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The gateway's orbit is elliptical in that has a very low periapsis (lowest point in the orbit) and a very high apoapsis (highest point). The key reasoning behin

The gateway's orbit is elliptical in that has a very low periapsis (lowest point in the orbit) and a very high apoapsis (highest point). The key reasoning behind this is ease of rendezvous with Earth departing space craft. While the gateway is at its apoapsis, the speed in which it is going is very very low, this means it stays there for a far longer time. This in turn makes it wayyy easier for incoming spacecraft from earth to plan a rendezvous maneuver. If you have Kerbal Space program try it out yourself. All you have to do is circularize your orbit around the Mun at the apoapsis of your lunar gateway's orbit and you are guaranteed get a rendezvous encounter within a couple orbits. If you are lucky you can even get a rendezvous on your initial approach. The downside to this is as there is high eccentricity of the orbit, the velocity that the lunar gateway is going at it's periapsis is going to be faaar higher than if the orbit was circular at that same height. This means any lunar decent craft needs to have extra fuel to cancel out that eccentricity generated horizontal velocity to be able to land. I would bet that the ease of the initial rendezvous and lack of fuel required to acquire that rendezvous greatly offsets the additional fuel required to depart the gateway and subsequently land.

comprambler 6y ago
The part that cannot be simulated by Kerbal Space Program is the fact that the Gateway is not technically orbiting the Moon. It is technically "following" an area of space which is a halo orbit (not really pure orbits in the sense of leave it and forget it, rather where you have to stationkeep (use RCS thrusters a lot)) in an area of 3D space that has nearly the same gravity influence as the L2 LaGrange point does. From: https://www.youtube.com/watch?v=X5O77OV9_ek https://www.youtube.com/watch?v=X5O77OV9_ek at 0:05 The red color orbit around the actual L2 point at 0:05 means that you don't need to stationkeep a whole lot to hold the orbit. As the video continues on, it keeps rendering orbits that are still L2 halo orbits that are unstable as you need to continually stationkeep more and more (the reason behind the color change as I understand it) up until the periapsis of the orbit approaches the moon, the last couple orbits turn red again. This means they are somewhat stable. This is the gateway's planned orbit. At 1:05 it shows what the orbit looks like in 3D earth centered. Now you can see that the station isn't really orbiting the moon, it is in an earth orbit flirting with the South Earth-Moon L2 point. This also means that craft going to the station don't actually need to circularize as you would in a traditional orbit to dock. As long as you time it right that both the gateway and the incoming spacecraft arrive at the gateway's apoapsis at the same time, all you have to do is match their relative velocities and dock. There really is no simple english explanation out there as much as I can find anyways.
- avmich 6y ago
  Thanks. I see something here, but they don't tell about the reasons to choose the orbit - which doesn't come too close to the Moon and has a period of a week (so in case of an emergency on a Moon base..?). And why it's called near-rectilinear. What's "South Earth-Moon L2 point"? There's just one Earth-Moon L2 point, right?

comprambler 6y ago

The part that cannot be simulated by Kerbal Space Program is the fact that the Gateway is not technically orbiting the Moon. It is technically "following" an area of space which is a halo orbit (not really pure orbits in the sense of leave it and forget it, rather where you have to stationkeep (use RCS thrusters a lot)) in an area of 3D space that has nearly the same gravity influence as the L2 LaGrange point does. From: https://www.youtube.com/watch?v=X5O77OV9_ek https://www.youtube.com/watch?v=X5O77OV9_ek at 0:05 The red color orbit around the actual L2 point at 0:05 means that you don't need to stationkeep a whole lot to hold the orbit. As the video continues on, it keeps rendering orbits that are still L2 halo orbits that are unstable as you need to continually stationkeep more and more (the reason behind the color change as I understand it) up until the periapsis of the orbit approaches the moon, the last couple orbits turn red again. This means they are somewhat stable. This is the gateway's planned orbit. At 1:05 it shows what the orbit looks like in 3D earth centered. Now you can see that the station isn't really orbiting the moon, it is in an earth orbit flirting with the South Earth-Moon L2 point. This also means that craft going to the station don't actually need to circularize as you would in a traditional orbit to dock. As long as you time it right that both the gateway and the incoming spacecraft arrive at the gateway's apoapsis at the same time, all you have to do is match their relative velocities and dock. There really is no simple english explanation out there as much as I can find anyways.

avmich 6y ago
Thanks. I see something here, but they don't tell about the reasons to choose the orbit - which doesn't come too close to the Moon and has a period of a week (so in case of an emergency on a Moon base..?). And why it's called near-rectilinear. What's "South Earth-Moon L2 point"? There's just one Earth-Moon L2 point, right?