Another Close Encounter!

KPLO as seen from LRO — Hard to find, but centered here, you can see a fuzzy view of the Korea Aerospace Research Institute (KARI) lunar orbiter, Danuri, as it flew just 8 kilometers below LRO. Image width about 3000 meters, NAC M1464323568 [NASA/GSFC/Arizona State University].

Three Danuri imaging opportunities on consecutive orbits occurred late on March 5th and into the early hours of March 6th. Due to the fast relative velocities (>3.2 kilometers per second), exquisite timing was required to point LROC to the right place at the right time. The flight paths of the two vehicles were nearly parallel but in opposite directions, resulting in extreme relative velocity. The LROC NAC exposure time was very short, only 0.338 milliseconds. But still, Danuri was smeared by a factor greater than 10x in the downtrack direction. As always, the talented LRO operations team at the NASA Goddard Space Flight Center enabled the LROC observations.

First Danuri image from LRO, raw geometry — On the first opportunity, LRO was slewed 43 degrees to capture Danuri from a distance of 5.0 kilometers; M1464309505, 2024-03-05 20:24:05.408 UTC, Danuri pixels 5 cm x 109 cm [ NASA/GSFC/Arizona State University].

Second Danuri image from LRO, raw geometry — During the next encounter, Danuri was closer to LRO, 4.1 kilometers, and the slew angle was 25 degrees; M1464316536, 2024-03-05 22:21:17.510 UTC, Danuri pixels 4 cm x 109 cm [NASA/GSFC/Arizona State University].

Third Danuri image from LRO, raw and corrected views — For the final image, LRO slewed 60 degrees when Danuri was 8.1 kilometers away; left, the NAC pair was corrected for parallax, and right, the Danuri pixels were unsmeared and aspect corrected. Here, the image was rotated 90 degrees into a standard panoramic format. The full image is shown below, M1464323568LR, 2024-03-06 00:18:29.651 UTC [NASA/GSFC/Arizona State University].

Below is the dramatic full NAC oblique (third image M1464323568LR) with Danuri near the center; the large bowl-shaped crater in the background is 12 kilometers in diameter [NASA/GSFC/Arizona State University].

An opposite encounter occurred last spring when the ShadowCam instrument captured an image of LRO as it passed beneath Danuri. ShadowCam was derived from the LROC NAC, the main difference being a much more sensitive detector that allows ShadowCam to acquire high-resolution images of permanently shadowed regions (PSRs). Malin Space Science Systems, San Diego, CA, designed and built the LROC suite and ShadowCam.

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Published by Mark Robinson on 5 April 2024