On 1 April 2026, four astronauts strapped themselves into a capsule called Integrity and did something no human had done since 1972: they left Earth's neighbourhood entirely.
NASA's Artemis II mission carried Commander Reid Wiseman, Pilot Victor Glover, Mission Specialist Christina Koch, and Canadian Space Agency astronaut Jeremy Hansen on a roughly 10-day journey around the Moon and back.
No one drove. No one steered. And yet the spacecraft ended up exactly where it needed to be, at exactly the right time, travelling at exactly the right speed.
How on earth - or rather, how off earth - does that work?
First, the Numbers That Will Break Your Brain
Before we get into the how, let's appreciate the what.
The mission lasted 9 days, 1 hour, 32 minutes, and 15 seconds. The crew travelled a total of 1,126,922 kilometres.
On April 6, the crew broke the record for the farthest any human has ever travelled from Earth - reaching a maximum distance of 406,773 km, some 6,600 km further into space than any human in history.
At reentry, the spacecraft hit a maximum speed of around 25,000 mph.
And they landed within a few kilometres of the target splashdown zone in the Pacific Ocean.
For a trip that covered over a million kilometres, that's the equivalent of throwing a dart from Sydney and hitting a bullseye in Perth. Blindfolded.
So how do you plan something like that?
Step 1: Figure Out Where the Moon Will Be
Here's the thing about the Moon: it doesn't wait for you.
By the time a spacecraft travels the roughly 385,000 km to get there, the Moon has moved. So scientists don't aim at where the Moon is - they aim at where it will be.
This is called trajectory planning, and it requires precise knowledge of the Moon's orbital speed (roughly 3,700 km/h), Earth's rotation, and the exact moment of launch. Miss the window and you miss the Moon entirely. Which is why Artemis II could only launch when the Moon, orbital paths, weather, and Earth's rotation all lined up safely - with a two-hour window each day.
Timing, in other words, is everything.
Step 2: Get Off the Ground
The Space Launch System's four RS-25 engines burn liquid hydrogen and liquid oxygen - kept at −253°C and −183°C respectively. The core stage carries approximately 730,000 kg of propellant and produces 2 million pounds of thrust at sea level. Two five-segment solid rocket boosters provide an additional 6.6 million pounds of thrust, burning out and separating after just two minutes.
The whole rocket stood 322 feet tall - roughly the height of a 30-storey building - and most of it was fuel.
This points to one of the most counterintuitive things about space travel: the hardest part isn't getting to the Moon. It's getting off the ground. Escaping Earth's gravity requires an almost absurd amount of energy. Once you're out of the atmosphere, things get much easier.
Step 3: Let Gravity Do the Driving
Once Orion reached orbit, the clever part began.
The rocket places Orion into a parking orbit, then a second burn sends the spacecraft on a translunar injection trajectory - a carefully calculated path that intersects with the Moon's position days later. This is orbital mechanics in action: using the precise geometry of gravitational fields, velocity vectors, and timing to navigate between worlds without burning fuel continuously.
For Artemis II, this translunar injection burn fired Orion's rockets for nearly six minutes and consumed roughly 1,000 pounds of fuel - just enough to loosen Earth's gravitational grip and set a course for looping around the lunar far side.
One burn. Six minutes. Then coast for days.
Step 4: Use the Moon as a Slingshot
Artemis II didn't enter lunar orbit. It did something more elegant - a free-return trajectory.
By approaching the Moon at the correct angle and speed, the spacecraft is captured by lunar gravity, swings around the far side, and is flung back toward Earth without requiring propulsion.
Think of it like a marble rolling around the inside of a bowl - gravity curves the path without any engine firing at all. The maneuver went so well that NASA skipped two out of three smaller corrective burns that had been built into the mission schedule.
There's also a safety reason for this approach. If something were wrong with the rockets, choosing a free-return trajectory means less risk for the astronauts - the Moon's gravitational pull acts as a natural slingshot back to Earth. It's the same type of path that saved the Apollo 13 crew in 1970.
Step 5: Come Home Without Burning Up
Returning to Earth brings its own challenge: speed.
After days of coasting through space, the spacecraft is falling back toward Earth fast. Very fast. At 25,000 mph, hitting the atmosphere at the wrong angle is catastrophic - too steep and the spacecraft burns up; too shallow and it skips off the atmosphere like a stone off water and drifts into space.
The target entry corridor is just a few degrees wide. Getting it right requires calculations that account for the spacecraft's exact speed, weight, angle, and atmospheric conditions on the day.
Orion splashed down in the Pacific Ocean southwest of San Diego on April 10, 2026. Right on schedule, right on target.
The Real Answer: It's a Lot of Maths, Done Very Early
None of this is improvised. The trajectory for Artemis II was calculated months - in some cases years - before launch. Teams of engineers ran thousands of simulations, modelling every burn, every gravitational nudge, every possible failure.
The actual mission is almost the easy part. The work happens long before anyone climbs into the capsule.
Which is, in a way, the same principle behind a good Fermi Problem: break the question down into pieces you can actually calculate, chain the answers together, and arrive somewhere surprisingly close to the truth.
The Moon is 385,000 km away. But with the right maths, four people can leave on a Tuesday and be home by the following Friday.
Think You Can Estimate Like a Rocket Scientist?
Artemis II required some of the most precise calculations in human history. But the underlying skill - reasoning carefully from what you know toward what you don't - is something anyone can practise.