India’s space programme achieved a significant milestone with the launch of the Space Docking Experiment (SpaDeX) on December 30, 2024, at 10.00pm IST aboard the PSLV-C60 rocket from the Satish Dhawan Space Centre in Sriharikota.
This ambitious mission, potentially culminating in a successful docking on January 7, 2025, is a crucial step towards demonstrating India’s capability in autonomous in-space docking, a technology vital for future space endeavours. The SpaDeX mission involves two small satellites, SDX01 (designated as the Chaser) and SDX02 (the Target), both placed into a low-Earth circular orbit at an altitude of approximately 470 kilometres.
The primary objective is to demonstrate the complex process of autonomous rendezvous and docking, where the Chaser satellite will manoeuvre and gently connect with the Target satellite.
The docking process is a meticulously planned sequence of events. Following their successful deployment, the two spacecraft were initially separated by about 20km due to slight variations in launch velocities. These variations are a natural outcome of the launch phase. Crucially, the Target satellite’s propulsion system was then used to halt the increasing separation, ensuring that the satellites remained at a stable 20km distance. This phase is known as “far rendezvous.”
Over several days, the Chaser satellite will use its onboard propulsion system to carefully reduce the distance from 20km to 5km. This is achieved through a series of precise orbital manoeuvres, calculated and executed by the Chaser’s onboard computer. A critical phase begins when the satellites are approximately 5km apart, at which point the inter-satellite radio frequency (RF) link will be established. This communication link is essential for sharing real-time data on their positions, velocities, and orientations, enabling precise coordination.
The Chaser satellite will then initiate the final approach, gradually reducing the distance to the Target satellite. This is where advanced sensors, including those developed by ISRO’s Laboratory for Electro-Optics Systems (LEOS), play a pivotal role. The Chaser satellite is equipped with a suite of sophisticated sensors, including high-resolution cameras which will provide visual feedback of the Target satellite, helping to determine its precise orientation and position. Then there are laser rangefinders, which emit laser pulses and measure the time it takes for them to reflect off the Target satellite, providing highly accurate distance measurements.
The Chaser likely carries additional sensors, potentially including star trackers for attitude determination and accelerometers to measure changes in velocity. The Target satellite has reflectors that help guide it. These sensors work in concert, providing the Chaser’s onboard computer with a detailed and accurate understanding of the relative position, velocity, and orientation of the two spacecraft. This data is crucial for the computer to make precise adjustments to the Chaser’s trajectory using its thrusters.
“The docking mechanism itself is a marvel of engineering, requiring extreme precision. As the Chaser approaches the Target, its speed will be meticulously controlled. Despite both satellites orbiting at an incredible speed of 28,800km per hour (about 36 times faster than a commercial aircraft), their relative speed will be reduced to a mere 10mm per second (0.036km per hour) for docking. This is akin to a slow crawl, ensuring a gentle and controlled connection. The Chaser will continue to close in, using its thrusters and the data from its suite of sensors for guidance, until it is just 3m away from the Target. It is at this proximity that the final docking manoeuvre will commence.
At this snail’s pace, a specialized “hugging” mechanism will be employed. Latches on both spacecraft will be released, and clamps will engage to secure a firm connection. Throughout this process, the thruster systems on both spacecraft will work in concert to maintain stability and prevent any unintended collisions. The Target satellite is equipped with a special docking mechanism that complements the Chaser’s, ensuring a secure and reliable connection,” explained space expert Girish Linganna.
Once successfully docked, the two spacecraft will demonstrate power transfer capabilities, with electricity flowing from one satellite to the other. “This capability is essential for future missions involving in-space refuelling, repairs, or the assembly of larger structures. Following the docking and power transfer demonstration, the spacecraft will undock and continue with their independent mission objectives, which include conducting scientific experiments using their onboard payloads,” added Linganna.
The PSLV-C60 mission also carries the PSLV Orbital Experimental Module-4 (POEM-4), a platform that repurposes the rocket’s fourth stage for in-orbit scientific experiments. POEM-4 hosts 24 payloads, 14 developed by ISRO and 10 from Non-Government Entities (NGEs), including universities and startups. This initiative, facilitated by the Indian National Space Promotion and Authorization Centre (IN-SPACe), underscores India’s commitment to fostering a vibrant private space sector.
Notably, four of the NGE payloads on POEM-4 originate from Karnataka, including RVSat-1 from RV College of Engineering, designed to study bacterial growth in space; the RUDRA 1.0 green propulsion system from Bellatrix Aerospace; the GLX-SQ payload from GalaxEye Space Solutions for testing Synthetic Aperture Radar imaging; and the BGS ARPIT payload from Adichunchanagiri’s SJCIT, a communication system designed to enhance amateur radio services.
The successful execution of the SpaDex mission will place India among an elite group of nations—the United States, Russia, and China—that have mastered autonomous space docking. This capability is not just a technological demonstration but a critical enabler for future missions, including human spaceflight, lunar sample return, satellite servicing, and the construction of India’s proposed space station, the Bharatiya Antariksh Station.
As we await the anticipated docking on January 7, 2025, the SpaDeX mission stands as a beacon of innovation, collaboration, and a bold vision for the future of India’s space program. The successful completion of this mission will not only enhance India’s capabilities but also inspire a new generation of scientists and engineers to reach for the stars.
Besides the technicalities, the private segment has played a major role in the SpaDeX mission. Ananth Technologies Limited (ATL), a trusted private partner of ISRO, played a key role in the mission’s success. ATL supplied critical components such as Rendezvous Processing Units (RPU) and DC-DC converters for the Chaser (SDX01) and Target (SDX02) satellites, enabling their precise orbital manoeuvres and docking operations. Additionally, ATL undertook the Assembly, Integration, and Testing (AIT) of the satellites at its world-class private satellite manufacturing facilities.
For the PSLV-C60 launch vehicle, which deployed the SpaDeX satellites into orbit, ATL delivered 29 vital subsystems, including Data Acquisition Units, Transmitters, Power Modules, NAVIC processors, and control modules. The company also developed telemetry and telecommand packages, ensuring precise tracking and control of the satellites throughout their complex docking process. Beyond ATL, private companies contributed cutting-edge technologies like high-resolution cameras and laser rangefinders, which are critical for the Chaser satellite’s navigation and docking precision.
These contributions underscore the growing technical expertise of India’s private space sector, which has evolved from being a supplier to becoming a vital partner in India’s ambitious space missions. Through collaborations like this, ISRO and private enterprises are building a thriving ecosystem that is propelling India’s space program to greater heights, paving the way for future advancements in human spaceflight, satellite servicing, and deep space exploration.
