Dawn Cruise |
Dawn Cruise |
Sep 27 2007, 12:31 PM
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#1
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Member Group: Members Posts: 100 Joined: 11-October 04 From: Oxford, UK (Glasgow by birth) Member No.: 101 |
Pushing out of Earth orbit now...........
-------------------- "There are 10 types of people in the world - those who understand binary code, and those who don't."
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Aug 27 2008, 04:44 PM
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#2
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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 |
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Aug 27 2008, 07:25 PM
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#3
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Junior Member Group: Members Posts: 47 Joined: 27-June 08 From: Ashford, Kent, United Kingdom. Member No.: 4244 |
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. -------------------- "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.
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Aug 29 2008, 06:02 PM
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#4
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Senior Member Group: Members Posts: 1018 Joined: 29-November 05 From: Seattle, WA, USA Member No.: 590 |
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 |
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Aug 29 2008, 09:15 PM
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#5
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Newbie Group: Members Posts: 17 Joined: 5-January 06 Member No.: 636 |
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 |
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