How the SLS rocket flings a crew around the Moon

HostWe see the moon every night, just hanging there, but it feels like this far-off place that's almost impossible to reach. Now we have this new giant rocket called the SLS that's built to take people back there for the first time in a long time, but on these first trips, they're not even going to touch the ground. How do you actually throw a ship that far, let it loop around a moving rock, and make sure it comes back home without just flying off into the deep dark?

GuestIt all starts with the biggest push we have ever built. The SLS is basically a giant tower of fuel with a tiny room for people on the very top. To get that room, or the pod, away from Earth, you have to hit a very specific speed. If you go too slow, the Earth just pulls you back down like a ball you threw in the air. You have to hit about twenty-five thousand miles per hour to break the grip of our world. Most of the rocket is just there to get that pod moving fast enough in the first eight minutes. Once they reach that speed and point themselves at the moon, they actually turn the big engines off. They're basically coasting across a huge empty void for days, just using the speed they gained in those first few minutes.

HostSo they're just drifting? That feels like you're leaving a lot to chance. What happens if you get out there and you realize your aim was off by a few inches? You can't exactly turn a steering wheel in space.

GuestWell, you do have small thrusters, like little puffs of air, to nudge the ship left or right, but you're right that you can't steer it like a car. There's no air for wings to push against. Everything is about the math you do before you even leave the ground. You're not aiming for where the moon is right now. You're aiming for where the moon will be in three days. It's like a quarterback throwing a long pass to a player who's sprinting down the field. If you throw it to where they're standing now, you miss. You have to lead the target. And the moon is moving fast, thousands of miles per hour. So the ship has to meet the moon at a very specific spot in space, almost like they're timing a jump onto a moving train.

HostBut why go all the way there just to turn around? It seems like a lot of work and a lot of fire just to see the far side of the moon and then head home. Why not just carry enough fuel to turn the ship around whenever we want?

GuestFuel is incredibly heavy. Every extra gallon of gas you bring means you need even more gas just to lift that gas. It's a loop that makes the rocket get too big to even fly. So, instead of bringing a huge engine to turn around, we use the moon itself as a u-turn. We call this a free return path. We aim the ship so it skims just past the edge of the moon. As the ship gets close, the moon’s own weight, its pull, grabs the ship. Instead of pulling it down to crash, it swings the ship around in a tight curve, like a tetherball swinging around a pole. That swing flings the ship back toward Earth. It's a way to change direction for free, using the weight of a whole world to do the heavy lifting for us.

HostThat sounds risky though. If that pull is so strong, how do we know it won't just suck the ship in? It feels like you're playing with fire by getting that close to something so heavy.

GuestIt's a bit like a balancing act on a ridge. If you go too close, you crash. If you stay too far away, the moon doesn't pull you hard enough and you go sailing off into deep space, never to come back. You have to hit a tiny window in space. But the beauty of this path is that if the engines stop working halfway there, the pull of the moon and the Earth will naturally bring you home anyway. It's like a giant invisible track in the sky. Once you're on it, you're locked into that loop. You go out, you swing around the back side of the moon where it's totally dark, and then you see the Earth rise over the edge as you get kicked back toward home.

HostOkay, so the moon does the turn for us. But when the crew gets back to Earth, they're still moving at that crazy speed of twenty-five thousand miles per hour. They're basically a falling star at that point. How do they stop without burning up or hitting the ocean like a bomb?

GuestThis is the hardest part. When they hit our air, they're going about ten thousand miles per hour faster than the people coming back from the space station. If they just dove straight in, the heat would be too much for even the best heat shield. So they do something called a skip. They hit the top of the air at an angle, like a stone skipping across a pond. They dip in, the air slows them down and heats up the bottom of the ship, and then they actually bounce back up into space for a second. That bounce lets the ship cool down for a moment before they make the final dive. It also lets them pick exactly where they want to land in the ocean.

HostThe skip makes sense, but it still feels like the margin for error is almost zero. If the ship bounces too high, do they just fly back out into space again?

GuestThat's the big fear. If they hit the air at too shallow of an angle, they'll just skip off and fly away forever, like a rock skipping off water and never sinking. If they come in too steep, they burn up. The window they have to hit is like trying to slide a piece of paper into a thin slot while you're running at full speed. Once they get through that skip and slow down enough, the air finally grabs them for good. Then it's just a matter of three big parachutes opening up to turn a twenty-thousand-mile-per-hour fall into a gentle splash in the sea.

GuestThe craziest part is that for most of the trip home, the crew is just coasting in the dark with no engines running, trusting that the math they did a week ago will hit a target in the ocean only a few miles wide.

HostThat big white light in the sky is more than just a view, it's a giant, moving anchor that lets us swing out into the dark and still find our way back to our own front door.