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

[OmCheeto]

And in case anyone didn't notice, a new Journal is out:

Dawn Journal | March 6
by Marc Rayman​

They posted a fun new video, reminiscent of our collected works.

​

At first I thought they'd boogered the trajectory, as it appeared that Dawn came in from the wrong direction. But I just watched it for the 5th time, and decided it was an optical illusion. Pay attention to the shadows of Ceres, and your brain probably won't go haywire, like mine did.

 

[OmCheeto]

After reading this comment from yesterday's journal:

Luis says:
March 6, 2015 at 5:43 pm
Marc,

Interesting that many of the impact craters on the surface of Ceres show what look like to be central peaks uplift, even in some of the smaller size ones. How important will this type of complex craters be to understand the subsurface characteristics of the dwarf planet, namely its upper crustal composition?

​

I went back and re-examined each of the frames of the moving* gif of Ceres rotating. I too see many craters with central peaks.
So, of course, I will now give my opinion of how they were created:

Upon impact, some asteroids will melt the subsurface ice layer, and through some low gravity, extremely cold mechanism, the water freezes before it can flow back.
This also implies that the ice layer is very, very near the surface.​

Or something like that.

*If you have a Mac, you can save the image, and the "Preview" application will display it as selectable static images.

 

[Dotini]

After reading this comment from yesterday's journal:

Luis says:
March 6, 2015 at 5:43 pm
Marc,

Interesting that many of the impact craters on the surface of Ceres show what look like to be central peaks uplift, even in some of the smaller size ones. How important will this type of complex craters be to understand the subsurface characteristics of the dwarf planet, namely its upper crustal composition?

​

I went back and re-examined each of the frames of the moving* gif of Ceres rotating. I too see many craters with central peaks.
So, of course, I will now give my opinion of how they were created:

Upon impact, some asteroids will melt the subsurface ice layer, and through some low gravity, extremely cold mechanism, the water freezes before it can flow back.
This also implies that the ice layer is very, very near the surface.​

Or something like that.

Yes, there is no shortage of craters with that silly little central peak. While your suggested mechanism sounds plausible, there are similar-looking craters on bodies thought to be entirely rocky.

 

[OmCheeto]

Yes, there is no shortage of craters with that silly little central peak. While your suggested mechanism sounds plausible, there are similar-looking craters on bodies thought to be entirely rocky.

I just posted, what will be my 4th attempt, on the Journal. I theorized as to why Ceres has only one set of headlights:

OmCheeto says:
Your comment is awaiting moderation.
March 7, 2015 at 9:29 am
I concur. I count at least 30 of these central peak objects. My guess is that the asteroids that created the craters, melted the ice, which must be very, very near the surface, and the water froze, before it could flow back. The “two headlights” peaks are possibly the newest, and haven’t had time to accumulate any cover dust. Just a theory, of course. ​

ps. If anyone else tries to comment on his Journal, do the "CAPTCHA" thing last, just before submission. It has a "timeout" feature, which will make you crazy.
Trust me. And don't forget the sunscreen.

 

[marcus]

Thanks for pointing out the new Dawn Journal entry, Om! I wasn't expecting one. Also for the approach trajectory video link.

I watched it and was puzzled by one detail. I understand the sun is off to the right. And Ceres N pole (and solar system/ecliptic north) is up.
So it seemed to me that Ceres should be rotating opposite from how the computer artists have it.
Maybe you can straighten me out. Since north is up, rotation should be "right hand rule" make a fist with your thumb pointing up and the turning is in the direction of your fingers.

The public outreach computer artists have her turning in the "left hand rule" direction.

Maybe I'm not getting something and somebody can explain, or maybe it is just that busy artists sometimes get a detail wrong.

Except for the direction Ceres is turning it all makes sense with that earlier trajectory timeline picture you adapted from Rayman's November Journal diagrams.

.

 

[OmCheeto]

Thanks for the link, Om. I watched it and was puzzled. I understand the sun is off to the right. And Ceres N pole (and solar system/ecliptic north) is up.
So it seemed to me that Ceres should be rotating opposite from how the computer artists have it.
Maybe you can straighten me out. Since north is up, rotation should be "right hand rule" make a fist with your thumb pointing up and the turning is in the direction of your fingers.

The public outreach computer artists have her turning in the "left hand rule" direction.

Maybe I'm not getting something and somebody can explain, or maybe it is just that busy artists sometimes get a detail or two wrong.

Except for the direction Ceres is turning it all makes sense with that earlier trajectory timeline picture you adapted from Rayman's November Journal diagrams.

.

I didn't notice the error in rotation before. Good catch!

But my head has been spinning for the last few days. I downloaded a dataset of the 300 brightest stars the other day, and plugged them into my graphing software, hoping to get a background for the Dawn trajectory we've collected so far.
Unfortunately, "galactic coordinates" appear to be completely backwards from the spherical coordinates my software wants.

The blue and pink stars are supposed to be the constellation of Orion.
The pink ones being Rigel, Bellatrix, and Betelgeuse.
Somewhat upside down when you get the belt pointed the right way.
And it doesn't get any better if you rotate it and view it from the other side.
​

When I saw wiki's list of what I would have to wade through to figure this out, I decided, once again, to just sit back and wait for the pictures.

1 Coordinate systems

1.1 Horizontal system
1.2 Equatorial system
1.3 Ecliptic system
1.4 Galactic system
1.5 Supergalactic system​

2 Converting coordinates

2.1 Notation
2.2 Hour angle ←→ right ascension
2.3 Equatorial ←→ ecliptic
2.4 Equatorial ←→ horizontal
2.5 Equatorial ←→ galactic
2.6 Notes on conversion​

​

 

[OmCheeto]

The Planetary Society posted an interesting image on Facebook yesterday. I traced it back to its origin:

What's Up in the Solar System diagram by Olaf Frohn (updated for March 2015)

Amazing how much is going on in the solar system. What a year for a nerd to be alive!

 

[marcus]

Dawn's distance from Ceres is continually increasing and the next step in getting into a useful near-Ceres orbit is for the distance to max out.
Petrich's numbers show this happening soon, around 19 March.
Here's the key section that shows the turnaround. Ceres is at (0,0,0) so basically we want all three coordinates to be small, to stay at or oscillate around zero, and the total distance to be small.

date      X          Y          Z        distance from Ceres
M14     68.4790    28.2510    -6.38793    74.35
M15     69.7223    27.7726    -4.80202    75.20
M16     70.7383    27.1819    -3.57051    75.86
M17     71.5002    26.8275    -2.34785    76.40
M18     72.0023    26.5440    -1.37327    76.75
M19     72.3581    26.2900    -0.12403    76.98
M20     72.4098    25.8411     1.05432    76.88
M21     72.2873    25.4277     2.33013    76.66
M22     72.0303    25.0319     3.22497    76.32

Z coordinate shows Dawn had been lagging behind Ceres in their race around the sun. It draws abreast with the planet on 19 March and even gets a bit ahead. Total distance also peaks on 19 March and declines thereafter. Y is the distance above Ceres' orbit plane---it is under control and has been gradually declining ever since 6 March.
The X coordinate is the distance outward from the sun, and it shows the troublesome overshoot. Dawn was forced by a fluke accident to approach from sunward too fast, causing it to overshoot by quite a lot. It is now using its thruster to slow that outward motion.

To provide context, here's the full table. It extends to 5 April.

date      X          Y          Z        distance from Ceres
F17   -45.9972    6.4086    -27.2882    53.86           
F18   -38.555    9.71627    -28.2185    48.75
F19   -32.3324    12.4392    -29.202    45.30           
F20   -26.169    14.8491    -29.9728    42.46
F21   -19.6171    17.2648    -30.4689    40.14
F22   -13.2794    19.4975    -30.6993    38.71
F23   -6.73346    21.6416    -30.593    38.07
F24   -0.502056    23.4431    -30.212    38.24
F25     5.62894    25.0851    -29.7158    39.29
F26     11.407    26.4613    -29.1488    40.98
F27     17.2899    27.6663    -28.1919    43.11
F28     22.8583    28.5286    -27.0313    45.46
M1      27.9985    29.1842    -25.6846    47.90
M2      32.8862    29.7513    -24.1873    50.51
M3      37.6439    30.1647    -22.7166    53.31
M4      41.9734    30.4246    -21.3167    56.05
M5      45.8274    30.5605    -19.8726    58.55
M6      49.5028    30.6491    -18.2955    61.02
M7      52.8252    30.4896    -16.7451    63.24
M8      55.7681    30.3242    -15.1946    65.27
M9      58.5427    30.0761    -13.6441    67.21
M10     61.1564    29.769     -12.0671    69.07
M11     63.2886    29.4796    -10.5963    70.61
M12     65.1256    29.1961    -9.32939    71.97
M13     66.8636    28.8417    -7.90296    73.24
M14     68.4790    28.2510    -6.38793    74.35
M15     69.7223    27.7726    -4.80202    75.20
M16     70.7383    27.1819    -3.57051    75.86
M17     71.5002    26.8275    -2.34785    76.40
M18     72.0023    26.5440    -1.37327    76.75
M19     72.3581    26.2900    -0.12403    76.98
M20     72.4098    25.8411     1.05432    76.88
M21     72.2873    25.4277     2.33013    76.66
M22     72.0303    25.0319     3.22497    76.32
M23     71.7734    24.5653     4.24385    75.97
M24     71.1281    23.9983     5.08554    75.23
M25     70.1580    23.3604     6.02468    74.18
M26     68.9840    22.7402     6.74232    72.94
M27     67.6521    22.1023     7.46883    71.56
M28     66.0440    21.553       8.08901    69.94
M29     64.1702    20.9978     8.57630    68.06
M30     61.9597    20.5548     9.23193    65.92
M31     59.6842    20.2240     9.60404    63.74
A01      57.1311    20.0527     9.85212    61.34
A02      54.4022    19.8283    9.81668    58.72
A03      51.2408    19.6570    9.97616    55.78
A04      48.0025    19.6393    9.84326    52.79
A05      44.5782    19.8637    9.69264    49.75

X Y Z are coordinates relative to Ceres, which is (0,0,0), measured in kkm---thousands of km.
X is directed out from sun, in Ceres orbit plane
Y is directed perpendicularly up off the orbit plane, in Ceres' north pole direction
Z is directed forwards in Ceres orbit plane, the direction Ceres is moving, a negative shows the probe trailing behind.

 

[Dotini]

The X coordinate is the distance outward from the sun, and it shows the troublesome overshoot. Dawn was forced by a fluke accident to approach from sunward too fast, causing it to overshoot by quite a lot. It is now using its thruster to slow that outward motion.

Two questions, please, doctor Marcus.

1) What was the fluke accident? Is it possible the overshoot was due to a mis-overestimation of Ceres's mass?
2) By using its thruster, does this mean that hydrazine is being expended, or are we using the ion propulsion, still?

 

[mfb]

Thrusting stopped for 4 days due to a software error (details). Ceres' mass was irrelevant for most of the approach phase and its uncertainty was just a few percent even before Dawn arrived.
Dawn is using the ion propulsion for all orbit maneuvers. Hydrazine is used to rotate the spacecraft only.

 

[marcus]

It can be informative to mentally put these two views together---you see Ceres (and Dawn's approach to it) in a larger context with the sun and other planets. It's a convenient time to try this because from the spacecraft 's perspective Ceres is nearly on the belt of Orion. And part of Orion (the two shoulder stars) show up near the bottom edge of the larger view.

The point, I guess, is that Dawn is now out beyond Ceres a little ways. So it can take both the Sun and Ceres in the same view. If Dawn looks down from the Sun and a bit over to the right it will see Ceres---just below one of the two shoulder stars of Orion ("Bellatrix") by about the same spread as that which separates the two shoulder stars themselves.

http://neo.jpl.nasa.gov/orbits/fullview1.jpg
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
The two links are:
Sun view: http://neo.jpl.nasa.gov/orbits/fullview1.jpg
Ceres view: http://neo.jpl.nasa.gov/orbits/fullview2.jpg
In the first, the Sun view, the two Orion shoulder stars are the two bright ones currently down where it gives the speed relative to Sun. Right above the "17.23 km/s"
In the second you can see Ceres as usual smack in the center, but dark because it is partly sunward of Dawn. Dawn is on the unilluminated side of the planet. You can tell what direction the sun is, from Ceres, by the thin illuminated rim on the upper left sector of the Ceres disk.

Om did this earlier. He merged the two views graphically. Now is an especially good time to do the merger mentally because Orion is in the picture and there is an overlap of the background stars. The star background especially of the Ceres view is always changing as Dawn swings around and sees Ceres from different angles.

 

[lpetrich]

It's now at 72.28 kkm and 0.03 km/s, about 108 km/h or 67 mph -- typical highway driving speed. From my calculations, it is getting close to its closest Ceres-relative speed in the mission.

 

[marcus]

Hi Om! Glad you approve, following your example of merging the two. But having a slight problem with the system. The system is giving a two-day old version of the second, the Ceres view. Or is for me anyway. This is not serious but it isn't quite as nice. In the current 11 March view, where Ceres is practically ON the belt. It is where people locate the "dagger" stars hanging from the belt. Right below the belt on the right from our PoV.
If this slight mismatch bothers anybody they can go to w: http://neo.jpl.nasa.gov/orbits/fullview2.jpg and see the current 11 March version.

 

[marcus]

It's now at 72.28 kkm and 0.03 km/s, about 108 km/h or 67 mph -- typical highway driving speed. From my calculations, it is getting close to its closest Ceres-relative speed in the mission.

Let me go find the table made with your numbers, Petrich. It has been very helpful! I think unless you have made a revised calculation that the X coordinate should reach a maximum of 77 kkm around 19 or 20 March, and should never get any bigger. It should start declining back down towards zero then, and eventually oscillate around zero when there is a close circular orbit.

Here's a relevant portion of the table. I've omitted Petrich's speed numbers---he has velocities in each of the coordinates, and combined speed relative Ceres. But this abbreviated table just has a few distance numbers. Ceres is at (0, 0, 0)

date      X          Y          Z        distance from Ceres
F17    -45.9972    6.4086     -27.2882    53.86       
F18    -38.5550    9.71627    -28.2185    48.75
F19    -32.3324    12.4392    -29.2020    45.30       
F20    -26.1690    14.8491    -29.9728    42.46
F21    -19.6171    17.2648    -30.4689    40.14
F22    -13.2794    19.4975    -30.6993    38.71
F23    -6.73346    21.6416    -30.5930    38.07
F24    -0.50206    23.4431    -30.2120    38.24
F25     5.62894    25.0851    -29.7158    39.29
F26     11.4070    26.4613    -29.1488    40.98
F27     17.2899    27.6663    -28.1919    43.11
F28     22.8583    28.5286    -27.0313    45.46
M1      27.9985    29.1842    -25.6846    47.90
M2      32.8862    29.7513    -24.1873    50.51
M3      37.6439    30.1647    -22.7166    53.31
M4      41.9734    30.4246    -21.3167    56.05
M5      45.8274    30.5605    -19.8726    58.55
M6      49.5028    30.6491    -18.2955    61.02
M7      52.8252    30.4896    -16.7451    63.24
M8      55.7681    30.3242    -15.1946    65.27
M9      58.5427    30.0761    -13.6441    67.21
M10     61.1564    29.769     -12.0671    69.07
M11     63.2886    29.4796    -10.5963    70.61
M12     65.1256    29.1961    -9.32939    71.97
M13     66.8636    28.8417    -7.90296    73.24
M14     68.4790    28.2510    -6.38793    74.35
M15     69.7223    27.7726    -4.80202    75.20
M16     70.7383    27.1819    -3.57051    75.86
M17     71.5002    26.8275    -2.34785    76.40
M18     72.0023    26.5440    -1.37327    76.75
M19     72.3581    26.2900    -0.12403    76.98
M20     72.4098    25.8411     1.05432    76.88
M21     72.2873    25.4277     2.33013    76.66
M22     72.0303    25.0319     3.22497    76.32
M23     71.7734    24.5653     4.24385    75.97
M24     71.1281    23.9983     5.08554    75.23
M25     70.1580    23.3604     6.02468    74.18
M26     68.9840    22.7402     6.74232    72.94
M27     67.6521    22.1023     7.46883    71.56
M28     66.0440    21.5530     8.08901    69.94
M29     64.1702    20.9978     8.57630    68.06
M30     61.9597    20.5548     9.23193    65.92
M31     59.6842    20.2240     9.60404    63.74
A01     57.1311    20.0527     9.85212    61.34
A02     54.4022    19.8283     9.81668    58.72
A03     51.2408    19.6570     9.97616    55.78
A04     48.0025    19.6393     9.84326    52.79
A05     44.5782    19.8637     9.69264    49.75

X Y Z are coordinates relative to Ceres, which is (0,0,0), measured in kkm---thousands of km.
X is directed out from sun, in Ceres orbit plane
Y is directed perpendicularly up off the orbit plane, roughly in Ceres' north pole direction
Z is directed forwards in Ceres orbit plane, approximately in the direction Ceres is moving, a negative shows the probe trailing behind. It draws abreast with the planet on 19 March and even gets a bit ahead.

Total distance also peaks on 19 March and declines thereafter. Y is the distance above Ceres' orbit plane---it is under control and has been gradually declining ever since 6 March.
The X coordinate is the distance outward from the sun, and it shows the troublesome overshoot. Dawn was forced by a fluke cosmic ray accident to make up lost thrusting time and to approach from sunward faster than originally planned. This made it overshoot and It now has it thruster pointed mostly in the positive X direction to slow that outward motion.

 

[marcus]

People who want to find out more about Petrich's table may wish to go back to the posts #90 and #97 and #100 on page 5 of this thread

I've measured the positions off of marcus's most recent diagram. They are as a tab-delimited spreadsheet text file that I've attached to this message...
...I wrote an image measurer for myself since I couldn't find a good one that enters a position with each click on the picture being measured. It's OSX-native, so to port it to Windows or Linux, you'll need GNUstep.

There is an attachment there which let's you get the whole table of numbers. I've only been excerpting small pieces of it appropriate to the current discussion.

... From my calculations, it is getting close to its closest Ceres-relative speed in the mission.

Petrich it interested me that you said some speed relative to Ceres would be reaching a minimum at around this time. Minimum for the whole mission.
I think you may mean the magnitude of the speed you labeled V_X. Perhaps the TOTAL speed relative to Ceres will not be reaching a minimum just now basically because of the Y and Z motion. (The probe is still gradually descending from above orbit plane and it is still catching up). I need to look at your expanded table, with the velocities, to see what you mean. But the absolute value |V_X| could be actually zero in a few days, as the X coordinate peaks and begins to decline.

 

[OmCheeto]

Let me go find the table made with your numbers, Petrich. It has been very helpful! I think unless you have made a revised calculation that the X coordinate should reach a maximum of 77 kkm around 19 or 20 March, and should never get any bigger. It should start declining back down towards zero then, and eventually oscillate around zero when there is a close circular orbit.

18th. I just looked: 14.14 mps = 32 mph.
It doesn't match the date of the apex, as Dawn will be thrusting, pretty much continuously until final orbit.

 Date   dist kkm    m/s
16-Mar    75.8     18.37
17-Mar    76.4     15.13
18-Mar    76.8     14.14
19-Mar    76.9     14.81
20-Mar    77.1     15.06
21-Mar    76.5     13.58
22-Mar    76.4     12.51

ps. Did anyone see the comment about the "late April" look for an atmosphere?

March 9, 2015 at 11:54 pm
Great to get into orbit. Are you at some future point in a position where a solar eclipse let's you get a peek at a possible atmosphere?

Reply

• 
  Marc Rayman says:
  March 10, 2015 at 9:35 am
  Hi Kamal,
  
  Dawn is designed to operate in sunlight, and we keep it out of eclipse. The last time it was in the shadow of a planet was the day it left Earth. Nevertheless, as I have written in several comments in recent months, including below, Dawn will look for evidence of an extremely thin veil of water vapor above Ceres when it is on the night side in late April.
  
  Marc

 

[mfb]

I merged the two images, neglecting the two days difference (the sun view does not change notably apart from the spacecraft orientation and status). The stars won't change, so you can easily imagine a new position of Ceres in other images.

 

[marcus]

A nice convenience, thanks Mfb. I can see you scaled the Ceres view down by 2/3 because it is a 30 degree vs a 45 degree view.
I measured the distance between Sun and Ceres in your combined view and by coincidence it was just half the width of the Sun view so corresponds roughly speaking to 22.5 degrees.

So I'm picturing your diagram curved onto a spherical shell, in such a way that Dawn is at the center of the sphere and there is an angle of 22.5 degrees between Sun and Ceres.

that then is a substantial patch of Dawn's sky-map or "celestial sphere".

It also tells us why such a thin sliver at the edge of Ceres is illuminated. cos 22.5 degrees is 0.92.
Viewed from far enough off, the maximum width of the illuminated sliver should be 8% of a Ceres radius or 4% of the Ceres disk's diameter.

 

[marcus]

Current status shows Dawn has stopped thrusting, and reoriented for either photoshoot or transmission. This is in both the Sun view and Ceres views.
DSN shows the Goldstone antenna standing by for communication with Dawn.
https://eyes.nasa.gov/dsn/dsn.html
Sun view: http://neo.jpl.nasa.gov/orbits/fullview1.jpg
Ceres view: http://neo.jpl.nasa.gov/orbits/fullview2.jpg

My understanding was they would not be taking any more pictures for a while, even for navigation, so maybe the reorientation just signifies the probe is communicating with Earth. Can anybody figure out?

 

[Dotini]

My understanding was they would not be taking any more pictures for a while, even for navigation, so maybe the reorientation just signifies the probe is communicating with Earth. Can anybody figure out?

I think they are looking for sun-backlit pix of the limb, this in order to detect the plumes reported in the interim since Dawn was specced and launched. They didn't really plan on looking for plumes.

 

[wabbit]

The next pictures coming our way will be early april, is that right ?

 

[OmCheeto]

Current status shows Dawn has stopped thrusting, and reoriented for either photoshoot or transmission. This is in both the Sun view and Ceres views.
DSN shows the Goldstone antenna standing by for communication with Dawn.
https://eyes.nasa.gov/dsn/dsn.html
Sun view: http://neo.jpl.nasa.gov/orbits/fullview1.jpg
Ceres view: http://neo.jpl.nasa.gov/orbits/fullview2.jpg

My understanding was they would not be taking any more pictures for a while, even for navigation, so maybe the reorientation just signifies the probe is communicating with Earth. Can anybody figure out?

Perhaps they're looking at lpetrich's numbers.

Numbers on the left are from the images.
According to lpetrich's numbers, Dawn is too far away. Perhaps Ceres isn't as massive as we thought?

And if Dawn did transmit data, it was pretty quick. Goldstone #14 now assigned to "Geotail".

 

[marcus]

EDIT: Hi Om, our posts crossed. I didn;t see yours when I was writing.
Wabbit,
That's what I think (no pictures for a while) but I'm not sure and would be glad to hear from others about this.
BTW just checked DSN and no antenna was assigned to Dawn. Goldstone, which had been assigned to Dawn an hour ago, was so no longer.
That was quick.
I think they may just listen to the probe's signal to get a doppler reading of the radial speed. That would help in navigation and they wouldn't have to go through the whole rigamarole of photographing Ceres against its background of stars and transmitting the picture. But I simply don't know

PS: just checked and current status indicates normal thrusting has resumed (after an apparently brief communication session)

 

[OmCheeto]

EDIT: Hi Om, our posts crossed. I didn;t see yours when I was writiThat's what I think but I'm not sure and would be glad to hear from others about this.
BTW just checked DSN and no antenna was assigned to Dawn. Goldstone, which had been assigned to Dawn an hour ago, was so no longer.
That was quick.
I think they may just listen to the probe's signal to get a doppler reading of the radial speed. That would help in navigation and they wouldn't have to go through the whole rigamarole of photographing Ceres against its background of stars and transmitting the picture. But I simply don't know

It's been two days since any posts have passed moderation on господин Rayman's blog. I'm guessing he's a bit busy, and dancing, with excitement.

I'm tempted to post the following, based on his well known, dreadful, punishness:

The Dawn Nerd Party at PF, has noticed, anomalies. Can you explain this?
​

 

[mfb]

They have an ion drive and days to plan its usage. A small deviation from the expected mass won't hurt.

 

[OmCheeto]

I'm sensing a recalibration.

utc                  dist kkm   mps    acc m/sd
3/6/2015  18:34:20    62.23    44.70  
3/9/2015  12:08:35    68.34    36.21    -3.11
3/11/2015 17:47:31    72.28    29.95    -2.80
3/13/2015 11:31:07    74.62    25.03    -2.83
3/14/2015 04:17:07    75.41    24.59    -0.03

 

[lpetrich]

Dawn is getting as slow as it will ever get relative to Ceres without crashing onto the asteroid. The spacecraft is also getting to its maximum overshoot distance.

Distance: 77.34 k km, 48.06 k mi
Speed: 10 m/s, 71 km/h, 44 mph

 

[OmCheeto]

Dawn is getting as slow as it will ever get relative to Ceres without crashing onto the asteroid. The spacecraft is also getting to its maximum overshoot distance.

Distance: 77.34 k km, 48.06 k mi
Speed: 10 m/s, 71 km/h, 44 mph

I think you may be off by a factor of two in your "m/s" speed.

        utc          mph  dist kkm  mps    Δt(sec)  Δs(mps)  Δd(kkm)  acc(m/sd)  lp kkm  lp mps
 3/6/2015 18:34:20   100    62.23   44.7              
 3/9/2015 12:08:35    81    68.34   36.2   236055    -8.5     6.11     -3.11      67.3   36.20
3/11/2015 17:47:31    67    72.28   30.0   193136    -6.3     3.94     -2.80      70.6   28.10
3/13/2015 11:31:07    56    74.62   25.0   150216    -4.9     2.34     -2.83      73.2   26.39
3/14/2015 04:17:07    55    75.41   24.6    60360    -0.4     0.79     -0.64      74.4   25.18
3/16/2015 14:53:03    42    77.49   18.8   210956    -5.8     2.08     -2.38      75.8   18.37

utc, mph, & dist kkm are from the NASA images. "lp" kkm & mps are lpetrich's predictions from Feb 20th(?). The rest are my calculations.
It looks as though the acceleration figure I posted on Friday is wrong. (I'm still getting used to "Numbers", after 8 years. )

I've been relying on Marcus to remind me to take my recalibration readings from the daily images, so I missed a few.
But I've added it to my calendar to check it every 12 hours.

From the dead on accurate speed reading on the 9th, and the off by just a smidgeon today, I'll start taking my readings at noon & midnight.

 

[lpetrich]

Oops, you're right.

 

[OmCheeto]

Oops, you're right.

At least you weren't as far off as I was:

-0.03 {meters/second/day}

WRONG!

-0.64 / -0.03 = off by a factor of 21.3333333!

What is it old Jim always says; "Never trust a computer", or something like that.
Especially if you're not aware of its rounding algorithms.

 

[lpetrich]

Distance: 77.93 k km, 48.43 k mi
Speed: 20 m/s, 61 km/h, 38 mph (17 m/s)

Likely the farthest and slowest, but I'll keep my eyes open.

 

[marcus]

Hi Om, Petrich,
I just checked the current status view and noticed that thrust was turned off and the craft was oriented for telecommunication. I think it is a brief planned "radio navigation" break where they measure the doppler to tell the radial speed relative Earth.

 

[OmCheeto]

Download carrier displayed on Goldstone dish #24.
I think I'll just stare at it all day, until it's done.
Those squiggly lines are hypnotizing.

[edit] two minutes later: done. Yay!
Now I'll go stare at http://neo.jpl.nasa.gov/orbits/fullview2.jpg , and click refresh every 30 seconds, for the update.

[edit] 11 minutes later: doh! Signal's back on. I swear, that Dr. Rayman does this, just to torment me...

[edit] 22 minutes later: done again. Yay! (actually, I didn't sit and stare at it the whole time, so consider this a sciency failure...)

ps. all times are relative to my original post.

[edit] 29 minutes later: Signal's back on. I hate Dr. Rayman. He is evil. I have to go make lunch now.

[edit] UTC March 17th, 2015, 06:25 am: Dawn is now sending data transmissions to Canberra dish #35 @ 125.01 kb/sec, on frequency 8.44 GHz. Signal strength at the dish is fluctuating around 3.5e-20 kw. And I don't really care, as lunch, makes me want to take a nap.

 

[marcus]

You pointed out something interesting, Om. (as you often do! without emphasizing it)
It occurs to me that the antennas are at different latitudes and moving at different radial speeds relative to Dawn/Ceres. So that has to be corrected for when they calculate radial speed by doppler.
They are trying to determine Dawn's radial speed to within about one meter per second, I would guess. It can't be all that easy.
I can see why one might want to repeat the measurement several times with one antenna, or use a couple of different antennas.

Personally I feel slightly tense about the mission, at this point. I thought turnaround was to be expected around 77 kkm and it has gone past that, still traveling outwards. Didn't you mention at one point the possibility that the Ceres mass estimate of 943 billion billion kg might be on the high side?

 

[OmCheeto]

You pointed out something interesting, Om. (as you often do! without emphasizing it)

More likely, I didn't realize it.

It occurs to me that the antennas are at different latitudes and moving at different radial speeds relative to Dawn/Ceres. So that has to be corrected for when they calculate radial speed by doppler.
They are trying to determine Dawn's radial speed to within about one meter per second, I would guess. It can't be all that easy.
I can see why one might want to repeat the measurement several times with one antenna, or use a couple of different antennas.

Personally I feel slightly tense about the mission, at this point. I thought turnaround was to be expected around 77 kkm and it has gone past that, still traveling outwards. Didn't you mention at one point the possibility that the Ceres mass estimate of 943 billion billion kg might be on the high side?

I have no recollection of saying such a thing.
But Dawn is still communicating with Canberra #35 after nearly 5 hours.
Could they be doing parallax measurements, to determine axial speed?
Or did they take a bunch of sliver pictures, for the impatient ones?
Or were they taking pictures of the headlamps, to find out when they actually go out?

As someone tweeted yesterday about the LPSC2015 conference:

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