The Indian Space Research Organisation (ISRO) is preparing to launch its Space Docking Experiment (SpaDeX) mission on December 30, 2024, at 21:58 IST from the Satish Dhawan Space Centre in Sriharikota. This groundbreaking mission aims to showcase in-space docking technology by deploying two small satellites, SDX01 (Chaser) and SDX02 (Target), each weighing around 220 kg, into a 470 km circular orbit.
The primary goal of SpaDeX is to develop and validate the technologies necessary for autonomous rendezvous, docking, and undocking of spacecraft. These capabilities are vital for India's future space initiatives, including lunar missions like Chandrayaan-4 and the construction of the Bharatiya Antariksh Station (BAS). Additionally, the mission will demonstrate the transfer of electrical power between docked spacecraft, composite spacecraft control, and payload operations after undocking.
SpaDeX mission being moved to the launch pad at Satish Dhawan Space Centre, Sriharikota. (DC Image)
The mission will begin with the PSLV-C60 rocket launching the Chaser and Target satellites into slightly different orbits. The Chaser will then use precise manoeuvres to approach the Target, progressively reducing the distance from 5 km to 1.5 km, 500 m, 225 m, 15 m, and finally to 3 m, culminating in a successful docking of the two spacecraft.
Following the docking and stabilization, electrical power transfer between the two satellites will be demonstrated. Afterward, the satellites will undock and separate, continuing with their planned operations, with an expected mission duration of up to two years.
ISRO has developed indigenous technologies for this mission, including a docking mechanism, a suite of rendezvous and docking sensors, power transfer technology, an autonomous rendezvous and docking strategy, an inter-satellite communication link, and a GNSS-based Relative Orbit Determination and Propagation processor. The space agency has also created simulation test beds for validating both hardware and software designs.
ISRO noted that the SpaDeX mission presents unique challenges due to the smaller size and mass of the satellites, requiring greater precision in rendezvous and docking compared to larger spacecraft.
The docking mechanism is a low-impact system with an approach velocity of 10 mm/s and features an androgynous design, meaning the docking systems are identical on both spacecraft. It also uses a peripheral docking concept, similar to the International Docking System Standard used in human missions by other space agencies.
An animation illustrating the docking process has been released by ISRO, providing a visual representation of the mission's objectives.
The docking mechanism is compact, measuring 450 mm with one degree of freedom for extension, and uses two motors, compared to the 800 mm IDSS used in other spacecraft with 24 motors. Multiple test beds have been set up to test hardware and software simulations of the docking kinematics and finalize the approach parameters.
The sensor suite includes a Laser Range Finder (LRF) and Corner Cube Retro Reflectors, working in the range of 6000 to 200 m for range determination (R). Rendezvous Sensors (RS) operate in the 2000 to 250 m range and 250-10 m range, providing relative position (x, y, z), while the LRF measures both relative position and velocity independently. The Proximity and Docking Sensor (PDS) provides relative position and velocity in the 30 m to 0.4 m range. Laser Diodes (LDs) act as targets for RS and PDS. A video monitor captures the docking event in the 20 to 0.5 m range, and a Mechanism Entry Sensor (MES) detects the Chaser's entry into the Target spacecraft during docking.
Both spacecraft are equipped with a differential GNSS-based Satellite Positioning System (SPS), providing Position, Navigation, and Timing (PNT) solutions.
After the docking and undocking demonstrations, the satellites will separate and perform their intended operations. They are equipped with a High-Resolution Camera, Radiation Monitor, and Miniature Multi-Spectral Payload for monitoring natural resources and vegetation.
The SpaDeX satellites were developed by the UR Rao Satellite Centre (URSC), with support from other ISRO centers such as VSSC, LPSC, SAC, IISU, and LEOS. The spacecraft will be controlled from ISTRAC using ISRO ground stations and other external stations during the orbital phase.
The success of SpaDeX will position India as the fourth country to master space docking technology, joining Russia, the US, and China. This achievement is expected to enhance India’s capabilities in space exploration, enabling complex missions that involve multiple launches and in-orbit assembly. Future missions include lunar expeditions, human spaceflight, and the development of India’s own space station. By mastering in-space docking, ISRO aims to facilitate longer-duration missions and more intricate space operations, marking a significant milestone in India’s space program.
The launch is set for 9:58 PM on December 30, 2024, from the Satish Dhawan Space Centre in Sriharikota. ISRO has opened registration for the public to view the launch from the Launch View Gallery at SDSC SHAR, starting December 23, 2024.
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