Dawn approaches Ceres, From opnav images to first orbit Options

[Astro0]

Can't wait for the next image to come in



What features will be revealed?

[Explorer1]

Well, here's one idea:

http://xkcd.com/1476/

[Habukaz]

Those replies about Ceres' formation is something to chew on; thanks.

From looking at the more carefully processed Dawn image in this MPS entry, it appears that PIA19167 has actually been rotated 90 degrees relative to the animation for some reason (the bright spot is also darker relative to the rest of the terrain, but is still very much there), so that's what threw me off. Confused no more.

[DrShank]

hey ho!
was busy at a New Horizons meeting all week. fun to watch the speculations. I was making my own, only to decide i couldn't make out anything for sure. some dark markings looked like craters but the animation makes some things look like arcuate structures. grrrr! frustratingly tantalizing.

as far as ppp goes, i can't say, but I'm sure that will change once we find out what the venting is related to. could bump it up high on the list.

[Sherbert]

The comparisons with Pluto and Charon will be interesting. With the musical chairs mentioned above I suppose its not beyond the bounds of possibility Ceres may have originated in the Kuiper belt. Another exciting adventure to a new world begins. Can't wait!

[Tom Tamlyn]

* * * Where does Ceres rate on the PPP scale? Is it on par with Mars, Europa & Titan, rated somewhat lower, or not even on the radar at this time?

Not directly applicable to Dawn, of course, save that it might be a factor in EOM planning.

Once google helped me to remember what PPP stood for , a couple of interesting items turned up with a quick additional search. A summary of planetary protection categories is here.

Io and asteroids of the "[u]ndifferentiated, metamorphosed kind" are in Category I, the category of least concern. Other asteroids are in Category II, along with the Earth's Moon, Jupiter, and a host of other bodies.

Titan is in Category II* (an intermediate category for icy satellites) for all types of missions. Mars and Europa are in Category III for flybys and orbiters, Category IV for landers and probes.

A NASA planetary protection powerpoint from November 2014 lists Dawn at Ceres as Category II (a body of “significant interest * * * where there is only a remote chance that contamination carried by a spacecraft could compromise future investigations”), based on a determination that "Dawn will not impact Ceres due to orbital mechanics constraints." The 2009 flyby of Mars, by contrast is listed as Category III.

A powerpoint specific to the Dawn Mission from January 2013 is a little confusing to the lay reader; a specific category isn't assigned with respect to Ceres. On a page that characterizes the Mars flyby as a Category III mission, it notes that the mission was required to demonstrate “Orbital lifetime around Ceres of greater than 20 years post-orbital insertion."

This restriction is subject to change, based on assessment of... let's just call it conditions on Ceres as discovered by the mission. Based on what the mission finds, it could apply for a release from the 20 year orbit requirement. On the other hand, the mission might (the presentation specifically mentions observations of recent melting as relevant here) instead be required to demonstrate (i) a stable orbital lifetime for an additional 20 year period, (ii) a "long-term stable orbit around Ceres" ["long term" undefined] or (iii) a plan to "leave Ceres orbit."

So the answer to your question seems to be "Category II," subject to change.

In the course of researching this post, I found the November 2013 minutes of the planetary protection subcommittee of the NASA Advisory Council. I'm confused by the highlighted words in the following sentence at p.7: "If the Dawn mission successfully enters orbit around Ceres, it will be going so fast that impact avoidance is assured." I can't figure out what "so fast" has to do with Dawn's method of gradually slipping into a stable orbit.

Can anyone interpret that, or is it just a (perfectly understandable) instance of an over-worked minute taker writing something down that's a little confused?

Edited for a typo, to fix a quote, and to avoid potential issue with forum guidelines.

This post has been edited by Tom Tamlyn: Jan 22 2015, 07:36 AM

[Explorer1]

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).

[Tom Tamlyn]

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.

[vikingmars]

.../... What features will be revealed?

...and here is the view from DeviantArt...

[belleraphon1]

This ESA press release on HERSCHEL discovery of Ceres water vapor release has a map of the regions where the water emanates from.

http://www.esa.int/Our_Activities/Space_Sc...rf_planet_Ceres


Phil or anyone, are either of these areas (Piazzi or Region A) covered in this Ceres image release? My eye cannot find a good match.

Is the bright spot in Piazzi?

Craig

[Phil Stooke]

My longitudes look like they are a bit off. I'll look into that later.

Phil

[belleraphon1]

Thanks Phil...

If Region A contains the bright spot, my guess is we are seeing a fairly fresh impact with watery ejecta blanket sublimating away. Perhaps adding to the vapor emissions spotted by HERSCHEL?

From the news release “We estimate that approximately 6 kg of water vapour is being produced per second, requiring only a tiny fraction of Ceres to be covered by water ice, which links nicely to the two localised surface features we have observed."

6 kg seems an awful lot to come from just an ejecta blanket and crater interior so would suspect this is just a contributor.

In about a month we will know if impact crater... cool

[Habukaz]

...and here is the view from DeviantArt...

Nice! There is something about it that makes it very believable.

Meanwhile, I tried to get an answer on when the next image(s) are released, and here's the reply:

hoping by the end of the month

so if we are lucky, maybe by the end of next week? (Sunday next week is 1 February)

[climber]

...and here is the view from DeviantArt...

Olivier, I remember the picture you show us on Philae landing day about what we "may be" were going to see once on the ground...
So, well, yes, we may see what you are showing here... may be not
La critique est facile mais l'Art est... très beau ici
(I love the view you're showing here...thanks so much.)

[vikingmars]

.../... (I love the view you're showing here...thanks so much.)

Thanks so much Climber !