The Apollo 10 mission is often forgotten, overshadowed by Apollo 11 and the first moon landing. NASA had very carefully planned out the Apollo missions to successively test new hardware and systems, one step at a time.
Apollo 7 was the first manned launch of the Saturn rocket, in this case the smaller Saturn 1B. This was an important step to test the rocket with a human crew, after several unmanned launches. The rocket didn't venture any further out than low Earth orbit.
Apollo 8 was next, the mission took the spacecraft around the moon for multiple orbits. This was the first time humans had ever left Earth's gravity. It was also the first test of the mighty Saturn V, to this day the most powerful machine ever built by man. This was the rocket that would take us to the moon.
Everything about the Saturn V was off the charts amazing. It was the height of a 35 story building and weighed as much as a fully-loaded naval destroyer. It was loaded with a million gallons of kerosene, which it burned at the rate of 20 tons per second at launch, an amazing number. The pumps that delivered all this fuel to the five main engines (each F-1 engine was 14 feet in diameter) were 300,000 horsepower...not the main engines themselves, just the fuel pumps! If you had to put a number on this thing, it would be around 7.8 million pounds of thrust.
When you see film of the Saturn V lifting off, it looks slow, even lumbering as it climbs into the sky. This is an illusion due to it's size. In reality, it lifted off with a g-force that pinned the astronauts in their seats. It was traveling over 100 MPH before it cleared the top of the tower, and in 30 seconds it was already supersonic.
Apollo 8 tested the engines that burn to take the rocket from it's initial "parking orbit" around the Earth to the moon. This is called TLI, Trans-Lunar Injection. Apollo 8 circled the moon multiple times and then came back. The Lunar Module (LM) that would land astronauts on the moon was not yet ready, so Apollo 8 didn't carry one.
Apollo 9 tested the LM in low Earth orbit. They extracted the LM (this one was named "Spider") and flew it around to test out its systems in the microgravity of orbit. This mission also tested out the new space suits that would be used for the moon landings. Apollo 9 never left Earth's orbit.
There was only one big test left before the glory of Apollo 11 and Neil Armstrong's moon walk. Apollo 10 was a full dress rehearsal for the moon landing. All systems would be tested. Everything would be done exactly the same as the upcoming moon landing of Apollo 11.
Except the moon landing itself.
The third stage of the rocket along with the CSM (Command and Service Modules) would go into Earth "parking orbit", circle the Earth until given the go ahead for TLI, and then fire the engine that would carry them to the moon. The ship would travel to the moon at about 17,500 miles per hour, almost 5 miles per second. They would arrive in lunar orbit and the astronauts would extract the LM (theirs was named "Snoopy") as was previously done around Earth orbit by Apollo 9. The LM would be flown down to within only 50,000 feet of the lunar surface, then turned around and brought back up and docked with the Command Module. The ship would then return to Earth, NOT having landed on the moon.
Apollo 10 was a very important mission, but a very anti-climactic one. History would have to wait for Apollo 11.
There is an interesting side-story to this. Astronauts are pretty adventuresome characters, and there was some speculation that the crew of Apollo 10 might just ignore orders and land on the moon. After all, they had all the equipment and training in place. Even the Apollo 10 commander said it was very tempting. As it turned out, "Snoopy" never landed on the moon.
And it's a good thing too. As the crew found out later, NASA had already thought of that eventuality and purposely short-fueled their lunar lander. Had the astronauts tried to land "Snoopy" on the moon, they would not have had enough fuel to leave.
They really do think of everything.
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