102 votes

Why is an airbag landing on the moon not viable?

During the Pathfinder landing the airbag system hit the ground at about 20m/s. This seems fast but compared to other space speeds, it's very slow. When the pathfinder rover arrived to land at Mars, it ...
  • 15.9k
98 votes

Why is it so hard to land on the Moon?

Does the US have some secret insight into landing on the moon? Yes: fail early and often. The US developed experience with uncrewed landings first, before attempting crewed landings in the Apollo ...
80 votes
Accepted

How did the Russians get moon rocks?

The USSR flew three successful automated lunar sample return missions: Luna 16, Luna 20 and Luna 24. The probes landed on the Moon, collected samples, and started a small rocket with the samples back ...
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76 votes
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Why is the ladder of the LM always in the dark side of the LM?

All the lunar landings were performed with the sun low in the sky behind the LM, between 5º and 14º above the horizon at the landing site. This provided several advantages: The sun wouldn't be in ...
71 votes

Was planting UN flag on Moon ever discussed?

No. Planting a flag was the idea of NASA's "Mr. Fix-It", Jack Kinzler, less than 4 months before Apollo 11's launch: Kinzler believed that the people of the United States would also want to see an ...
  • 47.1k
54 votes

ELI5: Why do they say that Israel would have been the fourth country to land a spacecraft on the Moon and why do they call it low cost?

Chandrayaan-1 hit the Moon at high speed and did not survive its "landing", which would have been much more difficult to engineer. (Its successor, Chandrayaan-2, which will actually land, is expected ...
  • 4,536
47 votes

Why did NASA send two astronauts to the moon instead of one?

EVA suits are very difficult for a single person to put on by themselves, so another person was required to help the other astronaut put on their EVA suit. At the very least, it is much easier to do ...
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47 votes
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Was there a technical reason why Apollo 10 didn't land on the moon?

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 ...
46 votes

Was it really necessary for the Lunar Module to have 2 stages?

After these 25 second would have ended, the LM still had enough fuel to ascend with both of its stages right back to the Service module. In other words, the LM was designed to be able to take off from ...
45 votes
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Is it easier to get to and from the moon than to the bottom of the Mariana Trench?

The Moon is much harder to reach. I'd suggest the biggest reason for more visits is popularity and politics. The Space Race was about tactical advantage (and to a huge extent posturing) between the ...
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43 votes
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Why is an airbag landing on the moon not viable?

Actually there’s no reason that an airbag system could not be used on the Moon, nor that it could not be a good design decision in some circumstances. The MPF and MER airbags took out the last 10 to ...
  • 58.1k
43 votes

Why were contact sensors put on three of the Lunar Module's four legs? Did they ever bend and stick out sideways?

Some pictures from Apollo 11 of the landing gear – struts, footpads and contact probes. Three Apollo 12 images: Two Apollo 14 footpad images: An Apollo 16 image: The two probes bend straight up on ...
  • 48.1k
42 votes
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What were the problems on the Apollo 11 lunar module?

These were the "1202" and "1201" program alarms, which were warning signals that the lunar module's computer was becoming overloaded. During Apollo 11's descent to the moon, the ...
41 votes

Why is it so hard to land on the Moon?

While engineering and available technologies have greatly advanced since the 50's and 60's, safely landing something on the moon is still a highly technical feat with a critically long list of ...
38 votes
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Why were contact sensors put on three of the Lunar Module's four legs? Did they ever bend and stick out sideways?

It is correct that the probe on the forward footpad was omitted to avoid interfering with the ladder: The probe located on the forward landing gear was deleted because of a concern that the failed ...
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38 votes
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Could the lunar module have moved to a different spot?

In addition to the "single-serving" mechanical features that Uwe's answer describes, the guidance programming for the LM's onboard computer doesn't support any ascent flight on the descent ...
36 votes

Flung debris in lunar landing

Great question. The exhaust velocity from typical landing engines is about 3 km/s. You can imagine good-sized particles being accelerated to a significant fraction of that, say 1 km/s, which is the ...
  • 58.1k
36 votes
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What equipment was taken from Earth to the surface of the Moon and back?

You can find what items were carried on each mission, and what was brought back, by reading the stowage lists. Each list is divided into six sections: Section A is the items placed in the command ...
  • 47.1k
35 votes
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Why did NASA send two astronauts to the moon instead of one?

Even before Kennedy's "we choose to go to the Moon" speech, NASA was working on concepts for Moon missions. Dr. Robert Gilruth, Director of the Manned Spacecraft Center: "Even before the President'...
  • 122k
35 votes

Could the lunar module have moved to a different spot?

The Lunar Module was designed for a single landing. The contact sensors under the footpads of the landing gear did bend and stick out sideways during a landing. They could not be reused for another ...
  • 48.1k
33 votes
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Were the Apollo lunar activities observed from Earth?

Was it possible to observe landings and/or extravehicular activities (EVAs) with Earth-based telescopes? Short answer: Not possible in 1969 In 1969, we had not the faintest possibility to see a man ...
  • 4,963
33 votes
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How did astronauts using rovers tell direction without compasses on the Moon?

The Lunar Roving Vehicle did have a (form of) compass. It was gyroscopic rather than magnetic, thus it needed calibration when first powered up using the sun angle as a reference. It's in the upper ...
32 votes

Why did the Apollo Lunar Module have four landing legs?

Three legs offers less safety margin for steep slopes and sideways landings. If a three-legged lander touches down on a steep slope such that the center of gravity of the ship is outside the triangle ...
32 votes

Could the Apollo lunar ascent stage have separated from the descent stage before touchdown in an emergency?

Yes. There is a lot of good information in this presentation from the June 1966 Apollo Lunar Landing Mission Symposium relative to landing flight design including abort planning. The crucial figure ...
31 votes
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Could an Apollo LM land uncrewed?

I purchased "The Apollo Guidance Computer: Architecture and Operation" to answer this question (an excellent read). And the answer is yes it might be possible to land on the moon unmanned. However, a ...
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31 votes
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Are there photos of the Apollo LM showing disturbed lunar soil resulting from descent engine exhaust?

Most of the Apollo photo libraries have a few shots of the surface under the descent engine bell; I think A14 has some interesting ones: The disturbance of the soil is very subtle; compared with the ...
30 votes
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Did any of the Apollo lunar modules land significantly off level?

All the LMs landed a few degrees off-level. Apollo 11 was closest to level at about 4 degrees. The design limit for LM ascent stage lift-off is variously stated as 12 or 15 degrees. Apollo 15 was ...
30 votes
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Were all the switches on the Command Module and LM control panels used during a mission?

Some switches were normally left in one position for the entire mission, and would only be changed in unusual situations. One particular semi-famous example is the switch controlling the power ...
30 votes
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Was there fuel consumption budgeting for Apollo 11 Lunar module?

The Apollo LM had three independent propellant supplies: tankage in the descent stage usable by the descent engine, tankage in the ascent stage for the ascent engine, and in the ascent stage for the ...
30 votes

Why did Apollo 11 land at the sea of tranquility?

The set of landing sites considered for all the Apollo missions was driven by a desire to sample a wide variety of lunar geology, but the order for them was chosen to make the first landings easiest ...

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