It’s International Moon Day! Let’s talk about Cislunar Space.
Cislunar space includes the region between the Earth and Moon, and is the next frontier in space exploration. Dr. George Pollock, Aerospace cislunar astrodynamics lead, has provided background on the physics of how we will get there.
Cislunar space, the region within the combined gravitational influence of the Earth and Moon, offers numerous opportunities for scientific discovery, technological innovation, resource development, and economic growth. Its unique physics allows for new missions and efficient spacecraft operations. As a result, many countries and companies are actively pursuing lunar and cislunar missions to tap into the scientific and economic potential it offers. Many nations see these achievements as a method to enhance their geostrategic position, boost domestic space industries, and extend their presence beyond near-Earth space.
But why is cislunar space so crucial for these goals? This region, encompassing Earth, the Moon, and several nearby orbits, serves as a critical pathway for future interplanetary and deep-space exploration, acting as a testing ground for missions to Mars and beyond.
Technological breakthroughs are being developed to support a diverse cislunar economy and a permanent human presence. The Artemis program, for instance, focuses on advancing navigation, control systems, propulsion, and autonomous technologies. Future missions will feature astronauts returning to cislunar space and will demonstrate commercial capabilities for launches, human spaceflight, and lunar landings.
We’re going back to the Moon, but unlike Apollo, we’re going back to stay. This will present new challenges and need new solutions. We will also need a workforce trained to understand how to live and work in the lunar environment — possessing a mastery of cislunar orbital mechanics and the ability to thrive on the Moon for extended periods, navigate the Moon’s terrain, and operate amid surface regolith and other features.
As explorers, we’ll need to understand where we’re going, whom we’re sharing the space with, and have a plan for a shared, sustainable future. Some of the considerations for establishing a presence on the moon include managing the massive size of the region, modes of transport, orbital dynamics between the Earth and the Moon, and navigating to our destination, so let’s get started.
Defining Cislunar Space
The sheer volume of the region is massive — cislunar space is a 3-dimensional region, not just an A-to-B line from the Earth to the Moon. The Moon orbits at an altitude of roughly 380,000 km — just under 9.5 times the circumference of the Earth. To include most orbits of interest, for this discussion we’ll use a definition of cislunar space that extends to an altitude of 550,000 km, or just under 14 times the circumference of the Earth.
Getting to the Moon
Travel to cislunar space is not much more costly than reaching geosynchronous orbit (GEO), where we traditionally operate satellites for communications and weather missions. Currently, there are a half-dozen or so spacefaring stakeholders at the nation-state level that can launch to GEO, and they all have the capability to launch spacecraft to cislunar.
When traveling to cislunar space, there are three approaches to reach a destination.
First is the Direct Transfer method, which is roughly a three-day trip each way, and is characterized by large rocket engine burns. The Apollo missions used direct transfers to get the first people up to the Moon and return them safely to Earth.
With propulsion technologies available now and in the foreseeable future, it takes a minimum of three days to travel between the Earth and the Moon. If you have a human crew or perishable supplies on board, it becomes crucial to reduce their exposure to radiation and other hazards while efficiently managing the necessary resources for the mission’s duration. Opting for the Direct Transfer method allows you to reach your destination in about three days.
There are other ways to get to the Moon or cislunar orbits that take longer but offer greater efficiency. The second approach is Low-Thrust Transfers, which use a steady thrust to spiral out slowly from Earth orbit to the Moon over several months.
Or we can take the scenic route via a Low-Energy Transfer and loft the spacecraft on a trajectory that takes it far past the Moon’s orbit, with a maximum distance from the Earth about four times the lunar altitude. Way out there, we can use the Sun’s gravity to alter the trajectory so that the spacecraft falls back toward the Moon on an approach that costs little propellant to enter the desired lunar orbit. The flight time for low-energy transfers is typically between 2.5 and 4 months.
If the mission is not time-critical, you’re likely to take the most economical means of transportation and fuel consumption on a per unit mass basis. You can save on fuel costs by taking far longer using a Low-Thrust or Low-Energy Transfer to reach the destination.
Interested in all things Moon? Check out our Moon 101 series on YouTube.
We are beholden to the current three-day minimum to get to or return from the Moon. As we look ahead to a more permanent human presence in cislunar space, we will have the practical need to quicken that pace for emergencies. Readily available on-orbit refueling in cislunar space would enhance the ability to respond to such contingencies, so we have some work to do to get fuel depots in place.
Gravity and Orbital Dynamics
Making these transits is more complex than an orbital ascent to the International Space Station and back. With Earth orbits, we’re accustomed to shapes that are generally near circular or ellipses. As we add in the gravity of the Moon, the orbits are no longer so simple and are not confined to a plane.
You may have heard this described as a “three-body problem”. In cislunar space, the three bodies are the Earth, the Moon, and the satellite. Imagine the Earth is the only massive body in the universe, and a satellite orbits it in a nice ellipse. Add in the Moon, and as you raise the altitude of that satellite, it’s no longer influenced by Earth’s gravity alone. This scenario becomes a tug-of-war over which body — the Earth or the Moon — will exert primary influence over the satellite.
When you stand on the beach and observe the tides, you are seeing the Moon’s gravitational influence, but it’s no contest with the pull of the Earth. At higher altitudes there is increasing influence of the Moon’s gravity. Eventually, there’s a point relatively close to the Moon where lunar gravity exerts primary influence.
In those areas between, neither the Earth nor the Moon is dominant. Both play a significant role, but neither have total control in the tug-of-war. We end up with a wide variety of ways satellites in this space can behave.
Designing and maintaining these “three-body” orbits requires high technical proficiencies, as many of these orbits are not stable and must be actively maintained with periodic maneuvers of the spacecraft. Though complicated, there is a lot of utility in these orbits. Mastering this dynamic environment unlocks potential to conduct new missions and pursue lunar surface resources.
Points L1–3, situated along the line connecting the Earth and the Moon are unstable. If you put an object in orbit about these points and it will wander away… slowly. To stay there will generate a recurring cost of maintaining the orbit with propellant.
Camping out in an L1 orbit between the Earth and the Moon, provides continuous access to the near side of the Moon. Loitering in an L2 orbit on the far side of the Moon, enables mission applications like communications relays to connect a lander or rover on the far side with communication dishes back on Earth.
Points L4 and L5 lead and trail the Moon in its orbit, respectively. These points are stable. Objects parked there can stay for quite a while, providing several opportunities. L4 and L5 are good potential locations for a cislunar space station or a spot to dispose of satellites at end of life. They are also possible placements for space situational awareness sensors, a critical capability for ensuring safe operations in the developing cislunar ecosystem.
Navigation and Landing on the Moon
You hear “orbit” and likely envision a circle or an ellipse. A halo orbit is an elongated orbit associated with either the L1 or L2 Lagrange points and facilitates our access to certain locations or approaches to the Moon. These halo orbits have what are called northern and southern families — essentially, this just describes whether your orbit is loitering over the Moon’s northern or southern hemisphere.
Recall that the Moon always points the same side toward the Earth, if you want to land and operate on the far side, you’ll need a comms relay satellite. The Chinese have done this using a halo orbit about the Earth-Moon L2 Lagrange point.
NASA’s Gateway space station will use what’s called a near-rectilinear halo orbit that is elongated and has a long dwell time high above the lunar south polar regions. The halo orbit comes near the Moon in the northern hemisphere and flies high above the southern hemisphere. This configuration is advantageous for getting crew to and from the orbit with launch and landing locations in Earth’s northern hemisphere. It also offers a great vantage point to many of the south polar region candidate landing sites — places where scientists have detected frozen water that could be useful for future space missions.
With all these orbital mechanics, landing on the Moon might seem the simplest part of the process. Just detach, drop down and buckle up. This is a very complicated dance, however, and the series of steps to land can take more time than traveling to the Moon from Earth.
As an example, to land on the Moon from the Gateway station, astronauts will need to board a transfer vehicle or a lander. The vehicle will separate from Gateway and conduct a burn, probably at the closest approach to the lunar surface to enter low lunar orbit. Then it will wait until the timing is just right to initiate a powered descent to the desired landing location. Low lunar orbits aren’t stable for long, so you only use them briefly before or after a visit to the surface. The vehicle will then need to fire its engines and thrusters and approach the lunar surface for a soft landing.
During this process, Gateway is orbiting the Moon on that elongated near rectilinear halo orbit, not in a fixed position relative to the landing site The return window is on Gateway’s timeframe, roughly 6.5 days per lap, and contingent on it being in the correct place.
On the surface of the Moon, you enter the ascent vehicle and wait for the return window to open. You then return to low lunar orbit at the proper time and do another series of thruster firings to get back into an orbit with the Gateway, then rendezvous and dock.
Low lunar orbits are not particularly stable in general, because the Moon’s composition is “lumpy” — featuring mass concentrations that can cause orbits to crash into the lunar surface. We observed this behavior on the Apollo 16 mission when a small satellite had to be released in an undesired low lunar orbit and crashed after a month in orbit.
Frozen lunar orbits are the mission designer’s response to this challenge, with tailored designs that mitigate the undesirable effects of the lunar mass concentrations. NASA’s Lunar Reconnaissance Orbiter is using a special type of this orbit currently. At higher altitudes about the Moon, Distant Retrograde Orbits are a family of stable orbits that have been envisioned for applications where long-term stability is paramount.
International Moon Day celebrates the significance of the first Moon landing on July 20, 1969. Our return to the moon holds immense potential for scientific discoveries, technological advancements, resource utilization, and economic growth. Cislunar space offers the promise of unlocking new frontiers and opening doors to unexplored territories, making it indispensable for humanity’s exploration and advancement. This orbital domain is complex, but there is sufficient volume for all to ensure a shared, sustainable future in space.
