Dawn approaches Ceres, From opnav images to first orbit Options

[hendric]

Just for my own curiosity, I looked at if Dawn could soft-land on Ceres, and I don't think it can. g at Ceres surface is .28 m/s^2, which is way more than the Ion thrusters can provide. Dawn also has RCS thrusters, but they likely can't thrust that hard either, or have enough fuel to reduce the speed from orbit before impact. A circular orbit at 2 R_ceres is about 250m/s, skimming the surface is about 350 m/s. Chances for a moon are probably slim with such a well studied object, but maybe Dawn will find a small 10m moonlet to land on.

[djellison]

Each of Dawn's 12 RCS thrusters are I believe 0.9N - 4 of which could be used if one were trying to 'land' it - but they lack both the thrust and the delta V, by probably an order of magnitude. The 747kg dry mass of Dawn would require >200 newtons - not <4.

Not to mention planetary protection.

[DrShank]

thats right. no soft landing. no way to slow down. i don't recall how long the end-of-mission orbit will last (i think 50 years but don't quote me), but we wont be going to really low altitude. the orbits become less stable, and fuel use becomes excessive, are among the reasons.

[algorimancer]

Wasn't there discussion of Dawn possibly going on to visit one or more other targets following mission completion at Ceres? Is the plan to remain in Ceres orbit fully committed now?

[Phil Stooke]

I don't know if that was discussed as a vague possibility, but it's not been a realistic plan for a long time, if ever. Ceres is big - 4 times the surface area of Vesta, so needing 4 times the number of images to cover it at any given resolution. Any extended mission which might be possible with the current limitations of the spacecraft will be most productively used adding data for Ceres, not starting off on a long cruise to some other destination.

Phil

[DrShank]

I don't know if that was discussed as a vague possibility, but it's not been a realistic plan for a long time, if ever. Ceres is big - 4 times the surface area of Vesta, so needing 4 times the number of images to cover it at any given resolution. Any extended mission which might be possible with the current limitations of the spacecraft will be most productively used adding data for Ceres, not starting off on a long cruise to some other destination.

Phil

That is quite right. more area requires more time. also the shift to using hydrazine removes any margin that we might have had for a hypothetical transfer to another object, if it ever was seriously an option (which i don't know). the other factor is that the GRAND gamma ray instrument requires really long integration times to accumulate enough signal to confidently detect elements on the surface. the final orbit is not as low as they would like so getting extra time to fully map out Ceres composition would be a logical and compelling justification for extending the mission. any exciting discoveries like volcanoes or well, who knows, would also be a compelling reason.

[K-P]

Wasn't there discussion of Dawn possibly going on to visit one or more other targets following mission completion at Ceres? Is the plan to remain in Ceres orbit fully committed now?

Pallas was considered as a potential target initially, but I don't know how seriously. Anyway, the delays in launch and also problems with ion thrusters after Vesta departure propably rendered that option impossible to achieve?

It would have been a flyby anyway, and made possible only by orbital mechanics of certain date when Pallas and Ceres were close to each other during their orbits (Pallas has different inclination).
Correct me if I'm wrong.

But yes, would have been a huge bonus for this mission, especially since Pallas is the third (or second) big one out there.

I'm more than overjoyed by the fact that Dawn got even this far. Ceres is a place to see, definitely.

[TheAnt]

also the shift to using hydrazine removes any margin that we might have had for a hypothetical transfer to another object, if it ever was seriously an option (which i don't know).

Yes a colleague of yours stated in a blog that any transfer to any third object was now no longer even considered due to the flywheel problem. Getting good data from GRAND is better science than just a snapshot from a quick flyby.

So the third lowest orbit is out of the question now then? I tried to find any information about that, since I figured that would indeed be more costly on hydrazine?

So as for algorimancer's question I know Pallas have been mentioned, but have no idea if that was ever on any extended planning or just ideas that were tossed around. [I toss 14 ideas or more around daily, something that have taken years for some people to getting used to not to take too seriously.] =)

One ancient ice volcano would be really cool, and could provide some information about both history and perhaps even the interior of Ceres. But only if it have survived large impacts since the early days of Ceres. That artwork vikingmars posted is actually very close to my hopes for a best case scenario, even a polar cap, of what Ceres would provide for us. Hmm arcuate structures, like Miranda? Temporary melting after one impact, who knows.....

[Habukaz]

So the third lowest orbit is out of the question now then? I tried to find any information about that, since I figured that would indeed be more costly on hydrazine?

I guess the information you are looking for can be found in this blog entry from last year (which also mentions why a landing is impossible, cf. debate above):

What fate awaits our stalwart adventurer following the completion of its primary assignment? There are several possibilities, but they all conclude the same way. If hydrazine remains at the end, and if the spacecraft is still healthy, NASA will decide whether to invest further in Dawn. NASA has many exciting and important activities to choose among — after all, there’s a vast universe to explore! If it provides further funds, Dawn will perform further investigations in LAMO, making GRaND’s gamma ray and neutron pictures even sharper, refining the gravitational measurements, collecting still more photos of the expansive surface, and acquiring even more spectra with VIR.

There is no intention to fly to a lower orbit. Even if the two remaining reaction wheels operate, hydrazine will be running very low, so time will be short. Following another spiral to a different altitude would not be wise. There will be no below-LAMO (BLAMO) or super low altitude mapping orbit (SLAMO) phase of the mission.

[TheAnt]

I guess the information you are looking for can be found in this blog entry from last year (which also mentions why a landing is impossible, cf. debate above):

Thank you Habukaz, what I thought, it would have been one tempting idea if all gyros had been ok since the low gravity field of Ceres might have allowed the spacecraft to orbit really close without being to fast to take really high resolution images and better data from GRAND as well.
Now DrShank mentioned 'very long' integration time I guess they hope to get equal results anyhow from a higher orbit.

[algorimancer]

It sounds pretty settled. Leaving Dawn in a stable orbit around Ceres seems like a nice end to the mission. Someday perhaps we can look forward to a subsequent probe coming along and snapping a picture of it -- which would be really cool.

[DrShank]

GRAND basically observes the whole planet at one time, so to improve resolution you have to observe, observe, observe, with multiple overlapping orbits. Its simply the more the better. Its all mysterious to me, but it works. which reminds me that to get the highest resolution gravity map and best constraints on interior structure (does it have a core, how large . . . ?) is also improved significantly by getting as many orbits as possible. of course both are improved by going as low as possible, but we are constrained.

the blogs by Marc Rayman on the Dawn website are excellent for describing the Ceres mapping mission and why things are done as they are. google should get u there. excellent posts throughout 2014.
p

[elakdawalla]

Continuing operations as long as possible in the lowest orbit will also likely mean more thorough high-res imaging coverage. They won't waste hydrazine slewing to point at gaps; instead, they'll exert patience, just waiting until an orbit happens to pass over a gap.

A lot of these questions are also answered in my Sky & Tel article on Dawn at Ceres, coming out in the April issue but I think I will also try to go through and prepare a blog with a Cliff's Notes version of Marc's posts on the Ceres mission, when I can find the time!

[Mithridates]

It sounds pretty settled. Leaving Dawn in a stable orbit around Ceres seems like a nice end to the mission. Someday perhaps we can look forward to a subsequent probe coming along and snapping a picture of it -- which would be really cool.

In one of the most recent Dawn journals describing the moment of capture they used the term "and they will be together forevermore" or something along those lines so yes, it sounds pretty settled.

[DrShank]

it sounds pretty settled.

very much so. Our final orbit is also determined, though I would not mind getting even lower. But as we will be covering the majority of the surface in our final orbit altitude at ~35 meter resolution I can't and won't complain! (That is almost three times as good as our effective global resolution of approximately 100 meters over most of Enceladus, our best mapped icy world at present.)