This is how ships manage to reach the Space Station at 28,000 km/h

space shuttle docked to iss

The International Space Station and the Chinese Space Station are orbiting laboratories, dependent on supplies from spacecraft regularly launched from Earth. Unmanned missions are responsible for carrying large amounts of cargo, whether life support elements or scientific experiments. While expeditions are continually replaced thanks to manned flights. The orbital maneuvers necessary to carry the vehicles are known as rendezvous and they date back to the early years of the space race.

GT 6 7 Rendezvous feature
Artistic recreation of the first orbital encounter in history. This was done by Gemini 6 and 7 in 1965.

Flying into space

Just over ten minutes pass between the takeoff of a rocket and its entry into orbit. Going from being at rest at one atmosphere of pressure to reaching up to 28,000 kilometers per hour or 8 kilometers per second of speed and practically zero pressure. Throughout the journey the air offers resistance to movement, once sufficient height is reached this is negligible.

take off crew 5
The Falcon 9 rocket with the Dragon capsule of the Crew 5 mission takes off towards the International Space Station. Credits: SpaceX

Reached the necessary speed; Whether in Earth, lunar, Martian or solar orbit, it is possible to know without much difficulty where the ship will be in the future. This thanks to our knowledge of gravity, which defines trajectories in space. So far all docking and encounters have been carried out in circular orbits. Once the target craft reaches the desired height, the hunter can take off and head out to begin the chase.


Takeoff cannot occur at any time, but there are instant launch windowsThese are calculated from the moment in which the target flies over the area and the pursuer can enter a similar orbit. This in order to optimize the amount of fuel needed as much as possible. If it occurs at any time, the energy requirements would be much greater than those available.

A dance in orbit

Once in orbit, the maneuvers begin, which could seem like a dance. To understand them, certain basic concepts of astrodynamics are necessary.. The first thing is to know the difference between our daily lives. For example, when driving a car, if you want to go faster you must simply accelerate, or to go slower you just step on the brake. Meanwhile, in orbit to reduce your speed you must accelerate, to increase it you must decelerate (accelerate is understood as turning on the engines in your direction of movement).

This counterintuitive movement is due to the behavior of gravity. More specifically to Kepler's second law of planetary motion, which can be applied to the orbits of ships. Where the smaller the distance from the planet, the greater the speed. Therefore, by accelerating you are achieving greater distance over time and, therefore, slowing down.

In the most common and simple way of rendezvous, the chaser enters an orbit with a slightly smaller radius and a higher speed. A maneuver is then performed to intercept the target's trajectory. Each moment is previously calculated computationally to optimize the use of available fuel., reduce the time between takeoff and docking, and have the relative speed between both ships as low as possible.

progress ms14 rendezvous 1
Diagram of the maneuvers carried out by a Russian Progress unmanned ship. This shows how the altitude changes with respect to the Earth and the duration of the engine ignitions.

The final approach stage, when both are in line of sight, is a slow process. The relative speed is a few centimeters per second, while the orbital speed is more than 27,000 kilometers per hour.. Using a series of lower power engines known as RCS; reaction control systems, coupling between both ships is finally achieved.

First steps

The procedures of rendezvous They were created during the race for the Moon. Since this would be necessary to transfer astronauts from the lunar module to the command module. In which they would return to Earth. To test all these techniques, the Gemini program existed, predecessor of Apollo.. It was first achieved in 1965, when Gemini 5 and 6 met in orbit, in addition to maintaining a constant and controlled separation from each other. Later, with Gemini 8, the first successful docking was achieved.

Demonstrating the possibility of achieving a space encounter was crucial for lunar missions. The lunar module would take off from the surface, meet the command module and transfer the cargo and crew between the two.. In the latter the three astronauts would return to Earth.

Once manned lunar exploration and the space race came to an end, the United States and the USSR carried out an international collaboration mission. This was known as the Apollo Soyuz Test Project, where a spacecraft from each nation would be launched into orbit and they would carry out a rendezvous and coupling. Shortly after the Space Shuttle entered service, the US would visit the Soviet space station MIR.

The flight to the Space Station

 The procedure for ships with and without crew is different. In the first case, the time it takes to carry out the meeting is sought to be reduced as much as possible. In the times provided by the instantaneous launch windows; The Soyuz, Dragon or Shenzhou spacecraft (in the case of the Chinese station), takes off towards an orbit very similar to that of the station. With the only difference of having a smaller radius and, consequently, a smaller period. 

Once the height of about 200 kilometers and a speed of almost 28,000 kilometers per hour has been achieved, verifications of the structure, systems and whether the orbit achieved is correct are carried out. Just as astronauts are given time to adjust to weightlessness and take their first looks at the Earth through the window. At a given moment, a correction maneuver is performed, and more speed is obtained, in such a way that it intercepts the station's trajectory. In addition to reaching 400 kilometers in height.

spacex docking
SpaceX offers a simulator of the final phase of docking a Dragon spacecraft to the International Space Station (click to try it).

Once in line of sight, the ships are delicately controlled to achieve a safe docking. For security reasons, the ISS has two areas that must not be crossed unless all systems are working correctly. The first is the approach ellipsoid and with a smaller size is the keep out sphere or sphere of exclusion. These will only be crossed when the final approach and docking are made. Once there is contact between the two, the pressure stabilizes and the crew can board the station.

Francisco Andrés Forero Daza