Rockets of Today


I briefly described the LOP-G in the SLS article, as this station is being developed in conjunction with the Artemis program to return America to the lunar surface. That program is now pressing ahead without the Gateway for the Artemis landings, as they don’t want it to cause delays, but the Gateway is still very much a part of America’s long term plans for developing infrastructure on and around the moon. They envision a growing role for it for exploration of Mars and beyond, hence its original name, Deep Space Gateway (which was much cooler than the stupid name they’re using now).

The plan is for something quite a bit smaller than the ISS, but still roomy enough for a crew of four to live there for months. With the right lander (not yet built), this might enable them to make many trips back and forth between the station and the lunar surface, at a relatively low cost per trip. (The delta-V from station to surface would be about 2.5 km/s, whereas coming from low Earth orbit would require over 5.6 km/s.) This would in turn support much more extensive exploration of the surface than could be done with missions directly from Earth.

The orbit that this station would be placed into is called a Lunar Near-Rectilinear Halo. And no, this does not mean the orbit is square instead of round — the term “near-rectilinear” just means it has a high eccentricity (about 0.9), which gives it movement which you could say is not all that far off from going back and forth in a straight line. It’s a long ellipse with the low end 3000 kilometers above the lunar north pole, aligned so that it never passes behind the moon and out of contact with Earth. The main part of the orbit has to be high up, because low orbits over the moon are unstable, due to how the moon’s gravitational field is much more uneven and lumpy than Earth’s is. So they put one end high up, where it’s easiest to reach from Earth without a monster rocket, and the other end lower down to make it easier to make surface trips. Like a Sun-Synchronous orbit around Earth, the orbit is not quite a proper ellipse, but a carefully chosen path where the various tidal deflections and distortions balance out to a desirable result — in this case, rotating the orbital shape so it is never edge-on toward Earth. Though it spends only brief hours at low altitude, this orbit will need active maintenance, so the station has propulsion, which is in the form of ion thrusters which require only a tiny amount of propellant. The period of the orbit would be almost seven days (so nine orbits around the moon will equal two around the Earth), and for about six of those days it would be overhead as seen from the moon’s south pole, since it moves much more slowly in the higher parts of the orbit than in the low part. A couple of tiny comsats sharing its orbit will keep it in touch with anyone working on the surface when it’s out of sight. At its furthest it would be 70,000 kilometers from the south pole — a quarter of a light-second.

Construction would begin with a Power and Propulsion Element. This would be equipped with solar panels, thrusters, and the ion engines. (A set of ion thrusters like those would make a nice upgrade for the ISS.) The PPE was originally designed as an experimental asteroid deflector, and then it was repurposed. For this reason it has more ion thrust than it probably needs. It would not have any pressurized space in it, but it still attaches to the other modules with an international docking port. (The larger squarish berthing ports used on the non-Russian side of the ISS will have no place on the Gateway.) The first habitat area would be called the Habitation and Logistics Outpost (HALO), — originally it was called the Minimal Habitation Module. This is based on the pressurized part of a Cygnus cargo capsule — the portion built by Thales Alenia — and it doesn’t have much room, accommodating no more than two people. It’s only about four meters long. I sure would not want to spend weeks in there, as astronauts will be expected to... as far as I can determine, it doesn’t even have windows. It will have four docking ports on the ends and sides. Both of these pieces will be put in place inexpensively by a Falcon Heavy. Finally, the European System Providing Refueling, Infrastructure, and Telecommunications (ESPRIT) would have extra fuel tanks, support a lot of external unpressurized gear, and have an airlock for EVAs. It’s short and disc-shaped. More modules, including possibly a Russian one, are supposed to be added later, but these three are the essential core. The dry weight might be as low as twenty tons for all three pieces.

The Russians have signalled that they will decline to participate, so the most likely optional add-on is the International Habitation Module, or iHab, built by the Europeans and Japanese in partnership. Though not much larger than the HALO, this would raise the habitable interior volume to 125 cubic meters — about the size of a living room. This would make it practical (though probably far from pleasant) for crews to put in six month tours. Since it takes a much bigger rocket to deliver a module out here than it does at the ISS’s altitude, and such rockets remain rare and exotic for now, any shipment to this destination that exceeds the capacity of a Falcon Heavy might be very pricey.

How the pieces will be stuck together looks like an open question. Some say the ESPRIT will be sandwiched between the PPE and the HALO, while others show it sticking out the side. It may be that the modules will be reshuffled into a new arrangement as the station expands, as has sometimes been done with the ISS. Longer term plans call for two larger habitable modules, first the iHab, and then a slightly bigger American one known as D-Hab. They would extend in that order from the forward end of HALO, possibly capped by a separate dock/airlock piece, or that might go on the side. We shall see how far this gets. Sierra Nevada has proposed a LIFE bubble for the American add-on, which would make things a whole lot roomier, but unless they scale that way down it would be awfully heavy.

Among all these modules there would be a wide selection of ports available for resupply canisters, Orion capsules, landing craft, and maybe small drones carrying stuff up from and down to the lunar surface.

Radiation out by the moon is much worse than it is around the ISS, which is below the Van Allen belts and partially protected by the Earth’s magnetic field. This means that the radiation shielding on the LOP-G will have to be a lot beefier than what the ISS has. (Hopefully this will not make it a problem to have a couple of decent windows.) In the event of a solar storm or flare, crew safety might be quite a challenging problem. I would like to hear more about how they plan to address this, as this is difficult to mitigate without adding a lot of mass, and the risk is worse there than just about anywhere people might go in space — worse than a trip to Mars, or even a stay on the lunar surface, as down there you might be able shelter yourself with regolith if you have to.

On the bright side, the risk of meteor impacts is much lower out there, because there’s no “space junk” to worry about.


Now it should come as no surprise that some see the construction of the Gateway station as pointless. As mentioned in the SLS article, it has been disparaged as a “lunar tollbooth”. And it certainly isn’t needed for an Apollo-like lunar mission where people spend a few days on the surface and then come home. The value is in scaling up operations, doing the kind of heavy work that requires weeks on the surface and many trips moving stuff back and forth. For instance, the kind of work it would take to build a base on the surface.

And there’s another circumstance in which the LOP-G could end up being very valuable indeed, and that is if it can be a fuel depot that is not supplied from Earth. This is why there is so much interest in the south polar region of the moon. Water ice exists there (as well as metal deposits), and if ice can be dug up in quantity, the LOP-G could be equipped to supply liquid hydrogen and oxygen to rockets at lower cost than fuel brought from Earth, and with a reusable lander, replenishing this fuel could be a self-sustaining operation. This could go beyond the Starship in opening up the rest of the solar system, if a big enough quantity of fuel can be made available at the Gateway. Trips to Mars and so on would only need enough fuel to reach the Gateway, and it could then give them all they need to reach their final destination. A second depot might be set up later on one of the small Martian moons. Eventually, the ice used to fuel these depots would be supplied from someplace further out where it’s far more abundant, such as Ceres, or a comet. The core of the interplanetary economy would consist of bringing outer system ice to where inner system solar power can make it into fuel, and making much of that fuel available close to Earth, and using some of it to bring more people and equipment outward. Space colonies could then be self-sustaining in terms of energy and propulsion, at as large a scale as anyone wants, even without developing any nuclear power source away from Earth.

Eventually the fuel manufacturing might shift to the asteroid belt, where ice and metals and solar power are all available in vast quantities despite the relative dimness of the sun out there, and probably most other natural resources that spacefaring people might want (except for noble gases). And if fusion power ever becomes practical, we could expand out to Jupiter’s moons and beyond. That future is what these lunar missions might eventually make possible, and the LOP-G could be a key stepping stone on that outward path — it could be the bootstrap that allows us to pull ourselves up to the belt.

Musk’s Starship plan is an attempt to shortcut this gradual path, and though the earlier parts might be quicker, we will eventually want to switch tracks back to this older vision for long term sustainable growth. The two could certainly compliment and support each other in the middle term, but the Gateway probably won’t be as much help for refueling the Starship as it wants methane instead of hydrogen, and the moon (unlike Mars or the asteroids) cannot yield methane as it does not have any ready supply of carbon. But with methane fueled craft, the lox is more than three quarters of the propellant by mass, so the depot could still supply that part. And to supply oxygen from the moon, you don’t necessarily even need ice: the lunar regolith consists in large part of oxide minerals, and though this material would be not nearly as easy to extract oxygen from as ice would be, it is far more abundant. Smelting useful metals such as aluminum and titanium could also yield oxygen. Also, trace amounts of water are now known to be embedded in the regolith in at least some sunlit areas, perhaps due to solar wind protons binding with oxygen in the rock.

Who would be helped more by a hydrogen depot is ULA, with their plan to use ACES or Centaur V stages as “space trucks”. With this depot, ACESes could go everywhere, and so could similar craft from other makers. NASA has begun initial funding to explore this whole “strategic propellant reserve” concept, and has signed an international agreement to support cooperation in exploiting lunar resources. (Russia and China have not signed — in fact, they now plan to collaborate with each other in a rival partnership. They might put up their own station, but it now sounds as if China plans to skip directly to trying to set up a base on the surface, which would be enormously expensive.) So though commercialization of the LOP-G might be twenty or thirty years away, they definitely see it coming. And if eventually we switch to using other substances as fuel or propellant, such as krypton or uranium or deuterium-tritium, having an established depot out there will still be of great value.