 If you look at a list of Apollo missions, it goes Apollo 1 and then Apollo 4 through 17. So what happened to the missing Apollo 2 and Apollo 3? Apollos 2 and 3 aren't really missing, they just kind of got buried in NASA's very bizarre Saturn Apollo launch nomenclature. So let's take a look at that. NASA adapted a pretty simplistic nomenclature for all of its Apollo Saturn missions way back in the early 60s when it was just beginning to get things off the ground, quite literally. Basically, every rocket was given a letter for the rocket and payload type, followed by a number standing for the launch. Of course, this is only the internal designation. NASA couldn't keep it simple, it had to give every mission, or at least most missions, external designations as well, something the media and the press could use to discuss a flight. The Saturn 1 launches were primarily a testbed for NASA to work out the hardware it would later use in later rockets to send missions to the moon. As such, the Saturn 1 launches were given the designation of SA for Saturn Apollo, showing that they were primarily a test for the Saturn hardware, and secondarily a test for Apollo hardware. They were also given numbers in the 100 designation, or the just simple 1 designation 1-10. The first 5 Saturn 1 flights were tests of rocket components and retained the SA designation, they were numbered 1-5. The last 5 missions were primarily tests of Apollo hardware, and so were designated AS for Apollo Saturn, showing it was primarily an Apollo mission, not a Saturn mission, and were given numbers 101 through 105. The same nomenclature was used for the Saturn 1B launches. They had the same AS letter designation, but were given a number in the 200 series. The same nomenclature was used on Saturn 5 flights, but because these were primarily Apollo flights, they had the letters AS, not SA, and because they were Saturn 5, they were predictably given a number in the 500 series. So let's go back to a Saturn 1B launch as an example, specifically launch SA-203. That was the internal designation, but the mission name was AS-203 to show that it was primarily part of the Apollo program. AS-203 was, incidentally, a mission to test liquid hydrogen behavior in orbit to understand the S-4B's ability to restart in space. Another example is flight AS-501, a primarily Apollo flight and 501, showing that it is the first launch of a Saturn 5 rocket. Externally, generally, we know this mission as Apollo 4. So how does all this play into the story of the lost or missing Apollo 2 and 3? Well, according to NASA's original plan, Apollo 1 was to be launched on a Saturn 1B rocket as a shakedown cruise of the new Block 1 Apollo spacecraft. Apollo 2 would be the exact same mission again, basically a chance to redo anything that didn't get done on Apollo 1, and Apollo 3 would be the first test flight of a Block 2 spacecraft, the Apollo spacecraft that was capable of supporting a lunar mission, specifically docking with the lunar module. Of course we know the Apollo 1 mission never got off the ground. The crew was killed during a routine pre-launched plugs-out test on January 27th of 1967, but this also disrupted the nomenclature. While the crew was still training for Apollo 1, the mission was internally known as AS-204, the fourth flight of a Saturn 1B rocket in the Apollo series. But after the accident, the astronauts' widows asked that Apollo 1 never fly in honor of their husbands, such that they could still have a flight, and NASA of course agreed. From there, there were two ways NASA could continue naming its flights, both of which were outlined in a letter from George Miller to George Lowe in 1967. One option Miller said was for the agency to proceed in sequence, naming the first mission to fly after the fire Apollo 2 and then 3 and so on and so forth. The other option Miller said was to retroactively name the Saturn 1B test flights that were flown in support of Apollo with Apollo designations. Under this new nomenclature, AS-201, an unmanned test of a Block 1 spacecraft would become Apollo 1A, AS-202 and S-4B restart test would become Apollo 2, and AS-203 would become Apollo 3. Future flights would then start with Apollo 4. NASA decided to split the difference, though it never formally renamed those Saturn 1B test flights with Apollo designations. So even though they could be called Apollo's 1A, 2 and 3, they're not known as such, they retained their internal designations. But NASA did pick up with Apollo 4 in the naming sequence for its first flight after the fire. So Apollo's 2 and 3 sort of exist, they just don't exist under those names. Does that answer the question? Because a lot of you guys have asked that one. Do you have other questions about odd nomenclature, whether it's from NASA or whether it's the Soviet Union's bizarre way of only naming missions that actually flew, not missions that failed to reach orbit? This became very complicated in the early 1960s. If you're interested, let me know in the comments and I can do a separate video on that. And of course, if you have other questions or other things you'd like to see covered in future episodes, leave me those down in the comments as well. Be sure to also follow me on Twitter and on Instagram for daily vintage space content, and with new videos going up every single week, be sure to subscribe so you never miss an episode.