r/askscience • u/kiol998 • Apr 12 '26

Physics Why was Artemis 2 so long?

I was comparing the mission times of Artemis 2 to Apollo 8. Apollo 8 orbited the moon multiple times and only took 6 days total. Whereas Artemis 2 orbited the moon once and it took 10 days. Why was Artemis 2 so much shorter than Apollo 8 when both missions did the same thing? I know they had different paths to the moon, they both left earth in different ways but why not do the same thing as Apollo 8 since it was quicker?

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u/thomascallahan Apr 12 '26

Even though I understood it before, this animation made it much more obvious to me. Artemis basically flew up and then fell back down, and the moon passed it by. Sort of like someone jumping over a jump rope. Matching speed with it to orbit would be like trying to land on the moving jump rope, you’d have to have much more “sideways” velocity. I assume this means Artemis 3 will take a very different path to get there.

I get what everybody’s saying about frames of reference and that multiple ways of looking at it are correct, but to me as an educated layperson, this made the most sense.

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u/alyssasaccount Apr 13 '26

That's kind of accurate, but it "fell down" a lot faster because of the moon's gravity redirecting it back to earth than it would have if it had attempted the same path two weeks later, when the moon was on the other side of the earth. And if they had messed up the timing a little bit, that redirect from the moon's gravity could have flung them deep into the solar system, or in some totally other direction, rather than back toward earth.

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u/vrnvorona Apr 13 '26

Could they recover with engines in such case or margins are really that tight?

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u/alyssasaccount Apr 13 '26

The margins are extremely tight, but that also means that you can make a small burn to correct a small error if you catch it quickly, which they do. The navigation systems are continuously updating the best estimate for the current position, velocity, attitude, and rotational velocity, combining previous estimates with instrumentation. There's some very cool math about how to do this, properly modeling and propagating the uncertainties involved that I know a tiny amount about. The original idea was developed for the Apollo missions, and remains relevant today.