Dawn approaches Ceres, From opnav images to first orbit |
Dawn approaches Ceres, From opnav images to first orbit |
Jan 12 2015, 12:10 AM
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Solar System Cartographer Group: Members Posts: 10183 Joined: 5-April 05 From: Canada Member No.: 227 |
On Tuesday (two days from now, for visitors from the future), the first optical navigation image will be taken... hopefully we'll have it in our hands soon after that. So it's time for a new topic. Over the next few months we'll have progressively closer images and full orbit characterization sequences, no doubt including multispectral image sets.
A new world... This is a bit of reprocessing I have been doing with the Hubble images from a few years ago. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Jan 22 2015, 08:24 AM
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Senior Member Group: Members Posts: 2089 Joined: 13-February 10 From: Ontario Member No.: 5221 |
I believe that what they mean by 'so fast' is that the initial orbit will be very high above Ceres.
This is mostly academic, at any rate; space itself does a plenty fine job sterilizing everything we send up sooner or later. The distances and times spent in those environments mean Dawn has nothing to worry about (unlike the onboard labs of Curiosity or Viking). |
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Jan 22 2015, 10:38 AM
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Member Group: Members Posts: 444 Joined: 1-July 05 From: New York City Member No.: 424 |
I believe that what they mean by 'so fast' is that the initial orbit will be very high above Ceres. Yabut, even my admittedly shaky understanding of orbital mechanics tells me that the higher the orbit the slower the orbital speed. (Kepler's Third Law). So I still don't grok "so fast." The orbital speed will increase as the whispering ion drive wafts the craft to closer orbits, but these orbits will continue to be stable -- and thus pose no measurable risk of impact -- so long as they keep their shape and stay outside of the influence of any vestigial "atmosphere" of Ceres and of the gravitational influence of surface and subsurface features. (I think that we can skip perturbation theory for the mission's likely length, especially since the ion drive is available to keep the orbits in trim.) To me, "so fast" might make some sense if the maneuver in question involved a conventional rocket firing to adjust a flyby or set up an orbital insertion, where the analysis might be that the delta-V available to the vehicle was inadequate to lead to impact even if the desired result (whether flyby or insertion) were to fail.* It's not a matter of great interest, particularly if (as I think) the confusing phrase was the result of the minute taker's misunderstanding of something said in passing by Dr. Conley. (And I doubt that it's important enough for me to impose on her by writing for an explanation.) But for me it's a rare pleasure to be able to summon up anything at all about Kepler's Laws, and I'm grateful to any members who will humor me. * The phrase could refer to the fact that Dawn is catching up to Ceres, rather than achieving a position on front of Ceres that would let Ceres catch up to the vehicle, the latter case creating, I suppose, the possibility of impact if Dawn suddenly ceased to be commandable. Hmmm. In any event, I don't have the chops to analyze that kind of situation. |
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