ISRO Achieves another milestone in Indian Space Technology

By R Anil Kumar

• ISRO successfully demonstrated the docking of the SPADEX satellites (SDX 01 & SDX 02) for the second time on April 20, 2025 at 08:20 PM. Subsequently, power transfer from SDX 02 to SDX 01 satellite as well as vice versa was also exercised and accomplished on 21^st April, 2025

• The demonstration of the fully autonomous second docking along with power transfer marks the completion of an important milestone in the SPADEX mission

This achievement follows the first successful docking in January and subsequent undocking in March, demonstrating ISRO’s growing mastery of orbital manoeuvres.

The operation, described as “smooth” by ISRO Chairman V. Narayanan, reinforces India’s position as only the fourth nation globally to develop satellite docking capabilities, paving the way for ambitious future missions including Chandrayaan-4, Gaganyaan, and India’s planned space station.

The SpaDeX mission began with the successful launch of the PSLV-C60 rocket on December 30, 2024, from the Satish Dhawan Space Centre in Sriharikota. The mission deployed two satellites, designated as SDX01 (Chaser) and SDX02 (Target), each weighing approximately 220 kilograms, into a 470-kilometer circular orbit.

These satellites were specifically designed to demonstrate the complex technology of space docking, a capability crucial for future space endeavours. The first successful docking was achieved on January 16, 2025, at 06:20 AM IST, marking a historic milestone for India’s space program.

After nearly two months of connected operations, the satellites were successfully undocked on March 13, 2025, at 09:20 AM IST.

This undocking operation was executed in a 460-km circular orbit with a 45-degree inclination, with both satellites subsequently orbiting independently while maintaining normal operational health. This initial success demonstrated ISRO’s ability to perform both docking and undocking manoeuvres, validating the complete operational cycle required for future missions.

Intervening Experiments and Second Docking

Between the undocking in March and the second docking in April, ISRO conducted an innovative “rolling” or “rotating” experiment in late March 2025.

This complex operation involved the circumnavigation of one satellite around the other before returning to its original position.

ISRO Chairman V. Narayanan emphasized the importance of thorough ground simulations before attempting such complex manoeuvres, stating: “We’ve completed the rolling experiment. The satellites have a lot of propellant… I have only told [the teams] one thing, please do everything [on] simulation on ground [first]. Let us be very careful.”

The second docking operation was successfully completed by April 21, as officially announced by Union Minister Jitendra Singh who posted on social media: “Glad to inform that the second docking of satellites has been accomplished successfully.

“According to reports, this second docking was executed with even greater precision than the first attempt, demonstrating ISRO’s rapid learning curve and technical advancement in mastering this technology.

Jitendra Singh also indicated that further experiments are planned in the next two weeks, showcasing ISRO’s commitment to maximizing scientific and technological returns from the mission.

Technical Aspects of The SpaDeX Docking Operations: Docking Mechanism and Procedure

The SpaDeX mission employs sophisticated technology for the precise manoeuvres required in satellite docking. The process involves the chaser satellite (SDX01) approaching the target satellite (SDX02) in a carefully controlled manner, with a relative speed differential of just 10 millimetres per second.

This delicate procedure requires exact alignment of the docking ports and a precise “handshake” when all parameters fall within acceptable limits. The operation is executed in incremental stages, reducing the distance between satellites from 20 kilometres to 15 meters, then to 3 meters, before the final approach and connection.

The docking system utilizes multiple advanced sensors including Laser Range Finders, Rendezvous Sensors, and Proximity and Docking sensors that provide precise measurements as the satellites approach each other.

Onboard processors continuously calculate the relative position and velocity between the two satellites, making minute adjustments to ensure perfect alignment. The successful execution of this complex operation twice demonstrates ISRO’s mastery of the propulsion systems and thrust control mechanisms necessary for such precise manoeuvring.

Significance of the “Rolling” Experiment

The “rolling” experiment conducted between the first undocking and second docking represents a significant technical achievement for ISRO. This experiment, likened to the Chandrayaan-3 “hop” experiment in terms of learning opportunities for future missions, involved one satellite circumnavigating the other while maintaining visual contact throughout the operation.

The successful execution of this manoeuvre allowed ISRO to validate multiple software systems, ground station control procedures, and satellite manoeuvring capabilities.

This experiment provided valuable insights into satellite control and positioning technology, demonstrating ISRO’s ability to precisely control a satellite’s orientation and trajectory in orbit.

The techniques validated during this experiment will inform the development of future missions requiring precise orbital manoeuvres, such as satellite servicing, formation flying, and complex rendezvous operations necessary for advanced space exploration and utilization.

Implications For Future ISRO Missions: Critical Technology For Chandrayaan-4 And Gaganyaan

The successful demonstration of docking technology through the SpaDeX mission has significant implications for ISRO’s future flagship programs.

The docking capability is critical for the planned Chandrayaan-4 sample return mission to the Moon, which would require precise rendezvous and docking capabilities in lunar orbit.

The experience gained from multiple successful docking provides ISRO with essential data and operational experience that will directly contribute to the development of the Chandrayaan-4 mission architecture.

Similarly, the technology validated through SpaDeX is crucial for India’s human spaceflight program, Gaganyaan. While initial Gaganyaan missions may not require docking operations, the extended program will likely include rendezvous and docking with space modules or platforms.

The precision demonstrated in both docking operations provides confidence that ISRO can successfully execute the complex orbital manoeuvres required for human spaceflight missions, where safety considerations demand extremely reliable and accurate systems.

Foundation For India’s Space Station

Perhaps the most significant long-term implication of the SpaDeX mission is its contribution to India’s ambition to develop its own space station.

A space station requires the assembly of multiple modules in orbit, necessitating numerous docking operations during construction and for crew and cargo transfers. The successful demonstration of docking technology represents a crucial step toward realizing this ambitious goal, positioning ISRO to pursue its space station plans with greater confidence and technical readiness.

The SpaDeX mission has demonstrated all capabilities required for rendezvous, docking, and undocking operations in a circular orbit. This complete operational cycle provides the foundation for more complex future operations, including those involving multiple modules and varying orbital conditions.

Each successful experiment builds upon previous achievements, creating a comprehensive body of knowledge that will accelerate India’s capabilities in advanced space operations.

India’s Position in Global Space Technology: Joining an Elite Group of Space-Faring Nations

With the successful completion of the SpaDeX mission, India has firmly established itself as the fourth nation worldwide to demonstrate satellite docking capabilities, joining the United States, Russia, and China in this elite group.

This achievement represents a significant advancement in India’s space capabilities and raises its profile in the global space community. The demonstration of such complex technology highlights India’s growing technological sophistication and its emergence as a major player in space exploration and utilization.

The successful execution of multiple docking operations, along with the innovative rolling experiment, showcases ISRO’s ability to independently develop and master advanced space technologies.

This achievement is particularly notable given the relatively modest budget of India’s space program compared to other spacefaring nations. It demonstrates ISRO’s efficiency and innovation, characteristics that have become hallmarks of India’s approach to space exploration and technology development.

Future Plans and Ongoing Development

Following the successful second docking, ISRO has announced plans for additional experiments over the next two weeks. While specific details have not been released, these experiments are expected to further test and refine the docking technology, potentially including docking under different orbital conditions or with varying approach trajectories.

Each experiment will add to the body of knowledge developed through the mission, enhancing ISRO’s understanding of the factors affecting successful docking operations.

ISRO Chairman V Narayanan has described the second docking operation as “smooth,” noting that his team managed to do it as if it were “child’s play,” indicating the significant progress made in mastering this complex technology.

The availability of significant remaining propellant in the SpaDeX satellites provides opportunities for extended operations beyond initially planned experiments, allowing ISRO to maximise the scientific and technological returns from the mission.

A Significant Milestone

The successful completion of the second docking operation in the SpaDeX mission represents a significant milestone in India’s space program and demonstrates ISRO’s growing capabilities in advanced space operations.

This achievement, along with the earlier first docking and the innovative rolling experiment, establishes India as a member of an elite group of nations with demonstrated satellite docking capabilities.

The technology validated through the SpaDeX mission has far-reaching implications for India’s future space endeavours, including the Chandrayaan-4 lunar sample return mission, the Gaganyaan human spaceflight program, and the development of an Indian space station.

The precision and reliability demonstrated in these operations highlight ISRO’s technical expertise and methodical approach to developing critical space technologies.

The continued experiments planned for the coming weeks will further refine this capability, generating valuable data and experience that will inform future mission design and operations.

As ISRO continues to build on this success, India is well-positioned to play an increasingly prominent role in global space exploration and utilization, with docking technology serving as a fundamental enabler for more ambitious and complex missions in the years ahead.