Dawn Cruise Options

[SkyeLab]

Pushing out of Earth orbit now...........

[elakdawalla]

loon reports from the Cape that AOS happened about 2 hours after launch, at 9:44 EDT. Yay!

Emily

[ustrax]

Horst Uwe Keller, Dawn's FC Team Leader reported to spacEurope an hour ago:

"Now we have telemetry! Everything looks OK.
Looks good. Cameras are responding (heaters)."

He added:

"We just talked about it. We (MPS) are now in charge of 6 cameras operating currently in space!
2 on Rosetta, 2 on DAWN, 1 on VEX, 1 on Phoenix and in addition the detector for the microscope on Phoenix.
Looks like a record to me."

He's happy...

[climber]

May be little OT...and that's the reason of this post actualy !
We have 8-11 spacecrafts that can be considered in "Cruise phase" at this time (including 2 bound for Mars : Phoenix and Dawn)
Do I miss any?

New Horrizons
Phoenix
Dawn
Roseta
Messenger
Hayabusa
Deep Impact
Stardust
and may be as well :
Voyager 1
Voyager 2
Ulyses

That's quite an achievment. I'm wondering if we ever had as many at the same time.
I'm also wondering if, instead of keeping them in their proper section (Mercury,Mars, Pluto, etc…), it'll not be worth to have a section in the Forum that'll show "Cruise phase Spacecrafts".

Advantage will be that we could go there and check down all status instead of having to remember which Spacecraft is where.
Just a thought.

BTW : Go Dawn, Go

[punkboi]

Godspeed, Dawn!

[dvandorn]

Well -- I missed the launch (6:34 am is a little early for me these days, and that's when she lofted by my clock), but I just saw a quick replay of the launch at the beginning of the post-launch news conference, and I have to say, that thing heeled over to the left (from the camera angle I saw) pretty good before straightening out and angling to the right onto its correct trajectory. Took off like a bat out of hell, though...

Four issues were just mentioned -- the RCS switched itself from the primary to the secondary system, for reasons yet unknown; the RCS thrusters are running colder than anticipated, which is making the software controls lock them out, but reversion to hardware controls is keeping them running -- the fix is a minor re-set of the software's criteria values; the RCS brackets are running a little warmer than normal, but are cooling down; and there is a slight difference in electricity being generated between the two solar panels.

Those are the only issues that have been discussed.

-the other Doug

[stevesliva]

Amazing! Given the incredible complexity of the post-launch deployments and sequencing in the Planetary Society Blog post by Marc Rayman, I'm amazed that so much goes on without human intervention...
http://planetary.org/blog/article/00001153/

That post has got to be one of the more informative discussions of autonomous operation that I've read. (Other than perhaps the explanations of the incidents when they have gone wrong)

[Rakhir]

We have 8-11 spacecrafts that can be considered in "Cruise phase" at this time (including 2 bound for Mars : Phoenix and Dawn)
Do I miss any?

Already forget Kaguya ?
She is still in cruise phase.
(As well as VRAD and Relay sat. unless you count them as part of Kaguya until they are released)

[Toma B]

Just as I thought...nobody in the press conference asked question that I am most interested in...
Dawn is going to visit two biggest asteroids Ceres and Vesta. It is going to enter orbit around these two but there was some words before that it can do few close flybys of some small asteroids as well.
I guess that planed flybys are all canceled because slips in launch but there should be new ones.
Does anybody know if there are any candidates?

[ugordan]

I guess that planed flybys are all canceled because slips in launch but there should be new ones.

IIRC, there never were any planned flybys. It will be determined in flight (based on current trajectory, fuel and ion engine performance) what is feasibly reachable and if it's worth the trouble. One of TPS blog entries mentions something about it as I recall.

[elakdawalla]

I guess that planed flybys are all canceled because slips in launch but there should be new ones.

Gordan's right, there were no flybys planned in advance, not only because of the uncertainty in launch date but because of uncertainty in performance of the ion engines. And there may well not be any flybys of any real quality unless they get very lucky. Unlike a flyby mission, Dawn can directly translate fuel reserves into a much longer mission at its primary targets. Which is a better use of the xenon, a relatively distant view of a small asteroid or another week spent in orbit at Ceres, or a closer orbit, or a different orbit, etc. etc.? It sounds to me like unless the orbital mechanics gods smile upon them with a really great opportunity that lies fortuitously close to the trajectory, the economics won't work out for any other flybys.

(The answer to this question was on a Planetary Radio show.)

--Emily

[stevesliva]

IIRC, there never were any planned flybys. It will be determined in flight (based on current trajectory, fuel and ion engine performance) what is feasibly reachable and if it's worth the trouble. One of TPS blog entries mentions something about it as I recall.

I recall that as well, nothing was planned or expected until launch. Similar to the New Horizons Jupiter flyby... they didn't plan the launch to coincide with anything but the primary targets.But I am definitely curious to see what they will pass by, and whether the instruments can do useful science on more distant fly-bys.

[jabe]

If specific impulse reigns supreme then why do they use Xenon instead of hydrogen

I believe it is the ionization energy as well as storage issues..An ion engine needs ions..Xenon ionization energy is lower than hydrogen so if you can use a little energy to ionize it the rest of the available energy can be used to "spit" the ions out...(As well hydrogen is a diatomic gas so electrons are used in the bonds..no free ones available to excite if my chemistry is right ) Helium would be next best but still ionization energy is high)

[Greg Hullender]

jabe: No, that's not it, but this has already been discussed at length here, and the actual answer is well worth reading.

http://www.unmannedspaceflight.com/index.p...3274&st=300

--Greg

[jabe]

jabe: No, that's not it, but this has already been discussed at length here, and the actual answer is well worth reading.

http://www.unmannedspaceflight.com/index.p...3274&st=300

--Greg

Thanks for the link..always wondered why they didn't use helium

[Del Palmer]

Latest Dawn journal:

http://dawn.jpl.nasa.gov/mission/journal_9_30_07.asp

[punkboi]

Interesting journal. Hopefully, a "Where is Dawn now?" page will be posted on the mission website soon.

[Stephen]

Interesting journal. Hopefully, a "Where is Dawn now?" page will be posted on the mission website soon.

Speaking of "Where is Dawn now?", where the "Dawn" subforum on UMSF.com? Or are we going to continue working out of this present thread from now until it reaches the asteroid belt?

=====
Stephen

[djellison]

It doesn't need a subforum until it starts doing science in a few years. It has ONE thread right now...maybe it'll get another - but a thread on it's own in a subforum would be a tumble-weed like situation. On that basis, the front page of the C & AM subforum would need to be split into 6 subforums which would be a pointless overcomplication.

Doug

[volcanopele]

I could see the creation of a new thread though

[ugordan]

I could see the creation of a new thread though

That might be a good idea, seeing as how it's finally happening and on its way. 30 pages to go through just to read the current news is a bit of an overkill

[Stephen]

It doesn't need a subforum until it starts doing science in a few years. It has ONE thread right now...maybe it'll get another - but a thread on it's own in a subforum would be a tumble-weed like situation.

I notice Phoenix isn't "doing science" thus far either yet it has a subforum all its very own.

And then there's MSL, which last I checked hasn't even been built yet, much less launched.

And of course there's New Horizons which will be spending much of the next decade in hiberation before it starts doing science at Pluto, and Messenger which is still heading Mercury-ward. True, both did do a teensie bit of science at Jupiter and Venus, respectively. On the other hand I do seem to recall them getting their own sub-forums (sub-fora?) even before they got that far.

On the other hand I notice Kayuga hasn't got a subforum of its own yet either so I guess that puts Dawn in good company.

=====
Stephen

[Greg Hullender]

I think it's simpler than that; it's because there is no "Where is Dawn" link on the page yet. Until then, they haven't officially lanched, as far as I'm concerned, and they don't deserve a subforum yet.

Perhaps we should start a new thread titled "Where is Dawn?"

--Greg :-)

[jamescanvin]

I notice Phoenix... MSL... New Horizons...

Yeah, but look at how many threads are in each of those forums, too many for me to count!

Once the threads start mounting then a case for keeping them all together in a sub-forum can be made, but that time isn't now.

James

[CAP-Team]

Here's where Dawn is now:




Earth as seen from Dawn:

[djellison]

Yeah - that pretty much sums it up. Subforums get made when they're required - not becaue of some line in the sand or event that takes place. They're made because one topic dominates another forum.

Doug

[punkboi]

Here's where Dawn is now:




Earth as seen from Dawn:

Cool- Did you create that yourself? Or which website did you get that from?

EDIT: Interesting that Phoenix has been near Earth ever since launch... I think the second TCM later this month should change that

[ugordan]

I think the second TCM later this month should change that

No, it won't. TCMs are correction maneuvers, not huge boosts.

[CAP-Team]

Cool- Did you create that yourself? Or which website did you get that from?

EDIT: Interesting that Phoenix has been near Earth ever since launch... I think the second TCM later this month should change that

Yes, I created the image with a program called xplanet (by Hari Nair) and you can use spice kernels you can download from the NASA NAIF site.
It's a bit like Celestia but for still images.

Phoenix is still relatively close to Earth because it went inwards first (thus moving faster than Earth) , it's just since last week Phoenix is starting to move out.
Phoenix and Dawn will stay relatively close to each other till Phoenix reaches Mars.

[volcanopele]

Wow, you're right, Dawn is available on the NAIF site Which means I can import it into Celestia, sweetness! Wish New Horizons was available...

[Stephen]

Yeah - that pretty much sums it up. Subforums get made when they're required - not becaue of some line in the sand or event that takes place. They're made because one topic dominates another forum.

Glad to hear there's a defensible logic behind such things. As opposed to ad hoc rules of a more arbitrary kind.

=====
Stephen

[belleraphon1]

Sweetness and LIGHT.....

First ion thruster test......

http://www.planetary.org/blog/article/00001186/

Craig

[Marz]

Marc cracks me up!!!

"The drama was captured in the stirring name of the file that was transmitted to the spacecraft: dz002e.scmf"

Truly excellent and enjoyable reading, and great news everything's checking out ship-shape. I hope he can continue keeping me entertained in the 7.4 years to Ceres... are we there yet?

[peter59]

Science Instruments Checked Out
October 15 - 19
Dawn's science instruments were powered on and given their first health checks this week. The gamma ray and neutron spectrometer, imaging camera, and visible and infrared mapping spectrometer all operated perfectly.

[peter59]

Second Ion Thruster Checkout Completed Successfully
October 22 - 26
The mission operations team completed the checkout of a second ion thruster this week. In one of the tests, the thruster was operated for 27 hours continuously at 5 different throttle levels, and in two other tests it was operated at maximum power for 4 hours each time. All spacecraft systems performed extremely well.

[nprev]

Thanks for the update, Peter. Green bird, baby, green bird...GO DAWN!!!

Slightly OT here, but has anyone else noticed that, provided that VEX, Chang'e and/or Kaguya survive, we should have active missions happening on or around all the major bodies of the inner Solar System in 2011? If the hangtime for them all extends to Dawn's arrival at Ceres, then that envelope gets pushed out to the Belt. What a heady time for UMSF!

EDIT: Forgot about Juno going to Jupiter...if everything converges right, then we've got it covered all the way out to Saturn at that time!!!

[brellis]

has anyone else noticed that, provided that VEX, Chang'e and/or Kaguya survive, we should have active missions happening on or around all the major bodies of the inner Solar System in 2011? If the hangtime for them all extends to Dawn's arrival at Ceres, then that envelope gets pushed out to the Belt. What a heady time for UMSF!

EDIT: Forgot about Juno going to Jupiter...if everything converges right, then we've got it covered all the way out to Saturn at that time!!!

New Horizons and the Voyagers will still be humming too

[punkboi]

New journal up:

http://dawn.jpl.nasa.gov/mission/journal_10_24_07.asp

[peter59]

More Ion Thrusting Tests Completed
November 12 - 16
The week-long systems test of interplanetary cruise thrusting completed successfully on Monday. The third ion thruster was tested this week, and like the other two, it performed perfectly. The thruster operated at 4 throttle levels, including full power. In a separate activity, the mission operations team powered off the reaction wheels to test pointing control with hydrazine thrusters during ion thrusting.

[punkboi]

A week late, but whatever:

http://dawn.jpl.nasa.gov/mission/journal_11_13_07.asp

Still no "Where is Dawn now?" page on the official website, eh?

[peter59]

Main Antenna Checked Out and New Software Uploaded
November 19-23
This week the spacecraft was commanded to use its main antenna for the first time and measurements showed that it is in fine condition. New software was installed in one of Dawn's computers (and its backup), correcting a minor bug that was discovered shortly after launch.

[punkboi]

"Where is Dawn now?" page finally up:

http://dawn.jpl.nasa.gov/mission/live_shots.asp

[peter59]

"Dawn has started IPS cruise thrusting to Vesta. The thrust came on at
4:08 PM pacific. We are on our way."—Project Manager, Keyur Patel 12/17/07

[punkboi]

http://www.planetary.org/blog/article/00001266/

New Dawn Journal up... Planetary.org posted calibration photos taken by the spacecraft's Framing Camera

[IM4]

Good news everyone!
Extended version of the Dawn trajectory is now available at HORIZONS (two additional segments till April 2008). I did some simulations and found that Dawn will pass within 0.048 AU from comet 79P/du Toit-Hartley somewhere around March 17, 2008. Exact date and distance depends on actual thrusting, nevertheless, upcoming event will be the closest of this kind till EPOXI encounter with comet Hartley-2.
Comet 79P was rediscovered last November in its 5th apparition and will pass perihelion in May 2008. That's a faint comet, but I hope not a smallest one. Even with coma/tail size of 20-30 thousands km Framing Camera and VIR spectrometer will be able to distinguish some details. I think that's a good opportunity to test and calibrate instruments on the real object, especially for VIR, which is a relative of Rosetta's VIRTIS spectrometer.

PS Attached image features comet orbit, Dawn trajectrory and planets positions on March 17, 2008.

Attached thumbnail(s)

 

[tedstryk]

That is still 7 million miles, but it could be done, at least as a test. It also flies by a really big object in February 2009.

Ted

[elakdawalla]

The latest Dawn Journal is now up at JPL and TPS websites. In it, Marc reports that Dawn did go into safe mode on January 14, 2008, the same day as the MESSENGER flyby and when Ulysses went haywire, but Marc told me that the Dawn safing at least had nothing to do with any of the other drama in the solar system; it was a routine, well-understood event, and the spacecraft is fine.

--Emily

[JTN]

(I'm not normally into fields'n'particles, so forgive my ignorance...)
So, we lost the magnetometer. Is there any way, from science or engineering data, that we'll be able to tell anything at all about the magnetic environment of Ceres/Vesta? Not quantitative, just enough to spot Mercury-level surprises.
I know they don't put those awkward booms on spacecraft for the fun of it... just asking.

[Holder of the Tw...]

In a word, no.

If Vesta appears unusually bright, it could be guessed that deflection of the solar wind by a field might have prevented weathering, at least over a significant portion of past history. If gravity data indicates a very dense core for Vesta, that would also be an indication that a magnetic field is more likely, but won't tell you anything about whether it is really there. Any realistic value for what you could expect at Vesta will not affect high energy cosmics rays very much, those which provide part of the signature for GLAST.

For Ceres, it was speculated that salty underground water reservoirs might have a magnetic signature, like they do on Ganymede. If they are there, and they do, then there is no way now for DAWN to know about it.

[ugordan]

deflection of the solar wind by a field might have prevented weathering

Isn't the majority of space weathering related to micrometeoroid impacts?

[Holder of the Tw...]

The weathering being referred to here is the general darkening of an airless surface by the implantation of solar wind particles. Local areas of relatively high magnetic strength on the moon are lighter colored than their surroundings because they prevent this weathering.

But you make a good point. Vesta might have more gardening from impacts, since it is in the main belt. That could lighten up the surface, too. Which might make it even harder to guess about the cause.

Edit - Wait, wait. Sorry. You were talking about micrometeorites. Yes, they do darken the surface. I really don't know by what proportion. However, if you look at a picture of Reiner Gamma on the moon, you can see what a difference a magnetic shield can make.

[punkboi]

New Dawn journal up:

http://dawn.jpl.nasa.gov/mission/journal_2_29_08.asp

[peter59]

I nearly forgot about these cancelled missions.

Pre-Dawn: The French-Soviet VESTA mission

Very interesting three proposed trajectory for two spacecrafts.

Trajectory 1:
-launch from Earth
-Mars gravity assist
-flyby of 2335 James (a 10 km X-type asteroid) (an Amor-asteroid)
-Mars gravity assist
-109 Felicitas (C-type, 76 km)
-739 Mandeville (EMP(?) type, 110 km)
-4 Vesta (V-type, or Vestoid. Has a diameter of 570 km) flyby with 3.5 km/s. A penetrator is released.
Total delta-v: 450 m/s

Trajectory 2:
-launch from Earth
-Mars gravity assist
-flyby of the P/Tritton short period comet
-Mars gravity assist
-2087 Kochera (30 km?)
-1 Ceres (flyby & releasing a penetrator)
Total delta-v: 1150 m/s

Trajectory 3:
-launch from Earth
-Mars gravity assist
-1204 Renzia (10 km?) (an Amor-asteroid)
-Mars gravity assist
-435 Ella (U type, 30 km)
-46 Hestia (F type, 165 km)
-135 Hertha (M type, 80 km)
Total delta-v: 350 m/s

[peter59]

Dawn Completes Another Month of Thrusting
March 31, 2008
Dawn thrust with its ion propulsion system for most of March, stopping once each week to point its main antenna to Earth. Almost 96% of the month was devoted to thrusting.

[Stu]

Dawn Completes Another Month of Thrusting
March 31, 2008

What is this, "Carry On Spaceflight"?!?!?! I could swear I heard Sid James laughing when I read that title...

[Greg Hullender]

I just noticed that Dawn has substantially upgraded their "Where is Dawn" page.

http://www.dawn-mission.org/mission/live_shots.asp

Now I have something to keep me entertained for the next 1213 days. :-)

--Greg

[Holder of the Tw...]

Yes, the diagrams are much better now, especially compared to the earlier monochome ones where you could barely tell the different orbits apart.

And ... the latest monthly thrusting report.

April 22

Carry on ...

[mps]

Meanwhile somewhere on the vicinity of Mars...

Dawn Journal, May 27

quote: be sure to visit the cool new feature "Where is Dawn Now?" at http://dawn.jpl.nasa.gov/mission/live_shots.asp. The site includes depictions not only of the craft's trajectory and location but also of its attitude

[punkboi]

New Dawn journal up:

http://dawn.jpl.nasa.gov/mission/journal_6_26_08.asp

[dmuller]

Pre-Dawn: The French-Soviet VESTA mission

I noticed the following paragraph in the above article:

I've heard it's not ruled out that Dawn will be directed to rendezvous with 2 Pallas (for a slow flyby) in 2018, after the main mission at Vesta and Ceres is completed and enough fuel is left.

Is that possibly still on the cards?

[nprev]

...Dr. Rayman is a hoot! He sure can write an entertaining update.

[ilbasso]

He is indeed quite an entertaining writer.

Maybe during the relatively quiet years of the cruise phase, ESA could contract out to him to write mission updates of the probes they purportedly have deployed around the solar system.

[Greg Hullender]

...Dr. Rayman is a hoot! He sure can write an entertaining update.

A joke or two can be funny, but not one or two in every paragraph. It's hard to tell sometimes what's serious and what's not. Also, for my tastes, very little of his humor is actually funny.

--Greg

[ugordan]

Also, for my tastes, very little of his humor is actually funny.

Sadly, this is often the case for me as well which is a shame because it distracts from otherwise detailed status reports he makes.

[MahFL]

He sounds like a typical slightly nutty scientist type.........

[djellison]

See this...

----------------------

That's a line being drawn under the debate regarding Marc's writing. Some like it. Some don't. End of debate.

[jasedm]

I'm just pleased that we get regular updates on the mission.
The possible post-main mission rendezvous with Pallas hasn't been mentioned for a while, but IIRC the mechanics of setting it up are difficult due to Pallas' orbit being appreciably out of ecliptic.
Maybe the mission planners are going to present the relevant trajectories as a fait accompli when they're lobbying for that mission extension...

[3488]

I'm just pleased that we get regular updates on the mission.
The possible post-main mission rendezvous with Pallas hasn't been mentioned for a while, but IIRC the mechanics of setting it up are difficult due to Pallas' orbit being appreciably out of ecliptic.
Maybe the mission planners are going to present the relevant trajectories as a fait accompli when they're lobbying for that mission extension...

I hope so. In December 2018, 2 Palles is on the descending node. In fact DAWN would not even have to leave the plane of 1 Cere's orbit to do this, as 2 Pallas will intersect that plane. The biggest issues will be the supply of Xenon, & the state of the solar arrays, will they still be producing enough power?

I really, really hope that the 2 Pallas option stays open. To bag all three of the Asteroid Belt's largest members would be a real accomplishment. However 2 Pallas would not be orbited, but could be a slow encounter, enabling much of the giant asteroid to be seen at a fairly high resolution.

Whilst 4 Vesta & 1 Ceres are primary mission aims, I think to not lose sight of 2 Pallas as an encore right at the very end, would be worthwhile.

No decent Hubble Space Telescope images exist of 2 Pallas do they, or have I not been able to find them?

Andrew Brown.

[ElkGroveDan]

No decent Hubble Space Telescope images exist of 2 Pallas do they, or have I not been able to find them?

from http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2502.pdf

"Figure 1: Pallas imaged by HST in 336nm UV filter."

Attached thumbnail(s)

 

[Del Palmer]

No decent Hubble Space Telescope images exist of 2 Pallas do they, or have I not been able to find them?

Depends what you mean by "decent." I don't recall seeing any press release images of Pallas from STScI, but there is a set of WFPC2 images in the MAST archive. Looks like they were taken using gyro-guiding, and so the targeting was a little off...

[Del Palmer]

"Figure 1: Pallas imaged by HST in 336nm UV filter."

Dan, great find! Have not seen that in the raw data archives...

[3488]

from http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2502.pdf

"Figure 1: Pallas imaged by HST in 336nm UV filter."

Thank you very much Dan,

I tried, high & low to find HST imagery of 2 Pallas. I had heard before that the triaxial shape had been determined from rotational light curves. That is a very good image & quite clearly shows a rounded triangular profile, the best I've ever seen of this gigantic asteroid.

I've downloaded the image.

Depends what you mean by "decent." I don't recall seeing any press release images of Pallas from STScI, but there is a set of WFPC2 images in the MAST archive. Looks like they were taken using gyro-guiding, and so the targeting was a little off...

Thank you very much Del also for your help.

The scientific case for DAWN to go onto 2 Pallas after the end of the primary mission is compelling.

It's great to be back here, hopefully I can contribute something of interest at some point.

Andrew Brown.

[tedstryk]

This is another Hubble view. During the 2001 observations, the Hubble missed Pallas with its Planetary Camera chip, getting the image with its lower-resolution wide field portion of WFPC2.

[Del Palmer]

Latest Dawn update:

http://dawn.jpl.nasa.gov/mission/journal_8_24_08.asp

[Greg Hullender]

Hmmm. He raises the point that Dawn will cross Mars' orbit before the gravitational assist but doesn't seem to explain why. Obviously this happens with Venus gravitational assists, but that's unavoidable. I'm trying to think why it would be better to do the assist from the far side of Mars, but I can't think of any -- other than the question-begging one of "it wouldn't work the other way".

Does anyone know?

--Greg

[djellison]

The Mars flyby is mainly about changing the orbital inclination of Dawn iirc.

[Greg Hullender]

That makes sense -- I'd wondered why the projected path didn't seem to show much change after the assist -- but it still seems you could just as easily change inclination on an outbound flyby as an inbound one.

Again, I'm sure there's a reason why this works out to be superior -- I'm just not seeing what it is. Why is this outer-planet flyby different from all others?

--Greg

[3488]

Also as Doug before you said, it is about changing inclination & also speed. The Mars encounter IIRC also prevents DAWN from coming much closer to the Sun again.

Andrew Brown.

[Greg Hullender]

Also as Doug before you said, it is about changing inclination & also speed. The Mars encounter IIRC also prevents DAWN from coming much closer to the Sun again.

I wasn't wondering "why is there a Mars flyby." I was wondering "why is this an 'inbound' (toward the Sun) flyby not an 'outbound' one."

Given the way ion propulsion works (long-term but weak), I'm actually surprised Dawn can do this maneuver at all. I'd have thought the thrust would always be close to parallel to the velocity vector and that the orbit (if they turned the engines off) would be nearly circular at any given point. But Dawn is actually falling back towards the Sun (a little) in order to make this flyby.

Maybe that explains it; the angle between the velocity vector of the spacecraft and the velocity vector of the planet needs to get smaller during the flyby or else energy will be lost, not gained. Perhaps it's easier to lift Dawn above the orbit of Mars and then drop it than it would be to get it to rise past it at any significant angle. In that case, though, I do wonder why they quit thrusting entirely for a few months before and after the assist. It'd seem you could get a steeper angle that way.

Of course I know you plan your orbits with the planets you have -- not the planets you'd like to have. :-) At a certain point, I suppose the answer is just "Yeah, you'd go faster, but you wouldn't get to Vesta."

--Greg

[BrianJ]

I wasn't wondering "why is there a Mars flyby." I was wondering "why is this an 'inbound' (toward the Sun) flyby not an 'outbound' one."

Hi, just wanted to point out that for earlier launch windows, the flyby was indeed "outbound", as per this image http://upload.wikimedia.org/wikipedia/comm..._as_of_2006.jpg

Also, just out of interest, here's a graphic comparing the orbit of Dawn to the orbit of Mars, before and after the (current trajectory) flyby.
http://img169.imageshack.us/img169/3606/dawn1copyzs2.gif

Regards,
Brian

[dmuller]

My two cents worth:

Dawn is picking up speed from the flyby. As this happens halfway between periapsis (closest point to sun) and apoapsis (farthest point to sun), both are raised. It also rotates the apsises clockwise, so Dawn will be at periapsis shortly after the Mars encounter. By thrusting then, it efficiently raises the apoapsis further. It also increases the inclination from 1.9 to 6.5 degrees, which I think would be costly to do without a gravity assist.

I dont think it makes a difference whether the flyby is inbound or outbound, you gain speed w.r.t. sun as long as you fly behind Mars.

Horizons gives the following orbital elements w.r.t. solar system barycenter (Mars flyby as inferred from Horizons data is around Feb 12):

CODE

                  1 Feb 2009     28 Feb 2009     Change
Eccentricity        0.160521        0.132016     orbit gets rounder
Periapsis (km)   180,738,278     203,950,288     +23 million km
Apoapsis (km)    249,858,224     265,989,955     +16 million km
Inclination           1.9249          6.5018     gets steeper

[tasp]

I will reiterate my ignorance about such matters, but will still post a question.

Let's say the Dawn ion drive accels the craft nominally at .001 G right now (neglecting slow increase in accel with fuel mass depletion) and let's say every one in the front office monitoring the engine is happy to sign off at running it in the range of .00085 to .00115 G.

And the office crew is ok with adjusting the throttle from time to time, so long as Mars encounter and Vesta arrival (or was Ceres first ? no matter) happen on schedule, and the average accel is .001 for optimal fuel utilization.

So, by tweaking the throttle only, in the narrow approved range, with a minimum impact to our amazing flight of discovery to Vesta and Ceres, how far off the 'nominal track' can the probe get, and does this resulting 'lens' shaped area in the graph (assuming frame of reference w/ Dawn) take us near anything known and interesting ??

For example, from Mars to Vesta, we accel at lower approved G limit 1/2 way and then go rest of the way at higher approved rate (allowing for the trajectory smiths to approve limits, of course) or we accel 1/2 way at higher G limit, then at lower limit the rest of the way. This gives us an area (instead of a line) along the way to Vesta we can explore for interesting objects to encounter for 'free'. (other than the brain wear and tear it takes to figure this out)

So is this already obvious to everyone and has been found to be undesirable for some reason I haven't figured out yet, or does this put a toe in the door for a free extra object to look at ??

( I am assuming the area potentially available with this technique compared to the known number of asteroids yields a figure of >1 for # of objects on average expected to be in an area of that size, but if it is more like .001, then it looks like I have my answer . . . )

[dmuller]

that the orbit (if they turned the engines off) would be nearly circular at any given point.

Greg, that does not seem to be the case. To start with, the original post-launch (pre-ion thrusting) was nowhere near a circle (150m km x 246m km). And the Mars flyby boosts both the periapsis and apoapsis quite a lot (as compared to the thrusting)

Again from Horizons:

CODE

Date        Periapsis         Apoapsis
1-Oct-07     150,019,330      246,071,890
1-Nov-07     150,128,927      244,454,659
1-Jan-08     151,655,060      246,166,277
1-Jul-08     165,446,119      251,548,401
1-Jan-09     180,583,781      250,621,283
1-Jul-09     204,715,862      269,844,671
1-Jan-10     224,351,731      293,297,300
1-Jul-10     260,729,882      309,582,209
1-Jan-11     299,787,522      326,511,305
1-Jul-11     317,551,998      368,001,819
1-Jan-12     320,554,935      382,753,035
1-Jul-12     319,854,770      382,797,552
1-Jan-13     338,984,080      382,275,315
1-Jul-13     363,921,049      408,392,742
1-Jan-14     364,440,390      457,493,646
1-Jul-14     364,604,289      449,257,439
1-Jan-15     378,673,307      445,436,420
1-Feb-15     382,279,514      445,557,925

[siravan]

So, by tweaking the throttle only, in the narrow approved range, with a minimum impact to our amazing flight of discovery to Vesta and Ceres, how far off the 'nominal track' can the probe get, and does this resulting 'lens' shaped area in the graph (assuming frame of reference w/ Dawn) take us near anything known and interesting ??

IIRC, the ion engines of Dawn are not exactly throttlable. In fact, they have such a low thrust that the main way to control them is to simply turn them on and off. Dawn has periods of "powered flight", separated by periods of cruise. I think each powered segment (days or weeks in a row) has essentially a fixed attitude. Then, then turn off the engine, determine the exact location, turn Dawn to a new attitude and turn on the engine again...

But, back to your main question. Essentially, Dawn does not have one fixed route from where it is to Vesta and in theory it possible to target it to some interesting object en route. However , there are two complications: the low thrust means that everything should be planned way ahead of time and Mars encounter. The exact time and location of the Mars encounter is still not determined, so there is still lots of uncertainty in the post encounter orbit. I believe that it is only after the encounter that Dawn will have a constrained orbit and the team can start planning for any possible extra targeting.

[djellison]

IIRC, the ion engines of Dawn are not exactly throttlable.

They are - dramatically.

http://dawn.jpl.nasa.gov/mission/journal_10_07_07.asp - lots of detail of the 112 different throttle levels they can use.

[Hungry4info]

@ tasp

They may not due that so as to conserve fuel. According to Wikipedia (which I realise may not be accurate), if the Vesta and Ceres investigations are successful, DAWN may go on to Pallas -- something I would really love to see.

[vjkane]

They may not due that so as to conserve fuel. According to Wikipedia (which I realise may not be accurate), if the Vesta and Ceres investigations are successful, DAWN may go on to Pallas -- something I would really love to see.

Per the Dawn Q&A page: "Q: Will there be opportunities to visit other asteroids, either en route to Ceres or as part of an extended mission?
Answer: Unlikely, because there is greater return by spending more of our resources on Vesta and Ceres." http://dawn.jpl.nasa.gov/mission/faqs.asp#

I personally have my doubts about this answer. If at the end of the *extended* Ceres investigations, there is still meaningful amounts of fuel left, I strongly suspect that Dawn will either 1) be sent to orbit another asteroid (a fairly expensive option because of mission ops) or 2) be sent to flyby one or more other bodies in similar solar orbits as Ceres (a less expensive option). NASA is really good at getting the most bang out of a working spacecraft. On the other hand, I don't think that anyone is working really hard on the question of what to do after Ceres -- that event is 7 years away and lots of things can happen to fuel levels, spacecraft and instrument health, and NASA budgets.

[Greg Hullender]

Hi, just wanted to point out that for earlier launch windows, the flyby was indeed "outbound", as per this image http://upload.wikimedia.org/wikipedia/comm..._as_of_2006.jpg

Also, just out of interest, here's a graphic comparing the orbit of Dawn to the orbit of Mars, before and after the (current trajectory) flyby.
http://img169.imageshack.us/img169/3606/dawn1copyzs2.gif

Well, I think the first graphic actually shows an inbound, not an outbound flyby. Did you mean to give a different link?

The second graphic is really cool, though. Does look like a pretty good bang for the buck from this flyby!

dmuller: On the question of the orbit always being circular, I didn't mean to imply that I thought that was the case with Dawn -- it's that it seemed to me that that would be the optimal way to use a low-thrust engine, and I'm curious why that's not the case in practice. Is it solely to make the Mars flyby work?

EDIT

Should have checked Vallado (Fundamentals of Astrodynamics and Applications) first. In Chapter 6.7 (Continuous Thrust Transfers) he works out that this type of transfer returns to circular only on integral numbers of orbits.

--Greg

[tasp]

Just wanted to comment how much fun it is to have a new 'toy' (ion drive) to 'play' with on the Dawn mission (I wasn't paying attention during Deep Space 1).

I think for folks brought up during the space age we all mostly have a 'feel' for Hohmann transfer orbits and their permutations, but 'learning the ropes' with an ion drive is new and exciting.

No shortage of ideas on things to try with it, and probably some good caution being exercised by the mission planners.

I really appreciate all the interest in this mission.

And of course, future applications of this drive technology seem to be promising all kinds of exciting possibilities. I'd love to wire up an ion drive to an RTG and see what we could get going . . .

[BrianJ]

Well, I think the first graphic actually shows an inbound, not an outbound flyby. Did you mean to give a different link?

Erm...I don't think so. I would call it "outbound" since Dawn approaches the flyby from within the orbit of Mars (assuming the link I gave shows you the same as it shows me!) I only comment since I spent quite some time trying to figure out the flyby trajectory for the earlier June launch window (as an "interested layman" who finds these things fascinating )

Best regards,
Brian

[dmuller]

On the question of the orbit always being circular, I didn't mean to imply that I thought that was the case with Dawn -- it's that it seemed to me that that would be the optimal way to use a low-thrust engine, and I'm curious why that's not the case in practice. Is it solely to make the Mars flyby work?

Greg, if you have the Orbiter simulator (http://www.orbitersim.com) you can simulate this in Earth orbit. Get a scenario where your ship is docked to the ISS, set your Orbit display to PRJ ship and DST altitude (will look more or less like a circle), undock, hit "prograde", get your engines on the lowest thrust setting (i.e. press and hold "Ctrl" and then literally hit your "+" button on the Numpad as short as possible), speed up the simulation to 100x (unless you have a lot of time - but dont go over 100x), and watch the PeA (perigee altitude) and ApA (apogee altitude) increase ... everytime you nearly get a circle (circle: PeA = ApA), youl'll find the ApA running away, half an orbit later the PeA starts to catch up.

[siravan]

Regarding ion engines and circular orbits, one may make a qualitative non-mathematical argument why that is the case. We all know that for an elliptical orbit, firing engines at periapsis modifies the apoapsis (rising or dropping the apoapsis depending on the direction of the thrust), whereas firing the engines at apoapsis changes the periapsis. Now, for the case of a continuous low-thrust ion engine, it affects both apoapsis and periapsis. Let assume the case where both increase (as in Dawn). The spacecraft spends more time in the vicinity of the apoapsis that periapsis, as it is slower farther in its orbit from the center. Therefore the effect on periapsis is larger than the periapsis. The net effect is that with continuous thrusting, periapsis starts to catch up with apoapsis, i.e. the orbit becomes more circular.

[dmuller]

BTW, whilst getting the Dawn data from the Horizons system, I noticed the following description of the propulsion system. I particularly love the bold part, leaves every high-performance car lover drooling!

PROPULSION
Dawn uses 3 ion thrusters to reach Vesta once separated from the Delta II. It
will use the thrusters to spiral to lower altitudes on Vesta, leave Vesta,
cruise to Ceres, and spiral to a low altitude orbit at Ceres.

Weight : 8.9 kg each
Dimensions : 33 cm long, 41 cm diameter
Specific impulse : 3100 s
Thrust : 19-91 mN
Acceleration : 0-60 mph in 4 days
Operational time : 2000 days of thrust (entire mission)

[dmuller]

Since there is much talk about the Dawn cruise, I have finally uploaded the Dawn Realtime simulation onto my website at http://www.dmuller.net/dawn

That's been 4 posts to this thread in 24 hours ... I'll give it a break now for a while

[Greg Hullender]

Erm...I don't think so. I would call it "outbound" since Dawn approaches the flyby from within the orbit of Mars (assuming the link I gave shows you the same as it shows me!) I only comment since I spent quite some time trying to figure out the flyby trajectory for the earlier June launch window (as an "interested layman" who finds these things fascinating )

I see it now. The graphic is a bit confusing because sometimes the dotted line is Dawn's trajectory and other times it's a planetary orbit. I was seeing the solid line as Mars' orbit -- but it's actually Dawn.

--Greg

[Toma B]

I still haven't found what I was looking for....can anybody help me?
Will there be any "non-targeted" flyby of any known asteroid or comet on the way to Vesta or Ceres?

[Phil Stooke]

Earlier discussions of this mission, before launch, suggested there could be several other flybys, but now they are not emphasized. Saving fuel is probably the big reason.

Phil

[kwp]

Earlier discussions of this mission, before launch, suggested there could be several other flybys, but now they are not emphasized.

I asked a mission specialist a few weeks ago and was told that, sadly (my word, not hers), no en route flybys are planned.

-Kevin

[Marz]

I asked a mission specialist a few weeks ago and was told that, sadly (my word, not hers), no en route flybys are planned.

-Kevin

The Mars gravity assist is coming up in a few months (Feb 09). Are any observations or kodak-moments planned?

Only 2.8 more years until Vesta is reached!

[djellison]

Yes - an extensive campaign is planned for Dawn at the mars flyby

http://cosis.net/abstracts/EPSC2008/00442/...8-A-00442-1.pdf

One extra detail I discovered at Europlanet is a colour movie of a full mars day on the departure leg.

Doug