What if Your Engines Stopped Working on the Way to the Moon?

No gas, no brakes, stuck in space. Here’s how NASA would get you home.

Jan 13, 2021

You’re cramped, cold, and wildly under-stimulated. The sun is mostly behind the Earth, it’s dark, and the only sounds come from the instrument boxes in front of you. You’re on a spaceship headed to the moon. Now obviously you’re prepared for this—part of your testing was in sensory deprivation chambers, so you’re used to the stillness. At this point in your flight the engines aren’t on. They’ve worked hard to get you going at the right speed and in the right direction so they can take a break for now. As you get closer, traveling about 7 miles every second [1], you recall your protocol. You’re a few hours away from turning the engines back on to slow down. You can’t help but wonder where you are in space and why you’re there.

Your journey began over 24 hours ago on a massive rocket in Cape Canaveral, Florida. The Saturn V, which took the Apollo astronauts to the Moon, was 363 feet tall, 60 feet taller than the Statue of Liberty, and weighed 6 million pounds at takeoff. [2] Nowadays, you’re probably on the Falcon Heavy, which comes in at a more modest 230 feet. [3] This launch brought you into what’s called a “parking” orbit, which is essentially a stopping point to evaluate how the launch went, and make sure everything is ready for the next portion. In this orbit, you’re about 120 miles above the surface of the Earth. For reference the ISS is about 250 miles above sea-level. If you’re thinking that’s not very high, that’s because it’s not. It’s technically not even fully out of Earth’s atmosphere. But you were only there for a short while, under 3 hours. Then, once mission control decided everything was okay, your ship started a second burn, this one lasting over five minutes and putting you on course for the Moon. In these 5 minutes you go from 17,000 mph to 25,000 mph. [1] At this speed you are going faster than Earth’s “escape velocity,” or the speed needed to completely overcome Earth’s gravity. Note that this assumes you follow the same mission plan that the Apollo 11 astronauts did over 50 years ago. [4] It worked out well for them so let’s use it.

Planning out a trip to the moon isn’t easy. Mapping orbital trajectories is an extremely important job, and unfortunately brains will only get you so far. In order to accurately predict the path to take between two locations in space, even Jimmy Neutron would need powerful computers to do the math for him. Similar to what we briefly touched on in the first newsletter, these types of problems are unsolvable directly, so approximation methods are used. The more computing power you have, the more accurate your approximations can be. Holding the two bodies still, this is roughly what your trajectory would look like. [5]

Keep in mind that the Moon is orbiting around the Earth. The Earth is also moving around the Sun, but we’re close enough to the Earth that we don’t notice that motion much (we’re inside what’s called Earth’s “Sphere of Influence”). As a result of the Moon’s motion, we can’t aim directly for the Moon—we have to aim for where the Moon will be when we reach it. Imagine a quarterback leading his receiver so they can catch the ball in stride. Think Tom Brady or Aaron Rodgers. (NOT Daniel Jones) This makes the actual trajectory look a little more complicated. Nonetheless, the above figure is all we’ll need for this scenario.

Okay, so you have a plan—once you get to the right spot near the moon, you’ll use your thrusters to slow down. This deceleration enters you into a lunar orbit. The principle is similar to how satellites orbit around Earth, only instead of coming up from the surface, you’re coming in from space. All of these maneuvers must be very precise to enter you into the correct orbit. Ensuring you’re in the correct orbit is important for you to be able to carry out the rest of your mission to get down to the surface as planned.

Some time has passed, and it’s time to slow down. You’ve been in your ship for over 2 days now, and you’re anxious for some action. You complete your checklist, and you fire the engines. Nothing. There’s no sound in space… maybe that’s it? But you don’t feel the normal jolt that comes with them. They’re not working. There’s protocol after protocol outlining how to restart them and how to troubleshoot the error. None of them work. Now the question arises: what happens from here?

In comes the beauty of the lunar free return trajectory. Your route was planned meticulously such that now, without thrust capability, you will follow a path around the dark side of the moon, slingshot out the other side, and begin speeding right back to Earth. Not only are you headed back to Earth, but you’re headed back into almost the exact same orbit you were in before you left. Without realizing it, when the thrusters were turned off over a day ago, you were already on a course home. NASA didn’t just save you—they saved you months ago. You can’t make a pitstop at the Moon anymore, but you can avoid dying a miserable, lonely death in a sensory deprivation chamber 239,000 miles away from anything you’ve ever known or loved. Bummer?

This all falls under a greater umbrella of orbital trajectory planning. It’s a freestanding discipline of aerospace engineering where you not only have to calculate the route, but you have to engineer it to meet various requirements, with the safe return without thrust being one of them. There are infinite different routes to take. Some require more fuel, some take more time, but all of them are technically viable options to get there. This is why the trajectory looks the way it does. It was chosen to use minimal fuel, while still getting the astronauts there in a comfortable amount of time. While the Moon is the closest body to Earth, it can often be one of the hardest to plan a route to. This is because while traveling there, you are subject to both the gravity of the Earth and the Moon. Contrastingly, when traveling to say, Mars, most of the flight you are far enough away from both of them that you are just subject to the gravity of the Sun. As such, you plan a way to get away from the Earth, then a way from Earth to Mars, then a way to get into a Mars orbit. When going to the Moon you are technically simultaneously in orbit around both of them. Thankfully, we have NASA engineers to make sense of the mess for us and get spaceships there effectively.

If you’re interested in how we get things to other planets, stay tuned for a future newsletter. In the meantime, thank you for reading and welcome home!

It's Not Rocket Science

Discussion about this post