An illustration of NASA’s proposed Lunar Orbital Platform Gateway, formerly known as the Deep Space Gateway. Does such a facility in cislunar space effectively support human missions to the Moon and, eventually, Mars? (credit: NASA)
by Eric R. Hedman
The Lunar Orbital Platform-Gateway (LOP-G), despite its inelegant name, has been described by its proponents as the next reasonable step in human exploration of the Moon and Mars. We have been to the Moon before without a gateway station, so it isn’t an absolute must for returning to the Moon. Several practicable Mars mission architectures have been proposed leaving from low Earth orbit, so it is not an indispensable option to put humans on Mars.
The technical justification—not to be confused with the political justification—for LOP-G comes down to whether or not there is a greater value in having a lunar gateway than just going back to the lunar surface in a manner similar to what was proposed in the Constellation program. It is not a simple question to answer.
LOP-G, if it goes forward, will live in the political reality that SLS, Orion, and the International Space Station do. All three have become large jobs program in addition to any other benefits that they provide. It is unlikely that any of these three programs are going away soon. The Senate is balking at ending or commercializing the ISS in 2025. The programs provide a lot of very good jobs in the states and districts of very powerful members of Congress. It would be political suicide for members from these places to allow these programs to end. Their existence leaves limited money for developing new systems.
When making any tough decision it is useful to make a list of pros and cons and then weight them to come up with a good opinion. I am going to go through what I consider to be valid factors in such a decision.
LOP-G is starting in a legacy environment that creates less-than-ideal limitations on where it can be used. The Orion capsule and service module were sized when it was intended that they would be part of the now-canceled Constellation project. In Constellation, Orion and its service module were to be launched on the Ares I rocket. That limited the mass budget for the service module and Orion to what the Ares I could carry to low Earth orbit. From LEO, the Orion capsule and service module were to dock with the Altair lunar lander on the upper stage of the separately launched Ares V. The upper stage was responsible for the translunar insertion burn. The Altair lander would handle the insertion into lunar orbit. The service module only needed enough propellant for the return to Earth. Now the same size service module must handle lunar orbit insertion and return to Earth. It is undersized for its mission. As a result, some lunar orbits are infeasible to reach with Orion, especially if it is carrying a Gateway module with it. As long as Congress requires the use of SLS and Orion, these limitations are in place and have to be considered.
The proposed orbit for LOP-G is a Near Rectilinear Halo Orbit (NRHO). A NASA paper on the rationale for selecting the orbit includes a table on page 8 that spells out the reasoning behind the choice. The ideal choice, if the only factors for landing and returning to lunar orbit were delta-V requirements for the lander and access to any landing site, is a polar low lunar orbit. This is ruled out because the Orion spacecraft and service module can’t reach it due to the limited delta-V capability of the service module. Given the constraints of requiring the use of SLS/Orion the NRHO is the only feasible choice for now.
If constraints on spacecraft delta-V change either through design changes or access to a fuel depot, the orbit of LOP-G can change. The Power and Propulsion Element (PPE) would give it the capability to move to whatever orbit is deemed ideal at the time.
Many people question the need for Orion and SLS given other options being developed that will probably be much cheaper and may end up being far more capable. Until these alternatives are in operation, though, Orion and SLS are continuing forward; end of story. Orion and SLS have strong political support for many reasons, including the large number of high-paying jobs supported by the in states with powerful members of Congress. There are also those who don’t believe that SpaceX and Blue Origin will be able to deliver on what they are promising, despite what they have done to date.
Congress has been increasing its support for SLS, most significantly by adding funding for a second mobile launcher to eliminate any delays when SLS switches from the Block 1 version to the Block 1B with the more powerful and larger Exploration Upper Stage (EUS). After the first eight flights, SLS will require the Advanced SRBs, which will bring the design to Block 2, because there will not be enough legacy parts to make more of the SRBs to be used on the first eight flights.
By the time the advanced SRBs are needed, SpaceX’s BFR should be flying. Blue Origin’s New Glenn should be flying. ULA should also have Vulcan flying, possibly with its ACES refuellable upper stage. This could make for an interesting scenario where equal or greater launch capacity at a much lower price than SLS is available. This might be the point where SLS is canceled.
Landers are going to be required to return to the lunar surface regardless whether LOP-G is built. The existence of LOP-G may change the specifications of the landers significantly. The specifications of landers used on the Moon are going to be driven by many requirements, including the mass and content of the cargo or crew being carried, what surface locations and orbits it has to serve, and if it is partially or fully reusable.
Unless a crewed lander is preceded by robotic landers carrying and deploying machinery to make fuel from lunar resources, landers are not going to be initially refuellable on the lunar surface. That probably means it will have an expendable descent stage. If there is no station to return to in lunar orbit, the ascent stage and crew cabin will be expendable. With a LOP-G station in place, it may be possible to have a reusable ascent stage and crew cabin. This would require a tanker to bring propellant, and a new descent stage launched from Earth, each time a landing is required. In the long run, a fully reusable lander refueled in orbit and on the surface will probably be the only affordable way to support a growing presence on the lunar surface. For a static small base with infrequent visits by a crew, disposable landers might be the way to go.
With the ACES upper stage being planned by ULA to be refuellable in space, and SpaceX planning on refueling BFS with tankers, fuel depots along the way can aid and support reusable lunar landers and spaceships that meet them in orbit. LOP-G doesn’t have to have a fuel depot as part of it. A fuel depot could be following LOP-G at a safe distance, or at another convenient location, allowing landers and other spacecraft to be used multiple times with greater capabilities. LOP-G doesn’t preclude fuel depots because they don’t have to be part of the station. Fuel depots could also support architectures that don’t include LOP-G.
It is probably unlikely, but not impossible, for a crew returning from Moon on a lander to discover that the Orion spacecraft has problem and can’t take them back to Earth. In such a situation, the crew is in serious potentially fatal trouble unless another SLS/Orion vehicle is ready to launch. With LOP-G, they would have a safe haven in lunar orbit until a rescue mission could be mounted.
There are almost always alternative choices to any plan. Instead of building LOP-G, NASA could support using SpaceX’s BFR to fly directly to the lunar surface without stopping in lunar orbit at all. NASA could develop a lander that is launched to lunar orbit and waits for Orion to bring the crew for landing. They could develop a lander like Altair that will handle inserting a stack with Orion into lunar orbit. NASA could pay to put Orion on Vulcan with an ACES upper stage to bring it to lunar orbit. However, I don’t believe any of these alternatives will be considered because of the support for SLS in Congress. NASA can’t bite the hand that feeds it.
Planning for Mars
Developing LOP-G and a surface base will include components that could be used to go to Mars. The PPE could be used to send a ship to Mars orbit and back. Rovers and habitats developed for the Moon, possibly with some modifications, should work on Mars. Nuclear power sources such as NASA’s new Kilopower reactor should be adaptable to both places. LOP-G will have a new airlock and habitation module design, possibly with better radiation shielding than the ISS. All of these developments will make going to Mars easier.
The role of international partners
There has been considerable interest by international partners in taking part in the return to the Moon. ESA, through its director general, Jan Wörner, is promoting the idea of a Moon village. The big question yet to be answered is what are they willing to do and when? International partners, to follow and commit to investing in a return to the Moon, will require a firm commitment from the US through NASA to getting started. It may take a few years of funding to convince Europe that we will follow through. Even though ESA is developing the service module for Orion, they have not yet made a large commitment in a return to the Moon, nor have the other partners in the ISS.
NASA is looking at using a approach for lunar exploration modeled on the Commercial Orbital Transportation Services (COTS) cargo program, starting with development of robotic missions to explore the surface in preparation for crewed missions. NASA believes this approach is more affordable. COTS sped up development of capabilities and reduced costs for the US. NASA administrator Jim Bridenstine has spoken about how it is a necessary approach to make the plan affordable. It would be interesting to see if our partners are able to embrace this approach and mindset, including getting involved with it across borders.
Europe, Japan, and Russia also have budgets they have to live within. The big unknown in the budget for all is the ISS. The Trump administration wants to privatize the station by 2025. Sen. Ted Cruz (R-TX) and others in the Senate do not. The ISS consumes more than $3 billion per year in NASA funding. The biggest part of that is transportation costs to orbit and back for crew and cargo. Russia spends a significant portion of their budget on ISS operations. Europe spends close to a billion dollars a year on ISS. A decision on the future of the ISS needs to be made so the partners will know how much they will be able to free up for new ventures. The problem is that there is no consensus on the future of the ISS. In addition, a recent NASA report on commercialization of the ISS or on new commercial stations doubts that they can become profitable by the middle of the next decade.
Until a decision can be made on the future of the ISS, I fear that LOP-G and lunar landers will be a mostly American venture. For the near future, I wouldn’t count on much of an international contribution. Hopefully this will help push for a credible plan for the future of the ISS.
The choices for NASA going forward with human spaceflight are constrained by legacy choices, budget realities, and political realities. If given a clean slate, a steady stream of money, and no political pressure, NASA may come up with a completely different approach to a lunar return. Given the political and legacy constraints, this plan may be the best we can do.
If BFR is developed within a reasonable timeframe and delivers on what Elon Musk says it can do, then the political pressure could reverse and could ultimately force the cancelation of SLS/Orion and LOP-G. Until SpaceX delivers, I don’t believe NASA, Congress, or the President will consider it in their decisions. NASA is considering using new capabilities that are now in place ,such as commercial rockets, to deliver station elements and logistics to lunar orbit, and it doesn’t seem like too much of a stretch to develop small to medium-sized lunar landers to deliver scientific instruments to a number of sites on the lunar surface.
Because of all the constraints in place on NASA by the various factions, I do believe the actions NASA is moving forward on is the right path to take. It beats what we have been doing prior to this plan. NASA’s Journey to Mars was vaporware. This plan may make it more likely that we will move on to Mars. There can always be a better plan if we continue looking for it. But at some point, you must make a decision. The better is the enemy of the good if it keeps you from never moving forward while you are looking for it. Therefore, I support what NASA is doing.
The stakeholders need to be willing to make changes if circumstances change such as significant new capabilities from commercial providers do come on line. International partners need to step forward and commit to capabilities they can bring to the table to make returning to the Moon a more robust adventure.
The purpose of the LOP-G is to move us forward and support returning to the Moon and eventually going on to Mars. It is designed to be a gateway to the rest of the solar system. When the American West was settled, St. Louis was a key gateway for all the people looking for a place of their own. St. Louis, on the Mississippi River, was an ideal transit point because of the supplies that could be brought to stock up the Conestoga wagons. One of my great great grandfathers arrived in St. Louis for this purpose around 1850. That role is commemorated today with the Gateway Arch there.
LOP-G is to be a gateway space station in lunar orbit. Perhaps LOP-G should be called the Lunar Orbit Gateway Space Station or LOGSS. If international partners commit to taking part early, maybe we can add International to the name and call it the International Lunar Orbit Gateway Space Station or ILOGSS. I also wouldn’t mind naming it Jules Verne Station if we wanted to honor someone who popularized the idea of going to the Moon. It’s a far better option than LOP-G.