According to the 3-D model of the Artemis 1 mission (click on the "Mission View" button and use the slider to zoom out a bit), the outbound trip was close to the plane of the moon's orbit, but the return trip is swinging way to the south.

Is there a reason for this? Is it perhaps to gather data on a different part of the radiation environment around Earth?


1 Answer 1


The return orbit has a high inclination to align with the planned Entry Interface (EI) target line shown in figure 4 from Trajectory Design Considerations for Exploration Mission 1.

Each short blue arrow is a possible initial entry point vector. Note they all point towards the planned landing site off of San Diego, which is "one skip away". Although any of the blue arrows are plausible trajectories, Artemis 1 is using an approach from the South. Red arrow added to show probable chosen trajectory.

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Example EI Trajectory

From Wikipedia "Skip entry trajectory"

enter image description here

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    $\begingroup$ OK… that diagram really confused me, but after skimming the linked PDF (and especially Figure 6) and googling a bit, I think I'm starting to get it. So they're aiming to first re-enter the atmosphere somewhere along the blue Target Line (ideally at the "CFP EI State" point, but I assume other points on the blue line, or presumably somewhere in the green area, would be OK too?), skip off the atmosphere along the blue arrows and land at the Landing Site just off the west coast. Right? $\endgroup$ Commented Dec 6, 2022 at 19:55
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    $\begingroup$ … I'm still a bit confused about why the "Allowable EI Zone" is almost (but not quite) entirely on one side of the target line. I assume there are good technical / geopolitical / safety reasons for the allowable EI zone being the weird shape it is, but you'd think that having a "target line" that's right on the edge of the "allowable" area would be… risky. Or maybe that's just an initial target, and it's easier the correct it in one direction, or something? $\endgroup$ Commented Dec 6, 2022 at 20:01
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    $\begingroup$ @IlmariKaronen ... good questions. Apollo splashdowns were so accurate there was serious concerns of the capsule hitting the recovery aircraft carrier. Bill Tindall (NASA) actually recommended that, in future, the recovery carrier be deliberately stationed 5 mile away from the splashdown target. $\endgroup$
    – Woody
    Commented Dec 6, 2022 at 21:51
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    $\begingroup$ @IlmariKaronen, they're doing a "skip" reentry. The near edge of the green zone is the closest they can get to the target point and still do a skip; any closer, and they'll do a direct reentry instead. The only risk that arises from overshooting the blue line is being unable to complete one of the mission objectives (testing a skip reentry). $\endgroup$
    – Mark
    Commented Dec 7, 2022 at 10:22
  • $\begingroup$ Answer and comments raised a question for me: "What's a skip re-entry and why did they decide to do that?" and Wikipedia has a good answer here: en.wikipedia.org/wiki/Non-ballistic_atmospheric_entry Recommend adding a bit of context to this answer: Skip-reentry allows the higher speeds of a translunar trip over an longer period of time. This decreases the max heating on the re-entering vehicle. $\endgroup$ Commented Dec 8, 2022 at 16:56

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