The Staging Disaster (That Never Happened)

Among people who celebrate the Apollo program, there is a group that can recite the details of a near disaster during Apollo 10. They talk about the Lunar Module spinning out of control, seconds from impacting the moon. A chaotic staging in the middle of engine firings. It's a dramatic story, no doubt. Fortunately, for the men on board at the time, the story is largely false. The dramatic events actually never occurred, at least not in the way they have come to be described.

A still from the 16mm film taken as the descent stage moves away during staging. Notice that the lunar horizon is "upside down".

I can't blame people for believing in this mythology. Take a look at this excerpt of a video that has been shown on the Discovery Channel. We see the LM apparently in the midst of a rehearsal for landing. The descent engine is running. Then suddenly we see the stages separate and the ascent stage goes in a rapid, wild tumble. These are powerful images.

I am sure that the Discovery Channel showed this animation over and over to attract attention to their show. I don't know if the people who produced it knew it wasn't real, but for sure, if they had spent a few hours researching the mission records, they would have known it was a false telling of the tale. 

Just to be clear, something did go wrong at staging. At the moment it occurred, in May of 1969, no one knew exactly what was happening, and there is no doubt it was a frightening incident. Both Tom Stafford and Gene Cernan, the astronauts aboard, deserve great credit for quickly recovering and keeping their mission on track. In this post I will look at some of the fictionalized accounts online, and then go back to the original records to piece together what really happened that day. The exaggerations that have been popularized in recent years don't honor the true courage and flying skills of these brave men.

First lets look at more of the false accounts.

Here is a YouTube video from "Seeker", which claims that "the spacecraft could have slammed into the Moon." In fact the astronauts were in a stable orbit, and slamming into the surface was simply not something that would have happened in any scenario. 

 

Sadly, even the Draper Corporation has joined in the fiction-fest. This company descended from the MIT instrumentation lab that designed the guidance and wrote the software for the Moon landings! But this web site, which Draper sponsors, joins the chorus of bogus descriptions of the staging incident. They claim that the crew was "seconds from crashing into the moon when they successfully regained control." Fortunately for the Apollo crews, Draper's standards for reality-checking were tighter back in 1969. No doubt Charles Stark Draper is spinning in his grave faster than an out-of-control LEM.

Here is one more misleading account of the event, and again one would hope for better fact-checking from the source...Smithsonian Magazine. While I appreciate their effort to draw attention to an overlooked Apollo mission, the claim that the staging gyration took place "after the ignition of the ascent engine" is simply false, as a few minutes of online research will quickly prove.

Surprisingly, it seems that the over-dramatized accounts of the staging incident tie back to someone who was there - Gene Cernan, the Apollo 10 Lunar Module Pilot. Back in1969 his descriptions of the events related well to the facts of the mission. By 2009, 40 years later, his sense of drama appears to have enhanced his memories, and the tale he tells has grown. He recounts "seeing the lunar horizon go by 8 times in 15 seconds". No wonder the event mythology has become so exaggerated. Eyewitness testimony! While I have great respect for Gene and all of his contributions to the space program, in this case we unfortunately need to doubt his words.

Let's go back to May of 1969 and uncover what really happened. There are a number of great sources to help understand the event, including film taken out the LM window, on-board audio recordings, telemetry data, the crew debrief, and the "anomaly" investigation report.

For starters, one fun video from the time is the live coverage that was being broadcast by CBS. They were "all in" on the Apollo program, and invested heavily in animations to complement the audio they were getting from NASA. Famously, there was no 7-second delay in those days, and we clearly hear Gene cussing on live television as things go south in the LM. In the video I linked above, Cernan recounts how his wife admonished him about his "salty" language when he got back to Houston. However even from the live coverage, it's clear something went wrong during staging, resulting in a "wild gyration".

Cernan's mic was "live", so his words went straight to Houston, and on to CBS, which is why he was tagged as the "salty" one. Meanwhile, an on-board tape recorder captured audio from both Cernan and Stafford. From the transcript of the tape produced by NASA, we see that Stafford (labelled "CDR" in the transcript) had a few choice words of his own. Fortunately for all concerned these words were not broadcast live. 

This excerpt from the LM transcript captures the tense moments of the staging incident.

To understand what really happened, there is one truly outstanding source - the Apollo Flight Journal. Robin Wheeler of the AFJ put together a video that combines the film, the on-board audio, and telemetry data, aligned to a mission timer. Anyone interested in the true story of this incident needs to watch this video. When I watch it I have to really admire the crew as they quickly and effectively dealt with a very unexpected situation. Clearly they are a great team, with years of experience training and flying together. Listen to the great communication as they keep each other apprised as events unfold.

From the video it's clear that the LM did not tumble or spin in the dramatic fashion of the Discovery Channel animation. The lunar horizon does not go by "8 times in 15 seconds" as Gene claimed. What we see in the video matches up much more closely to what Tom and Gene described in the crew debrief interview they gave when they got home. In that interview Tom states that "We got the vehicle under control after about, I'd estimate, a 360-degree maneuver." Gene's account is more conservative, saying that "We could have maneuvered 30 degrees or we could have maneuvered 90 degrees. All I know is that it was fairly slow." Then he added "whether we did a 360-degree maneuver is difficult for me to say." The account Gene gives years later, is very different, and is probably one of the the main reasons that the staging mythology grew. 

This chart shows the spin rates at the time of staging. The peak in the Z-axis (roll) was around 26 degrees per second...about 13 RPM.  Source

It's important to understand that neither the descent engine or the ascent engine was fired during the staging maneuver. The plan was to use the small "RCS" jets to speed up by 2 feet per second, kick off the stage, then slow back down to the original speed. This is basically what happened, but the unexpected attitude changes meant that the stage was kicked off more upwards than backwards. Robin Wheeler at the Apollo Flight Journal undertook a detailed analysis of the film and prepared this image showing the planned and actual staging attitudes.

The confusion about the engine firings probably arises because the LM was designed to be able to perform staging "hot", during descent. This "Fire In The Hole" capability had been tested with the first unmanned LM flight during Apollo 5. For Apollo 10, no such risk was taken with men aboard. The descent engine had been deactivated after the Phasing burn, two hours before. The ascent engine would be fired for the first time 10 minutes after staging, as the crew began their rendezvous with "Charlie Brown".

This table from the Mission Report shows the Phasing Burn that preceded staging and the Insertion Burn that occurred about 10 minutes after staging. We also see that the LM was in a 190-by-12 nautical mile orbit...with no risk of impacting the surface

Another point of confusion about the staging mishap concerns the altitude above the lunar surface when it occurred. Prior to the Phasing Burn, the LM had been in an orbit that took it to within 8.5 nautical miles of the surface. However, at the time of staging, one can see from the Mission Report that they were at an altitude of 31.4 nautical miles. They were never in danger of impacting the Moon.

Table 6-II from the Mission Report, showing the altitude of 31 miles at the time of staging

All of the original documents point to the staging event as much less dramatic than the one so widely depicted online these days. That bothers me. Although the mission was a success overall, there were plenty of challenges, and plenty of surprises for the talented crew to overcome, and they met them all in grand fashion. Thanks to the fantastic job by Gene, Tom, and John, Apollo 10 was an amazing success, which paved the way for the first landing on the Moon less than two months later. There is no need to exaggerate the mission to celebrate its triumphs.

Propellants

Snoopy's descent stage was jettisoned into lunar orbit with more than 8 tons of hypergolic propellants still aboard. If the stage orbit was long lived, what happened to all this fuel? This might be the thing that brought the stage out of orbit, so in this post I will examine the possible outcomes.

Descent Stage Cutaway View  Source

The mission was a rehearsal for the Apollo 11, without the landing. They undocked, then did the Descent Orbit Insertion (DOI) burn to drop the orbit closer to the surface, as Apollo 11 would. Then they did an additional "phasing burn" to get the right alignment with the command module for the rendezvous. One orbit later the stage was jettisoned. These two short maneuvers used only a small fraction of the fuel, leaving the tanks at about 96% of their capacity

This excerpt from the Mission Report shows the quantities of propellants loaded and consumed

The stage was not designed for long life, so no one knows exactly what might have happened. There are some facts in various NASA reports that offer clues, so we'll map out the clues and then make some guesses about the eventual outcome.

In a previous post I showed a simplified model of the Descent Propulsion System, or DPS. It was a simple, reliable system with tanks fed by pressurized helium. The tanks of fuel and oxidizer led to the combustion chamber, so when a valve was opened, the propellants mixed, ignited, and burned, creating thrust. In the earlier post I described how the main "supercritical" helium tank likely vented to space within a few days after staging. Then what?

Here is a complete schematic of the DPS plumbing. There are actually two helium tanks, plus various valves, burst disks, and so forth.

Source

In order to analyze what happened, its useful to simplify things, as I show below. The "Squib valves" were sealed during flight and then opened using small explosive charges when the engine was activated. Once opened, they never close, so it's cleaner to show them as open pipes. One set of squib valves was used to vent the propellants on the lunar surface, so for Apollo 10 these remained sealed. I believe they can be ignored. (I don't think it's possible that they could activate themselves.) So I eliminated those as well. The Supercritical Helium tank no doubt vented within a few days after staging, so I also eliminated that from the diagram, showing instead an opening to the vacuum of space. Here is the simplified schematic:

What do we know about this system when it was cast off? We know that the fuel and oxidizer tanks were pressurized to 247 PSI at 70 degrees. We know the tanks were still 96% full. The small Helium tank in the schematic is the high pressure "start bottle". (It was used to initially pressurize the tanks and start pushing fuel through the heat exchanger.) We know this tank had a slow leak. We know that the burst disks were rated to open between 260 and 275 PSI. We know the quad check valves might have had leakage rates up to 100 standard cubic centimeters per hour. We know that even without the main helium pressurization system, the DPS could operate from existing tank pressures, in "blowdown" mode, generating significant thrust.

We also know that the stage was slowly tumbling in orbit. The dramatic film taken during staging captures the unplanned attitude excursions, and a post-mission guidance report shows the rates at the moment of staging in this chart:

At staging, Snoopy's tail had yaw, pitch, and roll rates of -9, -4, and +7 degrees per second. So that's one full yaw rotation every 40 seconds, one pitch rotation every 90 seconds, and one roll every 52 seconds.

So what happened? I think there are four possibilities. 1) Slow leaks might have allowed the tanks to depressurize, until the stage reached a stable state. 2) The tank pressures might have increased until the burst disks failed, allowing things to vent to space. 3) Propellants might have leaked back through the check valves, into common helium plumbing, which might have led to combustion or even a catastrophic explosion. 4) Something might have caused the throttle to open, allowing the engine to start generating thrust. Let's take these one by one, in reverse order.

If something caused the throttle to open up, residual pressure in the tanks would have allowed for significant thrust in "blowdown" mode. For example, as Apollo 13 was headed back to Earth, it was noted that the DPS could provide an 800 f.p.s. velocity change to the full LM-CM-SM stack in this mode. In the case of Snoopy, the volume of gas in the tanks was lower, reducing the possible burn time. However the stage was much lighter than the Apollo 13 stack. If the throttle opened up, it might have knocked the stage out of orbit.

Detailed view of the throttle assembly (source)

I really doubt this happened, because several things would need to fail for it to occur. One of the two actuator isolation valves would have to fail hard open, allowing pressurized fuel to flow into the valve actuator, forcing the main shutoff valves open. Then the "thrust control actuator" would have needed to fail into the open state. Designing a thrust control valve that could fail open doesn't sound like something that would have passed muster during Apollo. If you have deeper insights, please leave a comment. In my view, a spontaneous blowdown burn is unlikely.

How about reverse leakage through the check valves? This could create big problems because it would allow both fuel and oxidizer to flow into the common helium feed plumbing. A high profile explosion during a ground test of the Crew Dragon spacecraft in April of 2019 was attributed to oxidizer leaking into helium pipes. NASA documents do state that early check valves had a higher leak rate than originally intended. So could this leakage cause the stage to explode?

For the check vales to leak, there must be reduced pressure in the helium feed. The helium system was still being fed by the small ambient "start bottle", so it probably remained at 247 PSI for some time. During the flight, the ambient bottle pressure dropped 35 PSI in 97 hours, apparently due to a leak that developed during launch. At this rate, the bottle pressure would have dropped to 247 PSI in about one year. After the other helium system vented, the leak rate might have increased.

So once the pressure was low enough for leakage to occur, does that result in an explosion? Honestly I don't know how to evaluate this possibility. If the leakage was in gaseous form, any reaction would probably be low-power. If liquids leaked, and eventually flowed together, the reaction would be more violent, perhaps even powerful enough to blow out the plumbing, or even trigger the complete destruction of the stage. Leave a note in the comments if you have insight into this possibility.

How about venting through the burst disks? These disks were designed to open up if the pressure in a tank reached 270 PSI, give or take. The tank pressures could have reached this level if they heated up, since pressure increases with temperature. The temperature in the tanks was 70 F after the second burn, and a rise of 43 degrees would have raised the pressure up to the nominal burst pressure. Could this have occurred? It seems unlikely. Orbiting the moon every two hours, half of each orbit in searing sunlight and half in freezing darkness, it seems that the heavy tanks would slowly reach thermal equilibrium. The surface of the moon in sunlight actually is quite hot, and radiates a lot of heat out into space, adding to the direct heat from the sun. But it doesn't seem that this would be enough to raise the temperature of those tanks to 110 degrees. Perhaps the hot/cold temperature cycles could lead to failure of the burst disks at a lower pressure? Again, I don't know how to evaluate this.

If the burst disks did open up, I don't think the orbit would have been significantly affected. Due to the "thrust neutralizers" and the tumbling of the stage, the net thrust would have been low. I believe this case would be similar to what occurred when the helium tank vented.

Finally, what about slow leaks that allowed the tanks to depressurize? For the fuel side, at least, this could have occurred through the isolation valves and pilot valves of the shutoff valve assembly. For the oxidizer this path doesn't exist. That sets up the possibility that the ox tanks, still pressurized, could have leaked back through the helium plumbing and into the depressurized fuel tanks. Kaboom!

It bothers me to say it, but I just can't make any solid prediction about what happened to these propellants. Ultimately there are two possibilities. Either the stage reached an inert state, and remained in orbit, or it didn't. The only way to find out is to look, and I hope someday the looking will occur.

The Stage Returns

On May 23, 1969, Tom Stafford was concerned. The day before, he and Gene Cernan had performed the first lunar rendezvous in history. Now back in the Command Module, fully rested, preparing for the Trans Earth Injection (TEI) maneuver that would bring them home, Tom saw something out the window that gave him pause. Snoopy's tail was back.

(This transcript comes from the wonderful Apollo Flight Journal, but it also happened to occur during a TV broadcast, which you can find on YouTube.)

The stage had been cut loose in a looping orbit, low on the near side above the Sea of Tranquility, and high on the lunar far side. This "phasing" or "dwell" orbit was designed to be slower than that of the CSM, allowing "Charlie Brown" to overtake the LM and set up the right timing for the rendezvous. When Tom and Gene in the ascent stage ran through the rendezvous they caught up to the CSM and docked. The descent stage was left in the slower orbit.

The difference in orbital periods meant that one day later, the CSM was lapping the stage. Stafford wondered if the stage would be safely out of the way in time for TEI. One fortunate consequence of Stafford's concern was a "hack" on the position of the stage that he called out to Houston. Again, you can hear this on the TV broadcast, about 8 minutes into the YouTube video.

He calls out the stage position "between Taruntius P and K...I'm looking down now at 257 He's right down below us." This is a very valuable clue about the stage orbit. The craters Tom mentions are close to the lunar equator, at about 51.55 degrees East longitude. (You can see them with the LRO Quickmap tool here.) The "257" refers to the angle towards the stage, relative to the local horizontal. It means the stage was about 13 degrees below and behind the CSM. Here is a diagram to illustrate the situation. From simulations, the stage was at an altitude of about 93 km, and the CSM was about 99 km high. From the geometry, this means the CSM was at around 50.82 East, and Snoopy's tail was at 50.86 East at that moment.

From the mission transcript this occurs at the Mission Elapsed Time of 132:16:10. This is really helpful to nail down the exact timing of the stage orbit. Think about checking the accuracy of a watch...you set it to a known time and then check it a day later...you can see how many seconds it has drifted ahead or behind. Same idea here. We simulate the stage orbit out to this time, and then look to see if the stage is coming up ahead or behind 50.86 East.

The process is easy. Start from data published in the Apollo 10 Mission Report, and run a simulation of the stage orbit. Stop this simulation at the time of the sighting, and check the longitude. The sighting time translates to UTC of 5/24/1969 05:05:10.00. When I run the stage simulation out to that time, using the nominal values from the Mission Report, the longitude from the simulation comes out at 49.85 East. Not bad! It's off by about a degree, which translates to about 30 km or about 18 miles at the lunar equator. Remember that these things were moving about a mile per second in their orbits, so that means the simulator ran the stage past the target longitude about 18 seconds early.

For this adjustment, I keep almost everything as it is in the mission report, adjusting only the initial velocity. This is where orbital mechanics gets fun! Since the stage is going too far, we need to...speed up? Yes indeed...we need to go faster to slow down!  It's about as counter-intuitive as it can be, but that's how it works. By adjusting the initial stage velocity upward, the stage is driven into a (slightly) higher orbit. Because it's in a higher orbit, it takes longer to go around. Since it takes longer to go around, when we stop the simulation at the right moment, the stage is farther back. It only takes a few tries to dial it in, and these simulations take only 20 seconds or so to run, so very quickly I have a simulation that puts the stage right at 50.88 East at 132:16:10. The tweak to the stage velocity to get this to line up is just 0.27 feet per second. Beauty!

After playing the same game with the CSM orbit, now I have two simulations that put the two spacecraft in the right place at the right time. Here comes some real fun! GMAT, the simulator, can co-simulate two different spacecraft. Now we can see the dance that was making Tom Stafford so nervous. Here is a link to the GMAT co-simulation script that I posted on GitHub.

First off, let's look at what happened while the crew was catching up on their sleep. The CSM, in the faster orbit, pulled away from the stage. Initially, around the time they jettisoned the LM upper stage, they were about 100 degrees of longitude ahead of the lower stage. Over the course of the night (it was night time in Houston) and next day, this lead increased, until they were 360 degrees ahead...they had caught up to Snoopy.

The "wiggles" in this line are due to Snoopy's eccentric orbit...down to within 20 or 25 km of the surface, then up to 340 km. When Snoopy dropped down lower, he would move faster than the CSM, and gain ground. When he went around the back side and went higher, he would move slower than the CSM, and fall behind. Overall he was losing ground. (Remember, that was the purpose of the "dwell" orbit.)

The plot below shows the position of Snoopy, relative to the CSM, and it makes clear how the stage would gain and lose ground during each orbit. The horizontal scale is compressed by 5x to fit several orbits onto the plot. Notice that Snoopy would mover farther ahead as he dropped down below the CSM, then fall farther behind as he climbed up higher. When the TEI burn started, Snoopy was safely away, about 1200 km behind, so it turns out that Tom's fears were unfounded.

Notice how close Snoopy came to the CSM, in the red circle marked "Close Encounter". (On this plot, by definition the CSM is at 0,0 in the center of that circle.) By zooming in we can see just how close they came:

As Snoopy dropped down behind the CSM, the stage seems to have passed within 3-4 kilometers before moving away. Nowhere near an actual collision, but too close for comfort!

Fortunately for all concerned, the stage drifted past harmlessly, and was quickly forgotten...until now. Perhaps we might hear some news about this long lost artifact someday if, indeed, it has remained in a stable orbit for all these years.

Supercritical Helium

In previous posts I showed that the Apollo 10 descent stage orbit was stable over decades and that the lunar atmosphere could not have slowed the stage significantly. Is the stage still in lunar orbit today?

It seems clear that an inert object would still be in orbit today, but the stage was hardly inert. It was much closer to a flying bomb, with over 8 tons of highly reactive propellants, plus a tank with 40-odd pounds of liquid helium, which was slowly warming up, slowly increasing its pressure, slowly approaching the breaking point. Let's look at what would have happened next.

Film taken as it was jettisoned shows the ladder and footpad of the stage. Note the "upside down" lunar horizon above the pad.

To understand what might have happened, we need to understand the design of the descent engine. To keep things simple, reliable, and light, the Descent Propulsion System (DPS) employed a pressure-fed system. Helium gas was used to pressurize the propellant tanks, so that when valves were opened the fuel and oxidizer would flow into the combustion chamber. The propellants were hypergolic, so they would burn as soon as they came into contact. The image at right, excerpted from this presentation, shows a simplified view of the system.

To save weight, the helium was not stored as a gas. That would have required a very heavy tank, able to withstand very high pressure. Instead it was stored as a "supercritical" liquid, at very low temperature and modest pressure, inside an insulated tank along the lines of a big thermos bottle. This worked as a lightweight way to store the helium, but it was not designed to work for a long time. Heat would leak into the helium tank during the mission, raising its temperature and pressure. Eventually this rising pressure could cause a "burst disc" safety valve to open, and the helium would vent out to space.

On a normal mission the engine would be fired long before the pressure reached the breaking point. Once on the lunar surface the extra helium would be vented. For Apollo 10 most of the original helium was still in the tank when it was cut loose. (The Mission Report states that 44 pounds were loaded at launch, and the DOI and phasing burns consumed only 4% of the fuel.) Sometime after staging, the pressure would have climbed to the breaking point. How long did that take?

The pressure inside the helium tank was monitored by mission control, and the Mission Report states that the pressure was rising at 5.9 psi per hour after launch. A report on the DPS showed that the tank pressure at the end of the phasing burn was at 1160 psi (and still rising, due to the way fuel was piped through the helium during the burn) and that the burst disc was designed to open at 1881-1967 psi. From this it seems very likely that Snoopy's descent stage vented the helium between 4 and 6 days after staging, sometime between May 26-28, 1969.

How did the venting affect the orbit? Could it bring the stage down? This document can help answer the question. Figure 9.1-3 in the document, copied below, shows the time and force for a full tank to vent, and the resulting impulse. The total impulse to vent a full tank is ~1700 lb-secs, over the course of 120 seconds. Let's ignore the fact that the stage is tumbling, and assume this impulse all contributes to a change in velocity. With the stage weighing a total of 21,000 pounds, the venting could change the velocity by 2.6 feet per second at most.

This graph shows the thrust generated by helium as it vented to space after blowing the burst valves

Keep in mind, the stage was moving about 5600 feet per second in it's orbit. So even if this 2.6 fps change was exactly aligned to the direction of motion, it's much less than the variation I simulated in my Monte Carlo run. I simulated velocity variations of plus or minus 65 fps, and all the runs stayed in orbit for at least 10 years with no sign of decay. Furthermore, since the stage was tumbling in orbit, the overall impact of the venting would certainly have been much less than 2.6 fps. As it spun around, forward thrust would cancel out backward thrust. It seems very likely that the overall change in velocity was less than 1 fps.

There is another data point to consider. During Apollo 13, as they were returning to Earth using the LM as a lifeboat, the descent helium tank vented. (In addition to the serious problem Apollo 13 faced, this tank apparently had an insulation problem, and warmed up faster than it should have. It vented about 109 hours into the mission.) Although the venting did affect the roll rate of the combined LM/CSM stack, it did not affect the trajectory in any substantial way, and no final course correction was needed. This is good evidence that venting the tank would not strongly affect Snoopy's orbit.

My conclusion is that the venting of the helium didn't bring down the stage. Now we are left with a mere 8 tons of hypergolic propellants, held back by aging valves and decaying seals, and exposed to pure hot sunlight for one hour, then to the freezing black of space the next, orbit after orbit after orbit. In an upcoming post I'll examine what might have happened to all this fuel.

A deeper understanding

There is a strong pattern in the behavior of the stage, which shows itself as soon as one simulates a month or two of its orbit. The orbit oscillates, becoming more circular for about 12 days, and then moving back towards being more eccentric...more egg shaped. What is driving this pattern? In this post we will dig in to understand it in more detail.

This is a plot I shared in an earlier post, simulating the first two months after the stage was placed in orbit, and the oscillation is visible as the waviness of the minimum and maximum altitudes. The peaks occur about once every 25 days.

A plot of stage altitude versus days in orbit shows a pattern of oscillating eccentricity. This pattern, repeating every 25 days or so, remains a fixture of the orbit even when simulated to the present day.

Here is another plot, simulating the stage orbit in the year leading up to the 50th anniversary of the mission. In this plot only the perilune points of each orbit are shown, but the same 25-ish day oscillation is still a major feature of the orbit.

The 25 day period is intriguing, since it is very close to the 27.32 days required for the moon to complete one rotation around its axis. But the difference between 25 and 27 is critical, since it means that the stage orbit oscillation is not directly tied of the moon's rotation. At least not in any obvious way.

When I first started running simulations of the stage, I was hoping to find an impact crater, and this required that the stage decay out of orbit quickly. Once I got going with the simulations, and the stage seemed to be long lived, there was one thing I noticed which kept me looking. I was interested in the lowest points of the orbit, the ones where the stage got below 20 km. Interestingly, these points always occurred at a similar location, around 30 degrees east longitude.

This plot shows low perilune points from a simulation of 3000 days. All the lowest points were centered around 30 degrees East longitude.

This result gave me some sliver of hope that I might yet find a crater...perhaps there was a lone mountain on the moon at 30 East, near the lunar equator, that acted as a giant catchers mitt, snagging the stage as it came whizzing by on a particularly low pass. So as I began to run longer simulations, I didn't just record the altitude of the perilune points. I also recorded their latitude and longitude.

One day, by accident, as I was plotting the results from a simulation run, I happened to plot both perilune altitude and longitude on the same graph, and something like this appeared:

The blue lines are longitude, while the orange line is altitude, from a simulation of a few months of 2019. What is important to notice is that there is a direct correlation between longitude and altitude. The peaks in perilune always occur near longitude 180 (or minus 180...same place) which is on the far side of the moon.

OK. Wow! That was a surprise to me. But something was not adding up. The plot is saying that the stage orbit perilune point moves all the way around the moon in 25 days. More correctly, what is happening is that the moon is rotating beneath the orbit, and it should take about 28 days to complete a full rotation. So how is the perilune point getting all the way back to the same longitude in less than 25 days? The answer is "precession".

Perhaps you have seen a gyroscope precessing as it balances on a pedestal. There is a nice video example here. Orbits also precess, and you can see a nice illustration (greatly exaggerated) here. In the case of the stage orbit, it is also slowly precessing, in a direction opposite to the rotation of the moon. The illustration below is not to scale, but shows how the major axis of the stage orbit changes over one month. The net effect is that the moon does not have to complete a full rotation for a given longitude line to come back under the low point of the stage orbit. It gets there a few days early. Just under 25 days.

So now the 25-ish day period of the perilune altitude oscillation makes more sense. It is a combination of the 27.32 day rotation of the moon under the stage orbit, combined with a slower precession of the orbit. This precession may also tie in to the slower cycles that are apparent in longer runs. The rate of the precession suggests that the stage orbit would precess all the way around the moon in about 10 months, which is about the period of the longer cycle.

I don't know what exactly is driving the change to the eccentricity of the stage orbit, but the fact that it is tied to longitude certainly points to a couple of possible explanations. It could be mascons along the lunar equator, tugging the stage to a greater or lesser extent. Or perhaps, since the same side of the moon is always facing Earth, and therefore the moon's rotation is closely related to its position along its own orbit, the relative positions of the Earth and Moon are somehow conspiring to tug the stage orbit in different directions. It remains a mystery to me. Nonetheless, I am happy to have a better understanding of the most prominent feature of the orbit of Snoopy's tail end.

Atmospheric Drag

Suppose it is true that Apollo 10 Lunar Module descent stage, aka "Snoopy", has not decayed out of orbit due to lumpy lunar gravity. What other factors might have brought the stage down in the intervening 50 years? That's a long time, so even subtle effects could come into play, including the nearly non-existent lunar atmosphere.

The space around the moon is nearly a perfect vacuum, by Earthly standards. But "nearly perfect" means that there are still thousands of molecules in each cubic centimeter of space around the moon. In fact scientists refer to the lunar atmosphere as an "exosphere" because there are so few molecules that they rarely collide with each other. They behave more like an army of tiny satellites, moving in response to gravity or electrostatic forces.

This instrument was placed on the moon's surface during Apollo 17 to study the moon's exosphere.

Source

According to this source, there are around 155,000 molecules in each cubic centimeter of the lunar exosphere. Yikes! That seems like a very big number. When you think that the descent stage is 12 feet on a side, and moving at almost a mile a second, that is a LOT of collisions with lunar molecules. Then take that out 50 years...now it's REALLY a lot. If we calculate the mass of all those molecules, and compare that to the mass of the stage, we can get an idea about whether the exospheric drag might have brought down the stage. If the mass of the gas is even close to the mass of the stage, that would be a real drag.

First of all lets estimate how many cubic centimeters the stage passes through every second. Just to get an upper bound on it, let's assume the stage is always moving with it's largest side facing forwards, sweeping through the largest area. We can approximate it as a square that is 12 feet on a side, which works out to an area of 140,000 square centimeters. (To get to this square I add the area of the foot pads into the missing corners of the stage, which is actually octagonal. Remember we are just estimating here.)

Now, the stage is moving about one mile every second. It moves faster when it drops down closer to the moon, and slower when it reaches it's "apolune" high point, but we can average that out and just assume a fixed velocity, as if it were in a circular orbit. One mile is 1.6 kilometers or 160,000 centimeters. Every second. Day after day. Month after month. Perhaps even decade after decade?

Well, now it seems like we might have a problem. If the stage is sweeping through an area of 140,000 square centimeters, and covering 160,000 centimeters per second, that works out to 22 billion cubic centimeters every second. And remember, if every cubic centimeter has 155,000 molecules, it means the stage is colliding with around 3.5 quadrillion molecules. Every second. Day after day. OK, then, perhaps all these collisions slowed down the stage, and it hit the moon after all?

Now we have to figure out how much all those gajillions of molecules weigh. Fortunately, remembering high school chemistry, this is easy. With a periodic table of elements, we take the atomic weight of each molecule, and then add them up. So for instance each Helium4 molecule has an atomic weight of 4 (or close enough for this estimate) so 40,000 of them total to 160,000 atomic units. Total up all the other molecules in the same way and it comes out to about 2.5 million atomic mass units.

OK, so now how much is that in units I can comprehend? It seems like a lot! Well, it turns out to be very little, because each molecule weighs very, very, very little. For instance one Helium atom, in grams, weighs about 0.00000000000000000000000166 grams. We better start using scientific notation, so that would be written as 1.66E-24. Now multiply that by the 2.5 million atomic mass units, and it come out to 4.13E-18 grams per cubic centimeter. Which is not much. (Yes, I am assuming the lunar exosphere is uniformly dense, which I know is not true. At this point I just want to get a feel for the problem.)

So now we have a war between big numbers and small numbers, that is, between the very large number of cubic centimeters (or cc's) that the stage sweeps through every second, 22 billion, versus the very small weight of the gas in each cc. So 22 billion times 4.13e-18 comes out to 9.2E-8 grams. Let's write that out as 0.000000092 grams. Every second. So far the small numbers are winning. That is the weight of the stuff the stage is running into every second, and it isn't much. And if we imagine each molecule as stationary when the stage strikes it, and moving away at the velocity of the stage after the collision, each collision will rob the stage of that tiny amount of momentum. (Think of a billiard ball striking another, stopping the first ball and sending the second one off with the momentum of the first.) So again, 9.2E-8 grams of collisions every second.

So now we just multiply by the number of seconds in 50 years. One year is 31.5 million seconds, and 50 years works out to 1.58E+9 seconds. Now the seconds help the big numbers to win the war, but not by much. The 9.2E-8 grams times 1.58E+9 seconds comes out to 146 grams. About one-third of a pound. Compare that to the weight of the stage. It's dry weight was 4700 pounds. So colliding with one-third pound of lunar exosphere over the course of 50 years could have slowed down the stage by 0.007%...i.e. nada!

So far, it seems that lumpy gravity didn't bring the stage down, and neither did atmospheric (or exospheric) drag. Perhaps Snoopy is still out there!

Simulating Uncertainty

Previously I posted about a simulation of the Apollo 10 LM descent stage which shows that the stage remains in lunar orbit to the present day. How robust is this result? The data for the initial stage orbit comes from the Mission Report...no doubt it was the best information they had. But fifty years in lunar orbit is a very long time, and lunar gravity is notoriously "lumpy". What if a slight change in the initial stage orbit state meant the difference between stability and decay?

To answer this question, I ran a set of 50 simulations, each with the initial conditions randomly varied to cover any possible miss in the initial state of the stage. I tried to keep the variation wide, to insure I covered the real conditions, but I also stuck to reasonable limits. In fact the variations I applied were so wide that many of the orbits were not viable. To cover this, I ran each parameter set through one orbit, recording the apolune and perilune...the low and high points of the orbit. I cut any set that was lower or higher by more than 20% from the values NASA reported. Only about one third of the random sets passed this test. To get 50 sets for the final test I passed more than 150 sets through the initial 1-orbit screen.

Here is a plot of the perilune points for all 50 random parameter sets, showing their minimum orbit altitude after 10 years in orbit. It's a bit messy, as these orbits show quite a bit of variation.

But the important thing to note is that in all 50 cases, the stage was still in orbit after 10 years. Each one of these plots is very similar to the "nominal" orbit I simulated initially. What if we just find the one of these, out of the 50, that got lower than any of the others, and plot it out by itself? Here it is.

You can see that this one did indeed make a rather low pass, in December of 1979, to about 12 km above the mean radius. (Still well above any lunar mountains.) And if you saw my earlier post about the stage orbit behavior, you see the same patterns here. The oscillation over a period of 25 days, and a longer oscillation with a period of around 5 months. Why did this one get lower than the others? It was one of the lowest initially, so it is hardly surprising. The real question is whether this one is any less stable over decades  than the "nominal" orbit that I showed before. What happens if we simulate this orbit out to the present? Here is the answer:

It is every bit as stable in it's orbit as the "nominal" case. There is no long term decay in evidence, and the simulated stage remains in orbit to the present day.

To me, this represents rather convincing proof. The result I got the first time I ran a simulation out to 50 years was no fluke. The nominal stage orbit is just one of a family of similar orbits that all exhibit long term stability. If something knocked the stage out of orbit during those 50 years, it wasn't the moon's lumpy gravity.