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2 Pallas
Explorer1
post Apr 24 2016, 01:26 AM
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It would be amazing to get Dawn to a 3rd object, I agree, but Pallas is a lot less massive than Jupiter, so a slingshot would be far less effective.
And Dawn would be running on (xenon) fumes by that point so if anything goes wrong, it's over.
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Paolo
post Apr 24 2016, 07:02 AM
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and don't forget that changing the inclination of an orbit is very delta-V consuming. much more so than is usually appreciated
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nprev
post Apr 24 2016, 09:29 AM
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Pallas is not nearly massive enough to effect a significant change in Dawn's velocity nor change its inclination. Recall that the only moon in the Saturn system massive enough to affect Cassini's trajectory is Titan...and Pallas is only about twice as massive as Enceladus.

If this happens at all, it's gonna be a flyby.



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HSchirmer
post Apr 24 2016, 12:45 PM
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QUOTE (nprev @ Apr 24 2016, 10:29 AM) *
Pallas is not nearly massive enough to effect a significant change in Dawn's velocity nor change its inclination. Recall that the only moon in the Saturn system massive enough to affect Cassini's trajectory is Titan...and Pallas is only about twice as massive as Enceladus.

If this happens at all, it's gonna be a flyby.


Ahh, good point, interesting point actually, seems that gravity assist maneuvers are limited to about 70% of the escape velocity of the object.
While Pallas is moving fast, (relative to Dawn) it's weak gravity only lets you take small bites of that gravity assist capability.

So, Pallas escapeV is 324 m/s, so the maximum deltaV from any single assist is around 230 meters per second.
As compared to Titan, the reported gravity assist numbers I could find for Cassini had a deltaV of 1,100 meters per second.
That initially seems promising, Dawn at Pallas might get around 1/4 of the slingshot effect of Cassini at Titan,
but part of the benefit to Cassini is that Titan has a 16 day orbit, so get to do multiple slingshot encounters.
Interesting that Titan's orbit has a relative motion available for slingshot manoeuvres of around 5.6km/s.
For Dawn in a solar orbit, the relative motion of Pallas around the Sun is much higher, around 17.6km/s,
but the orbit takes so much longer, 16 hundred days, you're not getting many slingshot opportunities.

So, at first glance, it seemed like Pallas' high speed might balance out its low gravity for gravity assist,
but the way the equations work out, the speed of Pallas doesn't translate into higher deltaV, at least for single passes.
With each orbit taking 4.6 years, it would take a long while to do multiple passes.
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jasedm
post Apr 24 2016, 07:01 PM
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It would be fantastic to think it would be on the cards to 'do' a flyby of Pallas by extending an existing mission - this is the largest remaining object unstudied at close range within the orbit of Pluto, but my feeling is that budget constraints along with shortage of consumables and required deltaV will nix the idea.

Would love to be wrong though.....
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TheAnt
post Apr 24 2016, 11:41 PM
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QUOTE (Steve5304 @ Apr 23 2016, 06:08 PM) *
Why cant we just go an even lower descent on Ceres. Or are we not calibrated for that?

A new target does not make a lot of sense imo, it wont be anything bigger than a few football fields unless there is something i am missing


It's not calibration of instruments, we would get even higher resolution for the neutron measurements from GRaND for example. But that bad flywheel which have to be compensated by Dawn using fuel to aim at the surface then to realign itself to send data back to Earth.
The craft would use up the remaining fuel in such an orbit. And as already pointed out, that's now something that can be allowed to happen.

Dawn have to be put in a safe orbit that will not pose any risk of collision in the foreseeable future at least.
Pallas would be the grand price on one already excellent mission, since that small world appear to be an eroded proto planet.
And we know that they did at least look into the possibility of such a flyby earlier. Pallas will pass trough the plane of the ecliptic in just a few years, it might be the reason for the wish to approve a mission extension soon.

Though I agree with Nprev, this appear to be a means to get a little more of the spacecraft that by necessity will have to do a final flight anyway.
Regardless of object it won't be anything but a flyby, but it's good enough in my book, even though it might not be Pallas.
So if not, lets hope they have identified something with odd properties, carbonates or minerals or something else that deserves a closer look where even just images might give us an insight.
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HSchirmer
post Apr 25 2016, 12:00 AM
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QUOTE (TheAnt @ Apr 24 2016, 11:41 PM) *
Regardless of object it won't be anything but a flyby,


Well, it could be a fly into, like LCROSS or the end of GRAIL.

Edit, quick check shows that Pallas crosses Cere's orbital plane in late Feb '19, nice timing as Earth's closes approach occurs in April '19.
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nprev
post Apr 25 2016, 01:58 AM
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Landing or crashing is not considered an option since the spacecraft is not sterilized. Apparently, this would violate NASA Planetary Protection Protocols for Ceres.

Of course, that begs the question of what the plan is should a flyby of any target of opportunity not be possible. I'd guess that would leave just escaping Ceres in a non-targeted way or at the very least raising the orbit as high as possible before the xenon is exhausted and/or a critical failure occurs that renders the spacecraft uncontrollable.


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HSchirmer
post Apr 25 2016, 02:30 AM
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QUOTE (nprev @ Apr 25 2016, 01:58 AM) *
Landing or crashing is not considered an option since the spacecraft is not sterilized.
Apparently, this would violate NASA Planetary Protection Protocols for Ceres.


Sorry, should have been clearer, impact on Pallas, not Ceres.
Get all the photos you can from the spacecraft before Dawn impacts on Pallas.
Then Earth-based telescopes coordinate to do spectroscopy of the ejecta.

Hmm, now that I'm thinking of it, not sure if Dawn would have enough bandwidth
to offload the data or be able to point the main antenna to stream all the date live.
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nprev
post Apr 25 2016, 03:33 AM
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I think that's about ten deep in "ifs" right there. Let's just see what they decide to do. smile.gif


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