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Dawn approaches Vesta, Approach phase, 3 May to 16 July 2011
dilo
post Jul 16 2011, 06:43 AM
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QUOTE (Greg Hullender @ Jul 16 2011, 03:39 AM) *
Why are you so sure there's a strictly linear relationship between velocity and distance from Vesta?

Greg, perhaps you're referring to a velocity that is relative to Vesta but isn't TOWARD it; the distinction could explain the incongruences between simulation and mission update velocities: if we assume the former is total velocity and latter is only the radial component, we can argue a 45° angle between these two directions (or, if you prephere, between Dawn trajectory and the Dawn-Vesta vector) in the last hours.
This makes sense! In fact, the meaning of "orbit capture" just occurred becomes: "spacecraft total speed went under 47 m/s, which correspond to a parabolic trajectory at current distance of 16000km".
An interesting fact is that, in spite of Vesta gravity, spacecraft continues to decelerate, even though at decreasing rate (3e-5 m/s2 in the last 20 hours); this confirms that motors are on and are struggling against asteroid's gravity... rolleyes.gif


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Littlebit
post Jul 16 2011, 02:44 PM
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QUOTE (Paolo @ Jul 14 2011, 07:16 AM) *
they must be joking, that's clearly Miranda...

blink.gif

As Emily has observed, it is sobering how much this asteroid resembles moons in other places.

Shes coming in a little hot captain, I'm not sure the engines can take it...

A asteroid orbital insertion could be boring, but this one isn't...just think of it in terms of le tour.
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Greg Hullender
post Jul 16 2011, 03:34 PM
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QUOTE (dilo @ Jul 15 2011, 11:43 PM) *
Greg, perhaps you're referring to a velocity that is relative to Vesta but isn't TOWARD it;

Actually, I was just confused by your graph. It only sounded like I was asking an intelligent question. Sorry about that! :-)

However, now that we're on the same page, I agree with you. They key isn't the circular orbital velocity at a given distance; it's the escape velocity for that distance. Weirdly, since that's a function of energy, it doesn't seem to matter what direction Dawn is going (as long as it doesn't hit Vesta). When it's below the escape velocity for its current distance, then we could say it's in orbit around Vesta.

A graph of the hyperbolic excess velocity as a function of time would look pretty cool. Can you make that one?

Depending on how we want to define "orbit" of course, but let's not go there; I still think we need a rule against astro-linguistics discussions. :-)

--Greg
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punkboi
post Jul 17 2011, 08:14 AM
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Welcome to Vesta!

NASA's Dawn Spacecraft Enters Orbit Around Asteroid Vesta

PASADENA, Calif. -- NASA's Dawn spacecraft on Saturday became the first probe ever to enter orbit around an object in the main asteroid belt between Mars and Jupiter.

http://www.nasa.gov/mission_pages/dawn/new...wn20110716.html



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ElkGroveDan
post Jul 17 2011, 05:05 PM
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Moving the discussion over HERE

http://www.unmannedspaceflight.com/index.p...ic=7011&hl=

Topic now closed, but still available for browsing.


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