The European Space Agency just signed a contract with Airbus to build the third European Service Module for NASA’s Orion spacecraft that will ferry the next astronauts to land on the Moon by 2024. This mission is referred to as Artemis 3. NASA’s Artemis programme is returning humans to the Moon with ESA’s European Service Module supplying everything needed to keep the astronauts alive on their trip in the crew module – water, air, propulsion, electricity, a comfortable temperature as well as acting as the chassis of the spacecraft.
The combination of using a Service Module (SM) and Crew Module (CM) isn’t new: the Apollo missions used a similar configuration with also the inclusion of a Lunar Module (LM) used for the landing. Upon arrival at the Moon, the SM fired the thrusters at the far side of the Moon in order to enter Lunar orbit. This orbit was almost a circle, with a minimum altitude of 69 miles and maximum altitude of 190 miles.
Circular orbits like most Low Earth Orbits and Geostationary Orbits, as well as elliptic orbits like Russia’s Moniya orbits, are common in space. But the Lunar orbits that the Artemis missions will use, are far from normal. You see, there is another type of orbit called Halo orbit. Halo orbits are orbits with an undefined shape (though may be similar to elliptic) around Lagrange points that are located at several places close to Earth and Moon.
Lagrange points are the points near two large bodies in orbit where a smaller object will maintain its position relative to the large orbiting bodies. This is due to the attraction of two celestial bodies onto a satellite, such as both Sun (yellow sphere above) and Earth (blue sphere above), or both Earth and Moon. At Lagrange points, the sum of forces such as gravitational, Coriolis, and centripetal forces, match up.
Much like mountains, the points L1, L2 and L3 in the figure above, represent saddles in the fields of forces acting on to the spacecraft. You can see that L1 is on a saddle in-between the two celestial bodies (for example in-between the Sun and the Earth). Orbiting this L1 Lagrage point allows a spacecraft to continuously be in-between the Sun and the Earth, and therefore act as a fantastic Sun observatory, such as ESA/NASA’s SOHO satellite.
The same type of Halo orbit can be applied to the Earth-Moon system. The figure below shows many different Halo orbits that are possible around the L2 Lagrange system of the Earth-Moon system.
Rotating the view a bit, we can see one particular Halo orbit, shown in white below, resembles a highly elliptic orbit with its closest point very close to the Moon and its furthest point far away. It is referred to as a ‘Near Rectilinear Halo Orbit’, as it looks like it’s orbiting the Moon instead of L2, and is the chosen orbit for the future Lunar outpost: the Lunar Orbital Platform Gateway known as LOP-G. This gateway is a possible stop-over before landing humans on the Moon.
It is remarkable that while the orbit seems far away from the L2 point, it is in fact considered an orbit around L2. There are several advantages to this orbit over normal circular or elliptic orbits: unlike normal orbits around the Moon which are often perturbed by the gravity fluctuations of the Moon, this orbit requires little maintenance, meaning that a small amount of propellant is required to stay in this orbit. Also, as seen from Earth, the orbit plane remains perpendicular to the Earth, as shown below). This means that the satellite does not go behind the Moon as seen from Earth, and communication is therefore always possible.
Another type of possible Halo orbit around L2 is called Distant Retrograde Orbit (DRO). The figure below shows, again, many types of possible DRO orbits around the Earth-Moon L2 point. The orbits are in the same plane as the plane in which the Moon rotates around the Sun, and as such the obstruction by the Moon is possible. However these orbits require both low propellant to maintain, but also low propellant to enter when arriving from the Earth.
In the figure above, one white orbit is highlighted which is a 14-day DRO orbit. The NASA/ESA Artemis 1 mission, a demonstration mission like Apollo 8 to test the European Service Module and Crew Module ORION, will use this orbit. It will stay in this orbit for 7 days, so half an orbit as shown below, before returning to Earth.
The image below shows the intended Artemis 1 flight profile. The mission is due for launch within a year from now. When it arrives at the Moon, it will perform a swing-by around the Moon at point 11 (igniting the thrusters at the closest point to the Moon) and arrive at the start of the DRO orbit and do a small insertion orbit (12), orbit half a DRO (13), inject away from the DRO (14) in order to perform another thrust-powered swing-by at the Moon (15) to return to Earth.
Exciting times are ahead… not only will we see the return of humans to the Moon, to be launched by the biggest rocket in the world, but also we will very groovy types of orbits being flown. These new missions are are actually orbiting saddles in the gravity well of the Earth-Moon system.
Robin Biesbroek is author of the book “Lunar and Interplanetary Trajectories“.
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