Representatives from SpaceX, Blue Origin, and United Launch Alliance participated in a forum last week held by NASA to determine the future of humans on the moon. This isn’t just how they will live, how long they will stay, or what they will do; no, this is far more interesting: this was how humans will travel from lunar orbit from the surface of the moon. The future of the next generation of lunar lander is being determined right now.
The plan right now is entirely unlike Apollo, which sent a pair of spaceships in orbit around the moon, sent one to the surface, then returned to the mother ship for the trip back to Earth. Instead of something somewhat simple, the next era of lunar exploration will happen from a gateway orbiting in cis-lunar space. What makes this so amazing is how weird the orbit is, and the reasons behind it.
It’s The Orbit That’s Special
By now, most of us should know how the Apollo missions went. The Saturn V took off from the cape, went around the Earth for an orbit or so, and then re-lit its engine to send it off to the moon. After a three-day journey, the Apollo command module entered a nearly equatorial orbit, sent the lunar lander on its way, waited a day or two or three, docked, and then sped back to Earth. The equator is the easiest place to land on the moon, and the first three Apollo missions to do so — Apollo 11, 12, and 14 — all landed within a few degrees of the lunar equator. Later missions ventured farther north, but only so far: Apollo 15 landed at 26° North latitude. Sure, we’ve explored the moon, but it’s like saying you’ve explored the Earth if you’ve only been as far North as Florida. There’s interesting stuff in more temperate climes, and on the moon especially so: there is water ice underneath craters on the lunar poles.
Part of this was due to the technology at the time. Apollo could, theoretically, land at the poles. It could also land on the equator. Mid-latitudes were challenging; to get to these latitudes, the command module would have to orbit at an inclination that’s equally far away from 0 as it is from 90. Sure, you could take an Apollo lunar lander down to the moon at 45° North, with the Earth low on the horizon, but to get back, you’d have to rendezvous with the command module. That too could happen, but then you’d be stuck. Apollo simply wasn’t built for landing at mid-latitudes with the Earth low on the horizon.
Sure, it’s rocket science, but so is playing Kerbal Space Program. With Kerbal space center on the equator, and the Mun in a zero degree inclination orbit, you don’t need to think too hard about getting to the Mun. If you want to go to latitudes other than the Armstrong memorial or the munar arch just north of East crater, it takes a bit more work.
Note the same also applies to past, potential landing sites of Apollo missions. When Harrison Schmitt suggested landing on the far side of the moon using a relay satellite to send transmissions around several thousand miles of rock, he suggested landing near the equator. It never happened, but the possible landing sites of the Apollo missions was based on the capabilities of the Apollo spacecraft.
A New Orbit, And A Gateway
NASA’s new plan for future trips to the moon don’t go directly to the moon. Instead, future moonwalkers will first visit a ‘lunar gateway’ in a Near-Rectilinear Halo Orbit. What’s a Near-Rectilinear Halo Orbit (NRHO)? That’s takes a little bit to unpack, but the benefits are worth it.
There are stable co-orbits in any three body system, and already there are a hundreds of objects in the solar system in such a configuration. As a thought experiment, imagine yourself on the surface of the sun. Directly above you, about five AU away, is Jupiter. If you were to draw an equilateral triangle with legs five AU long, you’d find a group of asteroids trailing and leading Jupiter. These are the Trojan asteroids, and thanks to Jupiter’s enormous mass, these asteroids are in an extremely stable orbit. They’re also orbiting a Lagrange point, in this case L4 (orbiting before Jupiter) and L5 (after Jupiter). There are three more points that are stable in any orbit, but the important ones are L1 and L2. In the Sun-Earth system, these points are for solar monitoring satellites, and eventually the James Webb Space telescope will be hanging around Earth-Sun L2, or in Earth’s shadow, about a million miles away.
While there is a mathematical reality of Lagrange points, in practice it’s much easier to orbit these points. That’s exactly what the lunar gateway will do. The Near-Rectilinear Halo Orbit is a special case of the Earth-Moon L2 orbit that at first appears to be a strange polar orbit of the moon, with one orbit taking between 6 and 8 days.
From a simplistic point of view, the NRHO looks like a highly eccentric polar orbit around the moon, with a periapsis of about 2,000 miles, and an apoapsis of about ten times that. This orbit isn’t a lunar orbit, though. Mathematically, it’s still an orbit around the Earth-Moon L2 point. It takes a bit more energy to get to than a lunar orbit, but it provides some fantastic benefits. It’s easy to access with launch vehicles either currently flying, or in testing, it’s in a favorable location for Earth, sun, and deep-space observations and communicating with Earth, and NRHO orbits can be used as relay stations for operations on the moon’s far side.
Since this is an orbit around Earth-Moon L2, it is inherently unstable, and will require a bit of fuel to maintain the orbit. That’s a problem, but as the Apollo science packages found, nearly all orbits around the moon are unstable and would require fuel to maintain orbit.
Soon, there will be a space station orbiting somewhere around the moon. It’s not actually in lunar orbit, although if you looked at it long enough it might seem like it. This is the orbit that will be the first stopping point on the way to the moon, and unlike the Apollo missions, this lunar gateway will be able to take us anywhere, whether it’s to the water ice trapped under the poles, to the far side of the moon, or to revisit the historic landmarks of the Apollo and Surveyor landing sites.