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The Artemis I mission will take 8-14 days to get from Earth to the Moon. Apollo 11 only took 3 days. Similarly the return will be 9-19 days for Artemis I and just over 2 days for Apollo 11.

Why is the transit time for Artemis so much longer and how can there be such a large margin between lower and upper estimates? Can this be explained from an orbital mechanics point-of-view?

Note that the time in lunar orbit is listed separately in the image, so it seems that the variation is not due orbiting the Earth/Moon longer before the transit.

Artemis I map

(Source: NASA)

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    $\begingroup$ A standard Hohmann transfer to the Moon's distance takes about 5 days, Apollo went a slightly more energetically to cut down on transit time for lower life support requirements and get the free return trajectory. Apollo also went for a low lunar orbit, instead of a brake into a long ellipse followed by a high retrograde circularization. $\endgroup$
    – notovny
    Commented Apr 2, 2022 at 16:45
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    $\begingroup$ @notovny That's an answer, and a good one. I suggest making that comment into an answer. $\endgroup$ Commented Apr 3, 2022 at 11:21
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    $\begingroup$ Note the re is a very large range in mission duration estimates for Artemis I because there is still a wide range of launch dates and the launch date will have a considerable effect upon the trajectory and time taken. $\endgroup$
    – Slarty
    Commented Apr 4, 2022 at 22:03

3 Answers 3

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The goal of the Apollo missions was to land humans on the Moon and return them to Earth. That requires a surprisingly large delta-V capability. (In terms of delta-V, is easier to land a vehicle on Mars and leave it there than it is to land a vehicle on the Moon and leave it there.) The Space Launch System plus the Orion capsule do not have the delta-V needed to land a vehicle on the Moon and return it to Earth. SLS plus Orion does not even have the delta-V needed to insert into and later return from low lunar orbit.

On the other hand, the Orion capsule has an extremely capable environmental control and life support system (ECLSS), capable of supporting a crew of six for 21 days. It can afford to make a leisurely trip. The Apollo vehicles did not have this luxury as they had a rather limited ECLSS capabilities. The outbound and inbound trajectories to and from the Moon were intentionally designed to be suboptimal with regard to delta-V so as to make the trip take less time. Had the Apollo vehicles used optimal trajectories (from the perspective of delta-V) the result would have been dead bodies returning to Earth.

The Apollo missions had the spacecraft immediately enter low lunar orbit upon getting close to the Moon. The Artemis-1 mission will instead use close lunar approach as a powered gravity assist and days later inject the vehicle into a distant retrograde orbit (DRO) about the Moon. The drift from closest approach to the Moon to DRO insertion will take a few days. Targeting lunar DRO as opposed to targeting low lunar orbit reduces the required delta-V to something the SLS+Orion can handle. (Subsequent missions will place the vehicle into a near-rectilinear halo orbit, with similar delta-V requirements.) The greater time needed for a fuel-optimal trajectory to lunar periapsis followed by a drift to DRO altitude combine to make the outgoing trip take eight days rather than a bit more than the three days used by the Apollo missions.

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    $\begingroup$ "SLS plus Orion does not even have the delta-V needed to insert into and later return from low lunar orbit." Wait, what? I'm grossly overestimating SLS, apparently. Seems that even block II barely equals Saturn-V's payload capabilities. $\endgroup$
    – Ludo
    Commented Apr 3, 2022 at 15:56
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    $\begingroup$ @Ludo. Even SLS Block II won't equal Saturn V. The Saturn V was an incredible beast of a machine. The only vehicles that might equal or exceed the capabilities of the Saturn V are SpaceX's Starship and China's Long March 9. I wrote might because both remain in development. $\endgroup$ Commented Apr 4, 2022 at 12:49
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    $\begingroup$ I don't get it. This article from today's launch of Artemis to the moon is specifically bragging that Artemis is "the most powerful rocket to ever fly" and "Indeed, the SLS isn't just the brawniest rocket in operation today; it's the most powerful launcher ever to fly a successful mission, beating out both NASA's space shuttle vehicles and the iconic Saturn V rocket". $\endgroup$
    – davidbak
    Commented Nov 17, 2022 at 0:40
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    $\begingroup$ @davidbak The first stage of SLS Block 1 has more oomph than did the first stage of the Saturn V. However, SLS is a two stage vehicle while Saturn V was a three stage vehicle. The Saturn V, which was designed to put a lot of payload mass into translunar injection (TLI), could put 43500 kg of payload into TLI. SLS Block 1, only 27000 kg. $\endgroup$ Commented Nov 17, 2022 at 8:30
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    $\begingroup$ @jwenting - "using metrics which are unfamiliar to most lay people" is exactly why it is misleading to use in a press release for lay people. $\endgroup$
    – davidbak
    Commented Nov 24, 2022 at 17:51
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Apollo burned a lot of fuel in exchange for a quicker mission. Back in the 60's the spacecraft had a very short lifespan, mostly limited by the capacity of the batteries. Nowadays battery capacity is much better, and so is solar panel technology, so missions can afford to spend an extra few days in exchange for saving on some fuel (weight).

If you look at unmanned probe mission to the moon, some take fuel economy to extremes, and take weeks to get there. Roughly speaking, you can get to most destinations in the solar system on very little fuel if you are prepared to wait (sometimes many years) for the right launch conditions, and can tolerate very long flight times.

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    $\begingroup$ Plus one, but ... only weeks? Months, sometimes over a year. It took the pair of GRAIL spacecraft over three months to get to their targeted low lunar orbit. It took SMART-1 17 months to get to its intended lunar orbit (which wasn't even a low lunar orbit). $\endgroup$ Commented Apr 3, 2022 at 19:21
  • $\begingroup$ A few months is also some weeks :p $\endgroup$
    – Innovine
    Commented Apr 4, 2022 at 16:17
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    $\begingroup$ Batteries were a constraint on how much time the astronauts could spend on the surface of the Moon, but were not as big a constraint for that quick translunar coast. The command and service module used fuel cells instead of batteries. Limited life support was a bigger constraint. $\endgroup$ Commented Nov 17, 2022 at 13:18
  • $\begingroup$ fuel cells also have limited operational life, both because they need fuel to operate which is in short supply and because they wear out @DavidHammen $\endgroup$
    – jwenting
    Commented Nov 24, 2022 at 10:00
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    $\begingroup$ @jwenting You are correct that fuel cells do have a life that is limited by fuel supply. However, unlike cars that have multiple devices intentionally designed to fail at more or less the same time, the Apollo vehicles had one key limit, which was life support capability. The last thing NASA wants is multiple independent failures. They assume that no more than two independent failures will possibly occur during a mission (this is possibly a bad assumption), and they work hard to rule out the possibility of multiple independent failures. $\endgroup$ Commented Nov 24, 2022 at 20:07
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Here is also a blog post with some info on the mission profile of Artemis I and how DROs work. There are some animations/figures that also compare the duration of the chosen moon-flyby maneuver to enter the DRO vs a direct flight towards the DRO. There is clearly a trade-off between duration of flight and DeltaV and Artemis 1. https://www.nperakis.com/post/artemis-1-mission-and-distant-retrogade-orbits

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    – Community Bot
    Commented Oct 5, 2022 at 6:52
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    $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$ Commented Oct 5, 2022 at 6:52

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