The International Space Station has two sets of incompatible docking ports, one designed for Russian spacecraft and Europe's space freighter, and another design for the now-retired US Space Shuttle. There also are berthing ports for vessels that cannot dock on their own but are first captured by the Station's robotic arm.

Adding to the complexity, Russian docking ports have a male and female version, like an electrical plug and socket. This means that a spacecraft can only dock with a vehicle that has the correct receiving port.

The Station's docking mechanisms are designed to handle large, 100-tonne craft such as the Shuttle. They grasp a spacecraft as it pushes into the Space Station - like a train coach connecting to its locomotive.

Given these premises, it is clear that connecting spacecraft in orbit will never be simple.

Docking spacecraft in orbit will never be simple

'Unfortunately, we don't always get to decouple difficulty from frequency.

'Docking is difficult because it is hazardous. It involves bringing two massive objects into a precise alignment while they are moving and subject to orbital mechanics.

'First comes the challenge of rendezvous. We don't have vast amounts of fuel to burn, so we need to conform to the natural orbital mechanics (as much as possible). The speed of an object in orbit is dictated by its altitude. So, a vehicle below another vehicle will be traveling faster than the other vehicle. For two vehicles to maintain a fixed distance from each other, they either need to be at the same altitude (and in the same plane) or one vehicle must burn a lot of fuel.

'When the Space Shuttle used to dock to the International Space Station (ISS), it would approach along the v-bar (x-axis) very slowly - a rate of about 0.1 m per second. But it was big. The Orbiter had a mass of around 100,000 kg. There's potential for a lot of kinetic energy, there.

'The two vehicles need to be near perfectly aligned. There are small mechanisms on the docking ring that can correct small errors, but if the alignment is not near perfect, the docking mechanism will be damaged.

Berthing

'There's no such thing as slamming on the brakes, either. A massive object like a Space Shuttle Orbiter has to decelerate by firing thrusters and those thruster burns can plume the ISS, damaging the solar arrays.

'Both vehicles must completely inhibit their thrusters at the point of contact to ensure that neither vehicle inadvertently fires a thruster and imparts a torque that can damage the docking mechanism. This is called being in Freed Drift.

'It is easier with a smaller vehicle. The much smaller mass of the Soyuz vehicle allows the use of a probe and cone docking mechanism.

'On the front of a Soyuz is a probe. The receiving port of the ISS has a cone. If the Soyuz isn't well aligned, the cone forces it to become aligned as the probe slides down the surface of the cone. But using a probe and cone requires a greater speed for the interface to attach.'

Frost concludes;

'Because cargo vehicles are uncrewed, we're even more cautious and tend to avoid docking and instead use berthing. Berthing means that instead of the vehicle flying into the ISS and docking, it pulls alongside the ISS and we use the robotic arm to grab the vehicle, pivot it around to the hatch, and connecting it.'