I could’t post anything last week because it was a busy time for space nerds like me. The launch of Artemis II and humanity’s first trip back to the Moon was big news. I spend time on different cable news channels talking about why this mission mattered so much.
Today I want do a deeper dive into something I said on CNN Friday morning. Here is a clip from that discussion.
So what did I mean about “surfing gravity”? You only get a few minutes to talk about anything in these appearances so I couldn’t unpack that very cool idea about how we travel through space.
The physics of rocketry demands you spend most of your fuel launching the other fuel you’ll need later. Since the fuel astronauts must lift into orbit is so valuable, once they’re up there they only turn their rockets on for short “burns” to speed up, slow down or change course. Here is a graphic showing Artemis’ trajectory including when the rockets are turned on for major burns (raise maneuvers, trans-lunar injection, re-entry)
The consequence of having so little fuel is that most of the time the Artemis II command module is just gliding along with its rocket motors off. Actually, the better way to think of it is that the capsule is “surfing”. What’s really happening is the spacecraft is working out a dynamic interplay between its own momentum (its motion) and gravity.
You know how, back in the day, skateboarders used to ride in people’s empty pools? They would drop in on one side, ride around the curves of the pool walls and then pop up and out the other side.
That’s kind of what Artemis is doing.
You can imagine the Earth and Moon as living at the bottom of their own deep gravity “wells”. The depth of each well at each point represents the strength of gravity there. Of course the two wells are actually orbiting each other so the whole set-up is constantly in motion.
Here is a nice visualization of the idea.
Visualization of Earth and Moon “gravity wells”. The well for Earth is deeper than that for the Moon because Earth is more massive. Artemis is launched at the very bottom of the Earth well, completes some orbits swinging around higher in that well. Then it executes an “insertion” rocket burn that sends it towards andover the “lip” separating the Earth and Moon gravity wells. Finally it swings around the Moon and back towards Earth.
When Artemis II blasted off, it first went into orbit using its launch momentum to keep it swinging around the “sides” Earth’s gravity well. Then, when it came to time to head towards the moon, the astronauts fired up the engines for a brief “insertion” burn which pushed them towards the “lip” separating the Earth’s gravity well from the Moon’s.
What they were inserting themselves onto was the right glide path that got them riding (i.e. surfing) over the lip from one well into the other. That path will also take around them around the moon and then back. The astronauts won’t fire their rockets again in a major burn until they are trying to slow down to re-enter Earth’s atmosphere.
This dance between gravity, momentum and rocket burns is very beautiful and described by beautiful (but complicated) mathematics. Somehow we humans have learned how it all works well enough to pull off mission like Artemis. In these difficult times, it gives us reason to celebrate. We can do amazing things when we work together. That’s humanity at our best.
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PS If you have specific questions or issues you want me to address leave a comment on the website or email me at [email protected]
— Adam Frank 🚀
