This is the Artemis 1 mission departing from Earth and heading to the Moon. This spacecraft left Earth in the same manner as all other spacecraft, but observe what happens when it approaches the Moon. The Orion capsule is launched on an erratic trajectory, leaping ahead of the Moon, falling behind, and then being hurled back to Earth. Even crazier is the fact that this journey required 26 days to complete. Compare this to the Apollo 8 mission, which lasted only six days and followed a much simpler path. Why then did Artemis not just do this? Getting to the Moon is difficult. To get there requires an enormous amount of energy.
In fact, landing on the Moon requires more energy than landing on Mars, despite the Moon being 500 times closer. The objective of Artemis 1 was to test the Orion capsule’s systems by sending it around the Moon. Apollo missions aimed to land humans on the Moon and return them to Earth. This requires a substantial amount of additional energy, which makes the Saturn 5 rocket even more impressive. However, SLS is significantly more powerful than Saturn 5. Why, then, did Artemis 1 need to take such an unusual route to the Moon? When planning a route to the Moon, everything hinges on delta-v, which represents the amount of velocity change required to complete the journey. Every rocket requires the same delta-v for a given trajectory. The amount of delta-v required by two distinct rockets traveling along the same path is identical. But what really matters is how much energy you have to expend to achieve that delta-v. This demonstrates the required delta-v for each phase of a typical Apollo mission to the Moon. The issue is that any change in velocity necessitates the combustion of fuel. To achieve the same delta-v if your rocket is significantly heavier or its engines are less efficient, you will need to expend more energy.
Although the SLS is more powerful than Saturn 5, the delta-v decreases significantly when comparing the command modules. Although Orion is lighter than Apollo, its delta-v is significantly less. This is because Artemis 1 opted for a command module with significantly more space at the expense of a smaller service module. Apollo utilized a smaller command module and a much more capable service module with increased thrust and propellant capacity. This had a significant impact on delta-v, but because the command module was so small and light, the life support systems could only sustain the crew for a total of 14 days. Apollo missions were therefore compelled to take the quickest route to the Moon. As soon as it arrived at the Moon, a lengthy burn was performed to slow it down and place it into orbit. This required a great deal of energy, but it allowed them to reach Lunar orbit in only 3 days, compared to 10 days for the Artemis 1 mission.
Orion lacked the necessary delta-v, so NASA had to execute a remarkable feat of planning in order to reach the Moon. In order for Orion to reach the Moon and remain there for an extended period of time, NASA aimed for a previously unattempted distant retrograde orbit around the Moon this year. This requires moving away from the Moon and in the opposite direction of the Moon’s orbit around Earth. In this orbit, the spacecraft spends equal time at lagrange points 1 and 2, which helps to maintain the orbit’s equilibrium. This means that the spacecraft can maintain its orbit without wasting fuel. In order to reach this orbit, a spacecraft could aim to encounter the Moon here and then perform a burn to complete the orbit. This is referred to as a direct insertion, and it is essentially what the Apollo missions accomplished. However, in order to conserve as much fuel as possible, Orion took a different route. Instead of targeting a point 70,000 kilometers from the Moon, Orion targeted a point 100 kilometers above the lunar surface.
From this point on, Orion utilized a gravity assist to slow down. When approaching the Moon from a different side, it is possible to use gravity assists to decelerate an object, contrary to what most people believe. Moon orbits Earth in a counterclockwise direction. If Orion approached the Moon from this side, not only would the Moon’s gravity pull it in, but because the Moon is moving away so quickly, it would also pull the spacecraft along and propel it at an even greater speed. If you approach the Moon from the opposite side, you end up traveling in the opposite direction of the Moon; therefore, instead of increasing your speed, it slows you down by pulling against you. Exactly this is what Orion did. However, this is insufficient to propel it into lunar orbit. When Orion reached its closest point of approach, it activated its engine to slow down further. This slowed it down just enough to enter an elliptical orbit with an apoapsis of 70,000 kilometers, the exact altitude it had been aiming for.
Due to the gravity assist performing the majority of the work, Orion was able to save a tremendous amount of fuel. However, Orion had not yet reached its distant retrograde orbit, necessitating an additional burn to circularize its orbit. After reaching its apoapsis, it performed a burn to accelerate and increase its periapsis to 70,000 kilometers. This allowed Orion to enter its distant retrograde orbit, allowing it to travel significantly farther into space than the Apollo missions. At this distance, a single orbit of the Moon required 12 days, and NASA had the option of extending it by another 12 days by simply circling the Moon once more. This orbit was ideal for NASA because it allowed for more time to test Orion’s systems. This path to the Moon may initially appear peculiar. However, when viewed from the Moon’s vantage point, it makes a lot more sense and we can see how the Moon captured Orion.
It becomes even more intriguing if we return to the perspective of Earth and remove the Moon. Technically, Orion was still in an elliptical orbit around the Earth, but because its orbital period was identical to that of the Moon, it appeared to be constantly switching sides. To return to Earth, Orion had to escape the Moon’s gravitational pull. This required performing the gravity assist maneuver in reverse. It slowed down so that it would come close to the lunar surface, and then performed a final burn to return to Earth. Due to the Oberth effect, it waited until its closest approach to perform this final burn. Even with the same amount of fuel onboard, the rocket gains significantly more velocity as it burns closer to the Moon, where it experiences the strongest gravitational pull.
Imagine two moving walkways, one moving at a slower speed than the other. If a person takes the slow path, they will emerge from the end still moving quite slowly. If a person walks on the fast one, they will exit at a significantly higher velocity. In addition, they will achieve a greater delta-v while consuming less energy because they will have spent less time walking. When a spacecraft is at its closest approach to a planet, it experiences the greatest gravitational pull, which causes it to accelerate as if it were on the faster walkway. These maneuvers play a crucial role in space travel. With Orion, they will continue to pave the way for future astronauts, having enabled humans to make the most incredible leap to the Moon.
