With launch behind them—the most dangerous part of the mission—there are now two critical milestones ahead: Trans-Lunar Injection (TLI) and re-entry into Earth’s atmosphere.
TLI is the burn that will take place tomorrow (assuming the crew can fix the toilet 😬), performed by the SLS upper stage (ICPS). This burn will accelerate Orion from roughly 17,500 mph in low Earth orbit to about 24,500 mph, placing it on a translunar trajectory.
If executed correctly, Artemis II will enter a “free-return” trajectory—a flight path that intersects the Moon’s orbit at precisely the right time. The Moon’s gravity will then pull the spacecraft around and send it back toward Earth. Once that burn is complete, Sir Isaac Newton is essentially in the driver’s seat.
But timing is everything. The burn must occur at exactly the right moment to ensure the spacecraft meets the Moon at the correct point in space—and returns to Earth at the right place and time for recovery in the Pacific.
If the burn is too short, the spacecraft won’t have enough velocity to reach the Moon. Too long, and it could miss the Moon entirely or approach it too fast for its gravity to bend the trajectory back toward Earth.
The good news: Orion’s own engine can perform course corrections. Apollo missions routinely used these to fine-tune their trajectories and target landing sites beyond the Moon’s equator.
The final—and most dangerous—phase is re-entry.
From low Earth orbit, spacecraft re-enter at ~17,500 mph and experience 3–5 Gs. From the Moon, that increases to ~25,000 mph and 6–8 Gs, with temperatures approaching 2,500°C.
The margin for error is razor thin.
Too shallow, and the spacecraft “skips” off the atmosphere and is lost to space. Too steep, and the heat and G-forces will destroy it.
To visualize this: imagine Earth as a basketball and the Moon as a baseball, about 8 metres apart. The re-entry corridor would be on the order of the thickness of a single sheet of paper.
NASA accomplished this safely nine times during Apollo—with computing power far less than a modern smartphone.
To me, those missions remain one of the greatest technological achievements in human history.
It’s incredible to see humanity pushing beyond Earth orbit again. The gap since Apollo wasn’t about capability—it was about will.
We were meant to explore.
And with any luck, we’ll see humans walk on Mars within our lifetime.
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