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

[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:

​

 

[OmCheeto]

This also in, from the Dawn journal:

Marc Rayman says:
March 17, 2015 at 11:41 am
Hi Andrew,

The 30 degrees for http://neo.jpl.nasa.gov/orbits/fullview2.jpg is the width of our viewpoint in looking at the simulator. It has nothing to do with Dawn’s trajectory but rather with how wide the picture is. I’ve asked my (busy) friend and colleague who maintains the Where is Dawn Now? simulation to give me some additional details to broaden my answer, and I will post an update here when he has time.

In the meantime, I should caution you that the simulator has a few quirks. In addition, he doesn’t always have time to incorporate the most up to date trajectory design, so you should be cautious about using it for detailed quantitative purposes. The numbers are close, but the distance indicated now is actually higher than the real value (and the speed is a little lower than what’s shown), as will be evident in the remaining part of my answer…

The following tweet though, kind of made my day:

Nature News&Comment ‏@NatureNews 4 hours ago
Dawn data suggest Ceres' bright spots are icy plumes http://bit.ly/1ABM9j6 #LPSC2015

A cosmic margarita, for Saint Patrick's Day. (hic!)

 

[marcus]

Valuable post by Rayman, Om!
http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/#comment-9108
Don't have to go strictly by "fullview2" figures. Turnaround expected on 19 March at 75 kkm and sideways speed of 35 mph.
All's well I think. At least as far as the turnaround goes.

Petrich's table originally showed turnaround occurring on 19 March, just that some distance and speed figures were different. He got the basic timing right, so bravo! 35 mph converts to 15.6 m/s.

I followed the link in the tweet you quoted and got this:
http://www.nature.com/news/bright-spots-on-ceres-could-be-active-ice-1.17139

It quotes Andreas Nathues of a planetary science MPI at Göttingen. I don't know how reliable the journalism is. It sounds like a quote from a speculative discussion session rather than a formal presentation. but it is datelined from a conference where Nathues spoke.

The MPI for Solarsystem-research at Göttingen is called MPS and it has a "Dawn Team" with responsibility for the main camera--the framing camera--on the spacecraft . Here are the people:
https://www.mps.mpg.de/3889370/Team.
The MPS Göttingen people play a part in the processing and release of photos---it is not all done at JPL.

Here is abtract of talk by Andreas Nathues at the recent LPSC (lunar&planetary society conference)
http://www.hou.usra.edu/meetings/lpsc2015/pdf/2069.pdf
Another Dawn talk given at the same LPSC session
http://www.hou.usra.edu/meetings/lpsc2015/pdf/1131.pdf
Link to the author index of all the conference talks
http://www.hou.usra.edu/meetings/lpsc2015/authorindex.cfm
(asterisk * denotes speakers)

 

[marcus]

Some of Marc Rayman posts that contain interesting information
http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/#comment-9108 (17 March)
Peak to come 18 March PDT (probably 19 March UT) that's today, at distance 75 kkm and lateral speed around 15.7 m/s (35mph)

http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/#comment-9099 (16 March)
Thrust level with the lesser sunlight is around 20% of what it was earlier.

http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/#comment-9019 (10 March)
Albedo (reflectivity) of Ceres is 9%, that of the "bright spots" is 40%. For comparison the overall albedo of Vesta is 40%.
The spots are just bright by contrast.

http://dawnblog.jpl.nasa.gov/2015/03/06/dawn-journal-march-6/#comment-9035 (11 March)
RC3 orbit plane will be skewed clockwise off the direction to sun, by between 5 and 9 degrees during the time RC3 is in effect.
It will start out 5 degrees and during the life of the RC3 orbit the direction to the sun will change by 4 degrees (counterclockwise) so the 5 will grow to 9. Then Dawn will spiral down to lower orbits, but presumably staying in the same plane, and that angle will continue to grow.
The sun direction angle to that plane has to grow because the idea is to keep Dawn out of Ceres shadow at all times. And the angular size of Ceres grows as the probe descends into closer orbits. The angle can't start out zero, because then probe would pass into shadow for some interval of its RC orbit. It has to start at some positive value like 5 degrees.

You can see that the planned-for Dawn mission at Ceres takes up about 90 degrees of a Ceres orbit.
http://neo.jpl.nasa.gov/orbits/fulltraj.jpg
At least 80 degrees. So if the RC3 orbit marks the beginning and the Sun direction makes a 5 degree angle off the RC3 orbit plane, then 80 degrees later the Sun direction will be nearly perpendicular to the probe's orbit plane---85 degrees---and that will be good because by that time the probe will have spiraled in really close.

 

[marcus]

when it's really close--e.g. LAMO (low altitude mapping orbit) at altitude 375 km above surface--the orbit plane seriously needs to be nearly normal to the sun direction. Otherwise probe might enter shadow and lose power.

when it is just in RC3 (rotation characterization) the initial orbit at altitude 13,500 km above surface, the planet has a small angular size in the sky so it is easy to stay out of shadow. Sun direction only 5 degrees off the probe orbit plane is plenty.

 

[mfb]

That is very interesting with the orientation of the orbit relative to the sun/Ceres axis. It also means Dawn will run into problems soon after the expected end of the mission.

 

[marcus]

I think that's right. It will be in the dark for a part of each orbit and need to hibernate. It's expected to be low on hydrazine for attitude control by that time too.

 

[OmCheeto]

Did anyone else notice yesterday, when the new image went up (Dawn's view of Ceres), that the time stamp was advanced 12 hours & 45 minutes?
I took that as a celestial hint, and went to bed a few hours later.
Hence, I woke up at the ungodly hour of 3:30 am this morning, and checked in:

DSN Madrid #54 just stopped talking to Dawn.
Mar 17, 2015, 06:25 UTC: Start
Mar 18, 2015, 11: 00 UTC: Stop [4:00 am local time]
28.5 hours?
That's a serious amount of data.​

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.

I'm predicting March 19, 2015 @ 20:37:58 UTC.
About 17 hours from now.

[edit] +/- 2 hours
new estimate: 3/19/2015 19:28:28
They just updated the image

 

[OmCheeto]

Not sure if anyone follows some guy name Phil Plait on Twitter, but this morning, about 14 hours ago, he shared a link to a stunning image of Orion, by someone named Rogelio Bernal Andreo. Knowing that Dawn was looking at Ceres, with Orion in the background, I copied and pasted:

I named the image; "if.astronomers.were.in.charge"​

 

[marcus]

Gorgeous.
I've often read Phil Plait's blog, in the past. It's called (or was when I was reading it) Bad Astronomy. I think the name might be an anatomical pun.
Om, Marc Rayman said something about being able to be in radio contact (using a small antenna) while continuing to thrust. I'm still unclear about that long communication session you reported. Were they verifying a turnaround in radial distance? I'm still somewhat confused. I think turnaround has happened and we are now on the way in!
In another month or so we will be concerned with the planned sequence of circular polar orbits. I'll bring forward info on the orbits. BTW RC stands for "rotation characterization". They want the orbits to go over N and S poles so first they have to FIND the rotation axis directions
==excerpted from earlier posts in this thread==
Here is the index of Dawn Journal entries so far:
http://dawn.jpl.nasa.gov/mission/journal.asp
Click on April 2014 for the sequence of closer and closer orbits (with some diagrams)
Click on August 2014 for discussion of how Ceres will be studied from the different altitudes.
When you click April 2014 you get:
http://dawnblog.jpl.nasa.gov/2014/04/30/dawn-journal-april-30-2/
And the sequence of orbits goes:
"RC3", "survey", "HAMO" and "LAMO" with nominal altitudes above surface (in km) being
13,500, 4400, 1740, 375 km.
LAMO is acronym for "low altitude mapping orbit".
If all goes well and they actually get down to LAMO before the hydrazine (attitude control propellant) runs out, then very small deviations in trajectory will reveal subsurface mass concentrations. The hope is to get a detailed gravity profile of the mini planet, as well as a visual map of the surface. there's an account of the various instruments at
http://dawn.jpl.nasa.gov/technology/science_payload.asp
See also: http://dawn.jpl.nasa.gov/technology/ for other topics like the spacecraft 's solar powered ion propulsion, navigation, general structure.
we can find the circular orbit speeds for each of these planned orbits, to get an idea of how much thruster work it will take to spiral down to the lower altitude ones. First, since the average radius is around 475 km, I want to convert these altitudes to orbit radii.
13975 km, 4875 km, 2215 km, and 850 km

(G*943e18 kg/13975 km)^.5 = 67.1 m/s
(G*943e18 kg/4875 km)^.5 = 113.6 m/s
(G*943e18 kg/2215 km)^.5 = 168.6 m/s
(G*943e18 kg/850 km)^.5 = 272.1 m/s

I want to estimate what the angular size will be, in degrees, from the planned orbit heights. The orbit altitudes are 13500, 4400, 1740, and 375 km. so adding the average radius of around 475 km, we get the orbit radii.
13975 km, 4875 km, 2215 km, and 850 km
2arcsin(475/13975) in degrees = 3.9 degrees
2arcsin(475/4875) in degrees = 11.2 degrees
2arcsin(475/2215) in degrees = 24.8 degrees
2arcsin(475/850) in degrees = 67.9 degrees
==endquote==

 

[marcus]

I want to know the orbit periods at each stage
2 pi*13975 km/67.1 m/s in days = 15 days
2 pi*4875 km/113.6 m/s in days = 3 days
2 pi*2215 km/168.6 m/s in days = 0.955 days
2 pi*850 km/272.1 m/s in hours = 5.45 hours

That is very interesting with the orientation of the orbit relative to the sun/Ceres axis. It also means Dawn will run into problems soon after the expected end of the mission.

The planned mission takes about a quarter of the Ceres year by which time Dawn is down into the closest orbit and its orbit PLANE is nearly normal to the sun direction.

Dawn will be lucky if it can continue its planned science activity through to the planned end, there could be various mishaps But supposing it does complete the planned mission, then it will just stay in stable LAMO orbit with nothing to do. Mfb makes the interesting point that not so long after that the sun direction will make a small enough angle with Dawn's orbit plane that the probe will be spending a substantial portion of its time IN THE DARK.

As I recall the Ceres orbit radius is about 2.8 AU and its year is about 4.7 Earth years.
So a quarter Ceres year, the time it would take for the sun direction to be 90 degrees off Dawn's orbit plane to zero degrees (sun direction right in line with the orbit plane) is a little over one Earth year.
And because the angular size of Ceres, seen from that low orbit, is almost 70 degrees it would take much less than one Earth year for the probe to find itself spending a lot of time in Ceres shadow.

 

[OmCheeto]

Gorgeous.
I've often read Phil Plait's blog, in the past. It's called (or was when I was reading it) Bad Astronomy. I think the name might be an anatomical pun.
Om, Marc Rayman said something about being able to be in radio contact (using a small antenna) while continuing to thrust. I'm still unclear about that long communication session you reported. Were they verifying a turnaround in radial distance? I'm still somewhat confused. I think turnaround has happened and we are now on the way in!
...

nope.
I've been quite diligent collecting the data, since I put it on my calendar.

        utc          thruster  mph   dist kkm       mps
3/18/2015 14:15:25      on      38      78.25      17.0
3/18/2015 20:14:24      on      37      78.32      16.5
3/19/2015 02:13:24      on      36      78.37      16.1
3/19/2015 08:12:23      on      35      78.4       15.6
3/19/2015 14:11:22      on      35      78.43      15.6

When I mentioned that I was up at 3 am yesterday, I probably should have also mentioned that I spent several hours digitizing(thank you again lp!) the collected images.
I was hesitant to share my findings, as I'm not really sure what I'm doing.
But, my work is always good for a laugh.
Anyways, I picked the nearest star to Ceres, and went from there:

left image: 3/17/2015 14:19:25 right image: 3/18/2015 14:15:25
I chose to center Dawn at the point where the solar panel arrays line up.

Then I looked at how fast Dawn and Ceres were moving, relative to the celestial background:
Ceres orbital period in days, per wiki: 1681.63
From which I was able to determine an angular motion of radians per second: 4.324E-08 (probably off by a bit, but close enough for govt work)
Which I had to correct for, by determining the radians per pixel in the "where is" images: 4.36E-04 (30°/1200 pixels)
Which gave me the number of pixels to subtract, due to that motion: 8.54

Then I did a whole bunch of trigonometry and math, and came up with the following:

speeds in m/s

om        om     lpet     lpet    sim
rec/apr  1.3928   x       2.36    1.3928
hor      3.79     y       4.07    n/a
ver     12.07     z      14.05    n/a
rel vel 12.73            14.82   13.38

My "rec/apr = receding/approaching" number came from the change in distance/time from "where is" images.

But I believe the answer to your "long communication" question, is that a single pixel measurement, changes things quite a bit.
Strategically changing the x & y pixel measurements by just one:

          raw     +/- 1
rec/apr   1.39     1.39
hor       3.79     3.80
ver      12.07    12.86
rel vel  12.73    13.49

switches my relative motion, from the low side, to the high side.

[edit] Canberra #35 is assigned now to Dawn. No signal.

 

[OmCheeto]

...
I think turnaround has happened and we are now on the way in!
...

But then again, Dr. Rayman probably knows better than I do.

Dawn Blog
March 19, 2015 at 1:21 pm
Hi Andrew,

For others, apoapsis is the most distant point of an elliptical orbit. (The term “apogee” may be more familiar. That is the most distant point in an elliptical orbit around Earth. Apoapsis is more general, applying to orbits around anything.)

As I described in my Dawn Journal above, Dawn reached that highest altitude on March 18, so, yes, its orbit is now taking it closer to the dwarf planet again.

In case anyone was wondering where I came up with my prediction, I plotted the change in distance/time vs time, since March 6th, and came up with the following graph:

x axis is time from 3/6/2015 18:34:20 UTC in seconds
y axis is meters/second

Strangely linear. But I suppose that's how ion drives do things.
I get 2.40 meters/sec/day deceleration
This is based on the distance between Dawn and Ceres, and not the relative velocity.

hmmm...

At today’s throttle level, it would take the distant explorer almost 11 days to accelerate from zero to 60 mph (97 kilometers per hour).

60 mph = 26.8 mps
26.8 mps / 11 days = 2.44 m/s/d

Ok. Dr. Rayman's math checks out. Also:

As I commented below, I will provide an update on the mass of Ceres in May. This will give you something else to look forward to!

Marc

Yay!

 

[mfb]

The animation suggests an end of thrusting on April 24, if the web page data is reliable enough then we can also make estimates what the mass will be ;).

@marcus: would you mind if I update the thread title to reflect the current distance?

 

[marcus]

...
@marcus: would you mind if I update the thread title to reflect the current distance?

That would be fine AFAICS. Please go ahead with whatever form of title update you have in mind.

 

[mfb]

The 1 lightsecond was a bit outdated.

78.37 Mm, certainly approaching Ceres again now. 33mph (14:07:19 UTC).

 

[marcus]

I think thousands of km is a good unit to track the gradually closing range in, better than fractions of a light-second. We are all used to kkm. Could use Mm (just saw your post).
The current status view range number doesn't always agree exactly with what Rayman says in his blog comments, but it's reasonably close.
Yesterday it was 78.43 kkm and today when I checked a moment ago it was 78.37 kkm.

So current status registered turnaround achieved on 19 March.

The speed given is mostly "sideways" speed at this point, not radial. But it might be interesting to watch change. in the headline. Up to you. I've been trying for pedagogical reasons to stick to metric units so I would put the 33mph in the form of a m/s figure.

 

[marcus]

I wonder what the angular size is now. 2arcsin(.475/78.37) in degrees
About 0.7 of a degree. Eventually the angular size should get up to around 60 degrees, if I remember right. That is about 120 times size of moon seen from Earth
What gradually changing headline makes sense to you from a communication standpoint, and perhaps journalistically?

Ah! I see you have already changed the headline. It makes good sense to me. Hope other contributors to the thread (Om, Petrich, Dotini...) agree as well

 

[Dotini]

I wonder what the angular size is now. 2arcsin(.475/78.37) in degrees
About 0.7 of a degree. Eventually the angular size should get up to around 60 degrees, if I remember right. That is about 120 times size of moon seen from Earth
What gradually changing headline makes sense to you from a communication standpoint, and perhaps journalistically?

Ah! I see you have already changed the headline. It makes good sense to me. Hope other contributors to the thread (Om, Petrich...) agree as well

Seems fine for now.

In view of the potential science discoveries, it might be good for the thread title to mirror those, if any. Otherwise, overlapping new threads may arise.

 

[marcus]

Dotini, glad you approve of the basic idea of updating the headline.
BTW here's a quote from Lakdawalla recent post:
==quote Emily about Andreas Nathues talk at LPS conference==
Then he focused on the bright feature. It is located in the floor of a crater 80 kilometers in diameter. From its behavior as the globe rotates, he said, the bright feature appears to lie in a depression. The images that have been released to the public from the rotation animation do not show all of the photos of the bright feature, so the next point concerns images that I can't show you. "What is amazing," he said, "is that you can see the feature while the rim is still in front of the line of sight. Therefore we believe at the moment that this could be some kind of outgassing. But we need higher resolution data to confirm this." What he is saying is that as Ceres' globe rotates and the 80-kilometer crater's rim rotates into view, that rim should block our ability to see the bright feature on the floor of the crater. However, the bright feature is already visibly bright as the crater begins to rotate into view. Therefore, it must be vertically above the rim of the crater: it must be some kind of plume. "During the day," Nathues went on, "the feature evolves: it brightens. At dusk it gets fainter; at late dusk it disappears completely. We see this for cometary activity."

He moved to color data, showing a global map of Ceres as seen through different-colored filters. There was a striking asymmetry to the color: one hemisphere was much more red and the other much more blue. The images were taken from too great a distance to resolve the bright spot; it is smaller than 4 kilometers across. So they can say that its albedo is at least 0.4 (meaning that it reflects at least 40% of the light that strikes it), but it could be much higher. The color information over the spot is consistent with an icy surface, but this is not a unique interpretation. The feature has variable brightness with time: its brightness increases strongly as seen through the 550-nanometer filter around local noon.

Obviously, active outgassing on Ceres would be a big deal, if it really exists. Fortunately, Dawn will get much closer and will take much better images, which will hopefully confirm this discovery!
==endquote==
http://www.planetary.org/blogs/emily-lakdawalla/2015/03191629-lpsc-2015-dawn-at-ceres.html

 

[marcus]

Yesterday current status gave distance 78.43 kkm today 78.37 kkm, so it confirms that turnaround happened even if numbers slightly off from what Rayman posted. Petrich was right about the timing.
Following a lead Om gave us I checked who Nathues is:
leader of the Dawn framing camera team at one of Germany's MPI
==quote from earlier post==
...I followed the link in the tweet you quoted and got this:
http://www.nature.com/news/bright-spots-on-ceres-could-be-active-ice-1.17139

It quotes Andreas Nathues of a planetary science MPI at Göttingen. ...
The MPI for Solarsystem-research at Göttingen is called MPS and it has a "Dawn Team" with responsibility for the main camera--the framing camera--on the spacecraft . Here are the people:
https://www.mps.mpg.de/3889370/Team.
The MPS Göttingen people play a part in the processing and release of photos---it is not all done at JPL.

Here is abtract of talk by Andreas Nathues at the recent LPSC (lunar&planetary society conference)
http://www.hou.usra.edu/meetings/lpsc2015/pdf/2069.pdf
Another Dawn talk given at the same LPSC session
http://www.hou.usra.edu/meetings/lpsc2015/pdf/1131.pdf
Link to the author index of all the conference talks
http://www.hou.usra.edu/meetings/lpsc2015/authorindex.cfm
(asterisk * denotes speakers)
==endquote==
I was skeptical at first when I read paraphrase and ostensible quotes from Nathues---sounded like outgassing---geysers, gushers, gassers, towering reflective clouds---I couldn't believe. But having come to respect Emily Lakdawalla's take on the Ceres news, I want to consider this a serious still-to-be-confirmed possibility.

 

[marcus]

Emily also reports that Ceres has been divided up into quadrants or tracts which have been named after other cultures' harvest gods.

I checked up on the Mayan god "Hobnil" http://web.raex.com/~obsidian/Mayapan.html#Hobnil
He apparently is associated with bountiful harvests and the direction East, and likes bees.
The Hobnil tract is where the double bright spot happens to be.
There are 15 tracts---two polar disks, four northern sectors, five equatorial rectangles, four southern sectors.

The gods' names are assigned roughly alphabetically. Hobnil is near Kumba the neighboring equatorial rectangle. One of the southern sectors is named Yum-Yum. (is that really the name of a deity?) Read more about it here:
http://www.planetary.org/blogs/emily-lakdawalla/2015/03191629-lpsc-2015-dawn-at-ceres.html

 

[OmCheeto]

I wonder what the angular size is now. 2arcsin(.475/78.37) in degrees
About 0.7 of a degree. Eventually the angular size should get up to around 60 degrees, if I remember right. That is about 120 times size of moon seen from Earth
What gradually changing headline makes sense to you from a communication standpoint, and perhaps journalistically?

Ah! I see you have already changed the headline. It makes good sense to me. Hope other contributors to the thread (Om, Petrich...) agree as well

I'd have thought you'd have learned by now, not to ask for my opinion.

Given the number of threads devoted to the Dawn mission, maybe we should number them.

06/15/2007 Liftoff for Ceres and Vesta (July 2007): Dawn mission, Pt 1, Liftoff, 2007, destinations: Vesta and Ceres
10/23/2010 https://www.physicsforums.com/threads/ion- spacecraft -sidles-up-to-vesta.440886/: Dawn mission, Pt 2: Ion driven spacecraft , sidles up to Vesta
07/06/2011 First orbit of an asteroid this month (around Vesta, with ion drive): Dawn mission, Pt 3: Pre 1st orbit of Vesta
08/16/2011 Flyaround video of Vesta as seen from low orbit by Dawn: Dawn mission, Pt 4: Video low orbit of Vesta
01/17/2015 Dawn distance to Ceres ≈ distance to moon: Dawn mission, Pt 5: Approaching Ceres
01/19/2015 Ceres at 78000km, 126% size of full moon: Dawn mission, Pt 6: The PF Dawn Nerd Party* of 11, assembles
01/27/2015 New closer shots of Ceres: Dawn mission, Pt 7: {we might want to merge this into part 6}

honorable mention:
01/22/2014 Water vapor observed venting from Ceres: Dawn mission, Pt 4.5: Herschel observes H2O venting from Ceres. Another task for Dawn.

For example, if the spectrogram shows sharp spikes at 557 GHz and 1113 GHz, that means you are seeing water.

*Current "Party", in order of appearance: marcus, _Adam, OmCheeto, phyzguy, mfb, mheslep, Delta31415, (Dr. Marc Rayman, in absentia. Though he said he visited this thread. Woo Hoo!), Dotini, lpetrich, wabbit

 

[marcus]

Yesterday current status gave distance 78.43 kkm today 78.37 kkm, so it confirms that turnaround happened even if numbers slightly off from what Rayman posted...

Just checked some 9 hours later and it said 78.25. Here's Om's annotated trajectory to have handy for reference:

You can see that the probe scarcely moves for several days at the crest of its first orbit. Only starts diving back towards the planet around the day labeled 24 March. Even then daily progress looks pretty slow until around 31 March.
At some point they will turn thruster around because they don't want to arrive TOO fast. I don't know when that flip around will come. Maybe someone knows, or maybe we can tell by looking at the current status view.

At this point current status has been showing Dawn "sun-outwards" from Ceres, with its blue-green ion "tail" pointed outwards---thrusting towards Ceres. Anybody know what day in April they will flip it around?

When they get within that 14 kkm "RC3" orbit range they have to be going slow enough that they can shape the trajectory into a circular polar orbit. And it is critical that the RC3 orbit plane be at least 5 degrees off the direction to Sun! Sun direction can't be IN the orbit plane or they lose sunlight power. Sun direction should be 5 degrees to the "left" or rearwards of Dawn's polar orbit plane. From the lower Om diagram, it is NOT the way things would naturally work out, because Dawn is approaching Ceres from slightly forwards (mainly from outwards clearly, but also from slightly forwards of the planet).

 

[marcus]

In this (impressionistic, not completely reliable in detail) youtube that Om posted earlier, I see them beginning to redirect thruster around 4 April, at first pointing tail downwards (Ceres south) and then by about 10 April having it fully forwards to slow down

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[marcus]

http://neo.jpl.nasa.gov/orbits/fullview2.jpg shows a tiny bit of Ceres beginning to emerge from behind the solar panel wing of Dawn.
It is emerging on the left side of the second panel from the end.

Distance is currently reported to be 77.55 kkm, which still rounds off to the 78 kkm we have in the headline.

The fullview2 simulated view is 30 degrees wide. When will Ceres completely fill the width of the simulated view?

At this point the interesting visual size comparison is perhaps simply the angular diameter.

I wonder if it wouldn't be a good idea, now or later, to simply forget about the full moon comparison because if the simulated view is at all realistic the Ceres disk is going to start filling more and more of the view frame. One could simply start giving the angular size in degrees.

2arcsin(.475/77.55) in degrees = 0.702 degrees