10 hours ago
How Shubhanshu Shukhla's Dragon spacecraft docked with ISS
The SpaceX Dragon spacecraft carrying Indian astronaut Shubhanshu Shukhla and three others docked with the International Space Station (ISS) at 4 pm IST on Thursday.
Docking is a process by which two spacecraft are joined in space. While a routine procedure today — the ISS witnesses around a dozen space dockings annually — this is nonetheless a complicated procedure, requiring extreme control and precision.
Axiom Mission 4 aboard the @SpaceX Dragon docked to the station at 6:31am ET today. Soon the Ax-4 astronauts will open the hatch and greet the Exp 73 crew live on @NASA+. More…
— International Space Station (@Space_Station) June 26, 2025
The challenge
The capability to bring together two spacecraft is necessary for missions involving heavy spacecraft that cannot be launched by a single vehicle. The ISS, for instance, comprises 43 different modules and elements that were assembled to form an integrated whole in space over the course of years, beginning in 1998.
The central challenge of space docking is that it requires two spacecraft travelling at speeds of thousands of kilometres per hour to align their orbits, come close to one another, before making contact and joining. This requires sophisticated algorithms and sensors that accurately calculate the position of the two spacecraft and determine the path to be taken, as well as the capability to precisely manifest these calculations into the actual manoeuvring of the spacecraft using thrusters.
In the case of the ISS, which orbits Earth at an altitude of more than 400 km, the procedure takes place while both the space station and the approaching vehicle are travelling at speeds of around 27,000 km/h.
Till date, only four countries — the US, Russia (and the erstwhile Soviet Union), China, and most recently, India — have demonstrated the capability to carry out space docking.
The process
Docking is not a single manoeuvre — it comprises tens of minute but essential manvouvres which eventually result in two spacecraft coming together. While specifics may differ depending on the spacecraft involved, here's a broad overview. Note that nowadays, the docking process is completely autonomous (although manual overrides do exist).
RENDEZVOUS: The first stage of the process is aligning the orbits of the spacecraft and the ISS. This is done over the course of many hours, through a number of thruster burns that slowly align the approaching vehicle's orbit to the ISS.
The SpaceX Dragon is equipped with 16 Draco thrusters placed across the spacecraft, each capable of producing 90 pounds of force in the vacuum of space which helps propel it to the desired trajectory and adjust its orientation. Its last major burn, which lasts one-and-a-half minutes, is conducted while the vehicle is 7.5 km from the ISS (horizontally).
This puts the Dragon on a rough trajectory towards the docking port of the spacecraft — only very precise, minor adjustments need to be made here on.
FINAL APPROACH: After getting in a range of a few hundred metres to the ISS, the approaching vehicle enters what is known as the 'final approach corridor'. Here, laser ranging and thermal imagers provide precise measurements of distance and alignment to the docking port, which is then processed by a suite of onboard computers to make minor adjustments to the spacecraft's trajectory, if needed.
The final approach sees the vehicle crossing multiple checkpoints at each of which the health of various systems and the status of the approach are assessed and a decision is made whether to move forward or abort the mission. (Abort sequences are pre-programmed, and as such, only a command to abort the mission must be given).
For the Dragon, the final checkpoint — 'Waypoint 2' or WP-2 — is only 20 m from the ISS. At this point, the Dragon can hold its position (relative to the ISS) if needed. At a distance of less than 5 m from the docking port (roughly 25 seconds to docking), there's what in ISS lingo is known as the CHOP (Crew Hands Off Point): there is no scope for manual intervention beyond this point, and an abort sequence, if needed, will be automatically triggered by the capsule itself.
CONTACT & CAPTURE: The Dragon's soft capture ring makes initial 'soft contact' with the International Docking Adapter (IDA) used by all American vehicles to the ISS. Soft contact coarsely aligns the two spacecraft for the 'hard capture' which sees the Dragon's 12 hooks extend and lock in with the IDA.
The whole process, from soft contact to hard capture, takes around ten minutes to take place, after which docking is said to be completed. But hatches on both the ISS and the capsule remain shut for another hour-and-a-half. Pressure between the spacecraft and the ISS is equalised, a number of checks (mainly for leaks) are carried out, and only then are the hatches opened, allowing the crew to enter the ISS.
