[X] My Assistant

[SkyeLab]

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

[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

[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