Dawn dead in Ceres orbit, ran out of fuel Oct 2018

[OmCheeto]

...the October 2014 Dawn Journal. ...

True classic!
I read that entry, word for word, start to finish. (I usually just skim the journals)

I think it would make a great new "pre-pre-Trekky" screenplay:

We were on the very first trip to the Centauri system, when we were attacked by the Omfoomulatorians, and their proton beam weapon damaged an auxiliary computer, shutting down the warp engines. Adrift in space for days, certain of our doom, we waited. Fortunately, as always, Scotty fixed the problem...​

It's kind of like that, only, well, real. And incredibly well written.

And the rebooting. OMG. Nightmares of past crashed computer systems filled my mind...

 

[marcus]

LOL!
Laughed so much at the Omfoomulatorians that it took me a long while to read the rest of your post. It kept jiggling.

Yes that October 2014 piece is pretty good. And it's informative. You may be right that it's a classic.

 

[mfb]

and you are quite right to note that it might have been SEVERAL particles hitting the same transistor at once. But the simplest assumption is it was just one high energy proton. the most common type of cosmic ray event.

Well multiple high-energetic particles at the same time are so unlikely we can neglect that chance.
A slow build-up of radiation damage can lead to failures, but if you monitor the critical pieces you can monitor it, and see effects long before something fails.

 

[mheslep]

and changes a bit, a

ok. When you said "disabled electronics" earlier I assumed permanent damage, not an upset event.

 

[marcus]

Ooops! sorry about the unclear language. I should have said "temporarily put out of commission" or something. I don't have a good engineering vocabulary at all :(

 

[OmCheeto]

I like Om's way of responding to Mheslep's question! I did not see it until I finished writing and posting my own response.
For sure, why not write to Rayman's blog?! but I'll leave my answer standing anyway since it suggests some partial answers to think about for the time being until Rayman or one of his group responds on the blog.

It looks as though Dr. Rayman's Journal moderators didn't like something. My comment no longer shows up, and may have been deleted.

Anyways, without knowing the exact details, it appears we would just be guessing as to what happened.
And after a bit more snooping, I'm now a bit suspicious.

Oct 31, 2014 entry; "The events to that point were virtually identical to a radiation strike that occurred more than three years earlier."
Jul 18, 2011 entry; "(A cosmic ray is the most likely culprit, but other explanations for the circuit's inaction are still being considered.)"

Perhaps this is Dr. Rayman's fiendish scheme to get me to read all of his journal entries.

ps. So far, I'm up to Sept 30, 2007, 3 days after its launch.

 

[marcus]

Since we just turned a page I should probably bring the basic material on the approach plan forward.
http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/
Here's an approach and navigational picture-taking schedule from the recent 25 Jan images to 15 April.
The numbers in parens are KILOMETERS: distance to Ceres and also kilometers PER PIXEL (to indicate resolution).
The number between them is another measure of resolution--Ceres diameter in pixels.
The percentage tells how much of the face is illuminated, whether full-Ceres, half-Ceres etc.
The number before the percentage tells how much better Dawn's image is expected to be than Hubble Space Telescope's.

Jan 25    (237,000)    43    (22)     1.3    96%
Feb 3     (146,000)    70    (14)     2.2    97%
Feb 12    (83,000)    121    (7.8)    3.8    98%
Feb 19    (46,000)    221    (4.3)    7.0    87%
Feb 25    (40,000)    253    (3.7)    8.0    44%
Mar 1    (49,000)     207    (4.6)    6.5    22%
Apr 10    (33,000)    304    (3.1)    9.6    18%
Apr 15    (22,000)    455    (2.1)    14     50%

for reference I quote the relevant column headings given in the original table:
Distance from Dawn to Ceres in (kilometers)
Ceres diameter in pixels
Resolution in (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk
http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/

If we do a linear interpolation (2*146+7*83)/9 = 97 we estimate that the distance to Ceres on 10 February will be 97,000 km. On the previous page I made a rough guess of 100,000 km which now seems about right.
Conceptually 10 February is an important date because that is when the spacecraft enters the approach diagram that has been drawn for us.
Other critical dates are 24 February, when it stops falling behind Ceres in solar orbit and begins catching up (gravity assist), and 6 March, which is marked "capture" on the diagram.

 

[marcus]

Here's the approach plan diagram, with day circles so you can locate critical dates like 10 Feb (entry from sunward at left edge) and 24 Feb when the probe (hopefully) stops falling behind and begins to catch up.
The plan of approach is viewed from above the north pole of Ceres, with orbital motion direction from bottom to top of frame. The sun is way far to the left of the diagram. Dawn approaches from sunward direction. Capture is on 6 March, if things go as planned.

The diagram shows the first sector of the first circular polar orbit, just getting started. the probe comes over the north pole of Ceres, swings around almost a half circle and starts to around under the planet's south pole. A circular polar orbit at altitude 13500 km is the initial one they want to get into, and then after a short pause start spiraling down to lower altitude circular polar orbits (for survey, and mapping).

 

[marcus]

Since in about 10 days the probe will be advancing on that post #43 approach diagram I also bring forward here the diagram which shows the same thing from another angle. where, as before, the sun is to the left, but now instead of looking down on Ceres north pole and solar orbital motion direction being from bottom to top of frame, in this new diagram the solar orbit direction is INTO THE PAGE and CERES NORTH POLE IS UP.
So the fact that probe is falling behind slightly is not shown in this diagram. To show it you would have to make the probe trajectory at first rise up towards you out of the page, and then around 24 Feb begin to settle down back onto and into the page. that would be putting the two diagrams together in one's imagination. Anyway it's good to have them together for reference.
In this figure, probe enters at the left edge on 16 Feb when the distance is 62,000 km.
(4*46+3*83)/7 = 62 , see the table of distances two posts back.

Let's get the current status (i.e. distance 175.24 thousand km, etc as of 31 Jan) here where it's convenient to refer to. We can have it as a thumbnail that you click on if you need to see it: Nope. Don't see how to do it. Anyway, current distance is that, so 75 thousand km to go by 10 February, when Dawn shows up at the left edge of the above diagram. Ceres angular size now 2arcsin(.475/175.24) = 0.31 degree.

 

[marcus]

BTW I tried to count the day circles from 6 March "capture" to arrival at RC3 orbit (in the second diagram) and I got that arrival is on 23 april. But the day circles are too close together to count properly over part of that. I also tried to count day circles in the first diagram (that shows gravity assist bringing Dawn up to speed) and got that it ends on or around 22 April. the RC3 orbit can't be drawn easily on the first diagram because its aspect is so tilted. It is a polar orbit and the perspective is looking down on Ceres north pole. So I just take for granted that the last day on the first diagram corresponds to arrival at RC3 orbit---IOW altitude 13,500 km, IOW orbit radius 13,975 km or approximately orbit radius 14,000 km.

So we could add a bottom row to the TABLE in post #42 that says something like "23 April...14000 km"

But that table, to be clear, is a list of navigational "photo opportunities" occasions when they plan to stop thrusting and turn the camera at Ceres and get a picture of it against the background of known stars (to use in locating Dawn precisely relative to Ceres, so as to steer the probe better).
The table indicates times when we might get to see a photograph of Ceres---assuming they share some of these navigational shots with the public.

that said, it's still tempting to add no-photo non-navigational rows to their table, just to keep track of the simple distance to the planet---without mentioning pixels or anything. :)
I just checked current status and distance to Ceres as of 1 Feb (now) was 168 thousand km.

Jan 25    (237,000)    43    (22)     1.3    96%
Feb 1     (168,000)
Feb 3     (146,000)    70    (14)     2.2    97%
Feb 10    (100,000)
Feb 12    (83,000)    121    (7.8)    3.8    98%
Feb 19    (46,000)    221    (4.3)    7.0    87%
Feb 25    (40,000)    253    (3.7)    8.0    44%
Mar 1     (49,000)    207    (4.6)    6.5    22%
Apr 10    (33,000)    304    (3.1)    9.6    18%
Apr 15    (22,000)    455    (2.1)    14     50%
Apr 23    (14,000)

Relevant column headings as given in the original table:
Distance from Dawn to Ceres in (kilometers)
Ceres diameter in pixels
Resolution in (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk
http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/

 

[marcus]

I tend to be checking "mileage" around 3 or 4 in the afternoon Pacific time. After all Dawn is a Pasadena spacecraft , run out of JPL
So I just checked, and the distance that current status gave was 157 thousand km. Which is right halfway between the 1 Feb and the 3 Feb figures, so that seems right.

 

[OmCheeto]

Mark another one off of my bucket list*:

Hi Om,

I decided to answer your question in email rather than on the blog, as it's a little specialized for our normal audience.

The conclusion that it was a cosmic ray was the result of a pretty extensive analysis. (In fact, you might be astonished at how thoroughly we investigate such anomalies.) Telemetry made it clear what particular circuit in the ion propulsion system controller was affected. It turns out that the phenomenon is a result of the way this circuit uses a certain part. It is known to be sensitive to a single-event-upset due to something causing an internal electrical glitch, for which the only probable cause is interaction with a high-energy cosmic ray. The physics is very well understood. The parts used in the circuit are designed and screened against failure due to cumulative radiation exposure (total ionizing dose) to a much higher level than that to which Dawn has been exposed, which makes failure due to low-energy radiation much less likely. Mechanical fatigue or stress is also not probable, as the circuit has continued to perform well over long periods of time between the two faults, and there were no precipitating events (such as thermal or mechanical shocks) that would have triggered a marginal mechanical connection to cause the observed fault.

I hope this answers your question.

Thanks for your interest!

Marc

I responded as briefly as Omically possible:

Hi Marc,

Thank you for the totally unexpected personal response.

Your answer is adequate with me, but the members of the forum will probably not be satisfied until they see the blue prints, schematics, and analysis of the two failure events.
We are nerds.
And I’m sure the particle physicists on staff would love to analyze, and devise a solution to the "Proton weaponry" used upon Dawn in her travels.
:D

If you are interested, here is the link to the post containing the question: https://www.physicsforums.com/threa...-size-of-full-moon.793140/page-2#post-4993023

Thanks again,

Om

I wanted to ask him about a source of better historical telemetry data, but decided against pushing my luck.
I wanted to know, mainly, because yesterday, I was looking at the images, and couldn't figure out why Dawn was approaching Ceres from not just the direction of the sun, but from an angle indicating Dawn was ahead of Ceres. I spent a couple of hours yesterday annotating the two images.

But then, I kind of decided, in a, "who cares" kind of way, that it was irrelevant.
Except for the rocket scientists, of course.*In the early days of the internet, I received email responses from Tim Berners-Lee, Max Tegmark, and John Baez. It always struck me as odd, that with 7 billion people on the planet, they would take time out, for little ol' me. :)

 

[marcus]

Nice annotations to Marc Rayman's diagrams! Thanks for sharing them with us!

 

[mfb]

I scaled some pictures of the moon to get an idea of the image quality at the different navigation points. 1 pixel here corresponds to 1 pixel in their estimates.

The first set of images corresponds to Jan 25 (what we saw), Feb 3 (what should come today), Feb 12, Feb 19, Feb 25, Apr 10, Apr 15.
The second set (shown in this post) includes the illumination fraction as rough estimate. I dropped the April 10. crescent there as the camera won't make a picture.

 

[OmCheeto]

Nice annotations to Marc Rayman's diagrams! Thanks for sharing them with us!

It would appear, that I have a pen pal. (God help Dr. Rayman, for he knoweth not, my endless curiosity...)(I warned him not to respond to my latest email)

Hi Om,

You're welcome. I appreciate your interest.

Well, we're nerds too :D In this case, we're nerds with deep knowledge of the details. We don't reach conclusions lightly, but this one is a pretty clear case. Nevertheless, NASA does not release the kinds of details you mention, although I know you didn't really expect that sort of information. Believe me, I know it's fun to get all the details. I've been a physics and space enthusiast my whole life. (You might get a kick out of this video tour of my private collection of space information and memorabilia.)

I browsed briefly through the forum, although I didn't have time to read it in detail. It looks fun! I'll mention just one thing that might interest you. I've explained in many Dawn Journals that we never use more than one thruster at a time. The spacecraft is not designed that way. They don't even point in the same direction, so even if we did, we would pay a cosine loss. In addition, the acceleration of 7 m/s/day was early in the mission, and today it is < 3 m/s/day. (See my February 2010 Dawn Journal.)

Thank you again for being so actively engaged in the mission!

Regards,

Marc

What an eye! I can't believe he caught my "full throttle, 7 m/s/day" mistake.

ps. To facilitate him from returning my pestilent correspondence, I only asked him one last question today; "You're not a fan of TMBG are you"?

 

[marcus]

The update just showed the simulated image of Dawn turned so as to be in picture taking mode.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg This happened at around 1:30 PM Pacific time as far as I can tell.
The distance range is given as 149 thousand km.
The table copied back in post #45 estimated that the picture would be taken at 146 thousand km, today.
(EDIT: actually the current status distance changed to 146 thousand km later today. Maybe there is no discrepancy. Unsure about this.)

A discrepancy could suggest that some unexpected factor entered in, but most likely I think it just says what we may have guessed already that the table distances are just estimates. And it's all more or less approximately consistent. I'm very glad we have both the table, and the current status simulated views, and they match up tolerably well.

This "picture taking mode" thing happened the last time Dawn took a picture, I noticed. For some period of time like several hours to half a day the main axis of the spacecraft , along the solar panel arms, is VERTICAL in the simulated view frame. And then when the probe gets back to thrusting it resumes the usual attitude with the solar panel arms nearly horizontal in the view frame.

It could also be that what I'm seeing in the simulated view is not exactly "picture taking" mode. It might be "data transmission"----the attitude has to be turned around so the big antenna is aimed at Earth. But either way it seems to have something to do with getting a navigation shot of Ceres against the background of known stars and transmitting it back to Control at Pasadena.

Damn, some of our family live in Pasadena and show no sign of excitement about this. They are walking on holy ground. If I didn't hate to travel I would go crash on their couch and try to hang around JPL some. Ceres is a PLANET, and it has a lot of WATER ICE. Don't people get this?
Do dolphin's bones lose calcium if their watery habitat is in low gravity? Or are they adapted to more neutral buoyancy and don't need so much gravity? Suppose you had an ice cavern with a lake of chilly water (which serves both to grow algae and to cool the power supply that provides light). Suppose there are fish in that lake. Do the fish suffer from 3% gravity the way today's humans would. The humans would have to do some pretty intense exercising to stay healthy.

 

[Dotini]

Video showing curious bright spot, etc.

This source describes the white spot as "flickering"!
http://www.space.com/28336-mysterious-white-spot-on-ceres.html

http://www.techtimes.com/articles/2...on-ceres-leaves-nasa-scientists-flummoxed.htm

They think it may be a pool of water at the bottom of a crater.

 

[marcus]

BTW if you have been checking out the simulated view current status http://neo.jpl.nasa.gov/orbits/fullview2.jpg from time to time you will have noticed that the brown dot in the center has grown a lot.
Another nice thing, the the cursor that you move around on the frame with your mouse or touchpad is a white dot with a + symbol on it (clicking enlarges, so "+")
The width of the frame is scaled to be 30 degrees.
If you look at the size of that white dot, it is about 1/60 of the width of the frame. IOW that white dot they give you is the size of the full moon in Earth sky
so you can drag it over to the brown image of Ceres in the center and compare.
You will see that Ceres dot is now about 3/5 or 2/3 of the full moon dot. It suggests to me that the guy who does the simulated views for them is thoughtful and consistent in unadvertised ways (this is a form of coolness, the guy has aspects of coolness, check it out.)

 

[OmCheeto]

The update just showed the simulated image of Dawn turned so as to be in picture taking mode.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg This happened at around 1:30 PM Pacific time as far as I can tell.
The distance range is given as 149 thousand km.
The table copied back in post #45 estimated that the picture would be taken at 146 thousand km, today.

That discrepancy could suggest that some unexpected factor entered in, but most likely I think it just says what we may have guessed already that the table distances are just estimates. And it's all more or less approximately consistent. I'm very glad we have both the table, and the current status simulated views, and they match up tolerably well.

This "picture taking mode" thing happened the last time Dawn took a picture, I noticed. For some period of time like several hours to half a day the main axis of the spacecraft , along the solar panel arms, is VERTICAL in the simulated view frame. And then when the probe gets back to thrusting it resumes the usual attitude with the solar panel arms nearly horizontal in the view frame.

It could also be that what I'm seeing in the simulated view is not exactly "picture taking" mode. It might be "data transmission"----the attitude has to be turned around so the big antenna is aimed at Earth. But either way it seems to have something to do with getting a navigation shot of Ceres against the background of known stars and transmitting it back to Control at Pasadena.

I don't think it's "data transmission".
I was just snooping through my Twitter feed, when I saw that https://twitter.com/NASA_Dawn had retweeted a picture of Dr. Rayman from https://twitter.com/NASAJPL, and I realized I wasn't following NASAJPL. There I found a tweet about an interesting website: http://deepspace.jpl.nasa.gov/dsnnow/

If you click on the "Enter now" button, you get a live feed of the Deep Space Network activity. I've been checking in on it, off and on, for about 2 hours now. It was fascinating watching the dishes turn off and on, and seeing the signals switch from each spacecraft to different dishes.

Anyways, I've seen no signals to or from Dawn, over the last two hours.

Damn, some of our family live in Pasadena and show no sign of excitement about this. They are walking on holy ground. If I didn't hate to travel I would go crash on their couch and try to hang around JPL some. Ceres is a PLANET, and it has a lot of WATER ICE. Don't people get this?
Do dolphin's bones lose calcium if their watery habitat is in low gravity? Or are they adapted to more neutral buoyancy and don't need so much gravity? Suppose you had an ice cavern with a lake of chilly water (which serves both to grow algae and to cool the power supply that provides light). Suppose there are fish in that lake. Do the fish suffer from 3% gravity the way today's humans would. The humans would have to do some pretty intense exercising to stay healthy.

I cannot answer these questions.

ps. Dr. Rayman says he is not a fan of TMBG. But, I warned him...

Hi Marc,

Sorry to be so assuming about you possibly being a fan of TMBG, but, your latest journal comment

The probe is much closer to Ceres than the moon is to Earth.
And now it is even closer…
And now it is closer still!​

looked just like something from their song “Older”

You're older than you've ever been
and now you're even older
And now you're older still​

You may want to hire a lawyer.

 

[OmCheeto]

The update just showed the simulated image of Dawn turned so as to be in picture taking mode.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg This happened at around 1:30 PM Pacific time as far as I can tell.
The distance range is given as 149 thousand km.
The table copied back in post #45 estimated that the picture would be taken at 146 thousand km, today.
(EDIT: actually the current status distance changed to 146 thousand km later today. Maybe there is no discrepancy. Unsure about this.)

A discrepancy could suggest that some unexpected factor entered in, but most likely I think it just says what we may have guessed already that the table distances are just estimates. And it's all more or less approximately consistent. I'm very glad we have both the table, and the current status simulated views, and they match up tolerably well.
...

Dawn induced panic attack #47.
Just checked "fullview2.jpg", and it's still in photo/data x-mission alignment.
Checked the DSN, and no data was being received.
Somethings wrong!

Just rechecked DSN, and data is streaming in at 125 kb/sec.

Phew!

Anyways, Dr. Rayman, in his last email said;

Hi Om,

... The Where is Dawn Now? feature is not accurate enough to warrant using so many significant figures. See this recent point. And I've explained in many Dawn Journals, sometimes we thrust and sometimes we coast, and the coast periods are neither regular nor of uniform duration. ...

I don't mean to be discouraging. Quite the contrary! I love doing the kind of thing you are having fun with, and I admire your creativity and insight. I would simply point out that it's worth being careful and recognizing the limitations. I'm too busy to provide details, but I hope you find some more in my prior Dawn Journals and, most importantly, that you continue to be so interested in the mission!

Sorry, I'm not a fan of TMBG. I love your PPS! Thank you for being considerate with my time. And now, I will indeed stop responding to your emails.

Marc

As if he could discourage us! And the limitations are only exacerbated by the fact that I lazily didn't bother to note the UTC times, so, in the following excerpt from my spreadsheet, corresponding to the published projected dates and distances, I included both the previous and following days data. Which, up until February 19th, is completely accurate, +/- a day.

Columns B, C, & D are my projections. Column E is Dawn published projections.
And as I mentioned, please ignore all of my data after Feb 19th.

I probably should have changed the "Distance 1000 km" functions after the 19th, as the "x vs y" components of the velocity get really big, inducing planetoid crashing results.
(The "x vs z" number was based on the published distance(hypotenuse) vs 20,000 km, if that makes any sense.)

Though, looking at the projected data and images again, I probably should have used a number closer to 40,000 km as my "x" orbit intercept value.

Oh well, I guess they're lucky I'm not flying the vessel.

And thank god for Emily:

Emily Lakdawalla @elakdawalla · 47m47 minutes ago
(Of course, when I tweet about DSN Now, it is undergoing scheduled maintenance. If you get a 404 on that link, please return in 2 hours)​

She's averted panic attack #48.

 

[marcus]

The pictures are up!
Images taken on 4 February are posted:
http://www.jpl.nasa.gov/news/news.php?feature=4475
That's just some of what's online. It is an ANIMATION that shows a large part of one rotation (not the full 9 hours but quite a bit of it) taken at a distance of 145 thousand km
and the resolution is around 14 km per pixel.
Here are some more links
http://www.jpl.nasa.gov/spaceimages/details.php?id=pia19174
http://www.jpl.nasa.gov/spaceimages/details.php?id=pia19179
I like the watching the animation, but am not sure how to paste it into this post, so here is a still:

Current status http://neo.jpl.nasa.gov/orbits/fullview2.jpg shows the craft has rotated out of picture-taking mode and has resumed normal thrusting, current distance is given as 133.91 thousand km.

That means an angular size of 0.406 degree (getting close to full moon size, which is half a degree)

There seems to be a variety of pictures to choose from, from yesterday's session. The link for this particular one I happened to post is:
http://www.jpl.nasa.gov/spaceimages/images/largesize/PIA19179_hires.jpg
in case anyone wants to forward it or something. If anyone finds other interesting images please share the links! And post them for us if you think appropriate. No naked grain goddesses or fertility symbols please.

 

[OmCheeto]

The pictures are up! ...

Yay!

...
in case anyone wants to forward it or something. If anyone finds other interesting images please share the links! And post them for us if you think appropriate.

This one is interesting: http://photojournal.jpl.nasa.gov/archive/PIA19174.gif
from the page: http://www.jpl.nasa.gov/spaceimages/details.php?id=pia19174

It seems to confirm my suspicion that Dawn was imaging Ceres for nearly a full rotation. Dawn stopped sending data last night around midnight. 10 hours!

No naked grain goddesses or fertility symbols please.

How about the "Einstein" image, which seems to have answered NASA's 10 year old question?

Was Einstein a Space Alien? (NASA)
March 23, 2005​

Confirmed!

https://pbs.twimg.com/profile_images/474989255773925378/0nhQR66C_bigger.jpeg Cody Healey‏@cody_healey
@NASA @NASA_Dawn Hey, I see a face on that planet. Looks like Einstein!

https://pbs.twimg.com/media/B9GFbgZCcAA0Vsj.jpg ​

I don't follow Cody Healey, and I still don't know how to properly use Twitter, but I think this showed up briefly on Dawn's feed, and I thought it was very funny.

ps. #lookingforwardtononanthropomorphizedpicsofCeres:)

 

[marcus]

Current distance 129.88 thousand km. approach speed 0.100 km/second, so 8640 km per day. When might the next photo shoot be?
The table is basically a list of navigational "photo opportunities" when they plan to stop thrusting and turn the camera at Ceres and get a picture of it against the background of known stars (to use in locating Dawn precisely relative to Ceres, so as to steer the probe better).
The table indicates times when they might share some of these navigational shots with the public.

Jan 25    (237,000)    43    (22)     1.3    96%
Feb 1     (168,000)
Feb 3     (146,000)    70    (14)     2.2    97%
Feb 10    (100,000)
Feb 12    (83,000)    121    (7.8)    3.8    98%
Feb 19    (46,000)    221    (4.3)    7.0    87%
Feb 25    (40,000)    253    (3.7)    8.0    44%
Mar 1     (49,000)    207    (4.6)    6.5    22%
Apr 10    (33,000)    304    (3.1)    9.6    18%
Apr 15    (22,000)    455    (2.1)    14     50%
Apr 23    (14,000)

Relevant column headings as given in the original table:
Distance from Dawn to Ceres in (kilometers)
Ceres diameter in pixels
Resolution in (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk
http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/
Click to get Om's dateline+trajectory diagram:
View attachment 78574

 

[marcus]

Let's see if I can get a copy of Om's dateline trajectory diagrams. Both have the sun way off to the left and Dawn coming in from the left. In the upper (respectively lower) Ceres orbit direction is into the frame (respectively straight up in the frame). I'll try to insert these diagrams from Marc Rayman''s Journal which Om made more useful by annotating them, tagging some of the day circles with dates to produce a dateline.

Let's see how that looks. Not bad. In the lower diagram, Dawn enters the diagram at the left edge on 10 Feb, when her distance to Ceres is about 100 thousand km.
For that to work, if distance is now 130 thousand, the probe has to go 30 thousand km between now and 10 Feb. Sort of 4 days, so it has to average sort of 8 thousand km a day. OK, seems do-able.

 

[lpetrich]

There is something interesting about the world Ceres, as I like to call it. It looks like Ceres's spin axis is now nearly perpendicular to the asteroid's current direction to the Sun. That means that it should be possible to map nearly all of it.

 

[marcus]

That's right! There is what appears to be an element of luck. Ceres orbit period is 4.6 years. That means its seasons, like e.g. Winter, last on the order of a year. Dawn might have arrived e.g. in the middle of Winter when the north polar regions are in arctic darkness. The probe's limited supplies only allow it to operate for a few months. So it would be unable to map those regions in darkness. I don't know if the planet's axis inclination was determined earlier by Hubble space telescope and this went into planning the mission, or if it was just luck. Maybe the inclination is a small angle anyway, or we simply happen to be in an "equinoctial" season just now (a Ceres spring or autumn.)

Om's annotated diagram showing the approach timeline from two different perspectives is useful for reference, and since we have turned a page I will bring it forward for convenient reference. Thanks to Marc Rayman for posting the original un-annotated approach trajectories in his April 2014 Dawn Journal.

The lower diagram (looking down on Ceres north pole, sun to the left, solar orbit motion "up" in the figure) shows the probe having deficient solar orbit speed and falling behind until around 24 February when Ceres gravity causes it to start catching up. By 23 April, probe is in a circular polar orbit and beginning to pass under Ceres south pole. The orbit would not look circular in the lower diagram because we are viewing it somewhat edge-on, from the north pole direction.

According to current status page http://neo.jpl.nasa.gov/orbits/fullview2.jpg the distance to Ceres is now (as of 6 February 9AM pacific ) 125.92 thousand km.
2 arcsin(.475 /125.92) = 0.432... deg, about 86% angular size of full moon .

 

[mfb]

Ceres' axial tilt is just 3 degrees, no matter when you arrive you can map nearly everything. The Wikipedia article cites a paper from September 2005, two years before Dawn was started. Hubble could see the bright spot, so it was possible to measure its axis of rotation directly.

 

[lpetrich]

Dawn got complete coverage of Vesta just before it departed for Ceres: Spring arrives to Vesta's north pole, as Dawn departs, plus a request for citizen scientists | The Planetary Society, Dawn Mission: News & Events > Shape of Vesta

For other Solar-System objects, one can get a clue from published maps -- where do they lack detail? Planetary Visions: Texture maps

Mariner 10 flew by Mercury 3 times, but saw only half of that planet's surface. That's from a Mercury solar day being two Mercury years and Mariner 10 being in an orbit with a period of about 2 Mercury years. However, the MESSENGER Mercury orbiter has succeeded in seeing all of that planet's surface (MESSENGER: Global Mosaics of Mercury).

Venus has 98% coverage from Magellan's radar, all except near its south pole. Likewise, the Moon and Mars have nearly complete coverage.

Jupiter's and Saturn's big moons are well-covered, except for near their poles. However, Uranus's moons only had their southern hemispheres visible. That is because the only spacecraft to visit them, Voyager 2, flew by near Uranus's southern summer solstice.

 

[lpetrich]

Here are some figures on Ceres, from its Wikipedia article.

Stable orbits around an object are all inside its "Hill sphere", and Ceres's Hill sphere has a radius of about 220,000 km.

Surface-satellite orbital velocity and orbital period: 360 m/s and 2.3 hours.

Dawn is currently 122,000 km away from Ceres and traveling around 100 m/s toward it. Escape velocity from Ceres at that distance is about 32 m/s. But it's a month before Dawn has slowed down enough relative to be Ceres to be captured.

We should be getting much better values of Ceres's mass and size before long, however.

 

[mfb]

At 105000km Ceres will look as large as the moon (using its semi-major axis). 2 days to go.

Ceres' mass is known to better than 1%, getting a better measurement know looks complicated - it would need a very good distance measurement to see the small effect of gravity (relative to thrust) with an uncertainty below 1%.

 

[Dotini]

Rayed craters and possible maria make this look like our familiar Moon, IMO.
"Here be dragons -- but not for much longer!"
http://www.planetary.org/blogs/emily-lakdawalla/2015/02061616-ceres-coming-into-focus.html

NASA / JPL / UCLA / MPS / DLR / IDA / Emily Lakdawalla
Dawn's view of Ceres on February 4, 2015 (animation)

This animation consists of 20 individual frames shot as part of Dawn's third optical navigation campaign on approach to Ceres. The images have been enlarged from the original size by a factor of two, and contrast enhanced to bring out more detail.

 

[marcus]

...
...
http://www.planetary.org/blogs/emily-lakdawalla/2015/02061616-ceres-coming-into-focus.html

NASA / JPL / UCLA / MPS / DLR / IDA / Emily Lakdawalla

Beautiful job! I like Emily Lakdawalla's comments at her blog.
Dotini, could you please explain how you transferred the animation from her blog (at the link you gave) to your post here? Did you use the "media" button, with the icon that looks like some 35mm film frames. And how exactly does one proceed?

 

[Dotini]

You're going to roll your eyes when I tell you. I have a 20 year old Mac that is so obsolete I don't have cursor control even typing in this reply block, tho I do gain a cursor when I edit.

Anyway, I just highlight, copy and paste.

 

[mheslep]

I see not just scant, but zero mention of Dawn's arrival at Ceres in the mainstream newspapers, which is disappointing. Though, perhaps they're waiting for closest approach. I don't really expect a front page piece every day between now and April.

 

[marcus]

...
http://www.planetary.org/blogs/emily-lakdawalla/2015/02061616-ceres-coming-into-focus.html
Emily says Dawn is approaching Ceres "from the south". That means we should be able to see more small-circle rotation detail at the bottom of the Ceres image. I think I do. This agrees with the upper Om-figure of post #62. Going by that figure, Dawn crosses the equatorial plane of Ceres around 15 Feb.
Or let's say the orbit plane determined by direction to sun (left) and orbit direction (into the page).
But the axial tilt is only about 3 deg, so the planes are nearly parallel. Until mid-February Dawn is looking slightly from the south and sees more of the south polar region. Increasingly after mid-Feb it will be looking more from the north.
Please correct if I'm missing something. In the animation, lighting also seems stronger near the north pole, with northern regions blanked out in glare. We are told the planet's axial tilt is only 3 degrees, could the current tilt (small as it is) be towards the sun?

Current angular size: 2 arcsin(.475 / 110.67) = 0.491832996 deg (as of 9AM pacific, on 8 Feb)

Om has some more figures tabulated in post #55 but here is the table of optical navigation photo shoots (with a few extra rows added) from 29 January D.J.
http://dawnblog.jpl.nasa.gov/2015/01/29/dawn-journal-january-29/

Jan 25    (237,000)    43    (22)     1.3    96%
Feb 1     (168,000)
Feb 3     (146,000)    70    (14)     2.2    97%
Feb 10    (100,000)
Feb 12    (83,000)    121    (7.8)    3.8    98%
Feb 19    (46,000)    221    (4.3)    7.0    87%
Feb 25    (40,000)    253    (3.7)    8.0    44%
Mar 1     (49,000)    207    (4.6)    6.5    22%
Apr 10    (33,000)    304    (3.1)    9.6    18%
Apr 15    (22,000)    455    (2.1)    14     50%
Apr 23    (14,000)

Relevant column headings as given in the original table:
Distance from Dawn to Ceres in (kilometers)
Ceres diameter in pixels
Resolution in (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk