact causing ejecta of rock, solid particulates and dirty snow/ice (possibly laden with the hypothesized salts), leaving behind temporary meltage in the crater itself. The meltage deposits would be deeper in the crater given the presences of thawables and salts in the mantle, and would linger longer, but the surface ejecta would be thinner and would sublime much faster, and thus if there's lingering long term reflectivity traces in said ejecta, that would SEEM more consistent with the salts theory.

I wish April were here already.
Darby
 


Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Watson on March 2nd, 2015, 3:28 pm 

Watson » Sun Mar 01, 2015 4:07 pm wrote:
Are there other potential bright spots that face away from us at an angle, like towards the lower edge at about 8 o'clock.


With the more recent views it appears there are several other spots visible, depending on the orientation. They almost seem to tinkle.

Interesting the two main spots are bright even as they fall into the dark side.
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Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Marshall on March 2nd, 2015, 4:22 pm 

Watson » Mon Mar 02, 2015 12:28 pm wrote:
Watson » Sun Mar 01, 2015 4:07 pm wrote:
Interesting the two main spots are bright even as they fall into the dark side.


That is strange. There were questions from audience at the news briefing about that. Towards the end one of the Dawn team who was in the audience (not the select panel in front) spoke up and said they had done some analysis focusing on those two very bright spots and they found that they actually do get dimmer as they go into the dusk.

What may be happening is that to get a readable image they have to adjust contrast. But the two very bright ones are so reflective as to be OFF SCALE given an adjustment which is optimal for everything else.
So with an otherwise optimal adjustment they come out as 100% white.

But that is not what is really going on. If you adjust contrast over a wider range, so that these spots come out GRAY rather than being off scale, then other stuff becomes harder to read but you WILL see the spots become darker gray as the sun begins to set and they get illuminated at a lower and lower angle.

As the sun goes down they are getting fewer and fewer watts of light per square meter, like everything else, so they are reflecting fewer watts per square meter----they have to be dulling out---but for some time (in the standard optimized contrast range) they remain off scale.

that might be what is going on. The team member who spoke up from audience seemed to suggest that.
Marshall
 


Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Watson on March 2nd, 2015, 4:31 pm 

That is strange. There were questions from audience at the news briefing about that. Towards the end one of the Dawn team who was in the audience (not the select panel in front) spoke up and said they had done some analysis focusing on those two very bright spots and they found that they actually do get dimmer as they go into the dusk.


I haven't had time to listen to the briefing, but have it cued up to listen to it later. Perhaps the spots are so tall that they reach up into the light, even though the lower landscape is in shadow.
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Re: Faxing new photo, speed rel. Ceres 57 m/s @ 51 kkm

Postby Darby on March 3rd, 2015, 9:04 am 

Here's the flat mosaic picture they released yesturday, which their description says was created from the source photos from Feb 19th. HOWEVER, looking at it closely, it seems to me that while creating it they also cleaned it up with some sort of post processing. For instance, zoom in close to the double bright spot, and then look at it side by side with an equally zoomed pic from 2/19 ... the mosaic version is sharper, indicating some sort of digital pixel re-interpolation/optimization.

Image

One factor I can see in favor of the reflective salts is the presence of some signs of reflectivity outside of just the bright craters.


Example: The bright crater in far center left appears to have some traces of reflective ejecta ringing it from the 5-10pm positions.
Darby
 


Re: Fax sent thrust resumed, speed rel Ceres 55 m/s @ 54 kkm

Postby Marshall on March 3rd, 2015, 1:06 pm 

Dawn has already made its speed reduction target for today. Approach seems to be going well.
I saw one of the 25 Feb photos included in a YouTube which was posted yesterday. For some reason the 2 minute YT was silent. The sound track either hadn't been recorded or for some reason was not coming through.

This YT, made for very wide audience especially for kids, is nevertheless visually informative and (I found) quite fun. It is only 3 minutes and I think really worth watching:
https://www.youtube.com/watch?v=uHj_kmYSpkI

That one has sound track. I don't know what the trouble is with the other, the 2 minute one.
The 2 minute soundless YT is https://www.youtube.com/watch?v=GM6Nm3ZLVsI
The picture of a crescent, nearly half, Ceres that was taken on 25 Feb is at 0:53.
You can pause and drag the time button to the 53 second mark and inspect it as a still photo.

Image

This is from 25 Feb. It is at http://dawn.jpl.nasa.gov/multimedia/Cer ... Shadow.asp
The pixel width is 3.7 km per pixel. As I recall the face is 44% sunlit.
Marshall
 


Re: Fax sent thrust resumed, speed rel Ceres 55 m/s @ 54 kkm

Postby Darby on March 3rd, 2015, 11:53 pm 

Image
Darby
 


Re: Speed rel Ceres 53 m/s @ 56 kkm (nearing capture)

Postby Marshall on March 4th, 2015, 2:10 am 

Lakdawala has a new (3 March) blog entry
http://www.planetary.org/blogs/emily-la ... guide.html

Current status as of 11pm pacific on 3 March gives the speed as 53.2 m/s @ 55.7 kkm

It bugs me slightly that I don't see anything labeled "the sun" in the Current Status view. Remember that this is a 30 degree view with Ceres at center. So there are 15 degrees on either side, to left and right. Dawn is currently show on the centerline also so the Dawn-Ceres direction is roughly into the page.
Something that is 9 degrees left of the Dawn Ceres direction would be 9/15 or 3/5 of the way from Ceres to the left edge. that is about where I expect the SUN to be.

there is a largish star about where I'd expect to see something labeled "sun" but there is no graphic indication that it is supposed to be the sun. just a quibble.

what is escape velocity at 55.7 kkm?

(2G*943e18 kg/(55700 km))^.5

47.54 m/s and Dawn is going 53.2 m/s only 6 more meters per second worth of speed to scrub.
Marshall
 


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