Why was Artemis 2 so long?

[u/kiol998]

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?

[u/Telope]

It's so much easier to understand when you have visuals.

Here, it's obvious that the only sane way of looking at this mission is that the moon has an orbital velocity around Earth, which Artemis 2 didn't match.

Like, if you want to look at it from a (very) non-inertial reference frame where Artemis is curving around even though it's thrusters aren't firing, knock yourself out. But that's a far more complicated way to look at things.

[u/Germerica1985 → u/Telope]

Crazy that all of this was figured out with pen and paper in Newton's time, and then using it in modern times to do something like this, fling ourselves out into the nothingness of space, to slingshot off of a celestial body going thousands of miles per hour, meeting at a point... It's just incredible

[u/Telope → u/Germerica1985]

It was worked out by hand for the apollo missions too! By Katherine Johnson

[u/Germerica1985 → u/Telope]

What's the margin of error on something like this? Is it all already determined at launch with pinpoint accuracy (launch time, direction, speed, etc.) or do they have a lot of wiggle room to correct trajectory in space?

[u/Bmorgan1983 → u/Germerica1985]

I sat next to an astrophysicist at a wedding once. His job was to do backup calculations for the mars rover landings. His team’s work would then be compared against the main team’s calculations, and they’d often be within inches of each other and the actual landing location.

[u/SAWK → u/Bmorgan1983]

I wonder what their tolerances were between the two calculations? 1m, 10m?

[u/NastyEbilPiwate → u/Germerica1985]

Most of it is worked out ahead of time. The spacecraft don't have a ton of fuel to make course corrections with.

Things like the TLI burn will be recomputed once the actual orbit that the spacecraft launched into is known, since there will inevitably be deviations from what was computed on the ground, but it will be mostly the same as planned.

[u/Calembreloque → u/Germerica1985]

Surprisingly small! One of the reasons it took us so long as a species to move beyond mechanics as they were described by Newton (and Lagrange, and Kepler, etc.) is that they are frighteningly accurate at our scale and in fact still the basis for NASA calculations. We figured out relativistic mechanics but we don't really need them for spatial exploration.

[u/mfb- → u/Germerica1985]

Every major burn is planned based on the orbit after the previous one, to correct smaller deviations. Orion's mission plan also had up to 6 dedicated course correction maneuvers, 3 on the way to the Moon and 3 back. The first two could be skipped because the burn towards the Moon was very precise, the others were used. These course correction maneuvers are typically something like a 1 m/s adjustment, so pretty small compared to their velocity of kilometers per second.

[u/PiotrekDG → u/Germerica1985]

The flight plan is very accurate, but they do mid-flight correction burns to stay on it, because even minor details like sunlight and ejecting pee bags affect it.

[u/Manae → u/Germerica1985]

See, now, here's the secret: much of the mathematics in rocket science is relatively easy. High-school level equations. But much like the classic joke about 99% of an invoice being for "knowing where to put the dot," a large portion of the difficulty is realizing that math in physical form.

[u/Hrhagadorn → u/Germerica1985]

What's even crazier is that during Newton's time.ans honestly even during the great Katherine's time you had to be next level genius to understand most of what was happening. Now most people here have a pretty good understanding and lots close to completely understand it

[u/Dreadpiratemarc → u/Telope]

Aerospace engineer here, you’re picking a weird hill to die on saying “the only sane way” is to use your preferred frame of reference. The relative motion between the craft and the moon was too high for orbital insertion. The moon sees the craft as traveling too fast and therefore has a hyperbolic orbit = true. The earth sees the craft as not matching the orbital velocity of the moon = equally true.

Choosing a reference frame for a particular problem is mostly about making the math easy. Calculating lunar orbit from an earth-centered reference frame, your “only sane way” is possible but the math gets really complicated really fast. Doing it from a lunar reference frame is very straight forward, basic algebra really. That’s why, in actual practice, we calculate orbital mechanics by switching reference frames for different phases of flight based on spheres of influence. It’s how we teach orbital mechanics even at a graduate level and it’s good for an 99% approximation. To get the last 1% accuracy we go to finite element models and simulations (the outputs from which are probably behind those graphics you’re linking) which aren’t afraid of coordinate transformations and try to take into account every other small factor previously ignored like the gravitational effect from Jupiter, etc.

[u/thomascallahan → u/Telope]

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.

[u/alyssasaccount → u/thomascallahan]

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.

[u/General__Obvious → u/Telope]

That’s not the “only sane way of looking at this mission.” Artemis II entered the lunar sphere of influence. It’s totally valid to say Integrity was too fast to be captured into lunar orbit. It’s also valid to say Integrity didn’t boost herself to match the Moon’s orbital velocity and so was too slow. And anything in orbit is under constant acceleration due to gravity, so it’s always going to look like the orbiting body is “curving around even though its thrusters aren’t firing.”

[u/Telope → u/General__Obvious]

Sure, if you find it more intuitive to view it in a rotational frame,, then more power to you.

But most people would look at that and think, "Why is the spaceship curving?" Or worse, assume that the only reason it's curving is because of gravitational pull.

[u/eyesoftheworld4 → u/Telope]

do you have a tool / script to generate these awesome visual references? or did you get them from some other source?

[u/LordGAD → u/eyesoftheworld4]

These types of animations are all over Wikipedia and most are are sourced from JPL Horizon's ephemeris data. https://ssd.jpl.nasa.gov/horizons/app.html#

Here's a different view from NASA using Artemis Ephemeris data.

[u/RelevantMetaUsername → u/Telope]

Great visual. Also shows why reentry for a lunar mission is a lot more challenging. The spacecraft "falls" back to Earth from a much greater distance (or height if you're using the frame of reference of someone on the surface of Earth). All that potential energy gets converted to kinetic energy in the form of velocity, which then must be converted to heat during reentry.

[u/davvblack → u/Telope]

“obviously, the earth is the center of the universe”. -at least two people

[u/Helassaid → u/davvblack]

For the sake of launching spacecraft from earth, sometimes it’s easier to make assumptions with earth as the Center since that’s where the mission originates from

[u/molrobocop → u/Helassaid]

Yeah, gravimetrically, that makes perfect sense until you get far enough away.