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

[Janus]

If anyone knows Dr. Rayman, can you please tell him to stop responding to my emails...
Just thinking, about thinking, about his attachment:
View attachment 83838

makes my head want to explode.

It's pretty much what you would expect from an eccentric orbit with a decreasing semi-major axis. If you plotted the Moon's orbit in the same way you would see the same type of pattern over the long scale, with the exception that the orbital energy would be increasing rather than decreasing You would still see the orbital radius distance increasing and decreasing periodically while the period increases. ( the Moon's orbital distance varies by 46000 km over an orbit, while its average distance increases by ~4 cm per year or ~ 0.3 cm per orbit.)

Notice how the "peaks" of the "waves" in the radial distance get closer together as the plot goes from May 7th to 28th. This is also what you'd expect as the period of the orbit decreases.

 

[mfb]

It is interesting that the opnavs happen at periapsis, where thrusting would be the most effective with conventional rocket engines. On the other hand, if they would not have the opnavs they would not have that structure I guess. And they get a better view like that, of course.

Where is the promised mass estimate? Well, I'll have to use 9.47E20 kg from the NASA factsheet.
The initial 368 hour orbit leads to 14100 km distance, that fits very nicely to the given initial distance.

Energy is $$\frac{-GM}{2a} \propto T^{-3/2}$$
The limit of the ion drive is force and therefore acceleration, but the efficiency is proportional to speed, which is (roughly, would be exact for circular orbits) proportional to 1/sqrt(a).
Thrusting against motion all the time, we would expect it to lose energy with $\frac{dE}{dt} = c \sqrt{E}$. This leads to a quadratic function for E, or $T^ \propto t^-{3/4}$.

We started at 368 hours at May 9.5 (why do we have 5 marks for 7 days on the x axis?? By the way, fractional days are totally a thing), reached 186 hours on May 16.7.
This would suggest $T \approx 1200 \cdot (x+4.85)^{-3/4}$ where x is in thrusting days after May 9.5.
With 1.4 days of coasting in the diagram, at May 28 (x=17.1) we expect an orbital period of 118 hours. The real value of 88 hours indicates the second part is more efficient in some way, or my calculations are wrong. The assumption of a circular orbit is certainly wrong, making them elliptically might help in some way.

Orbital energy and distance should allow to calculate the relevant orbital quantities (in particular, speed) for every point in time, but I don't want to do that now.

The orbital period just fell below 5 days.

 

[marcus]

Mfb, thanks for the analysis! I also noticed that the opnavs come at periapsis, curious. I don't how optical navigation works at this stage, earlier they were using the star background. Maybe they are now using surface features?

This was tweeted an hour ago around 3 PM pacific on 21 May
https://pbs.twimg.com/profile_images/3502834940/2f750377e236127f02d96e270510d727_normal.jpeg NASA's Dawn Mission @NASA_Dawn · 1h1 hour ago
Update: Today I am descending from... (7,200 to 5,800 km) in altitude w/ respect to #Ceres

 

[marcus]

Om's attached graph shows that the opnav shots during descent to survey orbit are 16 May and 23 May.
View attachment 83838
And that 23 May photoshoot may have started. Simview shows thruster turned off, and I'll bet there is some radio tranmission.
...
Well I just looked at DSN and Madrid antenna #63 was assigned to Dawn and standing by but it didn't show any transmission (as of 6:05 PM pacific 22 May)

Simview (6PM pacific 22May) says the range is now 5.05 kkm and speed 257 mph. For comparison, the latest tweets indicated 5.8 was achieved on the 21st and the speed would reach about 260 mph on the 22nd. As well as I can judge, that is consistent with simview figures.

I suppose they do the opnav imaging at periapsis because at that time the range is fairly constant and the photo interpretation is simplified. That could be one reason, anyway. Also FWIW it has the best resolution, it is a temporary dip minimum in the range.

This was taken 16 May, so a week ago, at the last opnav shoot, from a range of 7.2 kkm. We should be able to see an improvement in resolution when they post the opnav shots that are planned for tomorrow, from around 5 thousand km.

Ah! just looked at DSN again https://eyes.nasa.gov/dsn/dsn.html and Madrid#63 was receiving Dawn's signal at 125 kb/second. So that photo session has begun already.

 

[marcus]

Photo shoot apparently ended around 6:50 AM pacific 23 May.
I checked at 6:30AM pacific and Canberra#34 was receiving Dawn at 125 kb/s (also simview showed thruster off)
20 minutes later I looked again and no transmission to Canberra with no other antenna standing by (also simview showed thrust back on)

Om's diagram shows the optical navigation interludes happening at periapsis. It's not a circular spiral down to lower altitude, it is an elliptical spiral. I wasn't expecting that, but it makes sense. they were in circular orbit so wherever they BEGAN retrothrusting would define the sector of the circle where the apoapsis of successive ellipsyish loops would fall.

Two more weeks and she should be settled into "survey" orbit at 4400 km altitude.

Lovely craft, lovely path, to a fascinating planet. : ^)

 

[Dotini]

I think they've figured out that the central bright spot is the central peak itself, or whatever is covering it.

Comparison of the most recent photos of the white spots taken Dawn’s current 4,500 miles vs. 8,400 miles on May 4. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Umbriel, a moon of Uranus, 727 miles in diameter, with Wunda Crater and its bright internal ring of unknown origin. The moon’s equator is vertical in this photo. Credit: NASA

Read more at: http://phys.org/news/2015-05-ceres-bright-sharpen.html#jCp

 

[marcus]

Since we turned a page, I'll bring forward info on Dawn's currently spiraling down to survey orbit:

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,500)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     72     3.1 days    (1.0 meters)
HAMO    Aug 4–Oct 15     (1,450)      (140)     215    19 hours    (33 cm)
LAMO Dec 8–end of mission  (375)      (35)      850    5.5 hours    (8.5 cm)

The spiral loops are elliptical, so altitude oscillates up and down during descent. Mission director Marc emailed Om this graph, which Om shared with us in an earlier post.

The general idea of spiral descent, with the ion beam always aimed ahead to slow the craft down, is sketched here:

 

[marcus]

Dawn is about to pass over the N pole of Ceres, from nightside to dayside, at altitude around 4800 km. Thrust beam pointed ahead to apply braking.
Solar panels barely visible since tilted to face the sun, which is roughly ahead of the craft.

this simulated view gives the speed as 253 mph. The orbit speed will increase on average as the craft descends to the planned survey orbit altitude of 4400 km.

 

[mfb]

speed^2 ~ 1/radius for circular orbits. Going from 14000 km to 4900 km should increase the speed from 150 mph to 253 mph. Which is the current speed, just at a distance a bit too large (so Dawn still needs to brake more).

 

[marcus]

speed^2 ~ 1/radius for circular orbits. Going from 14000 km to 4900 km should increase the speed from 150 mph to 253 mph. Which is the current speed, just at a distance a bit too large (so Dawn still needs to brake more).

Good point about the proportionality. AFAIK the survey orbit altitude is 4400 plus the average radius of 470 gives
(G*943e18 kg/4870 km)^(1/2) in mph
gives 254 mph
so you are right we are temporarily at the survey orbit speed, just a bit too far out. Dawn will still need to continue braking. Speed will presumably oscillate around the eventual value until it settles down.

EDIT: just checked simview and it gave speed 274 mph at range 4320 km as of 6PM pacific 26 May.
there's 20 mph it has to cancel, right there, before it can settle into circular orbit. Nice to see it getting close though.

 

[OmCheeto]

A new photo was just published:

May 23, 2015​

I just posted the following, on social media, as, well, it's my own crackpot theory. One of these days, I'll do the math, and/or do some experiments, as this is just too much fun.

https://fbcdn-profile-a.akamaihd.net/hprofile-ak-xaf1/v/t1.0-1/c5.0.32.32/p32x32/1911943_10203123722364595_1106501404_n.jpg?oh=5e45c1adb896bb9575f69571295ab51e&oe=56029EBE&__gda__=1443468149_b908c87c9aff26d93e33f61b5927f47c
Om; I really wish I hadn't developed an interest in extraterrestrial geophysics starting with Philae bouncing around Churyumov–Gerasimenko, last November. I really haven't had time to absorb everything I'm seeing. And now, every time I see a linear set of grooves, like on Vesta, I imagine high momentum objects gouging these troughs in really low gravity. Orbital velocity at the surface of Ceres is 0.364 km/sec = 814 mph. Someone, please tell me I'm wrong!
​

---------------------------
Ok to delete, infract, and ban.

 

[Jimster41]

I think I see what you are suggesting. Seems kind of plausible to me. Those things certainly need some explanation... Easy to picture a rock, maybe from some collision over on the other side, settling that way. Are those lines parallel to the plane or Ceres rotation?

 

[OmCheeto]

I think I see what you are suggesting. Seems kind of plausible to me. Those things certainly need some explanation... Easy to picture a rock, maybe from some collision over on the other side, settling that way. Are those lines parallel to the plane or Ceres rotation?

Roughly, as far as I can tell.

Dawn Spirals Closer to Ceres, Returns a New View

​

Though, I've never been to the asteroid belt before, and don't know much about it. I would imagine that the lines wouldn't necessarily have to line up with her rotational plane. This is why I solicited theories about the massive gorges on Vesta earlier, as I really have no clue what I'm talking about.

 

[Dotini]

...I would imagine that the lines wouldn't necessarily have to line up with her rotational plane. This is why I solicited theories about the massive gorges on Vesta earlier, as I really have no clue what I'm talking about.

IMO, those lines resemble similar lines on Phobos, which are thought to be caused by ejecta from another body, i.e., Mars.

 

[OmCheeto]

IMO, those lines resemble similar lines on Phobos, which are thought to be caused by ejecta from another body, i.e., Mars.

I spent at least 3 hours researching the grooves on Phobos yesterday. It sounds as though there is still some debate.

How the Mars Moon Phobos Got Its Grooves (space.com)
May 21, 2014
...
However, even with the new data and arguments, Ramsley and Head still think Murray's hypothesis is implausible.

"Although the Murray hypothesis is notionally appealing, in fact the required precision and complexity is essentially identical to painting a bar code on a golf ball from across the length of a typical classroom using a hand-held ink jet cartridge," they wrote in an email to Space.com.

It's more likely, they said, that there is no single explanation for Phobos' grooves.

"Our modeling of ejecta from impacts on Phobos suggests that grooves may instead be produced in more than one process," they said.

Phobos and Ceres are really different creatures, so I would imagine they have little in common.

environment

Ceres is, I imagine, floating along at the same relative speed as the rest of the rubble in the asteroid belt, at it orbital distance from the sun.
Phobos is circling Mars at about 4800 mph (2.138 km/sec). I imagine it as a little orbital hammer, punching everything in its path.​

diameter

Ceres: 950 km
Phobos: 22 km​

gravity

Ceres: 0.28 m/s
Phobos: 0.0057 m/s​

I would continue, but I have to go fishing now.

 

[marcus]

New Dawn Journal is out!
http://dawnblog.jpl.nasa.gov/2015/05/28/dawn-journal-may-28-2015/

 

[marcus]

The mass estimate has been refined

===quote from latest Dawn Journal==
...measurements from Dawn have revised the size to be about ...963 kilometers across at the equator. Like Earth and other planets, Ceres is oblate, or slightly wider at the equator than from pole to pole. The polar diameter is ...891 kilometers. ...
Before Dawn, scientists had estimated Ceres’ mass to be ...947 billion billion kilograms. Now it is measured to be ...939 billion billion kilograms, well within the previous margin of error. ...
...
==endquote==
simview as of 7AM pacific 30 May says range 4070 km and speed 123 m/s (i.e. 275mph). lower and faster than what they eventually want in the next orbit

 

[marcus]

Lets make a kind of average of the new diameter figures (963^2*891)^(1/3) = 938 km divided by 2 is 469 km for the radius. I had been using 470 km as average radius. Not much difference. Let's calculate the circular orbit speed for a radius of 4400+470=4870 km using the new mass.
(G*939e18 kg/4870 km)^(1/2) = 113 m/s

 

[mfb]

Dawn is so close to Ceres that the behemoth fills the camera’s field of view. No longer charting Ceres’ location relative to background stars, navigators now use distinctive features on Ceres itself. It was an indistinct, fuzzy little blob just a few months ago, but now the maps are becoming detailed and accurate. Mathematical analyses of the locations of specific landmarks in each picture allow navigators to determine where Dawn was when the picture was taken.

That answers an earlier question.

113 m/s are 253 mph, in agreement with the 254 given in the journal.

MYSTIC allows to estimate the total energy as -6200 J/kg, whereas 113 m/s at 4870 km gives -6500 J/kg.
On the other hand, if they go for the original orbit, then they would have to accelerate at some point. MYSTIC is too imprecise, our Ceres radius subtraction has an error, the trajectory is weird or they go for a lower orbit.

Edit: Ah, the journal covers this:

The flight profile is complicated, and sometimes Dawn even dips below the final, planned altitude and then rises to greater heights as it flies on a path that is temporarily elliptical.

 

[marcus]

Simview shows Dawn really close to circular survey orbit! As of 9AM pacific on 31 May it shows altitude 4480 (near the target 4400 km) and speed 248 mph near the target 253 mph.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Remember to get the CURRENT version of this image click "reply" to this post. This updates the image without your actually having to reply and let's you read the current altitude and speed information at the bottom of the simulated view.
Dawn just passed across the S pole of Ceres from dayside to nightside. Curiously, simview seems to show the ion thrust turned off. I was not expecting that, at this point.

 

[mfb]

3970 km and 279 mph earlier today. A strange detour. But it certainly gives high-resolution pictures if they choose to take some this orbit.

They really could fix that black hole at the poles in the MYSTIC simulator ;).

 

[OmCheeto]

...
They really could fix that black hole at the poles in the MYSTIC simulator ;).

At least they finally hired Kip Thorne...
Can you imagine the sim-view of them orbiting my log, or worse yet, a basketball:

Anyone remember "Hardware Wars"?

"You bet your asteroid, kid"

 

[marcus]

Rahman says the plan is for Dawn to be in survey orbit, and conclude thrusting, on 3 June, in just a couple of days.
Today's brief "status report" was informative:
==quote==
June 1, 2015 - Dawn Closing in on Second Mapping Orbit

Dawn spent the weekend maneuvering with its ion propulsion system and is now almost in its targeted mapping orbit. Last night it completed its final ascent in this complicated trajectory. Today it is descending from ... (4,900 kilometers) to ... (4,600 kilometers). It is scheduled to conclude thrusting on June 3 at an altitude of ... (4,400 kilometers).
==endquote==
http://dawn.jpl.nasa.gov/mission/status.html

According to simview during the weekend it briefly switched around from braking to thrusting ahead, which I guess would have been to circularize the elliptical orbit it got into when descending from the earlier altitude 13500 km orbit.

 

[marcus]

What is the spell-checker thinking of? Noodle soup? I tried to type the Dawn mission director's name Marc Rayman and it corrected it to Rahman. I swear the whole thing was unintentional. Or was I thinking of spectroscopy?, or bee-less Brahmans?

Anyway there are indications that Om's correspondence with Marc Rayman has contributed to a beneficial effect. The almost daily status reports have become very informative as to things of interest to us here: orbit mechanics, orientation to sun, photo resolution, altitude, speed, thrusting schedule, the degree of reliability of simview. We are getting more of the prate stoop these days.

 

[OmCheeto]

What is the spell-checker thinking of? Noodle soup? I tried to type the Dawn mission director's name Marc Rayman and it corrected it to Rahman. I swear the whole thing was unintentional. Or was I thinking of spectroscopy?, or bee-less Brahmans?

Anyway there are indications that Om's correspondence with Marc Rayman has contributed to a beneficial effect. The almost daily status reports have become very informative as to things of interest to us here: orbit mechanics, orientation to sun, photo resolution, altitude, speed, thrusting schedule, the degree of reliability of simview. We are getting more of the prate stoop these days.

Prate stoops and Dr. Top Ramen. Ha! I bet he'd like that.
Was that supposed to be "Pirate scoop"?

hmmm... I've never heard of "prate" before.

prate: babbling (hmmm...)
stoop: to do something reprehensible (hmmmm...)

Reprehensible babbling!
ps. My last correspondence from him was from May 20th. I'd imagine he's a bit busy these days.
pps. Sorry I've been a bit silent lately. I've been out of town 7 of the last 11 days. And the pests(let's go out and get drunk!) of summer are swarming...

 

[marcus]

Reprehensible babbling!

It was a spoonerism for "straight poop"
more commendatory than derogatory.

 

[OmCheeto]

It was a spoonerism for "straight poop"
more commendatory than derogatory.

Phew!
I was afraid you and Dr. Rayman were corresponding behind my back.

Dr. Ramen; "Marcus, will you please tell Om to slow down on the prate stoop...".​

I do look back at some of my comments, and think; "That was reprehensible babble. It's obvious I didn't even bother engaging my brain".

But, that's nothing new.

ps. On a hopefully not true side note, I had an argument with someone at JPL a couple of weeks ago. I'm assuming it wasn't Dr. Rayman:

May 21, 2015 9:24 AM

Good morning Marc,

So that wasn’t you I was arguing with on Facebook?
Someone asked;

Joseph Johnson

Why are there never any stars in these pictures?
15 May at 10:32​

To which I responded, and appear to have started an argument:

Om

Here's one of Ceres and some stars taken on Dec 1, 2014.
Enhanced Early View of Ceres from Dawn
15 May at 21:30​

NASA Dawn Mission

When you see stars on an image from Dawn... they have been added to the image! Charming, but not the real deal, for all the reasons noted, not enough exposure time...
16 May at 17:39​

Om

Well... The link to the Dec 1 image does imply that there were some photographic shenanigans, but I do believe the stars were captured by Dawn's camera; "Ceres is the bright spot in the center of the image. Because the dwarf planet is much brighter than the stars in the background, the camera team selected a long exposure time to make the stars visible. The long exposure made Ceres appear overexposed, and exaggerated its size; this was corrected by superimposing a shorter exposure of the dwarf planet in the center of the image." Someone get the Max Planck Institute on the line!
16 May at 18:04​

I guess, technically, we are both correct.​

No word since.

pps. Followers
Facebook: 19k
Twitter: 84k

As is usually the case, 95% of the comments on Facebook, are prate stoop(not a spoonerism, by my favorite new phrase).
I wouldn't even bother checking it out, as most of the posts mirror the Twitter site.

 

[marcus]

Om, Canberra 45 is standing by to talk with Dawn! It is 2AM in the morning there (9AM pacific 2 June).

Maybe a prate stoop means a kind of humorous indirection where one arranges to stumble on the straight story seemingly by accident. A kind of serendipitous pratefall which lands on the essential fact.

Hey! Simview is using a beautiful new globe map of Ceres instead of the old Lumpy guess-ball

The 19:30 UTC simview even has the famous double bright spot showing. The new Ceres ball in simview is made of real photos taken by Dawn, with some latitude and longitude lines projected on it.

 

[marcus]

The new Ceres globe has number-coded REGIONS NAMED AFTER OTHER CULTURES' GRAIN/FERTILITY GODDESSES projected on the map of real craters and features and bright spots and scratches. If they actually wrote out the names, this will be very tedious to duplicate in Om's customary fashion either with a log or using a sportive ball of some nondescript type like volley or basket.
Most of these grain goddesses' names are strange and hard to spell.

 

[OmCheeto]

The new Ceres globe has REGIONS NAMED AFTER OTHER CULTURES' GRAIN/FERTILITY GODDESSES written on the map of real craters and features and bright spots and scratches. this will be very tedious to duplicate with a log or sportive ball of some nondescript type like volley or basket.
Most of these grain goddesses' names are very hard to spell.

"Yumyum" is so far the only one I've memorized.

LPSC 2015: First results from Dawn at Ceres: provisional place names and possible plumes (The Planetary Society)
Posted By Emily Lakdawalla
2015/03/19 23:29 UTC

The bright splash crater is in the quad named Hobnil.

Dawn breaks over distant Ceres … and perhaps reveals signs of habitability (theconversation.com)
March 18 2015, 8.44am EDT

I’m not sure just how many of these there are, or how memorable their names will turn out to be. But as the Dawn mission’s principal investigator Chris Russell pointed out, there is one Mayan deity named Yum (Yum Kaax, god of agriculture and the jungle), who should readily be remembered. One can only hope the mission scientists find a suitably delicious feature on Ceres to give that name.

 

[marcus]

Om, thanks for posting the grid of Ceres named regions. Some pretty strange names. Every culture from every part of the world seems to have had a grain deity or fertility spirit. Happy Hobnil to you! Don't forget to celebrate YumYum day next Tuesday!

I'm trying to make sense of the current simview. It looks like Dawn has thrust turned off, and I see Canberra is assigned to Dawn but no signal.
It looks like as of 3Jun 15:50 UTC the altitude is right 4400 km and the speed is very nearly right 254 mph

and simview shows Dawn apparently approaching the S pole terminator which I guess it should cross early on 4 Jun UTC or like 6PM this evening pacific time (just a rough guess)

Simview is telling us that Dawn has arrived in Survey orbit on schedule as planned

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,500)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     72     3.1 days    (1.0 meters)
HAMO    Aug 4–Oct 15     (1,450)      (140)     215    19 hours    (33 cm)
LAMO Dec 8–end of mission  (375)      (35)      850    5.5 hours    (8.5 cm)

Ready to start a mapping campaign on 6 June.
I guess they first have to verify that and measure all the fine details, so that would be what Canberra antenna is waiting to do.
EDIT :As of 8:41AM pacific, Canberra#35 is talking with Dawn!
https://eyes.nasa.gov/dsn/dsn.html
EDIT: Now as of 6PM pacific, Madrid #55 is talking with Dawn (it is 3AM in the morning there).
In simview Dawn seems to be just now crossing the Ceres S pole.

 

[OmCheeto]

...
ps. On a hopefully not true side note, I had an argument with someone at JPL a couple of weeks ago. I'm assuming it wasn't Dr. Rayman:

...

I think I've started another argument...

Peter Fries ‏@Peter_Fries Jun 2
@NASA_Dawn @b0yle Does Dawn have the capability to send back 'natural color' images?

NASA's Dawn Mission ‏@NASA_Dawn Jun 2
@Peter_Fries @b0yle yes, I can take data with which to make color images, but the team has not yet released any yet

OmCheeto 17 minutes ago
@NASA_Dawn @Peter_Fries @ridingrobots @b0yle I'm confused. http://blogs.egu.eu/geolog/2015/04/15/findings-from-nasas-dawn-mission-shed-new-light-on-ceres/ …

Have I totally lost my mind?

PIA19063 - Dawn's First Color Map of Ceres
This map-projected view of Ceres was created from images taken by NASA's Dawn spacecraft during its initial approach to the dwarf planet, prior to being captured into orbit in March 2015. Image Credit: NASA/JPL –Caltech/UCLA/MPS/DLR/IDA​

 

[OmCheeto]

Phew!

Hi [Om],

Sorry for my slow replies. Dawn keeps me busy, which comes as no surprise to you. We arrived in survey orbit this morning, and that will be tweeted and put on our mission status page.

You’re right that that was not I on Facebook. I mentioned about giving the information for tweeting. For Facebook, if they send a question back to me, I answer it, but most of the time they don’t. So I’m quite unaware of what gets posted there.

As usual, the topic is a little more complicated than it appears from what you quoted below. For the approach phase images, we used two different camera integration times (what most people call exposure times). One value was chosen to ensure Ceres was correctly exposed and the other was chosen to bring out the background stars. The images alternate, so we interpolate to get Ceres' location relative to stars. We did it the same way at Vesta. In at least one of the Ceres OpNavs, it just so happened that some stars showed up in the images exposed for Ceres. I don’t know what the Dawn person (who is not technical) had in mind with the comment about adding stars. We’ve never done that.

As for what Ceres would look like, you’re quite right that Ceres is significantly brighter than the background stars. That’s why we had two exposure values. I wrote in my March 31 Dawn Journal that Ceres’ mean albedo is about 0.09 and the Moon’s is about 0.12. So you’re also correct that the Moon’s is 1/3 higher. If it matters, remember that Ceres is farther from the sun. The Planetary Society reposts my Dawn Journals (as do some other sites), and sometimes (but not often), I respond to comments there. I did address this aspect of it in responding to a comment by Solon. That is, the intensity of the sunlight is around 12% at Ceres what it is at Earth or the Moon, so it would look darker to your eye. You’re also right that Ceres’ variation in albedo seems much lower, but, of course, there are those famous bright spots and others that are not so famous.

Regards,
Marc

I hope I'm not getting his staff in trouble.

On a trivial side note, I was curious at what distance Ceres would fully fill Dawn's framing cameras.
So I did some maths, and determined that it was around 3400 km.
It's a bit problematic, as Marcus pointed out that polar and equatorial diameters are a bit different: 891 & 963 km, respectively.

Field of vision    5.5°    vertical & horizontal (θ)
Field of vision    0.096 radians   
distance           3400    km   
1/2 field           327    km   tan(θ) * distance
full field          655    km   
1/2 diagonal        463    km   √ (2 * half field^2)
full diagonal       926    km   
polar diameter      891    km   96% of full diagonal
equatorial diam     963    km   104% of full diagonal

Close enough for government work.

 

[OmCheeto]

Oh. Wait. Peter said "natural color".
oops.

But as Dr. Ramen responded in the "comment by Solon" link referenced above:

DaGuz: 05/02/2015 06:07 CDT
Why no color pictures? The picture taken by Hubble showed the terrain was brown. The pictures released so far make Ceres look like our moon.

Marc Rayman: 05/04/2015 11:49 CDT
...
DaGuz: Most of our releases so far are only grayscale, but we have released maps based on our color data. Like the Hubble views, they are false color. We will release true color images as well. Ceres would look mostly gray (although darker than the moon).

hmmmm... Is it just me? What is it about people, that makes them ask to see full color pictures of a grey dwarf planet?

Ah ha! It's all Hubbles' fault. I distinctly remember that the images of both Ceres and Pluto were very colorful.
I wonder why they did that?

Ceres via the Hubble Space Telescope

Pluto via the Hubble Space Telescope​

Actually, I think I know why they do that. Never mind.

 

[marcus]

Om thanks for passing along the informative letter Marc Rayman sent you.
Today's Dawn Status report gives the latest news succinctly so I'll just copy it:
==quote http://dawn.jpl.nasa.gov/mission/status.html ==
2015

June 5, 2015 - Dawn Begins New Science Phase

As Dawn flew ... (4,400 kilometers) over Ceres’ north pole this morning, the spacecraft passed from the night side to the day side of the dwarf planet. That marked the beginning of the new mapping phase, and Dawn began taking photos and making other measurements on schedule. Circling Ceres every 3.1 days, Dawn will make extensive scientific observations when it is over the sunlit side and will transmit its findings to Earth when it is over the side in darkness. The pictures will be three times as sharp as those from the first mapping orbit. This mapping phase is scheduled to continue for eight revolutions, providing plenty of opportunities to gather a wealth of data.
==endquote, my emphasis==