By 7 am, Sunday, January 12, ISRO's Space Docking Experiment (SpaDeX) twin satellites travelling at speeds of more than 28,000kmph, came within three meters of each other in a precisely controlled and coordinated manner in a trial docking attempt. India is working towards becoming the fifth nation after the US, Russia, Europe, and China, to perform space docking- a process that involves mechanically attaching two spacecrafts fully autonomously.
Space docking is an enabling technology that helps perform sophisticated missions to return lunar samples and facilitates space station building. ISRO referred to the 3-meter close-in as a "trial attempt" and that the spacecraft were later moved back to a safe distance. The docking process will be done after further data analysis, ISRO added, without specifying the date and time for docking.
ISRO's PSLV rocket launched the SpaDeX satellites on December 30th, placing them 470 km above the Earth. While the satellites remained at the same altitude, they gradually grew apart by a little more than 10 km. Then ISRO commenced the process of sequentially reducing the distance between the satellites. After one instance of the satellites travelling further away from each other, ISRO managed to bring them as close as 230 metres by evening on Saturday, January 11.
SpaDeX Docking Update:
— ISRO (@isro) January 12, 2025
SpaDeX satellites holding position at 15m, capturing stunning photos and videos of each other! 🛰️🛰️
#SPADEX #ISRO pic.twitter.com/RICiEVP6qB
In the early hours of Sunday, the satellites began to approach each other for docking - 105meters, 15meters and then finally, as close as 3meters.
What is the objective of space docking?
The primary objective of the SpaDeX mission is to develop and demonstrate the technology needed for rendezvous, docking, and undocking of two small spacecraft - SDX01, which is the Chaser, and SDX02, the Target. Each satellite weighs 220kg and is fitted with cameras, sensors and thrusters that will enable precise docking.
After the primary mission of docking is accomplished, the two satellites will then separate (undock) and start operating as two different satellites, operating their respective payloads for the expected mission life of up to two years.
ISRO Chairman Dr S. Somanath earlier told WION that the twin satellites could attempt more docking and undocking, depending on the mission requirement and the availability of fuel on both.
Why is space docking so complicated?
When a chaser spacecraft is approaching the target spacecraft, the relative velocity between the two (the difference in velocity between the two) and the distance between the two must be controlled very carefully. If the relative velocity is too high, the docking attempt can fail, causing damage or collisions. If it is too low, it may take too long to complete the approach, which could lead to fuel inefficiencies or mission delays.
When the spacecraft dock, their relative velocity must be near-zero. This means that the two spacecraft must be moving at the same velocity and in the same direction at the time of docking to ensure a smooth mechanical connection without any impact forces. This requires careful coordination and manoeuvring, all done fully autonomously.
Initially, the satellites were over 10 km apart from each other. Gradually, the satellites began to autonomously reduce the distance between themselves. The inter-satellite distance had been lowered to 5 km, 1.5 km, 500m and so on.
WION had earlier reported that SpaDeX is ISRO's most complicated mission yet in orbit around the Earth. ISRO's twin SpaDeX satellites being of small size (and carrying commensurate fuel) also add to the complexity of the mission. It is widely regarded that docking larger satellites with larger quantities of fuel is relatively simpler.