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Images from the long-awaited Dawn Vesta data set

Posted By Daniel Macháček

29-11-2012 11:55 CST

Topics: pretty pictures, explaining image processing, amateur image processing, asteroids, asteroid 4 Vesta, Dawn

This blog entry first appeared at Macháček's own blog and is translated from Czech and reposted here with permission. --ESL

A few days ago, the Dawn mission finally published their archival data. The data were originally published at the turn of 2011/2012. Then the situation was complicated by what always complicates things: politics. If you are interested in details (especially regarding disputes over Vesta's coordinate system), you can read that here. Finally, the situation was resolved and the data were published. During the year of delay I often looked with anticipation to the Planetary Data System to check whether or not images were there, and I am delighted that they are finally available.

Was the wait worth it? Definitely!

Dawn spent more than one year at Vesta. During this year, the mission published many images from the onboard Framing Cameras (FC) to their website. The problem is that the images were published in such a manner that were difficult to edit, so they were not interesting for amateurs of my kind. Also, they were published as isolated frames, making it difficult to perceive the data in their broader context.

In the newly released data, we can see many sequences of overlapping images, which show Vesta as a unique body with interesting geology. Currently, the only images available were taken from a relatively high orbit (during the periods of approach, arrival, and the High Altitude Mapping Orbit or HAMO).

Here is one such sequence of images, taken on August 24, 2011, quickly assembled into an animation. It shows Vesta moving due to the relative motion of Dawn and Vesta and also due to the rapid rotation of Vesta. The rapid rotation is evident in the changing positions of the shadows. In the animation you can see part of the region around the south pole of Vesta, containing an impact basin with a tall central mountain.

Example image sequence of Vesta from Dawn spacecraft

Animated sequence of Framing camera images 0006236 to 0006314. The south polar region and Rheasilvia impact basin are visible. Taken on August 24, 2011 from an altitude of about 2750 kilometers, with a resolution of 270 meters per pixel.

Credit: NASA / JPL / MPS / DLR / IDA / Daniel Machacek

The images generally have excellent quality despite the fact that they have not yet been calibrated. Unfortunately, sequential images were often separated by a large time interval (of several minutes). For a rapidly rotating asteroid like Vesta, the change in geometry makes it very difficult to assemble a mosaic. But for short sequences, say four shots from 0006121 to 0006124, it is sometimes possible to assemble a mosaic.

Mosaic of the asteroid Vesta from the Dawn spacecraft

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Mosaic of the asteroid Vesta from the Dawn spacecraft
Images 0006121, 0006122, 0006123, 0006124 taken by the Framing Camera on August 23, 2011 from an altitude of 2747 kilometers and with a resolution of about 250 meters per pixel.

The mosaic above is the highest-resolution that can be created through simple methods. All of the images in the Vesta data set that are suitable for such mosaics are black-and-white, taken through a clear filter. So how to create a larger color image of Vesta? One option is to create a 3D model and perform a suitable reprojection of images on this model. This is the method chosen by the Framing Camera team. They have not yet published any larger mosaic, but in the future we can expect them to do so.

Because I do not have such experience yet with 3D graphics, I constructed my own color mosaic of Vesta in another way. I began with one of the last images of Vesta that contained (nearly) all of the asteroid, image 0003520. I magnified this by a factor of three, and used it as a background image. Then I took higher-resolution color filter images (taken through the red, green, and blue filters) and warped them to fit the background. Here is the result.  The brownish color corresponds to the color of Vesta as seen in visible light. At this point, I have to thank one of the members of unmannedspaceflight.com, Stefan, who recently published a small but useful composition of three asteroids in approximately true color. I used this composition for the final fine-tuning of my color mosaic.

Approximately true color mosaic of Vesta

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Approximately true color mosaic of Vesta
All images used are named in image. Color is from images taken through F7 (red), F2 (green) and F8 (blue) filters. Images were taken August 12-18, 2011 from an altitude of 2750 kilometers. The original data had a resolution of about 260 meters per pixel. It has been resampled to a resolution of 200 meters per pixel.

The attentive reader will probably quickly notice the strange shadows in this mosaic. The reason is quite simple. Although it approximates a true-color image, you would never actually see Vesta look like this. It is a mosaic of the southern hemisphere of Vesta, with the south pole approximately in the middle. Because Vesta rotates once every 5 hours (+ a few minutes), the shadows change so fast that there is not a set of images covering all of Vesta with the same lighting and from the same relative position of Vesta and Dawn. My most viable option was to use the frames with the best lighting and minimal number of shadows. The resulting mosaic shows the polar region and Vesta in almost full lighting, which is physically impossible.

What is the disadvantage for mosaics, is an advantage for stereoscopic imaging. Almost every two consecutively shot black-and-white images are so different that they can be displayed as stereograms. The pictures below are such stereograms, where you can see a relatively fresh crater named Cornelia.

Cornelia crater, Vesta, in stereo (red-blue anaglyph)

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Cornelia crater, Vesta, in stereo (red-blue anaglyph)
Stereo image of the crater Cornelia on the asteroid Vesta made from images 0010191 and 0010192. Date: December 13, 2011 from an altitude of 686 kilometers and a resolution of 64 meters per pixel.
Cornelia crater, Vesta, in stereo (crossed-eye stereo)

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Cornelia crater, Vesta, in stereo (crossed-eye stereo)
Stereo image of the crater Cornelia on the asteroid Vesta made from images 0010191 and 0010192. Date: December 13, 2011 from an altitude of 686 kilometers and a resolution of 64 meters per pixel.

Originally I had planned to do another mosaic as in Figure 2, but I finally (for now) decided not to, for reasons of time. The color of Vesta is in visible light is very monotonous. But the Framing Camera system is not limited to visible light. It can also take photos in near-infrared bands. If we look at Vesta in this part of the electromagnetic spectrum, its appearance changes.

Below you can see a total of six individual frames, three each in two columns. The right column is made up of almost identical pictures as the left. The only difference is extremely increased color saturation. Each row of images was created using images taken through a different set of filters. Dawn's color filters admit wavelengths of light in a zone about 40 nanometers wide. The top pair of images were created from images taken through filters F7, F2, and F8, corresponding to 653, 555 and 438 nanometers (which are red, green, and blue visible light). This is the same color combination as in my color mosaic above. The middle pair is made up images taken through filters F4, F7, and F2, so the red frame was replaced with an infrared-filter image (centered at 917 nanometers). In the bottom pair of images, the green channel has been replaced with one taken through the F6 filter, with a wavelength of 829 nanometers. With these infrared channels, there are suddenly visible differences in the colors of the different terrains that are indicative of different compositions of surface layers.

Infrared color imaging of Vesta

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Infrared color imaging of Vesta
Comparison of images of the region around Antonia crater on Vesta (it's in the top right part of the images) as seen through different filters. Images used: 0013824 (F2, 555 nm), 0013826 (F4, 917 nm), 0013828 (F6, 829 nm), 0013829 (F7, 653 nm), and 0013830 (F8, 438 nm). The top left image is approximately true color; all others are false-color images. Images in right column are highly saturated versions of the images in the left column. The sequence of images was taken on October 30, 2011, from an altitude of 725 kilometers and with a resolution of 68 meters per pixel.

To finish, I have two pictures of the south polar mountains, which further demonstrate the effect of filters on the appearance of the resulting images. The first image is approximately true color.

Rheasilvia central peak, Vesta

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Rheasilvia central peak, Vesta
Approximately true color image of the south polar mountain in center of the crater Rheasilvia. Images 0005756, 0005761, 0005755. Taken on August 20, 2011 from an altitude of 2713 kilometers and with a resolution of 258 meters per pixel.

Here is a much more colorful picture, using frames taken through filters F2, F3, and F4, on August 6, 2011, from a height of 2717 km, with a resolution of 256 meters per pixel.

Enhanced-color view of Vesta's southern mountain

NASA / JPL / MPS / DLR / IDA / Daniel Macháček

Enhanced-color view of Vesta's southern mountain
A view across Vesta's south polar terrain to the central peak of its Rheasilvia basin. The view is composed of green and infrared-wavelength images, emphasizing the color variations among materials on Vesta's surface.
 
See other posts from November 2012

 

Read more blog entries about: pretty pictures, explaining image processing, amateur image processing, asteroids, asteroid 4 Vesta, Dawn

Comments:

Hakan Koseoglu: 11/29/2012 12:52 CST

Excellent pictures and lovely work, thanks Daniel! I really wonder why Vesta pictures were such a trickle. Dawn was there for ages but not enough pictures were made available with enough fanfare. Galileo pictures were available immediately and they're always gorgeous and they make a good PR noise about them for good reason.

Benjamin: 11/29/2012 04:40 CST

Thanks for the cross-eyed stereo image! Emily never includes those and I don't cary with me colored classes.

Stephen Uitti: 12/03/2012 04:00 CST

I can hardly wait for a printable tab-a into slot-a 3D Vesta, like the Phobos and Deimos. This year on the rainy first day at Astronomy At The Beach, we managed to hand out our entire stack of 150+ moons. I'm sure part of it was the rain. I'd be happy if the IAU officially made Vesta a Dwarf Planet. They'd "sell" like hot cakes.

Marc Rayman: 12/04/2012 10:33 CST

Beautiful work, Daniel. I have seen your images at UMSF, and I'm glad Emily posted this more complete description here. I look forward to more results.

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