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

[OmCheeto]

A new status report is out.
They will be doing a small orbital correction maneuver, between the 11th and 13th.

December 9, 2015 -Dawn at Lowest Orbital Altitude at Ceres

 

[marcus]

Simview now shows Dawn performing TCM (trajectory correction maneuver). It is not simply retrothrusting as it was during the main descent.
Then the ion beam was in the direction of motion, to slow down. Now the beam is sideways the direction of motion.
As if the orbit was askew---not exactly over the poles---and a sideways push was needed to true it up and make the orbit more exactly polar.

Here, as I interpret it, we see the ion beam pointed at 9 o'clock while the probe is crossing the terminator at the S pole in about a 4 o'clock direction. The thrust is only partly aligned with the direction of motion.

 

[mfb]

Thrusting directly over the poles (as shown in the image) doesn't make an orbit more polar, it shifts the orientation of the orbit relative to the terminator. They ruin my earlier estimate of the maximal remaining probe lifetime!

 

[marcus]

Yes, I can see how it might change your estimate of how long the probe can remain in permanent sunshine before it begins to fall into Ceres' shadow for part of each orbit. That was a pretty robust estimate though, as I recall, and not apt to change too much. The dwindling supply of hydrazine is more critical---so much so that it would be tactful to avoid mentioning it.

Still, it is a great success that Dawn is finally down in LAMO, at target altitude! I feel jubilant about this--relieved and heartened. At last some gamma spectroscopy of the chemical elements in the surface! When my wife heard the news she proposed toasting Dawn with something fizzy. ==quote Rayman==

December 11, 2015 -Dawn Ion-Thrusting to Adjust Orbit

Dawn is now using its ion engine to adjust its orbit. This maneuver (explained in the November Dawn Journal.) will synchronize the spacecraft 's orbital motion with Ceres' rotation around its axis to fit with the plan for the extensive observations that will begin next week.

Yesterday while the flight team was preparing Dawn's flight plan, the spacecraft tested its backup camera. Controllers occasionally run the camera through a series of tests to verify that it remains in good condition should the primary camera have a problem. (The test of the camera was performed eight years to the day after its first operation in space.) Although the results have not been analyzed in detail yet, all indications are that the backup is in excellent condition.
==endquote==
12Dec 3:36 UTC, 385.44 km, passing over S pole (positive inline thrust)
Judging from Simview the main function of the TCM (correction maneuver) was to raise the average altitude from around 360-370 to 385
12Dec 4:23 UTC, 384.15 km, 55º past S pole
12Dec 5:09 UTC, 376.16 km, 80º bef N pole
12Dec 5:55 UTC, 366.25 km, 30º bef N pole
12Dec 6:19 UTC, 364.65 km, passing over N pole
You can see why they are applying positive inline thrust. They want to raise the altitude into the 380s instead of having it sagging down in the 370s. Higher altitude means longer orbit period. The orbit period wants to be finely adjusted to synch with Ceres rotation to get efficient coverage of the surface.

I suspect the main reason they raised the LAMO altitude from original 375 to 385 has to do with timing.

 

[marcus]

Judging by simulation, the TCM is complete. Simview shows the probe at [edit: or near] altitude 385 km with ion engine turned off.
12Dec 15:34 UTC, 385.48 km, 60º past S pole
12Dec 15:57 UTC, 381.97 km, 80º past S pole
12Dec 16:20 UTC, 377.11 km, 75º bef N pole
12Dec 16:44 UTC, 371.71 km, 50º bef N pole [edit: I added some more data points, still seems close enough]
12Dec 17:06 UTC, 367.77 km, 24º bef N pole
12Dec 17:30 UTC, 366.54 km, right over N pole*
12Dec 17:53 UTC, 368. 50 km, 30º past N pole
...
12Dec 18:39 UTC, 377.96 km, 85º past N pole
Now the long awaited gamma ray spectroscopy of Ceres surface material can begin.
http://dawn.jpl.nasa.gov/mission/status.html
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
http://dawn.jpl.nasa.gov/mission/journal.asp
https://eyes.nasa.gov/dsn/dsn.html (Madrid antenna #55 is in standby)
http://dawn.jpl.nasa.gov/mission/live_shots.asp

 

[OmCheeto]

Judging by simulation, the TCM is complete. Simview shows the probe at 385 km with ion engine turned off.
12Dec 15:34 UTC, 385.48 km, 60º past S pole
12Dec 15:57 UTC, 381.97 km, 80º past S pole
Now the long awaited gamma ray spectroscopy of Ceres surface material can begin.

Yay!

...
https://eyes.nasa.gov/dsn/dsn.html (Madrid antenna #55 is in standby)
...

Canberra #35 was also in standby @ 15:34 & 15:57.

ps. Has anyone else tried to analyze the specific orbital energy? My graph comes out sinusoidal, so I'm not sure what's wrong.

x axis = time in hours ____ t₀ = Nov 27, 2015 17:46 UTC
y axis = energy

I haven't really sat down to figure out why the energy gets higher the closer the orbit is to Ceres, but I checked around, and saw that's just the way it is.

[edit #1: Doh! Canberra #35 is now unassigned]
[edit #2: Madrid #65 in two way com @ 17:44 UTC, 10.00 b/sec?]

 

[mfb]

How did you calculate the energy?
Maybe you need a different value for Ceres' radius?

Where are the pole crossings in that graph?

 

[marcus]

Om, it looks like Simview thinks periapsis of the new orbit is right at the N pole. (see post#756). That would be neat, no?

 

[OmCheeto]

Om, it looks like Simview thinks periapsis of the new orbit is right at the N pole. (see post#756). That would be neat, no?

Yes, it does appear that way, but I'm still stuck on homework problems from 2 weeks ago.
I'm trying to find the answers to mfb's questions, but I have so many goofy graphs going on at once, I have no idea what is going on.
If anyone has any questions, about how NOT to be a scientist, just ask.

 

[marcus]

Let's compare formulas we use.
specific potential energy at radius R: -GM/R
specific kinetic energy in circular orbit at radius R: (1/2)v² = (1/2)GM/R

It seems like the specific total would be KE+PE = -(1/2)GM/R
so the deeper down in the well you go, the faster the orbit so the KE increases, but the PE gets more negative, and it's twice as big, so the total gets more negative. The smaller R gets, the more negative the specific orbital energy.

You probably have gone beyond this, to non-circular orbits, and have a more complicated formula. Can you write conveniently write down the formula you are using?

 

[marcus]

Since we turned a page, I'll bring forward the post summarizing some reasons one might be especially interested in studying Ceres and in the current exploration by Dawn.
==quote post #748==
Just as a reminder, what now happens has consequences for human history because Ceres is (by a wide margin) the nearest icy dwarf planet and offers an advantageous site for chemical and materials manufacture in low gravity. Whether or not this will be developed depends somewhat on Ceres layer structure and chemical composition
Thinking has changed about the layer structure (Lakdawalla report on recent AAS conference) because at least in some regions craters remain sharply defined and are slow to smooth out---suggesting they are supported on a slow-flow rock+ice mix that could be as much as 60% rock. The searchable online cutaway GRAPHICS go back to 2005 and 2006 when scientists had only Hubble space telescope images to go on. I couldn't find any updated cutaway diagram showing more recent guesses about layer structure.
There were also reports at that AAS conference of detection of ammonia-bearing clays in the hydrated minerals at Ceres' surface, by optical/IR spectroscopy, which would be important if confirmed. Nitrogen is a key chemical element for both manufacturing and biology--common in outer solar system bodies but unexpected on an asteroid belt body like Ceres.
Anyway if things work out as planned we now get to learn a great deal more about Ceres' chemical and layer composition. This kind of graphic will hopefully be revised:
View attachment 93140

The icy layer should be more indicative of a 60-40% rock+ice mix. Orbit tracking should be able to map subsurface irregularities in density. Gamma and neutron spectroscopy is to measure the abundances of various chemical elements in surface material, to a depth of about 1 meter.

The last (chemical abundances) is especially significant so I'll bring forward the graphic. Spectroscopy depends on activation by cosmic ray particles.
View attachment 93141
Impact by cosmic rays (high energy protons, mainly) causes a sparkle. The frequencies of the gamma-ray sparkle reveal the identities/abundances of the atoms giving off the sparkle. Moreover among the scattered neutrons the fraction of them which have been slowed by successive collisions with hydrogen nuclei reveals the amount of hydrogen (e.g. water ice) in the surface material.
==endquote==

Judging by simulation, the TCM is complete. Simview shows the probe at [edit: or near] altitude 385 km with ion engine turned off.
12Dec 15:34 UTC, 385.48 km, 60º past S pole
12Dec 15:57 UTC, 381.97 km, 80º past S pole
12Dec 16:20 UTC, 377.11 km, 75º bef N pole
12Dec 16:44 UTC, 371.71 km, 50º bef N pole [edit: I added some more data points, still seems close enough]
12Dec 17:06 UTC, 367.77 km, 24º bef N pole
12Dec 17:30 UTC, 366.54 km, right over N pole*
12Dec 17:53 UTC, 368. 50 km, 30º past N pole
...
12Dec 18:39 UTC, 377.96 km, 85º past N pole

12Dec 19:25 UTC, 385.33 km, 45º bef S pole
12Dec 19:49 UTC, 387.07 km, 24º bef S pole
12Dec 20:11 UTC, 387.88 km, right over S pole**
12Dec 20:35 UTC, 387.88 km, 30º past S pole**
12Dec 20:58 UTC, 386.84 km, 50º past S pole

 

[OmCheeto]

How did you calculate the energy?

specific energy = v^2/2 - mu/(altitude + imm.rad.)

imm.rad. = immediate radius of Ceres, where r(theta)=(a*b)/sqrt((b*cos(theta))^2+(a*sin(theta))^2)
a = major axis = 481,500 meters
b = minor axis = 445,500 meters
theta = angle in radians from the equator heading ccw
mu = G * (mass of Dawn + mass of Ceres) = 6.67e-11 * (1240 + 9.38e18)​

Maybe you need a different value for Ceres' radius?

I need a different brain...

Where are the pole crossings in that graph?

 

[mfb]

specific energy = v^2/2 - mu/(altitude + imm.rad.)

imm.rad. = immediate radius of Ceres, where r(theta)=(a*b)/sqrt((b*cos(theta))^2+(a*sin(theta))^2)
a = major axis = 481,500 meters
b = minor axis = 445,500 meters
theta = angle in radians from the equator heading ccw
mu = G * (mass of Dawn + mass of Ceres) = 6.67e-11 * (1240 + 9.38e18)​

The given height values don't look like MYSTIC takes the oblateness of Ceres into account. Does the agreement get better if you use a single value for the radius? I would also try different values for the radius to see if others fit better.

 

[marcus]

The 2 minute youtube records the moment when Laurent Fabius says he hears no objection and so
"L'accord de Paris pour le climat est accepté"
Climate day (12 December) was also "Ceres Day"---when Dawn was finally in near orbit and could get down to real business.

 

[Janus]

Let's compare formulas we use.
specific potential energy at radius R: -GM/R
specific kinetic energy in circular orbit at radius R: (1/2)v² = (1/2)GM/R

It seems like the specific total would be KE+PE = -(1/2)GM/R
so the deeper down in the well you go, the faster the orbit so the KE increases, but the PE gets more negative, and it's twice as big, so the total gets more negative. The smaller R gets, the more negative the specific orbital energy.

You probably have gone beyond this, to non-circular orbits, and have a more complicated formula. Can you write conveniently write down the formula you are using?

For none circular orbits, the first equation still applies for any point of the orbit. The second equation becomes -(1/2)GM/a, where a is the semi-major axis of the orbit.

 

[OmCheeto]

The given height values don't look like MYSTIC takes the oblateness of Ceres into account. Does the agreement get better if you use a single value for the radius? I would also try different values for the radius to see if others fit better.

Along with all of the previously mentioned derogatory phrases my mother used to call me, she also called me a "doffa henna". Which I believe means; "Idiot chicken". But don't quote me on that translation.

In my previous graphs, I was off by 2 orders of magnitude for the mass of Ceres.
Inserting the correct mass of Ceres, and using: average radius + altitude = r
I come up with the following graph:

This makes a tad bit more sense to me.
I spent several hours trying to figure out the gravitational field around an oblate spheroid, but could not comprehend any of the maths.
The only thing I could extract, was that the force of gravity at the poles is greater than at the equator.
I'm guessing that this might explain the shape of this new graph.

This apparently was previously such an obscure, aka hypothetical, problem, that there is little discussion about it here at PF. At least that I could find.

But it was fun looking. DH mentioned an article in one thread about "gravitational anomalies of the moon", with disastrous consequences.
Hopefully, the Dawn crew is familiar with gravitational potholes.

[edit]

For none circular orbits, the first equation still applies for any point of the orbit. The second equation becomes -(1/2)GM/a, where a is the semi-major axis of the orbit.

I'm still scratching my head about this, as "a" is a constant.

 

[mfb]

Okay, the equator energy is too high, so we are closer to the center than the calculation assumes (as the speed value is directly from MYSTIC). If your latest plot was done with a constant radius for r, using the adaptive radius would make it even worse. There are two symmetric oscillations per orbit, so there is no periapsis/apoapsis problem left (at least not dominant).

The gravitational force is stronger at the poles if you are on the surface (there being at the poles makes you closer to the center) - in orbit for a given height, it is stronger over the equator. We would have to estimate the quadrupole component of the gravitational potential to include that effect.

 

[marcus]

Just to keep us up to date (don't let this interrupt the orbital energy discussion) according to Simview the correction maneuver is accomplished and Dawn is in LAMO, with engine off.
DSN graphic shows Madrid antenna #65 in two-way communication

It seems pretty clear (at least to me) that what Simview calls "altitude" is altitude above an idealized spherical Ceres, not above the actual surface of the body. I'm not sure what sphere radius Simview uses---maybe 473 km, which I've seen quoted as the average R_av of the true oblate dimensions. In that case, in case some notation might be helpful: Let's have H stand for the "altitude" term.

Dawn's radial distance from center = D = R_av + H = 473 km + "altitude".

In a few minutes Dawn will pass over Ceres S pole and that might roughly coincide with D_max or apoapsis. We'll see. I'll record what Simview says at that point, and check later how well it agrees.

It's fascinating how much smoother the surface is in the wide equatorial belt, and how much sharper the craters are in polar regions. As if the proportion of ice to rock is higher in equatorial subsurface material, so that it flows more easily---e.g. 40-60% rock to ice. And as if the proportion of ice is lower in polar subsurface--e.g. 60-40% rock to ice--providing a stiffer foundation for longer-lasting craters.

13Dec 18:10 UTC, 376.33 km, right at S pole*

I think that is the H_max of the corrected orbit (simulation) so will tentatively mark it with an asterisk. Well see if H declines at the next reading, and whether H_min comes at N pole (if that is not too much to ask of Mystic

Postscript: Yes! H_max = 376 km and it came right at the S pole. I'm thinking we should be on the lookout for H_min about 3 or 3:30 hours from H_max: around 1 PM pacific or 21 hours UTC. Hope I don't get too busy with something else and forget.

Postscript:
13Dec 20:53 UTC, 356.27 km, right at N pole*
A spread of almost exactly 20 km. H was decreasing up to that point, so if it's the min as I guess it will increase at the next reading.
Yes it increased
13Dec 21:16 UTC, 357.70 km, 20º from N pole

It seems that Simview has fallen into a pattern where the "altitude" H is going to be between 356 and 376km, with H_min at N pole and H_max at S pole.

To check consistency I took a reading around H_min again. It occurred as expected
14Dec 02:17, 356.25 km, 8º to N pole
the readings immediately before and after were larger (358.55 and 357.20)

So we can say with reasonable confidence that the semimajor axis a = 473+366 = 839 km.
as far as Simview is concerned, assuming 473 is right for the Ceres average radius
2 pi ((839 km)^3/(G*938e18 kg))^(1/2) = 5.36= 5: .36*60 = 5:22

20:53 + 5:22 = 26:15 = 24+2:15 hours, which is pretty close to 2:17!

 

[marcus]

Since I haven't checked in on Dawn much so far today, I'll take a look at the new orbit according to Simview. See if it's consistent with our perceptions of it yesterday. It looked then as if the "altitude" (probe distance from center, minus something like 473 km, the average body radius) was going to be between 356 and 376 km with (approximately at least) the min coming at Ceres' Npole and max at Spole.

I'll tag the max and min with asterisks.

Here are some fresh readings:
15Dec 01:49 UTC, 373.70 km, 70º to S pole
15Dec 02:12 UTC, 375.77 km, 45º to S pole
15Dec 02:35 UTC, 376.32 km, 24º to S pole*
15Dec 02:58 UTC, 375.88 km, right at S pole
15Dec 03:21 UTC, 374.84 km, 30º from S pole

Assuming the estimate of 5:22 hours for the orbit period (see previous post) is right the time to periapsis (min "altitude") is about 2:41, so we might expect it around 15Dec 05:16 UTC, just adding 2:35 and 2:41.
That's 9:16 pm pacific time, hope I don't forget : ^)

15Dec 05:17 UTC, 357.65 km, 30º to N pole
15Dec 05:40 UTC, 356.68 km, right at N pole*
15Dec 06:03 UTC, 358.90 km, 20º from N pole

 

[OmCheeto]

A new mission status update is out.

December 14, 2015 -Dawn Preparing for New Observations

 

[marcus]

...
...
15Dec 02:58 UTC, 375.88 km, right at S pole
...
Assuming the estimate of 5:22 hours for the orbit period (see previous post) ...

Three times 5:22 is 16:06. So if I add 16:06 hours to 2:58 UTC and get 19:04 UTC that should be the next S pole crossing---just as a check on that figure of 5:22 for the period.
15Dec 18:47 UTC, 378.17 km, 35º to Spole*
15Dec 19:09 UTC, 377.04 km, 12º to Spole
15Dec 19:34 UTC, 374.88 km, 15º from Spole
well, it's not perfect. I wonder if the Sun's gravity has an effect that I should be allowing for. Apoapsis seems to have increased by a couple of km and to have rotated slightly sunwards. The period is slightly more than the 5:22 found yesterday.

 

[mfb]

I wonder if the Sun's gravity has an effect that I should be allowing for.

Completely negligible: at a level of 10 parts in a billion.
Inhomogeneous mass distributions in Ceres are much more important.

 

[marcus]

Thanks! Maybe inhomogeneous mass distribution accounts for the seemingly chaotic jitter. After all, every time Dawn comes south across the day side it is a different dayside, because Ceres is rotating. Rayman alerted us that subsurface differences in density would be being detected at this level by the probe speeding up and slowing down. So they hope that they can map some underground bodies of ice (lower density) or rock (higher density) by this kind of "Xray vision" based on precise tracking.
So the "inhomogeneous mass distributions" must be having an effect---and maybe there are even significant departures from the oblate spheroid model. Maybe there are significantly different equatorial radii, besides a polar one, and that could have an effect.

 

[marcus]

Anyway it looks like she crossed the Spole around 15Dec 19:20 UTC today. And stubbornly holding to that 5:22 underestimate of the period, for the time being, I'll guess the Npole crossing will come 2:41 after that 19:20+2:41 = 22:01 UTC. Or around 2 PM pacific. We'll see.

According to Rayman's Update that Om linked us to, observation by the GRaND instrument began 14
December and will continue---so information is already being accumulated about elemental abundances.
==quote Rayman==
Dawn thrust with its ion engine on Dec. 11-13 to fine tune its orbit. When it finished, it pointed its gamma ray and neutron detector (GRaND) at Ceres. GRaND measures the energies and numbers of these two components of nuclear radiation, from which scientists can determine the abundances of some elements on the dwarf planet.
...
... the intensive observation campaign that will begin on Dec. 18. In the meantime, the spacecraft will collect more radiation measurements as well as conduct some bonus photography and infrared spectroscopy on Dec. 16-17.
==endquote==
http://dawn.jpl.nasa.gov/mission/status.html

Update: Earlier I estimated 2PM pacific for periapsis and here it is 20 minutes before 2. So I'll check
15Dec 21:29UTC, 357.79 km, 46º to Npole
15Dec 21:52UTC, 356.59 km, 15º to Npole*
15Dec 22:14UTC, 358.33 km, 10º from Npole

Reply to Om's question in next post: Yes, I believe I noticed the same thing you did. I had been expecting the "altitude" to stay within bounds like 356-376, and it overshot the upper bound to around 378. You showed that with your graphic. I'm just getting used to a little chaos in the numbers--as long as the overall outlines seem OK.

So if 5:22 hours is the period (underestimate) and 2:41 being half that, then the next apoapsis could be expected around 21:52 + 2:41 = 24:33 = 16Dec 00:33 UTC, which is about 4:30 PM pacific, this afternoon.
Well here it is 4:30 pacific and I see:
16Dec 00:11 UTC, 377.43 km, 38º to Spole* , and then later
16Dec 00:34 UTC, 376.56 km, 14º to Spole
16Dec 00:57 UTC, 374.92 km, 9º from Spole

I may have missed it, was busy otherwise should have gotten down and looked a little before 4:30.

16Dec 22:22 UTC, 377.34 km, 20º to Spole (probably close to "apo")
17Dec 01:04 UTC, 357.18 km, 26º to Npole (probably close to "peri")

 

[OmCheeto]

Thanks! Maybe inhomogeneous mass distribution accounts for the seemingly chaotic jitter. After all, every time Dawn comes south across the day side it is a different dayside, because Ceres is rotating. Rayman alerted us that subsurface differences in density would be being detected at this level by the probe speeding up and slowing down. So they hope that they can map some underground bodies of ice (lower density) or rock (higher density) by this kind of "Xray vision" based on precise tracking.
So the "inhomogeneous mass distributions" must be having an effect---and maybe there are even significant departures from the oblate spheroid model. Maybe there are significantly different equatorial radii, besides a polar one, and that could have an effect.

My thoughts, exactly. *

*Ok. My thoughts, are really, that this is all way over my head.

But did anyone else notice the deviation of the max altitude at the south pole?

x = time from 1:49 utc Dec 15 2015 (hours)
y = altitude per Mystic (km)

 

[OmCheeto]

Another new status update:

December 16, 2015 -Dawn Begins Photography and Infrared Spectroscopy

Dawn is now taking pictures and infrared spectra of Ceres...

The JPL flight team is continuing to incorporate the latest orbital parameters into the plans for the more intensive observations that will start on Dec. 18.

On Dec. 14, mission controllers activated Dawn's two operable reaction wheels ...They are performing well...

Whoop! Whoop!

 

[rootone]

Reaction wheels, fingers tightly crossed.

 

[marcus]

Great news about the reaction wheels! If both continue to work well it will save a lot of hydrazine and extend the mission. In addition to "Dawn is now taking pictures and infrared spectra of Ceres from its new average orbital altitude ..." which Om quoted, I want to quote another line from the latest status update:
"Nuclear spectroscopy and gravity measurements began last week upon arrival at this low orbit..."
We've already been anticipating the nuclear (gamma emission) spectroscopy. The gravity measurements could discover below-ground regions of higher/lower than average density---subsurface bodies that are mostly rock or mostly ice instead of simply being an average mix. As I understand it, subsurface regions of varying density are to be spotted by observing the probe speed up and slow down as it passes over a concentration of mass---IOW by highly precise tracking.

BTW:
17 Dec 06:32UTC, 357.85 km, 29º to Npole (I should have checked a bit earlier, just forgot.)
17 Dec 17:24UTC, 357.30 km, 30º to Npole
Seems fairly consistent. The difference in times is 10:52, half of which is 5:26 not too different from the orbit period estimate of 5:22 we've been using.
17 Dec 03:24 UTC, 378.04 km, 43º to Spole
18 Dec 01:08 UTC, 379.55 km, 50º to Spole (this time I remembered to check the one before and it was less, as was the one after, so this indicates where the max comes, approximately since we have only discrete rather than continuous readings.)
18 Dec 06:33 UTC, 378.65 km, 56º to Spole. (the next altitude reading was less, that's apt to've been max)
18 Dec 06:56 UTC, 377.50 km, 34º to Spole.

 

[marcus]

http://dawn.jpl.nasa.gov/mission/status.html
==quote new Rayman update==

December 18, 2015 -Dawn Ready for More Observations of Ceres

While Dawn was taking preliminary pictures and infrared spectra on Dec. 16-17, the flight team was putting the finishing touches on commands the probe will use for further observations that start later today. After the trajectory correction maneuver that completed on Dec. 13, navigators measured Dawn's orbital parameters very precisely. Combined with their latest measurements of Ceres' gravity field, they formulated a new prediction of Dawn's orbital motion over the coming weeks. The detailed plans for observing the dwarf planet then were adjusted to account for this latest information.
==endquote==
18Dec 22:50 UTC, 379.61 km, 60º to Spole* (given that readings come at discrete intervals, not continuously, this does seem to be the best guess as to apoapsis--the altitudes before and after this one are lower)
19 Dec 04:18 UTC, 380.09 km, 60º to Spole*
This compares with
18 Dec 01:08 UTC, 379.55 km, 50º to Spole*
and is consistent---given the discreteness we can't narrow it down much better than 50-60º.
Also for the periapsis I just saw
19 Dec 01:57 UTC, 358.04 km, 53º to Npole*
and altitude was larger both right before and right after so given the discreteness probably the best we can say is that for the moment the line of apsides runs through 50-to-60 degrees to Npole and around 50-60 degrees to Spole. And the range is about 358 km to 380 km. Admittedly that is just for the simulation. We'll see if it is reasonably stable.

 

[OmCheeto]

Drumroll please...

The first images from LAMO are out!

Lowdown on Ceres: Images From Dawn's Closest Orbit [jpl.nasa.gov]
​

ps. previously, on twitter;

NASA's Dawn Mission ‏@NASA_Dawn 18 hours ago

Tonight we'll pause observations of #Ceres to point our main antenna to Earth for 27 hrs. Got to send home some data!​

38 retweets 105 likes ​

 

[marcus]

http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20188
This was taken near Ceres' South Pole. At Ceres' poles the sun is always down near the horizon (or below).
Upper part is dark sky, to see the landscape you may need to scroll down.

If for some reason image doesn't load it may help to press "reply".

 

[marcus]

http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20186
"...38.1 south latitude, 209.7 east longitude, around a crater chain called Gerber Catena.
Many of the troughs and grooves on Ceres were likely formed as a result of impacts, but some appear to be tectonic, reflecting internal stresses that broke the crust..."

 

[newjerseyrunner]

Wow.

 

[marcus]

Mheslep and Runner, so glad you like the images from the new low-down perspective!

Latest update from Rayman suggests that Dawn will have completed sending data sometime tonight (evening of the 22nd December) and will resume observations:

==quote http://dawn.jpl.nasa.gov/mission/status.html ==
December 21, 2015 -New Mapping Proceeding Smoothly

Since Dec. 18, Dawn has been taking neutron spectra, infrared spectra, gamma ray spectra, and photographs of Ceres from the lowest altitude orbit. As the spacecraft revolves around the dwarf planet, it points its sensors at the ground but also switches among its auxiliary radio antennas to use whichever is pointed closest to Earth. That allows engineers and scientists to use the radio signal to measure the orbital motion very accurately to map the gravity field.

Tonight Dawn will turn to point its main antenna to Earth for more than 27 hours. Tomorrow night, after transmitting most of its pictures and other data, it will resume observing Ceres.
==endquote==
"tonight" in that update was the night of the 21st---that's when the transmission started. Presumably the transmission is finished---in fact we're seeing some of the pictures! "tomorrow night" is the night of the 22nd, which is now, and presumably observation has been resumed or will be shortly.

I wonder when preliminary results from the gamma spectroscopy will be ready---and will they be willing to share some initial conclusions about the a few chemical abundance. I'd love to see some provisional estimates! AFAIK Om is the only one of us who corresponds with Marc Rayman by email.

Can we get Rayman to tell us what chemical elements in the surface material they will be measuring the abundances of? Silicon? Carbon? Oxygen? Aluminum? Nitrogen?
I'm eager to get an idea of what to expect to eventually see abundance numbers for.

 

[mheslep]

Anyone readily know the magnification of that imager? I looked a bit without luck. The imager appears to be, say, 1km above the surface. 200x?

 

[marcus]

Anyone readily know the magnification of that imager? I looked a bit without luck. The imager appears to be, say, 1km above the surface. 200x?

Mheslep, you probably saw this earlier table---they've since revised the final target altitude from 375 to 385 km. The "pixel size" here doesn't directly answer your question but might be relevant to our understanding.

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,600)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     73     3.1 days    (1.0 meters)
HAMO    Aug 17–Oct 23    (1,470)      (140)     217    19 hours    (33 cm)
LAMO Dec 15–end of mission (375)      (35)      850    5.5 hours    (8.5 cm)

385 is close enough to 375 so we can take the table as approximately right. So a pixel, in these pictures is about 35 meters.

Some links:
http://dawn.jpl.nasa.gov/mission/status.html
http://dawn.jpl.nasa.gov/mission/journal.asp

 

[marcus]

That figure of 35 meters per pixel is also repeated here---the news article that Om linked us to:
http://www.jpl.nasa.gov/news/news.php?feature=4802
BTW here's an interesting excerpt from that news article:
==quote==
..Dawn's other instruments also began their intense period of observations this month. The visible and infrared mapping spectrometer will help identify minerals by looking at how various wavelengths of light are reflected by the surface of Ceres. The gamma ray and neutron detector is also active. By measuring the energies and numbers of gamma rays and neutrons, two components of nuclear radiation, it will help scientists determine the abundances of some elements on Ceres.

Earlier in December, Dawn science team members revealed that the bright material found in such notable craters as Occator is consistent with salt -- and proposed that a type of magnesium sulfate called hexahydrite may be present. A different group of Dawn scientists found that Ceres also contains ammoniated clays. Because ammonia is abundant in the outer solar system, this finding suggests that Ceres could have formed in the vicinity of Neptune and migrated inward, or formed in place with material that migrated in from the outer solar system...
==endquote==

 

[marcus]

==quote Rayman update 23 Dec==
December 23, 2015 -Dawn Observing Ceres Again

Dawn sent its latest measurements to mission control at JPL as it completed five revolutions around Ceres, each lasting about 5.4 hours. Around 11:30 p.m. PST on Dec. 22, it once again aimed its scientific instruments at the rocky, icy surface beneath it and began collecting more data.
==endquote==

Some of us may have wondered how they can be mapping irregularities in Ceres' gravitational field (including variations in subsurface material density) while doing everything else. That involves listening for the Doppler shift in a constant radio signal back to Earth as Dawn speeds up and slows down in orbit while passing over variable density terrain.
The problem is Dawn can't have its main antenna (used to transmit data) aimed at Earth while it has its camera and sensors aimed at the ground. But as Rayman explained it has several small antennas on different sides of the craft. It keeps sending the tracking signal from whatever small antenna is aimed most nearly in Earth's direction.

It's surprising how many observation tasks the probe is now performing at once. Imaging with the main camera. Optical and IR spectroscopy. Gamma and neutron (counting and) spectroscopy. Mapping variations in gravity.

 

[marcus]

BTW:
24Dec 17:47, 360.47 km, 105º to Npole, speed 275 m/s
24Dec 18:09, 361:49 km, 80º to Npole, speed 274 m/s
It appears that the line of apsides keeps rotating---at least so far it has been with max altitude (apoapsis) progressing more sunwards as min (periapsis) accordingly more antisunwards.

 

[mheslep]

Is there any lower limit to orbital altitude around such a planetoid (above the surface)? I suppose there might be a few gas molecules near the surface that would drag on a the resulting orbital velocity of a couple km/s. But once all other mission objectives have been achieved, why not take the risk, say, a year or so from now, descend to 10m.
"Better to burn out than to fade away" - The Kergan

 

[marcus]

Is there any lower limit to orbital altitude around such a planetoid (above the surface)? I suppose there might be a few gas molecules near the surface that would drag on a the resulting orbital velocity of a couple km/s. But once all other mission objectives have been achieved, why not take the risk, say, a year or so from now, descend to 10m.
"Better to burn out than to fade away" - The Kergan

I approve the adventurous spirit of this suggestion. There are several considerations----as I understand it the plan is to keep on taking and transmitting data at this level until the hydrazine runs out.

You need hydrazine to adjust the orientation so as to keep instruments pointed at the ground, and then you need more hydrazine to turn the craft to point its main antenna at Earth when you are ready to transmit data.

So according to the current plan it wouldn't do any good to get in closer after the end of the mission (even if the probe COULD descend further). Without attitude control the craft couldn't do anything, couldn't make observations, transmit data, or even keep its solar panels facing the sun.

Another consideration is that various space agencies have adopted rules about contamination of solar system bodies which might later be investigated for chemical traces of earlier life. Someone else may know more about this and be better prepared to answer. I have an idea that they don't WANT the probe to crash into Ceres because that might raise the issue of contamination. Ceres is thought to consist of very old material, from near the time of the formation of the solar system---until we know more we want to keep it clean, which enhances its scientific value.

So my thought is that they are being careful to leave Dawn in a STABLE orbit around Ceres, when the hydrazine runs out and the mission is over. So that there is virtually no chance of its ever crashing.

there is still one possible question: why didn't they cut the current plan of observation short and leave a reserve of hydrazine so they could descend to an even lower stable orbit?

They have plenty of PROPELLANT left (that is the noble gas Xenon) so why not choose to go to an even lower orbit? The answer must have involved various trade-offs. To descend at this point takes a lot of solar powered thrust, it is a long arduous task and it requires a lot of ATTITUDE CONTOL. It is very costly in terms of hydrazine which is used to keep the solar panels facing the Sun while the ion engine is meanwhile aimed in the right direction. And then used periodically to point the main antenna at Earth so they can communicate and review the trajectory and revise instructions. So going to an even lower orbit would have been very costly in terms of the scarce hydrazine reserve (used for attitude control) and would have eaten into the main mission which is supposed to be accomplished at this current altitude.

Basically this is just guessing on my part, trying my best to answer. Rayman has probably addressed your question in one of his monthly journal entries---but there is no topic index so I can't easily find out which entries would be relevant. You might enjoy browsing though:
http://dawn.jpl.nasa.gov/mission/journal.asp

 

[mfb]

One of the previous journal entries discussed that orbit in more detail. Avoiding contamination was one of the main points. The current orbit is not stable - but its expected lifetime is at least 50 years, so even if the mission shows signs of habitability for Earth-based microbes we still have some decades to launch another mission to get Dawn out of the way.
If Dawn would go lower, figure out that Ceres might be able to support life coming from Earth, and then fail, we would have a problem.

We are used to stable orbits close to Earth, but it is more like a lucky coincidence to have them for basically every inclination, see this paper for details.

 

[marcus]

...
We are used to stable orbits close to Earth, but it is more like a lucky coincidence to have them for basically every inclination, see this paper for details.

Good reference! I'll quote the abstract:
http://arxiv.org/abs/1309.5244v1
Why do Earth satellites stay up?
Scott Tremaine, Tomer Yavetz
(Submitted on 20 Sep 2013)
Satellites in low Earth orbits must accurately conserve their orbital eccentricity, since a decrease in perigee of only 5-10% would cause them to crash. However, these satellites are subject to gravitational perturbations from the Earth's multipole moments, the Moon, and the Sun that are not spherically symmetric and hence do not conserve angular momentum, especially over the tens of thousands of orbits made by a typical satellite. Why then do satellites not crash? We describe a vector-based analysis of the long-term behavior of satellite orbits and apply this to several toy systems containing a single non-Keplerian perturbing potential. If only the quadrupole potential from the Earth's equatorial bulge is present, all near-circular orbits are stable. If only the octupole potential is present, all such orbits are unstable. If only the lunar or solar potential is present, all near-circular orbits with inclinations to the ecliptic exceeding 39 degrees are unstable. We describe the behavior of satellites in the simultaneous presence of all of these perturbations and show that almost all low Earth orbits are stable because of an accidental property of the dominant quadrupole potential. ...
10 pages, 1 figure; published in American Journal of Physics, Volume 82, Issue 8, p.769-777

In this case we are dealing with a polar orbit, inclination to the ecliptic WAY more than 39 degrees. I gather that if the Earth were perfectly round and uniform (i.e. Keplerian potential, no multipole moments) then merely the sun or moon's perturbation would be enough to make such inclined orbits unstable! It's just luck that Earth has the right quadrupole moment to nullify those effects.

 

[marcus]

The behavior of the eccentricity and line of apsides (as best I can judge them) of Dawn's simulated orbit is beginning to make sense.
The line of apsides now seems very roughly to pass through the equator, with apoapsis on the day side.
25Dec 18:16 UTC, 378.94 km, 106º to Spole
25Dec 18:36 UTC, 378.65 km, 93º to Spole
The max used to be down closer to the S pole. Now it has swung around by something like 90º---it is on the dayside near the equator. I would guess something like 100º to S pole. That Tremaine-Yavetz reference is instructive and helpful.

Dawn team has a pretty nice interactive graphic describing the GRaND instrument (gamma ray and neutron spectrometer)
Introduction is here:
http://dawn.jpl.nasa.gov/technology/grand.html
Then you click on the image and it gets you this:
http://www.jpl.nasa.gov/dawn/swf/GRAND/Grand_animation.swf
and that, after a few seconds brings up a diagram that you can interrogate to get a bunch of information including explanation of how the GRaND instrument works and what chemical elements it can "constrain" the abundance of. That is, get a handle on, sometimes by some fairly complicated inference, from the energies of gamma rays and neutrons coming off Ceres' surface.

Elements whose abundances they can get a handle on include H, C (light enough to slow the fast neutrons that collide with them)
and also O, Mg, Al, Si, Cl, Ca, Ti, Fe.
You can play around with the graphic and get pop-ups to explain some of how these elements are identified.

Also there are elements like K, Th, and U that have naturally radioactive isotopes whose decay involves a distinctive gamma energy. They are included in the diagram as well.

 

[marcus]

I gather Dawn has temporarily stopped observing so it can turn main antenna towards Earth and transmit data. Madrid antenna #55 is receiving data at 125 kilobit per second.
https://eyes.nasa.gov/dsn/dsn.html
The last time stopped observing to transmit was around 22 December. Now it is only 4 days later and it is transmitting again. this is more frequent than in HAMO, the previous orbit. Maybe it is collecting so much more data now, in low orbit, that it has to take transmission breaks more often.

Hydrogen and Carbon are the two lightest elements that can be counted on to be really abundant in Ceres surface material.
Light nuclei are better at slowing down neutrons. So Dawn can measure the abundance of H and C by what fraction of the escaping neutrons have been slowed down by successive collisions, compared with how many are still going fast.

Why are light nuclei better at slowing down neutrons? Because when you smack into something massive it hardly moves, so it absorbs little energy and you bounce off going nearly the same speed you came in with.

But when you bounce off something light, it gets a kick and takes some of your kinetic energy and you come off noticeably slower.

 

[marcus]

Here are some graphics that show the (log) abundance of chemical elements in various contexts---the sun, the planets of the inner solar system, the Earth's continental crust...
https://www.uwgb.edu/dutchs/PLANETS/Geochem.htm
The first one shows abundances in the sun. Scroll down for the others.

Some of the text should be read critically, the author is a geochemist, not an astronomer. Showing the log abundances by the sizes of circles, roughly superimposed on the periodic table ordering of elements, though, was helpful at least to me.

 

[marcus]

...
Dawn team has a pretty nice interactive graphic describing the GRaND instrument (gamma ray and neutron spectrometer)
Introduction is here:
http://dawn.jpl.nasa.gov/technology/grand.html
Then you click on the image and it gets you this:
http://www.jpl.nasa.gov/dawn/swf/GRAND/Grand_animation.swf
and that, after a few seconds brings up a diagram that you can interrogate to get a bunch of information including explanation of how the GRaND instrument works and what chemical elements it can "constrain" the abundance of. That is, get a handle on, sometimes by some fairly complicated inference, from the energies of gamma rays and neutrons coming off Ceres' surface.

Elements whose abundances they can get a handle on include H, C (light enough to slow the fast neutrons that collide with them)
and also O, Mg, Al, Si, Cl, Ca, Ti, Fe.
You can play around with the graphic and get pop-ups to explain some of how these elements are identified.

Also there are elements like K, Th, and U that have naturally radioactive isotopes whose decay involves a distinctive gamma energy. They are included in the diagram as well.

The probe is collecting such a volume of data that it has to pause every few days and dump to Earth. If you've been reading Marc Rayman's updates you know it completed a dump and resumed collecting data around noon on the 22 December. It looks as if it paused taking data and took another communication break sometime around 26 December, and resumed normal activity by morning of 28 Dec. I don't know this for sure, just judging from DSN activity. Around the 27 Dec I saw there was a lot of data transmission involving Dawn and several antennas of the Deep Space Network. Rates like 125 kilobit per second.
Today, 28 Dec, what I saw was mostly just the tracking signal they listen to detect variations in Ceres gravitational field as Dawn passes over various geological formations and types of terrain.

 

[marcus]

A worrisome detail: as of 29Dec 9AM pacific, checking DSN, I see that Madrid antennas #63 and 54 are both assigned to Dawn and there is no signal either way on either one. Has some trouble with communication developed? We haven't had a status update from Rayman for almost a week. The last one was 23 Dec.
http://dawn.jpl.nasa.gov/mission/status.html

 

[OmCheeto]

A worrisome detail: as of 29Dec 9AM pacific, checking DSN, I see that Madrid antennas #63 and 54 are both assigned to Dawn and there is no signal either way on either one. Has some trouble with communication developed? We haven't had a status update from Rayman for almost a week. The last one was 23 Dec.
http://dawn.jpl.nasa.gov/mission/status.html

They may have just been messing with us...

Via Twitter:

NASA's Dawn Mission ‏@NASA_Dawn 32 minutes ago
Update: Navigators are designing a small maneuver to adjust my orbit. Altitude at #Ceres ~240 mi.
​

 

[marcus]

...
Via Twitter:

NASA's Dawn Mission ‏@NASA_Dawn 32 minutes ago
Update: Navigators are designing a small maneuver to adjust my orbit. Altitude at #Ceres ~240 mi.
​

Thanks, Om. I'm glad to know the explanation.

Reminding people that a major new feature of Dawn's data gathering at this lowest orbit is the GRaND instrument, which can get a handle on chemical element abundances in the surface material.

...
Dawn team has a pretty nice interactive graphic describing the GRaND instrument (gamma ray and neutron spectrometer)
Introduction is here:
http://dawn.jpl.nasa.gov/technology/grand.html
Then you click on the image and it gets you this:
http://www.jpl.nasa.gov/dawn/swf/GRAND/Grand_animation.swf
and that, after a few seconds brings up a diagram that you can interrogate to get a bunch of information including explanation of how the GRaND instrument works and what chemical elements it can "constrain" the abundance of. That is, get a handle on, sometimes by some fairly complicated inference, from the energies of gamma rays and neutrons coming off Ceres' surface.

Elements whose abundances they can get a handle on include H, C (light enough to slow the fast neutrons that collide with them)
and also O, Mg, Al, Si, Cl, Ca, Ti, Fe.
You can play around with the graphic and get pop-ups to explain some of how these elements are identified.

Also there are elements like K, Th, and U that have naturally radioactive isotopes whose decay involves a distinctive gamma energy. They are included in the diagram as well.

http://www.jpl.nasa.gov/dawn/swf/GRAND/Grand_animation.swf
What this gives, after a few seconds, is a more elaborate and informative version of this simple graphic posted earlier

and some idea how the instrument itself works to identify characteristic radiation coming from the surface.