Wednesday, February 22, 2023

Orion's Impact?

There is exciting news from Dr. Phil Stooke about Orion, the Apollo 16 Lunar Module Ascent Stage. I’ll discuss it more below, but first I’ll share some additional simulation results for Orion.

This post showed that by concentrating simulations on the last known inclination of Apollo 16, all the resulting impacts are in a small area of the Moon, around 6 km on a side. That was a surprising and exciting result, but the database of impact points was rather small. Out the original 350 impacts I had simulated, only 28 were within the tighter (+/-0.5°) inclination range. So, I have generated a lot more simulations within that range.

From my original parameter set, I “squeezed” the initial azimuth variation into the range of interest, keeping all the other parameters the same. (Actually, I generated entirely new set of random azimuth values in the narrowed range, rather than compressing the old values.) Then I had to go through another round of nudging of these parameter sets to coax the impact times towards the right value, as defined by the seismic signal which seems to have been generated by the stage impact. What’s the result? All but one of the simulated impacts are within the same small area defined by the earlier set. The figure below shows how it looks. Each blue dot is one impact point. Everything is centered on 104.3° E and 10.0° N, within about +/- 0.1° in both directions. I’ve uploaded the data to GitHub as a csv file if you want to check it yourself.

Figure 1: More simulated impact points for the Orion Lunar Module 

With the additional data points, patterns in the data start to be clearer. The impact points are generally concentrated along a band that runs from the SW to NE. Why is that? You might think that the orbital paths are running along this track, but it isn’t true. All the ground tracks are very close to due West, and I added the black "Heading Angle" arrow in the upper right corner as an example. In fact, the banding of the impacts is a reflection of terrain contours in this area. I included the red contour lines (at 250 meter intervals) in the plot of figure 1, and it’s clear that the ridge where these impacts strike the Moon is running SW to NE. (The ridge is the rim of a nearby crater. This link takes you to a view of the Moon that I used as the background for the plot, so that you can zoom out and see what else is in the neighborhood.)

Now for the exciting news. In the middle of the area where all these simulations strike the Moon, Dr. Phil Stooke has located a candidate structure showing the hallmarks of a Lunar Module impact! Dr. Stooke has previously located several other LM impact sites and is expert in processing and enhancing images of the Moon from NASA and other sources. In particular one of his best tricks is to combine images taken with low Sun angle from East and West, so that he can eliminate the shadows of the craters, and then enhance the contrast on the remaining features. Using this technique, he happened upon a characteristic V-shaped feature, which is very similar to other LM impacts that he has identified. The figure below is one of the images he has enhanced, and you see an East-West gouge with a V-shaped rays emanating Westward.  It is located at 9.99° N, 104.26° E, as indicated by the green rectangle in Figure 1. Unfortunately, the available images of this site from the Lunar Reconnaissance Orbiter (LRO) are lower resolution than other parts of the Moon that NASA has studied more closely. Also, the main impact crater is oriented East-West, so it never gets shaded as the Sun moves across the lunar sky, making it hard to see its extent clearly. 

Figure 2 is one of the enhanced images Dr. Stooke shared with me, which he produced using his shadow-cancelling flow.

Figure 2: Combined images showing an East-West gouge with a V-shaped fan emanating westward. Could this be the location where Orion struck the Moon?

Until better images are available, we can only count this location as a candidate for Orion’s final impact site. Meanwhile, this work will be presented in a poster session at the 2023 Lunar and Planetary Science Conference in Houston on March 15th, so the scientific community will have a chance to weigh in. My thanks to Dr. Stooke for submitting the abstract, putting the poster together, and basically doing all the legwork to present this result. I hope someday one of the still-operating lunar imagers, or perhaps a future one, can be used to examine this location in greater detail. 

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