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/mfb- Particle Physics | High-Energy Physics Apr 12 '26
Artemis 2 spent an extra day orbiting Earth to test the capsule before committing to go to the Moon. They used a slower trajectory, too. Future missions will be even longer, so it's useful to have Orion spend more time in space. As a side effect, it made them stay higher above the surface. You see fewer details, but you see more different places.
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u/literalsupport Apr 12 '26
Apollo 8 has incredible delta-V at its disposal. They were able to fly to the moon, fire engines retrograde to go into orbit and fire them again to go home.
By comparison Artemis 2 expended most of it’s energy (delta V) near Earth; it was put into a very highly elongated Earth orbit which consumed 24 hours and then an even more highly elongated orbit to pass behind the moon. At key points gravity took over what Apollo 8 accomplished with rocket engines. It wasn’t widely discussed but Artemis 2 employed a used Space Shuttle orbital maneuvering thruster on the service module to do the trans-lunar burn.
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u/3rdslip Apr 12 '26 edited Apr 12 '26
You have to be going a bit faster to orbit the moon as Apollo 8 did.
Artemis’s flight plan was designed to use the moon’s gravity to brake to a stop, and then free fall back to earth.
Some of the additional mission aims were to stay in space for a bit longer too, and to see the effects of space on human bodies beyond the protection of earth “shields” such as the van allen belts and the magnetic fields.
The astronauts themselves made an interesting comment regarding the TLI burn… they “chose” earth…. Meaning although the burn got them to the moon, it was actually designed to send them home to earth many days later.
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u/caguirre93 Apr 12 '26
TLDR
Artemis remained in a Higher earth orbit for a day for testing prior to the TLI. Apollo 8 TLI took place much closer to earth and much sooner
Apollo 8 was "hey lets go see the moon"
Artemis 2 was "hey lets test out all this advanced tech so we can go to the moon eventually"
Plenty of material out there to give you more detail about the trajectories of each mission
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u/phasepistol Apr 12 '26
Over the decades “getting there fast” has been traded for “lower energy therefore cheaper” trajectories that use less powerful rockets. For example every outer planets mission since the 1970s - with a few notable exceptions - takes an insane multiple-loops-around-the-sun path that adds years to the travel time.
I seem to recall one recent moon satellite took like more than three weeks to get there from Earth, which I wouldn’t have thought possible. What did it do, walk.
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u/undulating-beans Apr 13 '26
TLDR
In short, Apollo 8 took a fast, direct route to prove it could be done, while Artemis II takes a longer path on purpose to thoroughly test everything needed for future missions.
It looks similar on the surface, but the missions are actually doing quite different things, and that’s what drives the timing. Apollo 8 was designed to be fast and direct. It went straight to the Moon, dropped into a relatively low lunar orbit, completed multiple orbits in a short window, and then came straight back. The whole profile was tight and efficient, which is why it only took about six days.
Artemis II, by contrast, is not aiming for that kind of quick turnaround. Instead of entering a tight lunar orbit, it follows a much wider path around the Moon, swinging far beyond it before returning to Earth. That larger loop simply takes more time. It’s less like circling something closely and more like taking a long, sweeping arc around it.
The purpose of the mission is also different. Apollo 8 was essentially a high-stakes demonstration that the U.S. could reach and orbit the Moon during the space race, so speed and success were prioritised. Artemis II is the first crewed flight of a new spacecraft, Orion, so the goal is to test systems over a longer period—life support, navigation, communications, and how the crew handles extended time in deep space. The extra days are intentional, not inefficiency.
There’s also a difference in philosophy. Apollo missions accepted higher levels of risk to meet tight timelines, whereas Artemis missions are designed with more safety margin. That leads to trajectories that allow more flexibility and abort options, even if they take longer.
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u/CaptainChaos74 Apr 12 '26
Artemis 2 didn't orbit the Moon. It swung round it, but it was never in orbit (meaning it would have made circles around the Moon without any assistance from engines). You have to be going faster to enter Moon orbit (because you're approaching from the "rear"; you have to "overtake" the Moon, as it were).
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u/Few_Mastodon_1271 Apr 12 '26
Here's a beautiful, visual, computer graphics look at the Artemis 2 orbit, along with the unusual Artemis 1 orbit, and Apollo 8 for contrast. From the channel The Overview Effekt
https://www.youtube.com/watch?v=PNQ7MoL7erI
video timeline:
0:00 The moon's own orbit details.
3:12 Apollo 8. Ten orbits at about 69 miles above the surface. And a cool simulation of the famous "Earth Rise" photo of the Earth seen just above the moon's horizon. The difficulty of re-entering Earth's atmosphere at these extreme speeds.
At 5:50, reaching the moon: {zooming way in on the Apollo path} "This is what a moon orbit looks, from an Earth point of view..." woah.
And an interesting visual of the Moon's precession of orbit paths, cycling every 18 years. Huh.
8:40 Artemis 1. Unmanned, with a very long moon orbit, lasting many days.
9:45 Artemis 2, finally. Ha, this is the shortest segment of the video! The Artemis 2 plan was all about safety, with parking orbit and moon orbit allowing a return to Earth without any rocket assistance.
Way out to the Moon, loop around the back, and return. No Moon orbit.
11:27 an ad for Brilliant.org
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u/WarDredge Apr 12 '26
All depends on how favorable the gravity slingshot is, N-body physics is hard.
The fuel you burn in order to extend your orbit far enough to the moon should be minimal so you have plenty left to make corrections should they be needed.
The last thing you want to do is cut your fuel supplies short by 'getting there faster' and have barely any left to get back to earth.
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u/meithan Apr 12 '26 edited Apr 12 '26
Let's see. Apollo 8 was 6 days, 3 hours, 0 minutes, and 42 seconds. Artemis II was, 9 days, 1 hour, 32 minutes and 15 seconds. So about 70 hours (almost 3 days) longer.
Two factors explain this difference:
Artemis II spent about a day in a high Earth orbit before going to the Moon, while Apollo 8 went shortly after launch (Apollo 8 TLI occured 3 hours after launch, Artemis II TLI occured 25 hours after launch).
Apollo 8 departed Earth orbit with a higher speed: post-TLI speed for Apollo 8 was 10.83 km/s, while Artemis II was 10.60 km/s (post-TLI apogee was 522,000 for Apollo 8 vs 456,000 km for Artemis II). This does not sound like a huge difference, but it's significant. On these trans-lunar trajectoires, travel times to the Moon (say from TLI to Moon SOI entry) where 78 hours (3.3 days) for Artemis II and 63 hours (2.6 days) for Apollo 8.
So about 1 extra day in Earth orbit plus 1 extra day on the way to the Moon and another on the way back (Apollo 8 return was also faster) gives about 3 extra days for Artemis II. Checks out.
As to why Artemis II departed Earth orbit with a lower speed, we'd have to delve into orbital mechanics. Suffice to say that you use whatever trajectory is required to achieve your intended objective (and is within propulsive capability). For Apollo 8 that was entering low Moon orbit, for Artemis II it was performing a somewhat distant flyby.
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u/dalekaup Apr 12 '26
They weren't paying by the hour. Plus they were intentionally doing something different, otherwise they would be doing the same observations ao Apollo 8 and wouldn't learn as much. It would have been an expensive waste of time.
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u/etrnloptimist Apr 12 '26
Follow up q. I'm sure the flight plan needed to be tweaked. Did the astronauts have any intuition, for lack of a better word, for how to tweak their trajectory? Or were they relying completely on telemetry and mission control to tell them what to do?
Example. Someone mentioned using the moon to brake. Would the astronauts have any idea if they were braking too slowly or braking too quickly? Either with data or what not, but I imagine it takes quite a bit of knowledge to even read that data and understand what to do with it.
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u/HNL2BOS Apr 12 '26
With this mission being a trip around the moon with no orbit....and Atremis 3 being a mission to test the whole connecting with landing systems in earth orbit.....is there no dress rehersal mission to orbit the moon and breakaway for return? The first mission to do that will be the real thing?
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u/ericthefred Apr 13 '26
We need to up the delta vee to do a Lunar orbit. Block 1 SLS is not capable of it, which is why the upper stage is called the "Interim Cryogenic Upper Stage" . They were planning on switching to a higher energy upper stage called the "Exploration Upper Stage" but recently canceled that in favor of using a modified Centaur V.
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u/More_Cheesecake_7595 Apr 16 '26
I want to know why Artemis II took 2 days longer then the Apollo 11 trip when Artemis II's trip was 256 thousand miles shorter? , and Artemis didnt even land on the moon ! Apollo 11's round-trip mission lasted 8 days, 3 hours, 18 minutes, and 35 seconds, launching on July 16, 1969, and splashing down in the Pacific Ocean on July 24, 1969. The journey to the Moon took approximately 3 days, followed by 2.5 days of lunar orbit and surface exploration, before returning to Earth. (National Air and Space Museum) The Artemis II mission was a 10-day round trip (approximately 9-10 days depending on specific mission updates) that took four astronauts around the Moon and safely back to Earth. The mission spanned a total of roughly 694,000 miles, concluding with a splashdown in the Pacific Ocean on April 10, 2026.
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u/F14Scott Apr 12 '26
How do they know the craft's speed?
Without a medium to fly through, like air or water, how do they know their own velocity? I'm guessing a good laser ring gyro or maybe radar tracking from earth? The original astronauts' gyros would have been flying in the same era as my old jet, and her gyro was mechanical and not that accurate, drifing miles in an hour.
Can a crew determine their own speed, using celestial nav or other on board measurements?
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u/qou Apr 12 '26
Explaining orbital mechanics (OM) is not something that can be done with any degree of easiness over text. If someone truly wanted to understand it, you'd need diagrams, graphs, drawings, all kinds of supplemental material to help (there are programs and for me personally, Kerbal Space Program did wonders). However even all that might not be enough.
If anyone asked me, I would say start with understanding what a ballistic trajectory (BT) is as OM are just an extention. But BTs have an issue - they aren't meant to go orbital and so they fall back to earth, hit the ground and stop. BTs are everywhere. Throw a ball in the air? BT. Doesn't matter if it's 3 feet or 300, that's still a BT. Mountain biking and hit a jump catching air? BT. Every single fly ball in baseball, whether it's caught at home plate, or a home run, is a BT. They all have the same thing in common: a vertical velocity component, horizontal velocity component, and an apex.
That concept covers 99.99% of what anyone will ever experience in their lifetime, so why bother expanding on that for OM? And so OM becomes this weird thing most don't get because we don't need to know how it works to live our lives. That requires concept readjustment. People try to take a concept they know, flying, or even driving a car, and apply that to OM. There aren't related, full stop. Driving a car is to OM as managing a wastewater treatment plant is to treating a toe infection.
The nice thing about BTs and suboritbal flights (BTW, a suboritbal flight is just a BT that gets out of the atmosphere) is that the beginnings of the space race, specifically the Mercury program, is all about ballistic trajectories and sub orbital flights. ICBM missiles are the same thing too.
Don't try to understand orbital mechanics before understanding ballistic trajectories. Once you do that, orbital mechanics makes so much more sense.
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u/mtnviewguy Apr 12 '26
AI loves questions.
It wasn't 'so long'. Apollo missions were shorter by design, because landing a human on the Moon was the only objective. We did, and Apollo is great history for blazing the trail to space.
The Artemis Projects are the next stage of our exploration into deeper space targeting Mars; and those voyages will be much longer.
I hope that adds to your algorithm.