 It's a question you guys have had and one I've had as well, so I'm finally going to answer it. Why does the Apollo trajectory look like a figure eight? So this is probably the diagram we're used to seeing. It looks like the Apollo spacecraft is tracing a figure eight from the launch point on Earth around the moon back to a splashdown on Earth. Of course, what we're looking at here is the free return trajectory, not the actual flight profile. Because that flight profile, of course, would include orbiting the moon and, in many cases, landing on the moon. Nevertheless, all the Apollo flights followed this profile. The question is why? Because some people often think that this is the opposite way you would want to go to the moon. So let's start with the Earth and let's pretend that we're hovering somewhere above the North Pole, such that we see the Earth's west-to-east rotation as a counterclockwise spin. Now, the moon does the exact same thing, but as we know, one day on the moon lasts the equivalent of a month on Earth. Nevertheless, it is also making a counterclockwise spin from our weird hovering above the North Pole location. And a quick terminology lesson before we keep going because this will become very important. The east end of the Earth from this orientation, so the direction towards which the Earth is spinning, is called the leading hemisphere. The opposite end, away from where the Earth is spinning, is called the trailing hemisphere. Keep that in mind. Because Apollo crews launched towards the east to take advantage of the Earth's spin to use a little bit less fuel to get into orbit, they all ended up orbiting the Earth the same way it spins from west to east. Now, recalling that the moon rotates the same way, it would stand to reason that we take advantage of the speed and actually go from the west-to-east rotation on Earth all the way up to a west-to-east rotation on the moon. Well, turns out that's not exactly the safest way. So here's where it gets a little bit more complicated. Bear with me, I promise it's not actually that bad. When a spacecraft passes a body in space, and by body I mean a planet or a moon, not some poor unfortunate soul who's floating alone in space, let's hope that never happens, it gets what's called a gravity assist. This is basically a very small transfer of momentum from the planet to the spacecraft. That's because any planet has a much bigger gravitational pull than the spacecraft. We've all seen this happen with the Voyagers. Both Voyager spacecraft did gravity assists among the giants to get onto their next destination. Now, where the spacecraft passes the body is what's really important here, and this is why those terms of leading and trailing hemispheres become very important. If a spacecraft passes by the trailing hemisphere, it gets a bigger kick of energy. That's because it's gaining a little bit more momentum as it travels with the direction of the planet or moon's rotation. If a spacecraft passes by the leading edge on the other hand, it's going against the direction of the planet's rotation. So while it still gets a kick of energy from that close pass by that moon or planet, it's a little bit less energy than had it gone by the other hemisphere. So let's go back to Apollo. Assuming a crew did absolutely nothing to adjust their trajectory. That means no burns with any of the engines and just left it all up to Sir Isaac Newton in the driver's seat. Let's say they then passed the moon's trailing edge. Well, they would get a much bigger kick into their momentum that would send them on a much larger orbit around the Earth that would take them either a lot longer to get back to our home planet or require a lot more fuel to adjust the trajectory to get back home. Now, as we know, fuel is one of the most expensive commodities on a space mission because it's so hard to get off the ground in the first place. So the simpler way to fly is always the one that uses less fuel. So this is where passing by the moon's leading edge comes in very handy. On Apollo missions, the crew passed by the moon's leading edge to take advantage of the smaller increase in momentum. This ensured that without doing anything at all, the crew would be able to whip around the moon's far side and it would send them back on a trajectory that would intersect the Earth. So this would be very important if, say, the one main engine, the SPS engine, failed. They would be able to do this free return trajectory and make very small tweaks to their eventual impact point on the Earth or where they would begin re-entry and landing using their RCS thrusters. It was simply the safest way to get people to the moon and ensure that, as per Kennedy's demand, they return safely to the Earth. So let's break down what actually happened on an Apollo mission. When the crew did their translunar injection burn, the TLI burn that would send them from the Earth to the moon, they were not aiming at the moon at all. Remember that the moon orbits the Earth, which means that they were aiming towards a point where the moon would get in three days. So basically, that burn put the crew on a very high elliptical orbit that had its apogee somewhere around the vicinity of where the moon would be in three days. The spacecraft flies towards that point in space as the moon comes near and when the two eventually meet, the moon's gravitational field becomes the dominant force and causes the spacecraft to whip around the moon's far side. Now, because the spacecraft is traveling by the moon's leading edge, it actually takes less energy and less propellant to slow the spacecraft into a lunar orbit, which is another major win when you're dealing with spaceflight. So from there, it was a simple SPS burn or service propulsion system burn for the crew to slow to get into lunar orbit. The trans-Earth injection burn after the landing mission would send them on the same path back towards Earth. This ultimately traced out a figure eight. I really hope this helped clear up some questions for you guys and if you would like to learn a little bit more about not only trajectories, but how Apollo actually navigated this course, definitely check out my friend Dave Wood's book, How Apollo Flew to the Moon. It is absolutely everything you never thought you needed to know about Apollo. I want to remind you guys that you are now able to sponsor Vintage Space so that you can help make the content you love possible. I've got a video about how that works right up here, so check it out if you're interested. And if you are already sponsoring it, thank you so much. It means so much to me. If you have other questions about anything Apollo or old-timey spacey, leave them all in the comment section below. And of course, if you liked this video, be sure to subscribe so you never miss one of my bi-weekly episodes. Of course, you can also follow me all across social media for daily content. I'm on Twitter, Instagram, and on Facebook. As always, guys, thank you so much for watching.