Saturday, May 14, 2022

Orion's Impact Area

In a recent post I showed that one event in the seismic catalog of the Moon seems to have recorded the impact of the Apollo 16 Lunar Module “Orion”. This event occurred late on May 29th, 1972, about five weeks after Orion was jettisoned. Then in my last post I described a way to “nudge” the initial conditions of a simulation in order to move the impact date/time towards the time of this event. Using this nudging technique, I have been able to generate several hundred simulations, all random variants of the nominal orbit of Orion, all of which impact the Moon within an hour of the target event at around 21:14 UTC. I have posted csv and Excel versions of the combined result files on GitHub. The files include the initial orbital state used for the simulations plus other initial state data, along with the impact location and time for each case.

We can’t have perfect knowledge about the initial orbital state of Orion. These simulations represent a set of initial conditions that vary randomly around my best guess at the nominal state, allowing us to get a reasonable picture of the possible outcomes for Orion given the uncertainties. What is exciting about the results is that the simulated impacts are concentrated in four “high terrain” areas of the Moon. These are the same four impact areas I found earlier with a smaller set of simulations. That’s good! The search area didn’t expand even though we have a larger database.

Figure 1: Impacts from the new database superimposed on a map of the Moon. There are over 350 simulated impacts, all striking the surface within an hour of the target event on May 29th, 1972.

Figure 1 shows the impacts superimposed on a map of the Moon. You can see that each impact cluster is in a place where the terrain is higher…mostly along the ridges surrounding craters. Again, this makes sense: as the orbit destabilizes, the spacecraft is on a flat trajectory at its low point, and it will strike the first piece of high ground it encounters. Overall the possible locations for Orion’s final impact seem pretty well constrained.

Could we tighten things up even more? In looking deeper at the data, it appears that we can. In the result files mentioned above, one extra parameter included for each parameter set is Orion’s initial inclination. Using this data, we can look for any correspondence between inclination and impact point, as plotted in Figure 2. Lo and behold, there is a pattern! The impact longitudes cluster into bands depending on the initial inclination. If we could determine the inclination more precisely, we could focus in on one or two of the clusters.

Figure 2: Orion's Impact Longitude versus Initial Inclination. All the simulations close to the nominal inclination value result in impacts near 104.3° East longitude. This leads to a very small area to search for Orion's impact crater.

As it happens, we can get a very good guess at Orion’s initial inclination, thanks to the Metric Camera database. Prior to casting off Orion, the Apollo 16 crew ran a camera pass, exposing a 70 mm film picture of the Moon’s surface every 10 seconds. Meanwhile another camera took simultaneous pictures of reference stars, so as to know exactly which way the mapping camera was pointing for each shot. This allowed NASA to determine the latitude and longitude of each picture with great precision, which works back to the latitude/longitude of the spacecraft. 

Inclination means how much the orbit is tilted away from the equator, so if we look at all the pictures and find the one that is farthest north or south of the Moon’s equator, that tells us the inclination. It turns out that during revolution 60, a few hours prior to when Orion was jettisoned, there was a mapping camera pass, and we can see from the image database that image AS16-M-2828 was the south-most picture in the run, taken from a point above 10.55 °S. Therefore, the orbit was tilted 10.55 degrees away from the Moon’s equator. Since the orbit was “retrograde”, or against the Moon’s rotation, we reference the inclination to 180°, so it is expressed as 180-10.55 = 169.45°.

Take a look at Figure 2 again. If we limit the inclination values from 169.4° to 169.5° All the impact longitudes are in a narrow band around 104.3°. Wow! That gives us a very small area to look for Orion. Figure 3 is a plot of the impacts from this narrow inclination range. They are clustered within +/- 0.1 degrees in both latitude and longitude. That translates to a square-ish area about 6 km on each side.

Figure 3: A plot of impact locations after applying the inclination constraint. This is an area roughly 6 km on a side. Based on all the evidence, this seems to be the most likely area where Orion struck the Moon in 1972.

To give a sense of scale, Figure 4 compares this impact area to a part of Pasadena, California that is similar in size. The California Institute of Technology is at the lower right corner and the Jet Propulsion Laboratory is at the upper left. The Rose Bowl stadium, along the left about 1/3 of the way from the bottom, gives a sense for the scale of the craters.

Figure 4. Comparison of the impact area to a section of Pasadena, California.

I'm really surprised at how far this analysis has come. When I started, I was hoping that perhaps one of those Mapping Camera pictures should show the impact area BEFORE impact, making it possible to compare with modern images, and perhaps identify any "new" crater. It turns out that isn't feasible. None of the pictures from the mission provide the needed coverage. Given the relatively small size of the area I have identified, perhaps an exhaustive search may turn up some craters or features of interest? 

I have been very impressed by the work of Dr. Phil Stooke, who has been able to identify Lunar Module impact locations for Apollo 12 and Apollo 15, among other notable finds. Perhaps, with the above analysis as a starting point, Dr. Stooke or others might be able to locate the final resting place of Orion someday. 


  1. Hi!
    Nice work!
    I think there is a typo here: "If we limit the inclination values from 169.4° to 169.5° All the impact longitudes are in a narrow band around 102.3°." Actually, figure 2 shows the longitude to be ~104ยบ, as later used in figure 3.


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