In honor of Stardust: The Annefrank encounter
Posted by Emily Lakdawalla
24-03-2011 16:32 CDT
Since Stardust is being decommissioned today I thought it'd be fitting to take a look back at one of its data sets. I hadn't fiddled with the Annefrank data set before, and it was small and easy to deal with. I found as I was working with it that its metadata was written by somebody with a sense of humor.
I do not remember the Annefrank encounter, which is kind of embarrassing because I was working at the Planetary Society at the time (but not yet as its Web writer or blogger, so I guess I wasn't paying attention to the news). Stardust flew past Annefrank on November 2, 2002, a bit more than a year before its encounter with its main target, comet Wild 2. Annefrank is on the smaller end of the scale of bodies that have been visited by spacecraft at about 6.6 by 5.0 by 3.4 kilometers. Although the science team was of course interested in the data that they'd get from Annefrank, the encounter was really intended as a shakedown one for Stardust to prepare for Wild 2.
Here's the best image I've seen of Annefrank, processed by (of course) Ted Stryk.
The data set is available at the Small Bodies Node of the Planetary Data System. Here are the 71 images of Annefrank in which you can see anything of the asteroid. A few of the early ones are smeared; all are slightly blurry due to the contamination of the imaging system, but it's not so bad that you can't see details on the asteroid. A bunch in the middle are saturated (meaning that a long exposure resulted in the image being brighter than the detector could handle, washing out detail on the sunlit face of the asteroid). The final set of images alternate in brightness. Those end ones were taken at a rapid enough cadence that there's not a lot of apparent differences from one image to the next, which means you can stack them to reduce noise and blur and to bring out detail, which is I think one of the things Ted did to make his cleaned-up version. What you see below is the full resolution; Annefrank isn't all that big, and at the 3,000-kilometer flyby distance (more than 10 times as far as the encounter distance between Stardust and the bigger Tempel 1) Annefrank never got more than about 35 pixels across.
There's a text file describing the acquisition of the data here, and it's really an interesting read, being more informative than the norm about the thought process that went into sequencing the spacecraft. My favorite bit is the use of the highly technical descriptor "horrendous" to describe smear in some of the images. Here are some lengthy excerpts:
2002-10-31: Image Sequence #34 (Images 346-350)
One of the goals of the engineering readiness tests on Annefrank was to exercise the optical navigation team and to attempt to improve flyby accuracy using optical data. The approach to Annefrank was from a phase angle of 150 degrees, unfortunately, which meant the asteroid would be poorly illuminated and be very faint. There were no asteroid data available for phase angles larger than 100 degrees. It was assumed that Annefrank would be about 1.5 magnitudes fainter than the nearly linear decrease of about 0.03 mag/deg that is common to asteroids at smaller phase angles. It seemed that we would have a fair chance of detection 38 hours before encounter, which time was used for this first attempt.... As we later found out, the asteroid was much fainter than expected and the spacecraft drift during the exposures (smear rate) much larger than we had previously experienced. Further the camera pointing was not asaccurate as we had expected (the geometric calibration was not yet solid)....This is why these tests were run, to make sure something like this doesn't happen to us on Wild 2. The bottom line is that the asteroid was not found in these five E-38 hour images.
2002-10-31: Image Sequence #35 (Images 351-355)
A second set of approach images was acquired at E-32 hours. The same windows and exposure times were used as for the previous set at E-38 hours. The problems were much the same, as were the results. Annefrank was not found.
2002-10-31: Image Sequence #36 (Images 356-360)
A third set of images was acquired at E-26 hours. The same windows and exposure times were used as for the previous sets
aken at E-38 and E-32 hours. The problems were much the same,as were the results. Annefrank was not found.
2002-11-01: Image Sequence #37 (Images 361-365)
Given the experience of the first three sets of approach images, the navigators decided to increase the window size to 181x181 pixels and make all of the exposures 5 seconds for this set at E-18 hours. The image smear can only be described as horrendous. This doesn't matter for measurement purposes, IF the target can be found. The asteroid was still several magnitudes too faint for detection when smeared over some 20 pixels, and it was not located....
2002-11-01: Image Sequence #39 (Images 371-407)
Twenty-five minutes before the closest approach, images were acquired to attempt autotracking. Pointing was based upon radio navigation of the spacecraft and the best ephemeris for the asteroid supplied by JPL's celestial mechanics specialists. By this time the phase angle was down to 130 degrees and the range was only 11,415 km. Annefrank appeared in the first image, though far from centered. [In fact, it was very nearly off the detector, at the extreme upper left edge. --ESL] The navigators chose an exposure of 65 ms to make sure they were going "deep enough," so the images were well exposed. After the first few images, only every third image was transmitted to the ground, the others being used only to initiate autotrack. After 15 minutes, at a range of 5434 km, exposure was reduced to 25 ms. In all, 15 of37 images taken with 65 ms exposure were received on Earth. Of these, the first two or three were partially on the periscope, and three show a large amount of smear, but several are of scientific use. Autotracking was initiated shortly before reducing the exposure, and image 410 and all subsequent Annefrank images are well centered in their frames.
2002-11-01: Image Sequence #40 (Images 410-445)
Exposure times on Annefrank were reduced to 25 ms beginning with image 410 at a range of 5088 km and a phase angle of 113 degrees. Images beginning with #420 started to show saturation. This was predicted, but these images were being taken to test the autotracking rather than for scientific purposes, and autotrack works perfectly well with saturated images. The images soon reached 80% saturation, so images 420 through 445 are of limited scientific use. Every image was transmitted to the ground beginning with #426, a total of 26 images with 25 ms exposure. Twenty-two of these have some to nearly total saturation. [These images are on the second, third, and fourth rows of the montage above. --ESL]
2002-11-01: Image Sequence #41 (Images 446-476)
Beginning with image #446, exposure time was reduced to 5 ms. In fact the characteristics of the shutter are such that alternate images are given exposures shorter by 1.5 ms than the set value, so in fact all even numbered images have an exposure of 3.5 ms and odd numbered ones 5 ms. [This is what causes the alternating brightnesses of the images on the final four rows of the montage. --ESL] It was intended that these images be of scientific as well as engineering use. If Annefrank had not been acquired by this time, there was little hope of acquiring it, so there was no need to saturate the images. The subsequent images (through image 476) taken at phase angles from 71.0 to 47.2 degrees constitute the best images for scientific use. During this period the range fell from 3133 km to 3078.5 km and increased back to 3162 km, so there is minimal change to scale.
These images beg to be animated, but they're very small:
I enlarged the images and then threw out the smeared ones and the brighter of the alternating-brightness near-encounter images to make an animation of the flyby: