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No one knows what became of the Eagle. That seems wrong.
After it carried Neil Armstrong and Buzz Aldrin back from the surface of the Moon in 1969, the ascent stage of the Apollo 11 Lunar Module "Eagle" was jettisoned into lunar orbit. The astronauts watched out the window as it drifted away. The NASA tracking network followed it for a few revolutions, until they lost the signal. Since then no one has seen or heard from the Eagle. Without question it is one of the most important machines ever created by humanity. Not knowing her fate is a terrible wrong which must be righted.
The assumption has always been that the Moon's lumpy gravity caused the Eagle's orbit to decay, and she impacted the Moon at an unknown location. In this post I will go through the last known orbital state of the Eagle, and show the results of simulating that orbit with the best gravity models available. Spoiler alert: as I found previously with "Snoopy", the orbit is quasi-stable. Lunar gravity alone may not have brought the Eagle down.
For the orbital state of the Eagle at the time it was jettisoned, we look to the Apollo 11 Mission Report. Table 7-II lists information about the spacecraft at various points in the mission, and in particular there is an entry for "Ascent stage jettison" as below.
|Orbital State of the Eagle at jettison, from the Mission Report|
As I have described in a previous post, I use a simulation tool developed by NASA, and gravity models derived from GRAIL data. It's fairly straightforward to plug in the values from the table and simulate the stage. There is one problem with the Mission Report, though. It's wrong! When you think back to 1969, a world where word processing does not yet exist, and data processing is cumbersome, it isn't shocking that there is a problem in the table. But if you know a bit about the Apollo 11 orbit, the error is rather glaring.
|Extrapolating to find the inclination at the moment of jettison|
|Simulation of Eagle to the present shows no contact with the Moon!|
|A still from the 16mm film taken as the descent stage moves away during staging. Notice that the lunar horizon is "upside down".|
|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|
|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|
|Descent Stage Cutaway View Source|
This excerpt from the Mission Report shows the quantities of propellants loaded and consumed
|Detailed view of the throttle assembly (source)|
|Film taken as it was jettisoned shows the ladder and footpad of the stage. Note the "upside down" lunar horizon above the pad.|
|This graph shows the thrust generated by helium as it vented to space after blowing the burst valves|
|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.|
|This plot shows low perilune points from a simulation of 3000 days. All the lowest points were centered around 30 degrees East longitude.|