 Greetings and welcome to the Introduction to Astronomy. In this week's Exploration of the Solar System, we are going to talk about some of the Apollo missions, and specifically Apollo 4 through 6. Now these were missions that were uncrewed, so no humans aboard, and were to again test out the Apollo systems. So let's take a look at these, and what we are going to look at is, first of all, they were delayed, the Apollo mission was all, was delayed by the tragedy of Apollo 1, which killed three astronauts on the launch pad during a test rehearsal. Apollo 4 through 6 were uncrewed craft. Now that's what went on during the investigation as to what happened in Apollo 1, was that all of the missions in that time would be uncrewed. So there would be no humans going out, but we could still test some of the components that were going to eventually take humans to the moon. Now Apollo 4 launched on November 9th of 1967. It was a test of the Saturn V rocket, an extremely powerful rocket, needed to lift the craft off the Earth and to put it into lunar orbit. This was again a command module then returned, so a test of that, and testing the heat shield that the astronauts would eventually need to be able to survive the trip back in through Earth's atmosphere. So this was able to successfully splash down out in the Pacific as future astronauts from lunar missions would do. Now Apollo 5 launched in January of 1968 was a test of the lunar module, you can see the lunar module here as it's shown in preparation, but it was a test of the lunar module in Earth orbit and how its capabilities would work. So a lunar module mock-up was used to be able to test exactly how that would work. And then finally we had the Apollo 6 mission, which was April of 1968. Again we're still on uncrewed missions to test and simulate re-entry. So this took the craft up a little bit higher, not all the way out to the moon, but a little bit higher and adjusted the trajectory and the speed for re-entry that would match what we would expect coming back from the moon. So it was a way to test how that would work a little bit better than was done with the earlier Apollo 4 mission. And again it would also test not only that, but how well would this craft be able to shield the crew from the Van Allen radiation belts. And this was a concern, we knew that there were radiation belts around the Earth that contained an intense amounts of radiation. And the astronauts would have to pass through these, so we wanted to know how much shielding there would be and how well the craft would be able to shield the astronauts. So, and it was found that certainly as long as the astronauts were passing through it in a relatively short amount of time, which they were, the shielding of the craft was sufficient to protect the astronauts. Now of course you wouldn't want to just stay within those radiation belts. So that was again another test of the craft coming down and here we see one of those lunar modules from the Apollo 6 mission that then splashed down in that test of how the re-entry would work from the moon and how well that heat shield down at the base would be able to survive the intense heat of re-entering Earth's atmosphere. So again these were some uncrewed missions to really test the capabilities of the craft in preparation for a lunar landing which is now, as we see, only a little over a year away. So let's go ahead and summarize what we've looked at here and we saw that Apollo 4 through 6 followed in the aftermath of the tragedy of Apollo 1. These were uncrewed missions, there were no crewed missions until about a year and a half after the Apollo 1 tragedy but we were able to test various components in preparation of these missions coming up and they were mainly successful, there were some small issues that could hopefully be worked on for the eventual launching of humans toward the moon. So that concludes this lecture on the Apollo 4 through 6 craft. We'll be back again next week for another exploration of the solar system. So until then, have a great day everyone and I will see you in class.