Will NASA put astronauts into a polar lunar orbit? If so, how?

Question

The NASA news item New Space Policy Directive Calls for Human Expansion Across Solar System says:

New Space Policy Directive Calls for Human Expansion Across Solar System

and the administration is working with NASA to put together a plan to get Americans back on the Moon in 2024.

The target landing site will be at the Moon's south pole, so a trajectory similar to the relatively planar Apollo era free-return-like trajectories can't be used.

Does there exist a slightly modified free-return-like trajectory that includes a polar fly-over, or will the likely orbit involve some kind of substantial plane change maneuver? Or will it be something else entirely?

Are there any other spacecraft trajectories from Earth to a lunar polar orbit that can be used for comparison?

Answer 1

Will NASA put astronauts into a polar lunar orbit?

Yes, but it's a bit complicated.

If so, how?

The current plan is not for an Apollo-like launch from Earth orbit to Lunar orbit, culminating in a landing descent from that orbit.

Instead, the plan is to launch a crew from Earth on the Space Launch System to the Lunar Gateway, a mini-space station of sorts, which will be in a "near-rectilinear halo orbit."

According to this plan, a "landing system" will already be docked at the Gateway. The landing system will be tripartite: "a transfer vehicle to move the lander from the Gateway to low lunar orbit, a descent stage to go to the surface, and an ascent stage to return directly to the Gateway."

Since the landing is planned to be at the south pole that orbit will necessarily be polar, but it will transfer there from the halo orbit of the Gateway, not from Earth.

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I write this answer largely to try and clear up apparent misconceptions about what is planned, in the question itself and the other answer, not because I have any detailed knowledge of the orbits involved.

Answer 2

In terms of the question "do there exist a slightly modified free-return trajectory..." I don't know (I know that's a weird way to start an answer, but bear with me). Getting to Lunar Polar orbit is not made any easier by starting in Earth polar orbit, that is overkill.

You basically need to fly over the poles of the moon. Given the moon's size and distance, this means that it can be achieved with a relatively low inclination Earth orbit, something probably less than a few degrees inclined with respect to the moon's orbit, probably lower. It's all about angles - the moon's angular size is small, and the distance is large, so if you are inclined just a little bit, then over the distance to the moon your orbit will go over the poles.

Performing a plane change in orbit is prohibitively expensive no matter which body you are orbiting. To change just a few degrees in low Earth orbit would take several hundred m/s of delta v. To change from equatorial to polar in a lunar orbit would be insane (I can't find a good number, but there is an equation here. You'll probably find that the vast majority of the delta-v budget of a mission would be taken up by the plane change.

NASA has studied such orbits extensively, since mapping satellites are best placed in such orbits in order to get as much surface coverage as possible. Here is a NASA trajectory study of this very thing - low energy transfers.

Whether a free-return polar trajectory exists or not is an interesting question. I strongly suspect not, but I'm happy to be proven wrong here. Free-return in the sense of return to the surface without expending fuel would require a small gravitational assist from the moon, which is not possible for a purely polar orbit. Perhaps a slightly less-than-polar orbit, where some small in-plane velocity change can occur, is possible, so don't quote me on the impossibility of such an orbit yet.