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Mercury Orbit Insertion, Events and Discussion leading up to MOI
Phil Stooke
post Mar 13 2011, 12:16 PM
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Ingenious idea but impossible! Remember, we didn't have a rocket big enough to get Cassini to Saturn without several gravity assists to help it, so the reverse is bound to be impossible with a little bit of residual fuel. And Cassini's fate is decided - burn up in Saturn's atmosphere.

Phil


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centsworth_II
post Mar 13 2011, 04:39 PM
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QUOTE (Phil Stooke @ Mar 13 2011, 07:16 AM) *
Ingenious idea but impossible!...

Yeah. I know next to nothing about the mechanics of getting from one planet to another but look at how 'hard' it's been getting Messenger from Earth to Mercury. I don't see why it would be any easier to get from Saturn to Mercury and with a craft not designed to do that.
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nprev
post Mar 13 2011, 04:52 PM
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If I understand things correctly (and I probably don't), it might actually be easier in terms of delta-V to crash into Mercury from the outer Solar System. Saturn's heliocentric orbital velocity is much lower than that of the Earth, so presumably that means less thrust would be required to negate it & 'fall' into the inner system.

However, we're still probably talking about a change in velocity of several (if not tens) of km/sec, plus escaping from Saturn orbit. I doubt that Cassini could have done this even if it was fully fueled at the beginning of the maneuver.


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Explorer1
post Mar 13 2011, 07:13 PM
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The sun-grazer and long-period comets are an extreme example of this right? A tiny nudge in the Oort cloud is more than enough to send them in a more or less straight line sunward.
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nprev
post Mar 13 2011, 09:25 PM
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Yeah, basically. The stuff way out there is barely moving in comparison to the planets; probably doesn't take much to negate their orbital motion at all (e.g., gravitational nudges from passing stars over long periods of time, perhaps occasional outgassing from the cometary bodies themselves?)


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siravan
post Mar 14 2011, 03:14 AM
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If I have calculated correctly, you need a delta-V of 5.4 km/s to go from Saturn's orbit into a Hohmann transfer orbit intersecting Mercury. Dawn could have done it (ignoring distance from Sun issue), but I doubt if Cassini ever had that much of delta-V.
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MarkG
post Mar 14 2011, 06:06 AM
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At the present time (this Sunday Evening), Mercury would appear from Messenger to be about the same size as the Moon from Earth, with the Sun looming 3 times that diameter.
I think Messenger must halve its current distance to Mercury to enter the Hill Sphere... Soon!
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dmuller
post Mar 14 2011, 08:54 AM
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The cheapest (in terms of energy) way to get a spacecraft very close to, or into, the Sun is indeed a Jupiter flyby. Scientifically not very interesting because the spacecraft would not spend much time near the Sun during periapsis. See the Solar Probe Plus trajectory options at

http://solarprobe.jhuapl.edu/mission/docs/...018missions.pdf

pages 3 and 4. The Jupiter flyby option requires by far the least C3.


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Astro0
post Mar 15 2011, 04:16 AM
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Not far now! smile.gif
Attached Image


BTW - I love Eyes on the Solar System. Best space Outreach tool ever!
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mchan
post Mar 15 2011, 04:43 AM
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The orientation of the day side is to the right vs. to the left in Where is Messenger? page on the Messenger website. I had wondered about the latter since Messenger is ahead of Mercury in its orbit waiting for the planet to "catch up" to it. EOTSS appears to have the view in accord with the convention of North pointing up.
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djellison
post Mar 15 2011, 04:55 AM
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QUOTE (mchan @ Mar 14 2011, 08:43 PM) *
EOTSS appears to have the view in accord with the convention of North pointing up.


You could have north any way you want smile.gif
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MarkG
post Mar 15 2011, 03:05 PM
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QUOTE (mchan @ Mar 14 2011, 09:43 PM) *
The orientation of the day side is to the right vs. to the left in Where is Messenger? page on the Messenger website. I had wondered about the latter since Messenger is ahead of Mercury in its orbit waiting for the planet to "catch up" to it. EOTSS appears to have the view in accord with the convention of North pointing up.


Actually, Messenger is catching up with Mercury, with higher ellipticity in its current (not for long!) orbit, Messenger's speed at perihelion is greater than Mercury's.
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ElkGroveDan
post Mar 15 2011, 07:57 PM
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QUOTE (Astro0 @ Mar 14 2011, 09:16 PM) *
Not far now! smile.gif

If it were green, Messenger would look just like that little android character that keeps popping up on my new phone.
Attached thumbnail(s)
Attached Image
 


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mchan
post Mar 16 2011, 03:54 AM
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QUOTE (MarkG @ Mar 15 2011, 07:05 AM) *
Actually, Messenger is catching up with Mercury, with higher ellipticity in its current (not for long!) orbit, Messenger's speed at perihelion is greater than Mercury's.

Agree Messenger velocity at perihelion is higher than Mercury. But, it appears to me that Messenger's orbit angular velocity is less than that of Mercury at the rendezvous. The Mercury flybys and DSM burns are tailored to put Mercury and Messenger in near-resonant orbits with each flyby and DSM burn reducing the ratio of the resonance. After the last flyby, Mercury completes 6 orbits and Messenger completes 5 orbits before the rendezvous for the orbit insertion burn. So it appears to me that Mercury is catching up with Messenger.

Would someone who knows the astrodynamics please correct this?
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nprev
post Mar 16 2011, 04:03 AM
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A minor milestone just occurred to me: Not only will Messenger become the first spacecraft to orbit Mercury, but for the first time we will have orbited every terrestrial planet in the Solar System...in fact, we will have active spacecraft orbiting every major body in the inner Solar System.

Maybe that's not such a minor milestone, actually... smile.gif ...wow!


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