The end of year 2024 is approaching and Indian Space Research Organization (ISRO) is set to accomplish an elite feat on the second last day of the year, which only three nations has achieved to date. Only Russia, USA and China have mastered the technology of docking spacecraft in space. The success of this challenging mission will be a significant milestone for the nation in achieving its ambition of establishing its own space station. Let’s have a brief discussion about ISRO’s ambitious mission called “SpaDeX“.
PSLV C60/SPADEX animation of docking and power transfer video. Source-ISRO
The SpaDeX Mission Objective
The main goal of the SpaDeX mission is to create and test the technology required for the rendezvous, docking and undocking of two small spacecraft in a low-Earth circular orbit. The spacecrafts are named SDX02, referred to as the Target and SDX01, known as the Chaser. Furthermore, the SpaDeX mission aims to demonstrate composite spacecraft control, conduct payload operations after docking and showcase the transfer of electric power between the docked spacecraft. This capability is essential for future applications, such as in-space robotics.
The Concept of Mission SpaDeX
The two spacecraft, the Target and Chaser (each weighing approximately 220 kg) will be launched independently and simultaneously by PSLV-C60 into a 470 km circular orbit at a 55° inclination. they will follow a local time cycle of approximately 66 days. When the Target and Chaser spacecraft separate from the launch vehicle, a small relative velocity will be provided by the PSLV vehicle’s proven accuracy. The Target spacecraft will create a inter-satellite separation of 10–20 km from the Chaser within a day relying on this incremental velocity. At that point, the Target spacecraft will use its propulsion system to compensate for the relative velocity between the two spacecraft.
This drift arrest maneuver will result in a “Far Rendezvous,” where both the Target and Chaser are in the same orbit and moving at the same velocity, separated by approximately 20 kilometers. The Chaser will then approach the Target, reducing the distance between them step by step to 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m, until the two spacecraft dock. This will be accomplished by introducing and compensating for a small relative velocity between the two spacecraft. Once the two spacecraft are docked and secured, they will demonstrate electrical power transfer before undocking to start their payload operations for the planned two-year mission.
The New Technologies Developed for the SpaDeX Mission
ISRO with it’s associates has developed many state of the art technologies to accomplish the complex SpaDeX mission-
- Docking Mechanism,
- A suite of four rendezvous and docking sensors,
- Power Transfer Technology,
- Indigenous novel autonomous rendezvous and docking strategy,
- Inter-satellite communication link (ISL) for autonomous communication between spacecraft, incorporated with inbuilt intelligence to know the states 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.
Furthermore, the SpaDeX mission faces significant challenges due to the heightened precision required for rendezvous and docking maneuvers, especially given its small size and mass compared to larger spacecraft. This mission will serve as a prototype for autonomous docking, which will be essential for future lunar missions like Chandrayaan-4, especially without the assistance of Earth-based GNSS.
The Docking Mechanism behind the SpaDeX Mission
The docking mechanism is designed to be Low-impact, with an approach velocity of of 10 mm/s. It is androgynous, meaning that the docking systems are the same for both spacecraft, the Chaser and the Target. The design is peripheral, similar to the International Docking System Standard used by other agencies for human missions.
Unlike the larger International Docking System Standard (IDSS), which measures 800 mm and operates on a hexapod with 24 motors, this mechanism is more compact at 450 mm. It features one degree of freedom for extension and utilizes only two motors. The docking approach parameters have been verified and finalized through extensive testing of both hardware and software simulations of the docking kinematics across multiple test beds.
What after the Docking & Undocking Exercise
After the successful docking and undocking exercise, the two spacecraft will be separated and used for application missions as follows:
- A High-Resolution Camera (HRC) with a 4.5 m IGFOV and a swath of 9.2 x 9.2 km (snapshot mode) and 9.2 x 4.6 km (video mode) from a 450 km altitude is mounted in SDX01. This is a miniature version of the surveillance camera developed by SAC/ISRO.
- A Miniature Multi-Spectral Payload (MMX) is mounted in SDX02, developed by SAC/ISRO. This has four VNIR bands (B1/B2/B3/B4) at 450 nm to 860 nm and a 25 m IGFOV with a swath of 100 km from a 450 km altitude. The imaging is useful for natural resource monitoring and vegetation studies.
- A Radiation Monitor (RadMon) payload is mounted in SDX02, which will measure radiation dose encountered in space. This will help in generation of a radiation database for future Total Ionization Dosimeter (TID) and Single Event Upset (SEU) measurements for space science studies, with applications in human spaceflight.
Conclusion
India’s aspirations for space exploration will advance significantly with the successful completion of the SpaDeX mission. This mission marks an important milestone that elevates India’s status in the international space community, serving as more than a technical achievement. The success of SpaceX enhances India’s long-term space goals, which include crewed space missions, making its own space station, satellite maintenance and interplanetary exploration, all will be made feasible by improved space docking capabilities.SpaDeX guarantees that India remains at the forefront of the next phase of space exploration and is more than just a step for the country into space. It is a leap towards a more promising and technologically advanced future.
