Pre-Launch Preparations
The Artemis II mission, scheduled for April, will mark the first crewed voyage to the vicinity of the Moon since the Apollo program concluded more than five decades ago. Over the course of ten days, four astronauts will travel farther into deep space than any human has previously ventured. For those following the mission, understanding the terminology used by launch controllers is essential to grasping the significance of each milestone.
In the hours leading up to liftoff, the crew will proceed to the White Room—an environmentally controlled staging area adjacent to the spacecraft. There, they will don their helmets and gloves before ingress, the formal term for boarding the Orion crew module, which will serve as their home for the duration of the mission.
The spacecraft stack—comprising the Orion capsule and the Space Launch System rocket—is assembled atop the mobile launcher, a ground platform responsible for transporting the vehicle to the launch pad and facilitating final testing and servicing.
The Countdown Sequence
As the countdown progresses, the term nominal indicates that all systems are functioning as expected. The fueling process involves loading the rocket with cryogenic propellants, referred to as LOX and LH2—liquid oxygen and liquid hydrogen, respectively. This procedure proceeds through several phases: slow fill, fast fill, topping, and replenishing.
Two distinct timing references are used during the countdown. L Minus denotes the actual time remaining before liftoff in hours and minutes, while T Minus corresponds to the sequence of pre-launch events, such as the retraction of the crew access arm or the ignition of the engines in the final seconds. If the launch team announces a hold, the countdown pauses to allow for additional tasks or to adjust the timing for an optimal launch window. During a hold, the T Minus clock stops, whereas the L Minus clock continues to run.
The final phase of the countdown, known as terminal count, is initiated by the ground launch sequencer—a computer system that governs the last ten minutes before liftoff. Following engine start and booster ignition, umbilical separation occurs, disconnecting the fuel lines and power cables that connect the rocket to ground support. This marks the final step before the vehicle lifts off.
Ascent and Initial Orbit
Shortly after launch, two solid rocket boosters provide the majority of thrust during the initial ascent. The launch abort system, mounted atop the crew module, serves as a safety mechanism. In the event of a malfunction during ascent, two of its three engines can propel Orion to safety; the third engine is used to jettison the system once the vehicle has cleared the hazardous phase of flight.
Approximately eight minutes after liftoff, the core stage of the Space Launch System reaches main engine cut-off. At this point, the core stage separates from the upper portion of the vehicle—the interim cryogenic propulsion stage—and the Orion capsule. Shortly thereafter, the zero gravity indicator, a plush toy selected by the crew, will float into view, signaling that the spacecraft has entered the microgravity environment.
Orbital Maneuvers and Lunar Injection
Following separation, the interim cryogenic propulsion stage assumes control. About 49 minutes into flight, it executes the perigee raise maneuver, a burn that lifts Orion’s altitude and places it into a stable low-Earth orbit. Approximately one hour later, a second burn—the apogee raise maneuver—propels the capsule into a higher orbit. Following this burn, the stage separates from Orion.
Before the interim cryogenic propulsion stage is intentionally deorbited and burns up over the Pacific Ocean, the Artemis II crew will conduct a Proximity Operations Demonstration. This exercise involves piloting Orion toward and around the stage, allowing NASA to evaluate the maneuvering capabilities required for future missions that will involve docking with other spacecraft.
On the first day of flight, an additional perigee raise burn positions Orion optimally for the translunar injection burn, which occurs on flight day two. This critical maneuver, executed by Orion’s service module—which provides propulsion, power, and thermal control—accelerates the capsule sufficiently to escape Earth’s circular orbit and transition onto an elliptical trajectory toward the Moon.
Lunar Flyby and Return Trajectory
Following translunar injection, a series of smaller orbital trajectory correction burns ensure the spacecraft remains precisely on course for its lunar flyby. On flight day five, Orion crosses into the lunar sphere of influence—the region where the Moon’s gravitational pull becomes dominant over Earth’s.
After passing behind the far side of the Moon, the spacecraft exits the lunar sphere of influence. Three additional small burns refine the trajectory for the return journey, with the final burn occurring on flight day ten. The mission profile follows a figure-eight pattern, looping around the Moon before heading back to Earth.
Reentry and Splashdown
The service module, which has housed the engines responsible for all trajectory adjustments, separates from the crew capsule prior to reentry, exposing the heat shield. As Orion reenters Earth’s atmosphere, the heat shield endures extreme temperatures generated by atmospheric friction.
Once the capsule has slowed sufficiently following the reentry phase, a sequence of parachute deployments begins. Drogue parachutes deploy first, initiating deceleration, followed by pilot parachutes that unfurl the three main parachutes. This system reduces the capsule’s descent speed from approximately 200 kilometers per hour to about 30 kilometers per hour before splashdown in the Pacific Ocean off the coast of California.
Continuing the Artemis Program
Artemis II serves as a precursor to subsequent missions planned throughout the remainder of the decade, including a crewed lunar landing. For those following these historic flights, familiarity with the terminology provides a framework for understanding the technical precision required to extend human presence beyond low-Earth orbit.