What Is Distant Retrograde Orbit, And Why Is Artemis 1 Using It?

foreign Scott Manley here it has been a few weeks since SLS made its in-flight debut carrying Artemis 1 towards the moon now we all know that this is an overpriced mission that it is there for politics rather than science but boy some of this footage is pretty darn spectacular the engineering cameras don't have the depth and the quality of the medium format film that was used on the Apollo program but they cover a multitude of angles and they're coming down in real time we've been able to watch this live and track its every move as it headed out from the earth flew past the moon and then used its engines and the lunar gravity to insert itself into an orbit that took it far behind the moon now it went into a type of orbit called a distant retrograde orbit and a lot of people were sort of confused by this because over the last few years we've all been talking about Artemis using a near rectilinear Halo orbit so much so that I had to spend a lot of time explaining exactly what that was so people have been asking what is a distant retrograde orbit and why would they do uh why would they put Artemis 1 into a different retrograde orbit when say Capstone is already demonstrating the nrho and future missions will have to enter into this orbit to Rendezvous with the Gateway now as it stands if Artemis 1 is doing a distant retrograde Artemis 2 which will have the first crew is going to do a free return trajectory so that it can come straight back to Earth if there's any problems and Artemis 3 the plan is to meet the human Landing system in nrho so that it can perform the first landing of humans at the lunar poles so here's what the trajectory looks like from NASA's eyes on the solar system you can see Artemis there it's about to have an encounter with the moon and it gets kicked around on the inside going faster than the moon now from this point it's slowing down because the Moon is applying its gravity to it the moon pools around the inside Lane passes between the Earth and the moon that's what we saw now Artemis is behind it and it's getting accelerated again back towards the moon but at this point it makes a maneuver to send itself back to the moon so that it can swing around the moon and head homewards so it's kind of hard to appreciate from this angle it actually sort of flips itself down below the plane of the earth so it then comes up sort of over the Pacific so it can be recovered so here's the same orbit for a bit from a different Viewpoint holding the moon in the middle and take note of Capstone orbiting in its near rect linear Halo orbit so Artemis after flying by the moon and making its insertion burn into dro it is far from the Moon far enough that the Moon is not dominating the gravitational environment like the lunar sphere of influence is like tens of thousands of kilometers Artemis is Way Beyond that that is the distant part the retrograde part comes because the Moon from this point of view is orbiting the Earth anti-clockwise but if you look at the Moon to Artemis you know line it looks like Artemis is moving around the Moon clockwise which is retrograde with respect to the apparent motion of the Moon around the earth what's really important to realize is that this isn't a simple kepler-style orbit where you've got a single force and a single particle this is a three body problem and while the Earth is dominating the forces of gravity at some time the the moon can dominate at others but most of the time it's a delicate balance with both the Earth and the moon strongly influencing the orbit and this is also the case for Capstone in its Halo orbit so both orbits are three body problems they spend a lot of time near the moon the big difference is that distant retrograde orbit is vastly more stable so in the Halo orbit Capstone has to make orbital Corrections every time it reaches Apple loon so that the orbit doesn't degrade so that it doesn't end up eventually colliding with the moon or potentially getting ejected off into space and leaving the moon behind so this was important when they were planning the Artemis Mission they knew that it was an experimental vehicle that was potentially problems that could happen and so by choosing the distant retrograde orbit it meant that if there was a problem that meant they couldn't make a burn at a specific time they had a lot more leeway to re-time things or move these around so that they could actually complete the mission as planned this paper pretty much covers a lot of the planning for it and you know they explain how they would find these orbits by essentially just varying the velocity until they found it coming back to its Dart position they obviously did this using trajectory planning tools like GMAT but I wanted to show that it was possible using the rather more toy-like Universe Sandbox which is basically a game that you buy on the steam store and can play around so I start out with a basic Earth Moon system my plan is I gotta put an object just beyond the moon so what I'm gonna do is line these up so that I know roughly where the Earth Moon line is zoom in on the moon now my initial object should be 70 000 kilometers from the moon but not actually orbiting the moon in a regular circular orbit the LaGrange points of the Moon are about 68 000 kilometers out so here we go this is beyond the LaGrange points of the Moon uh come on I want to get 70 000 as close to 17 that's too high there we go that's it no this by default is going to have an orbit which is technically around the moon right so if I go into the motion page here and look at orbital parent it says it'll be the moon so I want to change that to say orbiting the Earth so it tells us the speed around the earth is 1.25 kilometers per second I have a magic number that I have calculated using you know the latest in envelope and Pen technology it is 679 meters per second or 6.679 kilometers per second and that's it so if I start this thing off it should remain in orbit let me just switch to show the orbit from the Earth's point of view and get the speed going because this is going to take a very long time to cover a month at one hour per frame here we go so yeah look it falls inside the moon and uh is basically again it's being influenced by both the Earth and the moon it's a distant orbit it doesn't come close enough that the moon's gravity is the dominant Force at any time but the moon its interaction with the orbit actually helps stabilize the object and uh yeah you can see like this is a couple of months it's already made several orbits that looks kinda stable right okay so like 13 years later it's still going that seems like a pretty good job now what does this look like from the moon's point of view so I'm going to pause it I'm going to zoom in on the moon and the other thing we're gonna do is we're going to make the trails Center on the moon rather than on the earth so say as Trail Center and we're gonna now zoom in to the Moon oh and well that looks like a very interesting orb it's almost like a square Let's uh accelerate things up a bit now again these are very rough values I you know the only thing I did math for was the speed so yeah you get this really strange square like orbit it is kind of processing around a little I hear that if you get it just right you can make a very nice square but as I said I just totally you know spitballed this so another advantage of this orbit with respect to Orion is that it doesn't take that much Delta V to get into this specific orbit it's a lot less Delta V let's say than getting into a low lunar orbit you do fly past the moon at lower altitudes and make maneuvers you then make Maneuvers to enter into the distant retrograde orbit but you're not making huge burns like Apollo did to slow down into that low orbit from this orbit It's relatively easy to get to the lunar surface like you don't pay much of a penalty however it's very hard to access the lunar poles from here that's why near rectilinear Halo orbit that's one of the reasons why that's preferred another reason why nrho is preferred is because it guarantees continuous communication with the Earth and you can twist it in such a way that it will never experience an eclipse so its solar panels never have to worry about that however a decade ago the plan for SLS wasn't to go to the Moon well at least not initially the plan for SLS in it was the asteroid redirect mission where they would go out to deep space they would find a boulder from the surface of an asteroid and using a spacecraft they would carry it back to an orbit near the moon right and of course the orbit near the moon that's best for this is a distant retrograde orbit because it's so wonderful and stable and once it's in that orb it doesn't need much maintenance in fact if the spacecraft dies it's probably not a big deal but from there astronauts could take off in SLS they could fly up to this rock and they could investigate the surface off it they could do some geology setting foot on the smallest world ever this was of course uh back in the days when NASA hadn't been given any budget to build a human Landing system or hadn't been you know authorized by the president to pursue a landing on the moon SLS had on the long time scale the idea that it would be used to go to Mars but you know there was no moon emission in there Orion was no longer going to serve as a ferry for crew to the International Space Station those responsibilities would ultimately be handed off to SpaceX and uh Boeing and that meant the SLS had to find something to do that involved going into deep space something that these commercial companies wouldn't be able to do but didn't include landing on the moon because they didn't have the budget for this and that's what the asteroid redirect mission was all about I kind of wanted to see that happen but you know it's certainly a lot less ambitious than the current Artemis 3 plan so as of right now Artemis is on its way back to the Moon It's Gonna Fly past on December the 5th and basically fall back to the Earth for a landing on the 11th I'm Scott Manley fly safe [Music] foreign [Music] [Music]

Channel: Scott Manley

Views: 247,280

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Id: XFIvKSVRtZ0

Length: 11min 7sec (667 seconds)

Published: Sun Dec 04 2022