Among the milestones achieved by the ISRO (Indian Space Research Organization), since its inception in 1969, the SpaDex mission successfully completed last January is a unique and an interesting one. The mission SpaDex (Space Docking Experiment) was basically to dock and undock i.e. join and detach two small satellites in the space so as to accomplish the ability for such maneuvers for other spacecrafts in the future. In space missions, it is necessary to dock or undock spaceships, satellites so that astronauts and equipment can be carried from one vehicle to the other. This becomes necessary to carry out experiments, in repair and maintenance activities, exchange from one vehicle to another. Like in the International Space Station (ISS), astronauts move in and out of space shuttles while entering ISS or returning back to the earth.
While becoming only the fourth nation after USA, Russia and China, India has now the indigenous capacity to move people and equipment between satellites or spaceships required for its future space expeditions. Launched on December 30th, 2024, and successfully executed on January 16th, 2025, and fully autonomous docking on April 20th, 2025, ISRO had successfully docked and undocked two small satellites namely SDX01 and SDX02 launched by the PSLV 60 rocket from Sriharikota station in Hasan, Karnataka. Controlled from the ground station at Sriharikota, the docking and undocking process worked out perfectly thereby establishing our capabilities for similar maneuvers in the near future.
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| Photo Credit: PICRYL |
The Chandrayaan-4 mission, the Gaganyaan mission as well as the Bharat Antariksh Station (BAS), a permanent space station of India like the ISS are the prominent future space expeditions of the ISRO. Given the need for the convenient and easy passage of equipment and astronauts between spacecrafts and satellites, the SpaDex mission was needed to be accurate as well as successful for us. The Bharat Antariksh Station (BAS) will be a permanent station for experiments in space, the stay of the astronauts as well as serve as a platform for carrying people and equipment to and from it and the earth. Similarly, the Gaganyaan mission that aims to put Indian astronauts to space through an indigenous mission first time ever, will also require such capabilities. Thus, the SpaDex mission will be a milestone in that regard. The docking and undocking of satellites in space is a very complex one. It involves a rendezvous i.e., staying aloft at a position so as to align accurately to the other craft and the docking maneuver i.e., perfect and precise movements to lock to the target spacecraft (since inaccurate docking can be expensive as well as dangerous), and commence the movement of goods and people from one craft to another. This was followed reversely by the undocking process soon. A large number of indigenous technologies were incorporated in the mission. That included the docking mechanism i.e., the whole process of identifying, communicating, connecting and ultimately docking, a suite for four rendezvous and docking sensors, power transfer technology, Indigenous novel autonomous rendezvous and docking strategy, Inter-satellite Communication Link (ISL) for
autonomous communication between spacecrafts, incorporated with inbuilt intelligence to know the state of the other spacecraft. GNSS-based Novel Relative Orbit Determination and Propagation (RODP) processor to determine the relative position and velocity of the other spacecraft. Simulation test beds for both hardware and software design validation and testing. ISRO built the two satellites and the associated equipment at a cost of ₹125 crore while the launch vehicle cost ₹250 crore. The main goals of the SpaDex mission were - develop and demonstrate technology for rendezvous and docking using two small spacecrafts, showcase controllability in the docked condition, demonstrate the potential for extending the life of the target spacecraft, test power transfer between the docked spacecraft. The ISRO chief S Somnath said at a press conference that additional SpaDex missions with greater size and complexity would be launched in the near future for larger docking systems for the space missions. Thus, India has the self-reliance in cutting-edge technology and the capability of the ISRO for such critical operations. Along with it, the SpaDex was a cost- effective technology demonstrating mission costing about ₹375 crore to show in-space docking using two small spacecrafts lifted by the 62nd PSLV flight.



