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I know that a rocket needs to launch at the right time in order to be able to reach a specific point in orbit (e.g. the ISS). This launch window is quite small (correct?).

But why does Artemis I target a specific launch window?

Sure, Artemis has to arrive at the moon at the right time in order be on the right flight path to perform the planned fly by. However this only explains why a quite specific time for the TLI burn is necessary and not why a quite small launch window on earth is required.

Sure, while the earth spins Artemis might launch while the moon is right above or on the other side of the earth. But does this matter? Could this not just be corrected by an extra orbit (or a half) before performing the TLI burn to the moon? Is the window for TLI this small?

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There is an alarmingly thorough article about this on NASASpaceflights’s website (not affiliated to NASA itself).

Lunar illumination at arrival plays a part; so does the maximum length of time for which the spacecraft can be allowed to be in the Earth’s or Moon’s shadow. So does the limited Δv currently available and the need for a daylight splashdown.

They have also imposed a 120-minute limit for the rocket to be in a “fuelled and ready” condition, and indeed if a particular launch window is more than that length, they pick the “best” 120 minutes and use that as the window.

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    $\begingroup$ The link you have is to to the website of NASASpaceFlight, not of NASA. NASASpaceFlight is a narrowly focused media group that provides coverage and commentary on space launches and other related things. They are a reliable source, but they are not NASA. $\endgroup$
    – Οurous
    Nov 17, 2022 at 1:12
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From an orbital mechanics standpoint, remember that:

  • The Artemis launchpad is not located on the equator and does not launch into perfect 0-inclination equatorial orbits
  • The planet Earth is inclined at approximately 23 degrees
  • The Moon's orbit is also inclined at approximately 5 degrees

Performing an orbital inclination change maneuver is extremely expensive in terms of delta-v (fuel) so that in the worst case one would need to wait a month (or maybe two weeks) for the optimal Hohmann window to line up, all without even considering solar illumination, weather, business hours, etc.

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    $\begingroup$ Suggesting there is a launch window of a few days every about 4 weeks? $\endgroup$
    – Peter Mortensen
    Nov 17, 2022 at 4:18
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    $\begingroup$ I'm not convinced. The outward trajectory is essentially radial. You can reach that from LEO with nearly the same Δv regardless of inclination. You do of course need to select an appropriate anomaly, which in turn restricts the choice of inclinations for the parking LEO that can be reached efficiently, but there's still a lot of leeway. It's definitely not an “all variables need to align” situation, more a “some launch times would require a polar parking orbit and are therefore unsuitable” situation. $\endgroup$
    – leftaroundabout
    Nov 17, 2022 at 9:12
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    $\begingroup$ @PeterMortensen Well, since the rotates once a day and thus "sweeps" all possible orbits, I'd expect at least some form of launch window once per day--although the orbits required might be weird. Now, if you are in an arbitrary near-earth orbit, I'd expect that you'd have a window at least once every lunar month. $\endgroup$
    – Dragongeek
    Nov 17, 2022 at 12:03
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If you look at the flight path animation (too large for SE), you'll see it makes two gravity assists from the Moon. So the moon needs to be in a specific point in its orbit so that they line up.

https://en.wikipedia.org/wiki/Artemis_1#/media/File:Animation_of_Artemis_I_around_Earth.gif

enter image description here

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  • $\begingroup$ Not exactly "gravity assists" - those are the periapses at the start and end of the "distant retrograde orbit" Artemis is going to be in while orbiting the moon (once) - see the right panel of the animation you referenced, it makes a lot more sense to view it in the frame rotating with the moon. $\endgroup$
    – FKEinternet
    Nov 18, 2022 at 13:54
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The current version of the vehicle does not have enough performance to initiate TLI from a low-earth, circular orbit. Instead the the launch has to be done into an elliptical parking orbit to give the craft a bit more energy.

The ellipse has constraints on the launch point, and the TLI must be done near perigee. So currently, no you can't go TLI at any time after launch.

Later versions are supposed to have more performance. They will use a circular parking orbit and will have larger launch windows.

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