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I understand Dawn has a mission to undertake geologic ("Cereologic"?) mapping of Ceres, but its final orbit will be at 22,000 km / 49% disk illumination.

Surely, there would be some value for adding (or finalizing) on a much closer orbit for some greater detail of specific features.

Is Dawn not equipped for closer study? Is it because the reaction wheel failure, we only have enough energy for this view? Am I not understanding something about the physics of such an attempt? Are we just not interested, in this particular mission?

Why will Dawn not go closer to Ceres?

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    $\begingroup$ I should clarify that I am motivated by finding out what those d#$% white spots are, among other things - even if they're just ice or an inspection sticker. $\endgroup$ – Mikey Mar 8 '15 at 1:17
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Its final orbit will only be 375 km above Ceres, but you have to give it time.

Dawn is powered by xenon ion engines, which are extremely efficient, but very weak. The usual comparison is that they are pushing the craft forwards about as much as a sheet of paper pushes down on your hand. Their advantage is that they can do this for a very long time. This is why Dawn has been able to visit Vesta and Ceres.

A probe with conventional chemical rocket engines can change its speed very quickly, but it only has fuel to fire a few times, briefly. What has been done in the past is that probes have been carefully aimed at their destinations, and then when they arrive, they swing in really close to the planet and fire their engines when grazing past at the closest point. This is the best way to slow down. By doing that over a few passes, it can be done with only brief firings and that saves on fuel. This is called an Oberth maneuver.

Dawn doesn't have to fire engines at the closest approach to Ceres. Its engines just keep firing, and firing until finally it slows down enough to orbit much closer. In fact the word 'firing' doesn't really apply. The engines have been running, continuously, for thousands of days. (Well, there's three, and they switch which is on periodically, and there were some periods of coasting.)

Braking this way takes weeks. It has already been braking for weeks as it got closer to Ceres. Then at first it was captured only into a highly elliptical orbit whose far point was very far away from Ceres. Still it runs its engines over a large portion of its orbit rather than applying much greater force briefly at the orbit's lowest point, and thus the Oberth effect is much less important in its maneuvers.

But because the engines are so very efficient, still less fuel is needed that if chemical rockets had been used. The luxury of having engines that run on and on may also have helped mitigate the problems of dealing with the failed reaction wheels, and also the (temporary) failure of an entire engine did not doom the mission.

(Note - Probes like Cassini and Voyager managed to visit several destinations by making extremely clever use of gravity assists as well, carefully entering into the gravity well of a planet or moon in just the right way so that they got swung around them and thrown out again in just the right direction to proceed on to the next destination. It is only because they were set up to do this that they managed to make it to several awesome destinations. But they wouldn't have been able to slingshot very much with these low gravity destinations.)

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    $\begingroup$ "The engines have been running, continuously, for thousands of days" - A certain youtuber is going to have a lot of "fun" reproducing that mission, then :-) $\endgroup$ – John Dvorak Mar 8 '15 at 4:14
  • $\begingroup$ Apologies: I mis-understood the wikipedia article. 375km is very satisfactory. $\endgroup$ – Mikey Mar 8 '15 at 12:42
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    $\begingroup$ This answer has some misleading stuff in it. First, the Oberth effect does not depend on the mass of the object. A slow spiral in to a low orbit from a C3 of zero will take about 2.4 times as much $\Delta V$ as an impulsive maneuver to do the same thing, regardless of the GM. Second, a chemical orbit insertion would not have "had to fire much longer". Longer than what? The total $\Delta V$ to insert directly into LAMO from a C3 of zero would be about 150 m/s (done in a few minutes), much less than ~1000 m/s at Mars. The smaller body makes orbit insertion faster, not slower. $\endgroup$ – Mark Adler Mar 8 '15 at 17:05
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    $\begingroup$ Also the time to get to the low orbit is driven just as much by the science as the thrusting. Dawn will spend about three weeks in the survey orbit, take five weeks to get to HAMO, then spend two months in HAMO, take two months to get into LAMO, and then three months observing in LAMO, plus extended mission. $\endgroup$ – Mark Adler Mar 8 '15 at 17:24
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    $\begingroup$ It definitely still makes use of the Oberth effect, I doubt they're firing halfway between the perapsis and apoapsis. If they're firing while passing over the lowest orbital points, the Oberth effect is still applicable. $\endgroup$ – Magic Octopus Urn Jul 11 '18 at 20:52
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Fear not. If all goes well, Dawn will get down to 375 km. You can read more details in the blog entry, but Dawn's lifetime will be limited by its use of hydrazine, which is in fact driven by the failure of two of the four reaction wheels.

Update 3+ years later: Dawn is currently on it's way to a 35 km altitude orbit! (Not a typo.)

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    $\begingroup$ You didn't want to go for 'Patience, grasshopper'? $\endgroup$ – kim holder Mar 8 '15 at 2:09
  • $\begingroup$ "Dawn's lifetime will be limited by its use of hydrazine" -hopefully $\endgroup$ – Rikki-Tikki-Tavi Mar 8 '15 at 18:41

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