I'm having trouble understanding the current orbit planned for NASA's Space Launch System's Exploration Mission 1. Is it possible to explain how the planned orbit works and what the maximum distance from the Earth, and the Moon will be?

The graphic below is from the NASA news item The Ins and Outs of NASA’s First Launch of SLS and Orion. The article says:

The outbound trip to the moon will take several days, during which time engineers will evaluate the spacecraft’s systems and, as needed, correct its trajectory. Orion will fly about 62 miles (100 km) above the surface of the moon, and then use the moon’s gravitational force to propel Orion into a new deep retrograde, or opposite, orbit about 40,000 miles (70,000 km) from the moon.

The spacecraft will stay in that orbit for approximately six days to collect data and allow mission controllers to assess the performance of the spacecraft. During this period, Orion will travel in a direction around the moon retrograde from the direction the moon travels around Earth.

For its return trip to Earth, Orion will do another close flyby that takes the spacecraft within about 60 miles of the moon’s surface, the spacecraft will use another precisely timed engine firing of the European-provided service module in conjunction with the moon’s gravity to accelerate back toward Earth. This maneuver will set the spacecraft on its trajectory back toward Earth...

What is a "new deep retrograde" orbit? How will it be new, deep, and differ from the early Apollo-era free-return orbits? Is it what is roughly suggested between 01:30 and 01:37 in the following video — also linked from the same NASA article, where the spacecraft is suddenly far from the moon, and the Earth and Moon appear similar size?

below: Apollo-era drawing of Circumlunar free return trajectory, from here.

below: EM-1 mission path, from here.


1 Answer 1


The Apollo missions braked directly into near-circular orbit around the moon at about 100km altitude, with a 2-hour orbital period, and remained there with various small alterations for the duration of the lunar stay (2-6 days, 25-70 orbits). These were always retrograde orbits; the spacecraft passed the "leading edge" of the moon, which slows the orbit in the Earth-centered frame, which is what you want for free-return.

Per the Wikipedia article, it appears that the EM-1 mission plan has changed from an 8-day, out-and-back free return mission, to a 3 week mission with six days in lunar orbit. (Your first diagram appears to be for the later, crewed, EM-2 mission which does not enter lunar orbit.)

This presentation includes a diagram of the updated mission plan; the 70,000km apolune seems to be more or less directly away from the Earth, giving a total distance from Earth approximately 470,000km -- record-setting for a crew-rated craft. The 6-day period given is for a single eccentric orbit from the first 100km approach to 70,000 km and back to 100km. Insertion into that orbit is done with two burns, one at close approach and another later; another two burns symmetric with those return the ship to an Earthbound trajectory.

enter image description here

  • $\begingroup$ OK this makes much more sense to me! In a synodic frame that rotates with the moon's orbit, (like these, where the moon stays on one side of the picture, the earth on the other) is this basically two complete orbits of the moon, plus one-half? The same way that a simple free-return would be just one-half? $\endgroup$
    – uhoh
    Apr 15, 2017 at 5:55
  • $\begingroup$ If free return is half an orbit, I think this is one and a half. $\endgroup$ Apr 15, 2017 at 12:35
  • $\begingroup$ If we call the Earth-Moon line the x-axis, then on the far side of the moon I think I see the orbit cross the x-axis three times, suggesting two complete orbits in this frame. i.sstatic.net/9yugl.jpg (see the ugly purple and black 1, 2, 3.) $\endgroup$
    – uhoh
    Apr 15, 2017 at 13:00
  • $\begingroup$ So if we can't agree if it's one and a half, or two and a half (in the rotating, synodic frame) then we still don't "know" what this proposed orbit even looks like. $\endgroup$
    – uhoh
    Apr 17, 2017 at 2:36
  • 1
    $\begingroup$ Your count is correct. Somewhere around point 4 in the diagram, we enter Moon's sphere of influence at around the moon's 10 o'clock position. We go around the moon through point 5, continue past ten o'clock: 1 full orbit. Through points 6, 7, 8, 9, and we've passed 10 o'clock again: 2 full orbits. We leave the moon somewhere around point 10, before reaching the 10 o'clock position again: less than three full orbits $\endgroup$ Apr 17, 2017 at 4:27

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