How does a rocket fly to the International Space Station?
Getting to orbit is not the hard part. The hard part is arriving where the station will be, at the moment it gets there, moving at the same speed.
The International Space Station travels around Earth about once every 90 minutes. By the time a rocket has lifted off and reached orbit, the station has already moved a long way along its path.
So a journey to the station begins with a prediction: where will it be when the spacecraft arrives? The flight has to bring them together at that future place, moving in the same direction at almost exactly the same speed.
First, get into the right orbit
As we saw in How a rocket gets into orbit, a rocket climbs and gradually turns, building the sideways speed needed to fall around Earth. For a trip to the station, that launch also has to line up with the station’s route.
Imagine the station’s orbit as a hoop around Earth. The spacecraft needs to travel in the same flat plane as that hoop. Earth rotates beneath it, carrying the launch site into a suitable position. This is one reason launches happen at carefully chosen times: changing the tilt of an orbit later takes a lot of fuel.
The rocket places the spacecraft into orbit, and the spacecraft separates to continue the journey using its own thrusters. It will often begin in a lower orbit than the station, with some distance still to close.
Catching up from below
A spacecraft in a lower circular orbit travels faster and has a shorter distance to cover on each lap. It therefore goes around Earth more quickly than the station. If the station is ahead, the spacecraft can gradually gain on it.
That lower orbit gives the mission a way to adjust the timing. The spacecraft can spend time catching up before making the climb to the station’s height.
The climb itself takes time, too. A short forward push from the thrusters changes the spacecraft’s orbit into an oval that reaches farther from Earth. It then coasts upward along that curve while the station continues around its own orbit. The manoeuvre begins while the station is still ahead, with the aim of bringing both to the meeting point together.
Real flights use a sequence of carefully planned manoeuvres, but this is the basic relationship: the spacecraft changes its orbit to change where and when it will arrive.
The two orbits here are circular and in the same plane, which sets aside the part a real mission cannot set aside: Earth turning the launch pad under the station’s path. That, more than the phasing, is why launches to the station happen at almost exactly one instant. The figure holds the flight plan fixed to isolate the timing; a real flight adjusts its later burns instead.
Arriving together
Reaching the same place is only part of the task. Two spacecraft arriving with different speeds could simply pass each other—or collide.
As the visiting spacecraft approaches, further thruster burns bring its motion into close agreement with the station’s. Sensors measure their separation and relative motion, allowing small corrections during the final approach. The spacecraft lines up with a docking port and closes the remaining gap slowly.
Seen from Earth, both are still travelling at roughly 28,000 kilometres per hour. Seen from the station, the approaching spacecraft can appear almost still. Their shared motion around Earth leaves only a small movement between them.
Docking completes a journey that began with the launch timing and continued through a series of changes to the spacecraft’s orbit. The spacecraft has gradually arranged to share the station’s path, position and motion, allowing the two to connect while they continue falling around Earth together.