The Dawn spacecraft successfully observed Ceres from a distance of 238,000 miles on January 13, 2015, capturing over half of its surface at a resolution of 27 pixels. The mission aimed to enter a polar orbit around Ceres, with a planned descent to an altitude of 375 km, but faced challenges due to limited hydrazine propellant for attitude control. A cosmic ray event in September 2014 had previously disrupted the propulsion system, complicating the approach trajectory. Despite these issues, the spacecraft was expected to achieve a stable orbit around Ceres, ultimately becoming a "perpetual satellite" as it ran out of fuel. The mission's success would provide valuable data on Ceres' physical characteristics and surface mapping.
#181
OmCheeto
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marcus said:
Yes! Orion is getting more completely visible---three of the four corner stars and the belt.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Considering the phase of Ceres and our looking approximately from Dawn's perspective, it seems to me that the sun ought to be in the picture---towards the upper left corner of the picture. Because the thin sliver of Ceres that is lit is on the upper left. http://en.wikipedia.org/wiki/Orion_(constellation)
Another possibility is that the Ceres-Sun line makes a 20+ degrees angle with the Dawn Ceres line, which would put the Sun out of the box, to the upper left. The greater that angle the more of Ceres should be shown as lit. (If it is zero degrees the sun is directly behind the planet.)
The very thin edge phase doesn't quite make sense to me.
Current status (11am pacific on 4 March) gives the speed relative to Ceres as 51.4 m/s at 57 kkm.
From my eyeball attempt, here's where I think the Earth, Sun, and Mars should be in the (lower) image. All based on relative positions to the stars in Orion from the upper image.
The sun is a bit off, as I placed Mars first, Earth 2nd, and I knew the Sun should fall in a straight line between them.
But close enough for me.
I might be off but it looks to me like the illuminated part goes something like 10% of full diameter, if true that would be a phase angle of ~acos(0.8)~37° which would indeed place the Sun well outside the box to the upper left.
Om, it's a bold attempt! Wabbit, good thanks.
In the simulated view of the sun
http://neo.jpl.nasa.gov/orbits/fullview1.jpg
there are two bright stars down near where it gives the 17 km/s speed
I think they are Betelgeuse and Bellatrix. Over 20 degrees would then be right.
Pretty sure they are Betel and Bella because the horns of Taurus are up and to the right of them.
That frame is a 45 degree view. So half width would be a bit over 20 degrees.
Agreed, that's right where they should be in relation to Taurus. This appears to confirm the "off, upper left Sun" relative to the previous picture unless I'm linking them together wrong.
Your figure of 37 degrees is now making very good sense. I put http://neo.jpl.nasa.gov/orbits/fullview1.jpg
and http://neo.jpl.nasa.gov/orbits/fullview2.jpg
together and see a straight line from the Sun through Bellatrix to Ceres (from Dawn's perspective).
Bellatrix is in both pictures, in http://neo.jpl.nasa.gov/orbits/fullview1.jpg it is the bright star near the "s" in "17 km/s" at the bottom edge of the frame.
Very roughly there is about a 20 degree angle between the Sun and Bellatrix in http://neo.jpl.nasa.gov/orbits/fullview1.jpg which is a 45 degree width view.
Then if you continue from Bellatrix to Ceres in http://neo.jpl.nasa.gov/orbits/fullview2.jpg there is about another 20 degrees (in that 30 degree width view). So it makes a roughly 40 or as you said 37 degrees, from the Dawn Ceres line. Beautiful, I'm glad to see some consistency in the simulated views!
I'm glad you set me straight on the phase angle. I wasn't thinking
Since we turned a page I'll bring forward the currently relevant part of the Petrich table shown in post#148, for easy reference.
The probe has been "retro" thrusting in an effort to slow itself down to around 46 m/s by 6 March
In current status view it looks to me like the thruster beam is pointed straight out ahead in the direction the probe is going.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Current status says speed is now 50.5 m/s at distance of about 57.7 kkm (as of 5pm pacific on 4 March) so that actually agrees fairly well with what the table says for 4 March. The distance is a bit more than the table projected it would be. That makes escape speed lower and makes it harder for probe to get UNDER escape speed, and thus achieve capture, but things still look OK to me as nonexpert onlooker.
Some explanation:
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.
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#188
OmCheeto
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marcus said:
Om, it's a bold attempt!
...
Well, for someone who's never even heard of Auriga before, I have to agree.
This paper makes the case for further exploration of Ceres--after the Dawn mission. http://www.lpi.usra.edu/decadal/sbag/topical_wp/AndrewSRivkin-ceres.pdf
Andrew Rivkin is the lead author. plus some 28 or so others. It's a report to a planning committee:
"We recommend Ceres be considered a candidate for a New Frontiers mission in the 2015-2022 timeframe, with mission architectures to be studied based on results from Dawn and other sources..."
Nearing CAPTURE! speed 46.5 m/s at 61 kkm (where escape is 45.4, so only 1 m/s left to scrub)
Current status as of 7am UT on 6 March (11pm pacific on 5 March) shows speed rel Ceres 46.5 m/s @ distance of 61.0 kkm. This is very close to what is required for capture into orbit.
Escape speed at that distance is
(2G*943e18 kg/(61000 km))^.5 = 45.4 m/s
So if the probe were going a mere 1 meter per second slower, at this point, it would already have achieved capture.
The constellation orion is fully visible in the background to the upper left of Ceres.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Downlink has begun! We're analyzing the signal to confirm that the spacecraft is healthy and in orbit at #Ceres
Goldstone #14 has activity.
fullview2.jpg gives 45.6m/s which is slightly above the escape velocity estimate (now 45.2m/s extrapolating from marcus' post), but with the 3% mass uncertainty we had for the calculations the difference is meaningless. Also, the image is two hours old and still shows Dawn thrusting.
It just took us a while to hear the news. Dawn had to communicate with the DSN antenna at Goldstone and so on.
See Mfb's post #196 of 6:15am pacific time. He was right on top of it.
It just took us a while to hear the news. Dawn had to communicate with the DSN antenna at Goldstone and so on.
See Mfb's post #196 of 6:15am pacific time. He was right on top of it.
It just took us a while to hear the news. Dawn had to communicate with the DSN antenna at Goldstone and so on.
See Mfb's post #196 of 6:15am pacific time. He was right on top of it.
Well, that'll teach me not to take a nap in the middle of the morning.
I was up doing calculations, and predicted it wouldn't take place until about 10 am pacific time, based on our predicted numbers and an equation at wiki, Precise orbital speed, which I was able to manipulate to calculate the length of the semi-major axis of the elliptical orbit, a.
a = \frac {1} { \frac {2} { r} - \frac {v^2}{ \mu } }
Which switches from negative to positive at the moment of capture.
y axis = semi-major axis length in thousand of kilometers
I'm not sure why the graph gets kind of goofy from here on out.
March 10th thru April 23rd
Probably has something to do with rocket science.
I'll have to document speed and distance, from here on out, to see if this is correct.
OmCheeto, you are better off calculating a's reciprocal, or else the total energy per unit mass, - μ/(2a). At the capture point, it will be zero instead of infinity.
Nice work Om! Petrich it is a credit to your table of numbers that Om could predict within a few hours. The difference between the projected and the actual seems to have been within reasonable uncertainty---not "statistically significant". that's great. I'm really happy with your and Om's numerical work which (albeit a small part) is, for me, a part of the accomplishment. Also Mfb's astute commentary. Cheers all round!
escape velocity (2G*943e18 kg/(62230 km))^.5 = 44.97 m/s
so probes actual speed is just a hair under what it would need to get away from Ceres. It has achieved orbit.
This view records the moment of capture. I uploaded it from desktop instead of inserting it as an image so that it would not get updated further and that particular moment lost.
If you want this image to share with friends and family just go to
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
and drag it to desktop. Ceres is a really interesting world and will IMHO become more so as we learn more about it. Also what you see in the picture is one of ourkind's first solar electric powered (ion drive) spacecraft .
Om, nice star map! It shows the sky from the Dawn probe's perspective, with Ceres in it AND the Sun in the position it would be against the background stars seen from the PoV of Ceres and Dawn. And there's Orion. I wish the NASA Dawn public outreach people had included a wide angle view like that which puts it all together.
I've been wondering about the possibility that Ceres might have some recoverable amounts of nitrogen. Moon doesn't seem to have nitrogen
but Mars atmosphere has 2.7% N2. https://books.google.com/books?id=YQ3rAQAAQBAJ&pg=PA324&lpg=PA324&dq=nitride+abundance&source=bl&ots=2hE3hrgoSG&sig=Uuo9HqhkYCuw5GwAXaSdacfGPao&hl=en&sa=X&ei=ikb6VNjREdKyogT2goLwBA&ved=0CC0Q6AEwBA#v=onepage&q=nitride abundance&f=false
Silicon Nitride (Si2N3) occurs as pre-solar dust particles in meteorites, apparently the result of supernova explosions.
It would have formed in surroundings that were carbon rich instead of oxygen rich. Oxygen would displace the nitrogen in the various nitride compounds.
So Si and N are abundant elements and you get a mineral dust under the right conditions. Then when a planet forms and heats up, nitrogen can OUT-GAS. and form part of the atmosphere. but if the atmosphere is hot (Venus) or the gravity is weak (Mars) most of the nitrogen can get escape velocity and blow off into space. You outgas it from bound mineral form (if the rocky material melts) and you keep the bulk of it in atmosphere if conditions are just right.
I suppose there is a chance that because some Ceres rock didn't get hot enough to outgas it may still contain MINERAL NITROGEN.
Could there be ammonia or ammonium salts dissolved in Ceres water?
Om it looks like your view (of the inner solar system from Ceres and Dawn standpoint) is a 50 degree by 40 degree frame (very rough, round numbers)
So that would make the spread between Sun and Mars a little over 1/3 of 50? Like a little over 17 degrees? 20 degrees?
So one half of the vertical height of the frame, which SAY is 40 degrees? Let's check that against the inner solar system diagram they give us. If you draw lines from the white dot labeled Ceres (down at the bottom) to the Sun and to Mars, would the angle between the lines be about 20 degrees? If so that sort of confirms the guess that the view you made is 50 degrees wide.
http://neo.jpl.nasa.gov/orbits/fulltraj.jpg
At the moment it looks about right to me. Of course in a few weeks the planets will have moved and they update these views so the angles will change, so it won't be 20 degrees any more. But that's irrelevant.
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#210
mheslep
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The AP has a nice photo of the Dawn spacecraft pre-launch. Ion drive, panels, etc all shown closeup.
