Dawn Survey Orbit Phase, First orbital phase Options

[tfisher]

another thing to question is the size of the error bars on the Hubble measurements of Vesta's size. The Thomas et al 1997 paper is online at http://www2.keck.hawaii.edu/inst/people/co...wg/icarus97.pdf. Skimming through that, they use images with a resolution of 52km per pixel. They estimate errors in the semi-major axes of +/- 5km, or 1/10 of a pixel. I don't know details of their fitting, but it doesnt seem crazy to me that they might have missed by a bigger fraction of a pixel than that. Getting sub-pixel info from images is hard.

[elakdawalla]

What surprised me about that paper was that their definitive reference for Vesta's size based on occultation data was from 1989. My 5 minutes of trawling in ADS didn't turn up anything obviously better. Given Vesta's huge size and interesting shape I'm surprised there haven't been more, better-quality occultation studies since then.

EDIT: Looking at fig 2 of that paper, I think it can be stated pretty strongly that no semi-axis of Vesta is at all likely to be longer than 290 km.

[Juramike]

New image! http://www.nasa.gov/mission_pages/dawn/mul...0721-image.html

[elakdawalla]

OK, now it looks like Hyperion. Vesta's an asteroid of many guises

[ugordan]

So *that's* what it was reminding me of!

[tfisher]

Looking at fig 2 of that paper, I think it can be stated pretty strongly that no semi-axis of Vesta is at all likely to be longer than 290 km.

I'm not so convinced. My understanding is those numbers are for best fit ellipses. But Vesta has a big chunk missing on one side. Fitting an ellipse to a shape which isn't an ellipse could give a systematic bias. The missing chunk would make all the best fit ellipses underestimate the true maximum dimension.

[Mongo]

Has anybody looked at the effects of geometric perspective? Dawn is now close enough to Vesta that it should show a somewhat greater angular diameter as seen from Dawn than you would expect by simply dividing Vesta's diameter by its distance from Dawn. Vesta's edge, as seen by Dawn, is actually slightly closer to Dawn than the "great circle" perpendicular to the line connecting the centre of Vesta with Dawn, which would be slightly smaller as seen by Dawn (if it could be seen through solid rock). Therefore the number of pixels from one side of Vesta to the other would be slightly more than calculated using Vesta's diameter, and its distance from Dawn.

I do not know if the difference would be enough to account for the discrepancy described earlier, but somebody could check?

[jsheff]

I have a question about Vesta's reference grid. A few days ago an image release specified as "The original image was map-projected, centered at 55 degrees southern latitude and 210 degrees eastern longitude." OK, the latitude is determined by Vesta's spin axis, of course, but how is the system of longitude determined? What feature on Vesta defines 0° (or some other standard) longitude?

[elakdawalla]

The answer to that question is in the paper that tfisher linked to: http://www2.keck.hawaii.edu/inst/people/co...wg/icarus97.pdf

We propose a prime meridian centered on the most prominent visible feature. This is a nearly circular area, about 200 km across, with a lower albedo than its surroundings. Its origin is unknown at this time. Because it is the largest visible feature on Vesta, the name Olbers Regio has been proposed in honor of the asteroid’s discoverer; the name is not yet official. We have adjusted the longitude system to put 08 longitude at the center of this feature....

[jsheff]

The answer to that question is in the paper that tfisher linked to: http://www2.keck.hawaii.edu/inst/people/co...wg/icarus97.pdf

Thanks, Emily ...

[Greg Hullender]

Dawn is now close enough to Vesta that it should show a somewhat greater angular diameter as seen from Dawn than you would expect by simply dividing Vesta's diameter by its distance from Dawn.

Well, it's the difference between 2*r/d vs 2*arcsin(r/d), right? So if I take 10,500 km as the distance and 289 km as the radius, then I get an angular diameter of 3 degrees, 9 minutes, 16 seconds. As I figure it, your approximation is 2 seconds smaller.

In survey orbit, at 2700 km, the difference will amount to about a minute and a half. (Someone should double check me, though.)

--Greg

[Mariner9]

I think we have a new image just posted at JPL website. Taken on July 18th.

Not sure how to embed a thumbnail, so I'll just have to put in a link.

http://www.nasa.gov/mission_pages/dawn/mul...0721-image.html

[djellison]

I think we have a new image just posted at JPL website. Taken on July 18th.

Already being discussed for a page of this thread - first cited here

http://www.unmannedspaceflight.com/index.p...mp;#entry175945

[dilo]

Another interesting stat might be energy/kilogram. Although kinetic energy is increasing, potential energy is decreasing.

Nice idea, Greg; this is the output in terms of energy per mass unit (on the left), with a zoom around the insertion time:

On the right, a plot of ratio between the two energies vs distance (obviously, I considered absolute vaue of potential energy, which is increasing while approaching Vesta); target is a 2700 km distance with a 0.5 ratio (a circular survey orbit).

[Phil Stooke]

I have been playing with a very rough map of Vesta from the released images. Please bear in mind this is VERY approximate and not controlled by any shape model or pointing information. It is intended just to show approximate image coverage and locations of major features. The tie to more distant images is very rough. Zero longitude in the Hubble map/shape model coordinates would be at the left end (and the right, I guess). A much earlier version of this, posted here earlier, used a different (arbitrary) zero longitude.

Phil