Artemis II used what is called a free return trajectory (FRT) when it flew around the Moon. The graphic shows NASA’s description of the voyage, which is a bit TMI. You can click on it to enlarge the details. What I want to focus on this week is the astrophysics of FRT’s.
An FRT requires no additional engine burns (other than small course corrections) after trans-lunar injection (TLI). So it’s a far less risky mission than a landing, which requires additional engine burns to enter lunar orbit and descend to the surface, re-ascend to lunar orbit, and then do a trans-Earth injection. After TLI, Artemis was in a state of free fall, essentially coasting through the rest of its mission. The only other crewed mission that used an FRT was the ill-fated Apollo 13, who had no choice but to abandon their planned lunar landing given critical equipment failures.
Artemis II was never planned to land on the Moon. It was a demonstration of the technology needed to do so. Artemis II allowed NASA to test several key systems for functionality and reliability, but they also did some real science, reporting visual observations and taking photos of lunar features never before seen.
The official NASA graphic at the top of this post contains a lot of information that isn’t essential for understanding the physics of an FRT. Here’s a simplified version with only essential features:
The main reason for an FRT is the simplicity of less engine burns, and the astrophysical fact that when launched on an FRT, you know you’ll get back to Earth even with a total engine failure. One prominent astrophysicist compared it to “surfing the gravity of curved spacetime with little energy input of your own.”
The Orion spacecraft was sent into a high Earth orbit with a period of roughly 24 hours. The core stage burned for about eight minutes before main engine cutoff, leaving Orion in a highly elliptical orbit with an apogee of roughly 1,400 mi, nearly five times higher than the International Space Station. During this time the crew performed various checkouts of the spacecraft’s life support systems, as well as an in-space rendezvous and proximity operations demonstration using the spent Interim Cryogenic Propulsion Stage (ICPS) as a target. When Orion returned to perigee, it fired its main engine to execute the TLI maneuver.
The graphic above simplifies, but greatly distorts, the scale of the mission. Here’s a properly-scaled diagram of the Earth and Moon. You can visualize what an FRT would look like at this scale:
The upcoming Artemis III mission will fly in low Earth orbit (LEO) practicing docking maneuvers with a lunar lander (from either SpaceX or Blue Origin). Artemis IV will be the ultimate Moon landing near the Moon’s South Pole. This is where NASA is planning to build a base and establish a permanent presence. Artemis IV is currently scheduled for 2028, but the exact date is still highly uncertain.
Next Week in Sky Lights ⇒ What the Chicxulub Impact Really Looked Like