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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)}

Related:

• What were the differences on the LM ascent stage engines on Apollo 9, 10, and 11?


• Apollo 10: Where's Snoopy?

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.

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 just about as safe as Apollo 11; it was simply a healthy degree of caution that kept NASA from making a landing attempt with 10.