Dawn taking chem makeup data at 385 km

Discussions ranging from space technology, near-earth and solar system missions, to efforts to understand the large-scale structure of the cosmos.

Re: Ceres 280% moon-size, speed rel. Ceres 80 m/s

Postby Darby on February 23rd, 2015, 4:53 pm 

Sorry about the idiomatic English jargon.

CoB = Close of Business ... aka, the end of the typical daytime working day on a weekday. For most, that's usually something like 5pm local time.
Darby
 


Re: Ceres 280% moon-size, speed rel. Ceres 80 m/s

Postby Marshall on February 23rd, 2015, 5:21 pm 

Darby » Mon Feb 23, 2015 1:53 pm wrote:Sorry about the idiomatic English jargon.

CoB = Close of Business ... aka, the end of the typical daytime working day on a weekday. For most, that's usually something like 5pm local time.


Of course! I was just a bit slow, not to catch it.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Marshall on February 23rd, 2015, 5:35 pm 

Since we just turned a page, I'll bring forward the current status view for easy reference
Image

As of right now, 1PM pacific on 23 Feb (or 21 UT), Dawn is at right angles from the Ceres-Sun line so it sees a "half-Ceres" like the half moon seen from earth except looking about 3 times bigger than our half-moon---filling more of the sky.

the sun is off to the left. Because the probe's solar panels are facing the sun, we view them edge-on, not illuminated, so effectively we don't see the panels.

In the past few hours the speed has dropped from 175 to 174 mph, IOW it is down to 77.8 m/s.

It has to get down to around 46 m/s by 6 March.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Darby on February 23rd, 2015, 9:31 pm 

The MD confirmed what I'd speculated last week about single shift non-weekend scheduling ...

Image releases will occur only on normal NASA/JPL work days, which do not include weekends. As I have explained in comments below, there are many steps in the process to release them, both in the US and Germany, so RC2 pictures will not be available this week. They have not yet even made their way from the asteroid belt to Earth yet (and it is after 11:00 pm PST on Thursday).

Dawn is budgeted for single shift operations.
Darby
 


Re: Ceres 280% moon-size, speed rel. Ceres 80 m/s

Postby BurtJordaan on February 24th, 2015, 12:43 am 

Marshall » 23 Feb 2015, 22:47 wrote:To get from 175 down to 103 by 6 March.

Yup, and that's the job of gravity assist now that Dawn is climbing away from Ceres. I think thrusting plays only a minor role in this 'scrubbing' of relative speed.
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Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Marshall on February 24th, 2015, 3:58 am 

Yup is right! :^) as you suggest, obviously gravity will do a lot of the scrubbing. in combination with thrust of course. since you raise the issue of comparative importance let's calculate how much has to be scrubbed.
right now I see current status says 76.9 m/s speed and 38.7 kkm . that has to get down to about 46 m/s at 61 kkm, as I recall.
So the scrubbing work is (76.9^2 - 46^2)/2 joules per kilogram = 1899 J/kg

On a per unit mass basis, that is the amount of kinetic energy that has to be dumped somehow.

On the other hand the change in gravitational potential is
943e18 kg (G/38700 km - G/61000 km) in J/kg which gives 594.5 J/kg

That is the part of the scrubbing that gravity will do and it is substantial, almost a THIRD
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Marshall on February 24th, 2015, 4:09 am 

Correct me if I'm wrong, but I wouldn't call the part that thrust plays minor either.

Since I can't always trust memory let's see what escape velocity is at distance 61000 km which I think is where Dawn will be on 6 March.

as always, the planet mass is 943e18 kg

(2*G*943e18 kg/61000 km)^.5 = 45.4 m/s well close enough
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby BurtJordaan on February 24th, 2015, 7:12 am 

Yea, right. I completely underestimated the capability of that long-playing ion drive!

Thanks for doing the sums. Goes to show that gut-feels are often dangerous...
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Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Marshall on February 24th, 2015, 2:23 pm 

I had more or less the same intutive assessment as you until I put the relevant stuff in google calculator.
And I'm still a little surprised and suspicious ("did I do something wrong? leave something out?") that the probes own propulsion system contributes the larger share of the slow-down.

Be that as it may, I wanted to say something a bit off-topic about google calculator. I think it's kind of "democratic" or and "equalizer", because people who don't know the values of G, or the boltzmann constant k, or c, or the mass of the sun, or how big a Megaparsec is in kilometers, can still easily do calculations and convert units just by typing into the google box that we use for other stuff as well---searches , translation, news, images.

So it is a familiar universally accessible thing that brings hands-on physics calculation closer to everybody. You don't even have to own a slide-rule or pocket scientific calculator.

But look! see what happens when I type in: "68 km/s per Mpc"

I get:
68 ((kilometers / second) per Mpc) = 2.20372992 × 10-18 hertz

In other words, when I type in the conventional Hubble constant----a common type of physical quantity--a growth rate---I get 2.2 attohertz.

In other words the metric system thinks that an important type of physical quantity, growth rate, is measured in hertz. Or at least the google calculator does :^)

So is this something to object to? should we urge the Metric governors (CIPM, BIPM) take it up at their next meeting? Or should we just accept it? and broaden our feeling about what a hertz means.

Maybe it is a measure of any kind of count per unit time.

There is ALREADY A MAJOR AMBIGUITY in the hertz unit because sometimes it means "cycles per unit time" and sometimes it means "radians per unit time". In a lot of physics angular frequency dominates over cyclic frequency. That is what "h-bar" is about and h-bar seems more commonly used than h, in a lot of contexts. h-bar relates angular frequency to energy.

Maybe the CIPM governors should declare separate names? gertz for growth rate, hertz for cyclic frequency, jertz for events per second, and kertz for angular frequency.

Should we take this up with google? Tell them to stop using Hertz in their calculator and aways say "s-1" instead. Everything becomes "per second".
But then when you try to use metric prefixes you get "kilo per second" and "atto per second" Maybe they could call it "pertz". then in metric-speak the Hubble growth rate is currently 2.2 attopertz :^)

Or should we just mentally adapt to a more flexible usage? Maybe both---adapt for the time being and hope for some kind of resolution of ambiguity at the CIPM level, eventually.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Marshall on February 24th, 2015, 5:14 pm 

geekC.jpg
this just posted on Twitter. Is it from RC2 shoot on 19 Feb? Or is it just a repost of a picture from RC1 on 13 Feb with maybe a little numerical sharpening?
In any case the real RC2 images should be on line by close of business today 24 Feb.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby Darby on February 24th, 2015, 9:11 pm 

Looks like a cropped repost of the 2/12 pic to me. Still no sign of the 2/19 pics.

Hopefully tomorrow.
Darby
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 24th, 2015, 11:40 pm 

Dawn passed its closest to Ceres (for a while) a couple of days ago and for the next couple of weeks the distance will be increasing slowly. The aim now is to achieve capture---to get its speed down to 46 m/s by the time it is some 61 kkm out from Ceres. As distance increases, the speed (now 74.7 m/s) will slow down. Dawn's speed is now the thing to watch.

In the current status picture you can begin to see the dark blue UNILLUMINATED side of the solar panels.
A couple of days ago Dawn's view of Ceres was 90 degrees from the Ceres-Sun line, so with the panels facing the sun we saw them edge on, and they were barely visible.
Now the Dawn Ceres Sun (DCS) angle is MORE than 90 degrees and we see slightly less than half of Ceres lit. Accordingly we see the backside of the solar panels.

Whoever does these simulated views is currently taking considerable care with them to make them realistic.

As Dawn dawn moves in the direction that its bluegreen ionized xenon tail is pointing (to the right in the picture) we should see Ceres move left against the field of background stars. Equivalently, since Ceres is always in the center of the picture, we should see the field of stars shift right, day by day. the stars on the left of Ceres should get closer to it. this is not heliocentric orbit motion, but rather Dawn's changing angle of view as it swings around Ceres.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby BurtJordaan on February 25th, 2015, 1:05 am 

Marshall » 24 Feb 2015, 20:23 wrote:I had more or less the same intuitive assessment as you until I put the relevant stuff in google calculator.
And I'm still a little surprised and suspicious ("did I do something wrong? leave something out?") that the probes own propulsion system contributes the larger share of the slow-down.

Another calculation as a cross-check: the ion thrusters deliver about 90 mN thrust for a craft around 1200 kg. This gives an acceleration (google calc'd) of (90 x ((10^(-3)) newton)) / (1200 kg) = 7.5 × 10-5 m / s2.

Ceres's gravitational acceleration at 38700 km is about (G * 943e18 kg) / ((38 700 km)^2) = 4.2 × 10-5 m / s2. Same order of magnitude. Although not accelerations quite inline with each other, orbital mechanics above escape speed are such that it boils down to the same thing.
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Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 25th, 2015, 1:25 am 

Good to have the cross check! thanks. and as the distance increases up to 61 kkm the gravitational slowing will decline, whereas now it is fairly comparable to the ion thruster's contribution. It makes sense. And since we turned a page maybe I'll bring forward that dateline labeled trajectory to have handy for reference
Om.jpg

The upper panel is a view from behind Ceres in the direction of its orbit motion, with sun off to the left.
The lower panel is a view looking down from above Ceres orbit plane, with sun off to the left.
Marshall
 


Re: Ceres 282% moon-size, speed rel. Ceres 78 m/s

Postby BurtJordaan on February 25th, 2015, 1:29 am 

Marshall » 24 Feb 2015, 20:23 wrote:There is ALREADY A MAJOR AMBIGUITY in the hertz unit because sometimes it means "cycles per unit time" and sometimes it means "radians per unit time". In a lot of physics angular frequency dominates over cyclic frequency. That is what "h-bar" is about and h-bar seems more commonly used than h, in a lot of contexts. h-bar relates angular frequency to energy.

Maybe the CIPM governors should declare separate names? gertz for growth rate, hertz for cyclic frequency, jertz for events per second, and kertz for angular frequency.

I would rather support more description in the text, while sticking to hertz. Otherwise it becomes like learning a new language with all the variants for the same basic concept. Many times it comes from the context, but where it may be ambiguous, I think it is better to add the extra words (e.g. expansion rate in hertz).
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Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 25th, 2015, 1:46 am 

That's not a bad point! Just add a few words parenthetically to explain what we're doing. Keep all the vocabulary familiar, and after all I don't foresee having to use hertz a lot to stand for a "s-1" expansion rate.
So much of the time we just say things like "1/173 % per million years" rather than "1.83 attohertz"

and "1/144 percent per million years" instead of "2.20 attohertz"

Everybody knows percents and can picture a distance growing by some percentage. And it immediately makes clear how slow it is (compared with growth we are used to seeing)

I got attracted to the metric unit and the metric prefix for billionth of a billionth (ie.10-18) when I read a research paper mentioning the cosmological constant's value of 1.0 x 10-35 s-2

Maybe it is the sort of thing you only get into in an optional section of a chapter in an introductory textbook. It is extra reading nothing in later chapters depends on it. But it's interesting. Have to go, wife wants to watch the news.
Marshall
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Darby on February 25th, 2015, 11:25 am 

From 2/19:

Image

You can definitely see its some sort of ice (water or CO2) in the bottoms of craters now.

Oh, and check out the very large and old impactor in the lower front face of the left image ... old enough to have almost completely filled back in over the eons, and large enough to have very nearly destroyed the entire body. Reminds me a bit of Mimas. They're definitely going to name that one. If my eyes dont deceive me, but there appear to be hints of a heavily worn rift canyon structure protruding from the 2pm and 7pm positions on that crater as well ... possibly signs of huge impact cracking that has since filled in. Too soon to tell.
Last edited by Darby on February 25th, 2015, 11:35 am, edited 2 times in total.
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Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 25th, 2015, 11:30 am 

Today is one of the planned optical navigation photoshoots.
Code: Select all
Jan 13  (383,000) 27  (36) 0.83 95% OpNav 1
Jan 25  (237,000) 43  (22) 1.3 96% OpNav 2
Feb 3   (146,000) 70  (14) 2.2 97% OpNav 3
Feb 12  (83,000) 121  (7.8) 3.8 98% RC1
Feb 19  (46,000) 221  (4.3) 7.0 87% RC2
Feb 25  (40,000) 253  (3.7) 8.0 44% OpNav 4
Mar 1   (49,000) 207  (4.6) 6.5 22% OpNav 5
Apr 10  (33,000) 304  (3.1) 9.6 18% OpNav 6
Apr 15  (22,000) 455  (2.1) 14 50% OpNav 7

http://dawnblog.jpl.nasa.gov/2015/01/29 ... anuary-29/
I just checked the current status view and it showed the probe with thrust off, oriented for picture-taking:
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Presumably we should see an antenna receiving data later today at DSN
https://eyes.nasa.gov/dsn/dsn.html
Marshall
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Darby on February 25th, 2015, 11:31 am 

Crossed posts. ;)
Darby
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 25th, 2015, 11:50 am 

Thanks for posting the new pictures! Sorry about the crossed posts. Now that we have the 19 Feb images my news seems irrelevant. I think what they are taking now is just for navigation purposes.
The table does give some numbers about the RC2 shoot however.
It says that the resolution is 4.3 kilometer per pixel
So that refers to the two images you just posted.
I tried enlarging them on the computer screen and it began to look quite interesting as I zoomed in.
Satisfying amount of detail.
The white stuff in some of the craters does look like ice

I see in the table that today's photos are supposed to have even higher resolution, something like 3.7 km per pixel. a modest improvement. But the phase angle means the planet is only 44% illuminated. I don't know if they will post today's shots for us to see, may only use them for navigation. What would you guess?
Marshall
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Darby on February 25th, 2015, 12:04 pm 

No idea.
Darby
 


Re: Dawn's speed rel. to Ceres 75 m/s at range 39 kkm

Postby Marshall on February 25th, 2015, 12:13 pm 

The canyon projections at 2pm and 7pm from the big old crater in the left picture do seem to line up in a single straight line as if parts of an ancient giant crack.
Marshall
 


Re: Dawn taking pictures, speed rel. to Ceres 74 m/s at rang

Postby Darby on February 25th, 2015, 12:31 pm 

I'm also seeing signs of a long Y-shaped ridge in the upper right quadrant of the left most image, at about the 12-2pm position, just sunward of where things fall into shadow ... could be another ancient crack or crustal shift structure dating back to the same or a different giant impactor. Again, too soon to tell, at this resolution, and from these angles.

I'd be interested in seeing a real planetary geologist weigh in. I'm just an armchair quarterback on this subject.
Darby
 


Re: Dawn taking pictures, speed rel. to Ceres 74 m/s at rang

Postby Marshall on February 25th, 2015, 1:04 pm 

Mission Director Marc Rayman just posted his Dawn Journal for February, featuring the new pictures,
http://dawnblog.jpl.nasa.gov/2015/02/25 ... bruary-25/

There's an updated table that lists the photoshoots:
with minor updates. Here are the column headings.
Beginning of activity in Pacific Time zone
Distance from Dawn to Ceres (kilometers)
Ceres diameter in pixels
Resolution (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk
Activity
Code: Select all
Dec 1  (1.2 million)  9   (112)  0.25   94%   Camera calibration
Jan 13   (383,000)   27   (36)   0.83   95%   OpNav 1
Jan 25   (237,000)   43   (22)   1.3    96%   OpNav 2
Feb 3    (146,000)   70   (14)   2.2    97%   OpNav 3
Feb 12   (83,000)   122   (7.8)  3.8   98%   RC1
Feb 19   (46,000)   222   (4.3)  7.0   87%   RC2
Feb 25   (40,000)   255   (3.7)  8.0   44%   OpNav 4
Mar 1    (49,000)   207   (4.6)  6.5   23%   OpNav 5
Apr 10   (33,000)   306   (3.1)  9.6   17%   OpNav 6
Apr 14   (22,000)   453   (2.1)  14   49%   OpNav 7
Marshall
 


Re: Dawn taking pictures, speed rel. to Ceres 74 m/s at rang

Postby Darby on February 25th, 2015, 1:59 pm 

Looks like we're at the closest approach until mid april, so this may be it until then on dramatically improved imagery.

We'll just have to content ourselves with watching the talking heads jabber about it until then.
Darby
 


Re: Dawn sending pictures, speed rel. Ceres 73 m/s @ 40 kkm

Postby Marshall on February 26th, 2015, 2:53 am 

http://www.planetary.org/blogs/emily-la ... ology.html

lengthy discussion comparing ceres geology (preliminary guesses) with that of other bodies (Rhea, Oberon, Mimas, Tethys...)
remarking on various features, the large flat crater, ...
==excerpts==
Ignore the bright spots, and what do I notice next? The craters. There are lots of them, as there are lots on all the icy moons. Many of the smaller craters on Ceres are bowl-shaped, but the bigger ones have noticeably flat floors, like the ones on Tethys and Rhea. In the picture above, the two craters on the left, (e) and (f), look a lot like those on Rhea or Dione: they have steep rims, but flat floors, a hint of a central peak, and a hint, also, that the craters aren't perfectly round. On Rhea and Dione, we think that the non-roundness of the craters is caused by the existence of fractures in the crust; those fractures were zones of weakness, and the rupturing of the impact crater followed those weaknesses, making straight rim segments.

But there's one crater on Ceres that made my jaw drop: it's the big one at the center of the view above, (g). It's incredibly flat. You can barely see its rim. Its interior is very smooth and lacks any mid-sized craters, though there are many small ones. On the Max Planck website, they go so far as to say that the large crater is relatively recent, because it contains no middle-sized craters, only small ones.

The main question about such a flat crater, other than its age, is: was it born that way, or did it become flatter over time? On icy worlds, initially deeper craters become shallower when, over geologic time, the warmer ice in the world's interior flows from high places to low places, raising crater floors. But a layer of ice at the surface is so stiff that it still preserves the pre-impact topography. That's how Tethys' huge Odysseus basin still has a rim, a peak ring, and a central peak, even though its floor has risen over time so that it is now convex, continuing the curvature of Tethys' globe rather than concave, like the original bowl shape of an impact crater:

But Ceres' largest visible basin is much, much smoother than Odysseus. There is no pre-impact topography present -- no peak, no peak ring. What happened to the topography? Was it covered up by a volcanic flow, like lava flows filled in the lunar impact basins? Or was there never much any topography in the first place, because the impact happened long ago into a Ceres with a thin crust and a subsurface ocean? These kinds of features are called palimpsests; they don't exist on the mid-sized outer planet moons, but you see them on Europa and Ganymede. The round basin on Ceres is so incredibly smooth that I'm inclined to think it's been filled in, like the terrain in the photo of Triton below, but -- like I said before -- I'm going to remain doubtful of volcanism until I'm left with volcanism as the only interpretation. And we haven't gotten there yet.
===endquote===
Marshall
 
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Re: Dawn sending pictures, speed rel. Ceres 73 m/s @ 40 kkm

Postby Darby on February 26th, 2015, 4:29 am 

Yes, it seemed like a given that the big impactor was going to stoke some discussion. Nobody's added anything really insightful yet however.
Darby
 


Re: Dawn back to normal, speed rel. Ceres 71.5 m/s @ 41.2 kk

Postby Marshall on February 26th, 2015, 8:43 am 

An additional view, the original "bright spot" resolves into two:
Image
Darby » Wed Feb 25, 2015 8:25 am wrote:From 2/19:

Image

You can definitely see its some sort of ice (water or CO2) in the bottoms of craters now.

Oh, and check out the very large and old impactor in the lower front face of the left image ... old enough to have almost completely filled back in over the eons, and large enough to have very nearly destroyed the entire body. Reminds me a bit of Mimas. They're definitely going to name that one. If my eyes dont deceive me, but there appear to be hints of a heavily worn rift canyon structure protruding from the 2pm and 7pm positions on that crater as well ... possibly signs of huge impact cracking that has since filled in. Too soon to tell.
Marshall
 


Re: Dawn back to normal, speed rel. Ceres 71.5 m/s @ 41.2 kk

Postby Marshall on February 26th, 2015, 7:34 pm 

Faradave, Darby, Jorrie,
can anyone say offhand how high one could jump, on Ceres? I assume radius 475 km and mass 943 billion billion kg. So surface gravity is 1/35 of our 9.8 meters per s^2. The simple answer is however high you could jump in standard Earth gravity you could jump 35 times higher.
Several sites on WWW use that simple reasoning applied to various solar system bodies. jump height inversely proportional to surface gravity. But I also found some differences of opinion as to how to analyze the problem.

On a different topic, over the next ten days the critical thing to watch is the meters-per-second speed of the probe. In order to achieve capture by 6 March it has to get its speed down to around 46 m/s ---then it will actually be bound in orbit rather than just a "fly-by" visitor.
From today (26 Feb) to 6 March, its distance increases from 41 to 61 kkm the speed should decrease roughly as an acquaintance named Petrich calculated. I've adapted his numbers

The table goes as far as 12 March, but the window only shows a limited segment, so you have to SCROLL DOWN.

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, approximately 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.
distance from Ceres continues increasing for a while because the probe has some excess momentum
vescape is the escape velocity at that given distance
vprobe is the predicted velocity the probe will actually have that day. It must fall below vesc to achieve capture.
Code: Select all
date      X          Y          Z        distance  v_esc  v_probe
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    56.67    71.36
F26     11.407    26.4613    -29.1488    40.98    55.48      69.67
F27     17.2899    27.6663    -28.1919    43.11    54.10     68.44
F28     22.8583    28.5286    -27.0313    45.46    52.68    64.25
M1      27.9985    29.1842    -25.6846    47.90    51.32     60.73
M2      32.8862    29.7513    -24.1873    50.51    49.98     58.67
M3      37.6439    30.1647    -22.7166    53.31    48.65     55.28
M4      41.9734    30.4246    -21.3167    56.05    47.44     50.18
M5      45.8274    30.5605    -19.8726    58.55    46.42     46.96
M6      49.5028    30.6491    -18.2955    61.02    45.47     44.35
M7      52.8252    30.4896    -16.7451    63.24    44.66     40.49
M8      55.7681    30.3242    -15.1946    65.27    43.97     37.71
M9      58.5427    30.0761    -13.6441    67.21
M10     58.5427    30.0761    -13.6441    67.21
M11     63.2886    29.4796    -10.5963    70.61
M12     65.1256    29.1961    -9.32939    71.97


The table starts 17 Feb and it shows the probe overshooting Ceres in the X direction (it has not had time to slow its X motion and will need Ceres gravity to pull it back in line)
likewise it shows the probe overshooting in the upwards Y direction.
Ceres' orbit inclination is 10 degrees and it just recently passed its descending node, so it is sloping down relative to the ecliptic and to Dawn's prior orbit.
This gives Dawn some unwanted speed in the "up" or Y direction. Again Ceres gravity will help draw the probe in.
Marshall
 


Re: Normal thrusting, speed rel. Ceres 69 m/s @ 42.5 kkm

Postby Marshall on February 26th, 2015, 11:17 pm 

The pictures we just saw, a post or two back, were from the 19 Feb shoot. The pictures from the 25 Feb shoot have been taken and seem likely to be shared with the public. I think we can expect to see the next few shots even though they are only taken for OPTICAL NAVIGATION purposes. The positions of known stars in the background of Dawn's view of Ceres tells crew the probe's direction relative to planet.
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Feb 12   (83,000)   122   (7.8)  3.8   98%   RC1
Feb 19   (46,000)   222   (4.3)  7.0   87%   RC2
Feb 25   (40,000)   255   (3.7)  8.0   44%   OpNav 4
Mar 1    (49,000)   207   (4.6)  6.5   23%   OpNav 5
Apr 10   (33,000)   306   (3.1)  9.6   17%   OpNav 6
Apr 14   (22,000)   453   (2.1)  14   49%   OpNav 7

So at some point, either tomorrow (Friday 27 Feb) or perhaps early next week e.g. Monday 2 March, we can expect to see the results of the 25 Feb shoot, a 44% illuminated "crescent" shape.

I was trying to figure out what the "RC" designation might stand for. It might indicate that the purpose (besides navigation) is to accurately determine the planette's axis of rotation. So RC could stand for "rotation coordinates" or the like.

The calculated speed is already down to 69.3 m/s at a distance of 42.5, according to current status

BTW check out the Wikipedium on "Colonizing Ceres" if you are at all interested in that sort of thing.
there are arguments to the effect that Ceres could have important advantages over either Moon or Mars as a colony outpost. And the article reviews some of the desirable features.
Marshall
 


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