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Apollo 10's lunar module Snoopy descended toward the lunar surface, but (as planned) didn't quite make it, returning to orbit only 8.4 nautical miles above the surface of the moon. Was there a technical reason why Snoopy couldn't land on the moon?

By technical, I mean...

  • equipment not yet designed, manufactured, or delivered.
  • software not yet written.
  • NOT the fact that the objective of the mission was to be only a "dress rehearsal" for landing (not a technical reason).
  • NOT a lack of training (not a technical reason).
  • NOT the fact that NASA intentionally under-fueled the LEM, to deter the astronauts from making a landing attempt (a deliberate reason):

Craig Nelson wrote in his book Rocket Men that NASA took special precaution to ensure Stafford and Cernan would not attempt to make the first landing. Nelson quoted Cernan as saying "A lot of people thought about the kind of people we were: 'Don't give those guys an opportunity to land, 'cause they might!' So the ascent module, the part we lifted off the lunar surface with, was short-fueled. The fuel tanks weren't full. So had we literally tried to land on the Moon, we couldn't have gotten off."

(Apollo 10 on Wikipedia)

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    $\begingroup$ It seems odd to suggest that a justification based on safety and risk is not considered a "technical" reason. Surely safety and risk management are highly technical factors? $\endgroup$
    – Michael Kay
    Commented Dec 4, 2018 at 14:30
  • $\begingroup$ @MichaelKay: Safety and the progressive testing of a very complex mission certainly were the main reasons. But what if we accept those risks... could it still have been done? That's what this question asks. $\endgroup$
    – DrSheldon
    Commented Dec 4, 2018 at 18:50
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    $\begingroup$ I think the point here is that Apollo/Saturn was technically ready for a Moon landing at that point, but the Apollo 10 mission was required to fully demonstrate its readiness and minimize Apollo 11's risks and shorten the list of "firsts". Remember that Apollo 8 was about the most audacious of the Apollo missions - a lot of firsts - first Saturn V launch to carry a crew, first crewed flight to travel beyond LEO/circumnavigate/orbit the Moon, farthest any human had traveled from Earth, fastest crewed re-entry (then), only the second time the Apollo command module had flown a crewed mission. $\endgroup$
    – Anthony X
    Commented Oct 6, 2019 at 1:41

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Grumman hadn't reached the weight targets for the LM at the time of Apollo 10. Snoopy weighed 197 pounds (89 kg) more than Eagle, according to Apollo By The Numbers.

However, this would not have made safe landing impossible. Apollo 11's LM carried a 300-pound descent fuel margin above and beyond all the specific contingencies that were accounted for (stuck valves, wishy-washy pilots, etc.) and landed with more than 600 pounds of usable descent fuel remaining. Cutting the fuel margin by 200 pounds wouldn't have been crazy. (Ascent-stage dry weight wasn't the problem; the ascent stage gained 20 pounds between A10 and A11!)

Irregular lunar mass concentrations were another issue; while they wouldn't directly interfere with landing, they could complicate rendezvous.

Personally, I believe Cernan was mostly joking about the reason Snoopy's ascent stage was short-fueled. Apollo 10 commander Tom Stafford himself was in favor of the "dress rehearsal" mission, according to Mike Collins in Carrying The Fire:

Tom Stafford, a rendezvous expert if there ever was one, was very hip to these arguments [the lunar mascon issues]. Since he had always been on the conservative side in our astronaut office discussions, and had insisted that a variety of rendezvous situations be demonstrated in flight before committing to a lunar landing. Tom wouldn't, or couldn't, reverse himself now and display great enthusiasm for an Apollo 10 landing, even if it meant he would be first to walk on the moon.

Snoopy's ascent stage was about 1 ton short of fuel; that weight reduction would provide some insurance against underperformance of the Saturn V or the SPS engine.

As far as I know, landing would have been technically possible, and almost as safe as Apollo 11; it was simply a healthy degree of caution that kept NASA from making a landing attempt with 10.

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    $\begingroup$ The 'short fuel' thing was definitely not a joke -- the Apollo 10 lander had the amount of ascent fuel that it would have had if it had landed on the moon and then ascended back up to the 15 kilometer altitude they had come down to. That is to say, they came down to 15 km, then dumped the descent engine and kicked off the ascender, which had the amount of fuel a real landing would have expected to have at that attitude, and flew the return and docking as if they'd just lifted from the surface. $\endgroup$
    – Darth Pseudonym
    Commented Jun 15, 2023 at 19:27
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    $\begingroup$ That implies that it definitely was a joke; A10 was short-fueled to make the dress-rehearsal's ascent fuel profile match the landing-mission's ascent fuel profile, not to prevent A10's crew from going off plan. $\endgroup$
    – Russell Borogove
    Commented Jun 15, 2023 at 22:19
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    $\begingroup$ Sorry, I misread what you wrote. I thought you said the fact that they were short fueled was a joke rather than the reason they were short fueled. $\endgroup$
    – Darth Pseudonym
    Commented Jun 16, 2023 at 13:23

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