Explained: Why is ISRO's SPADEX docking postponed to 9th Jan?

Since the launch of the mission on 30th December 2024 (last Monday), the Indian space agency ISRO has been working towards carrying out the complicated Space Docking Experiment, a process of mechanically attaching two satellites that are travelling around the earth at speeds of 28,000 kmph. On Monday, 6^th January, ISRO announced the docking of its twin SPADEX (Space Docking Experiment) satellites is being pushed to 9^th January, Thursday, from the initial expected date Tuesday, 7^th January. ISRO said that the docking process requires further validation through ground simulations based on an abort scenario identified.

"While performing precise autonomous docking of two satellites, there are many scenarios where the on-board system has to decide and abort the docking process (based on the real-time measurements). These are tested and validated using simulation. Today, we saw a new case that has to be handled. The updated algorithm has to be tested before being implemented in the satellite," ISRO Chief Dr. S. Somanath explained to WION.

Typically, when a spacecraft (carrying either crew or cargo) is launched from earth, depending on the approach taken, it takes around six hours or thirty-six hours for the craft to dock (mechanically connect) with the international space station. The former approach is known as the fast-track approach, while the latter is known as the traditional approach. However, India's space docking experiment (SPADEX) to mechanically attach two satellites in space requires slightly longer than a week. So far, the US, Russia, Europe, and China have performed and mastered space docking technology, and India is to soon join this elite list. India's SPADEX satellites were launched on December 30^th 2024, and they are expected to dock by January 10th.

"The foreign spacecraft that dock in less than eight hours or in a day are heavy ones that weigh several tonnes and carry massive quantities of fuel. Those crafts are built to carry lots of fuel that help them fire their thrusters and manoeuvre in such a way that they can align with the space station and dock in a very limited period of time. In our case, performing docking rapidly is not the goal. Our goal is to test the homegrown docking technology for the first time in space. We are relying on the natural drift of the two docking satellites and consuming minimal fuel, as our satellites weigh just about 220 kg and carry only a commensurate amount of fuel," M. Sankaran, Director of ISRO's UR Rao Satellite Centre, explained to WION.

Docking is a process that is traditionally used to transfer goods or astronauts between a spacecraft and the International Space Station. Docking is a technology that has various applications; it is critical for performing sample return missions from the moon and other celestial bodies, and it can be used to transfer fuel from one space vehicle to another. Docking can also be used to de-orbit a craft that is circling the earth. Various commercial entities are exploring methods to extend the life of satellites by refuelling via docking, providing in-orbit servicing, etc.

ISRO Chief Dr. S. Somanath added that ISRO will attempt even more sophisticated space docking missions in the future. He emphasised that the future Indian docking missions would be more complex and the satellites would be bigger in size and mass. India needs to master docking technology to accomplish future missions such as Chandrayaan-4 (lunar sample return) and the proposed Bharatiya Antariksh Station (Indian Space Station). Space stations are built by launching and assembling (docking) large components in space. 

At the time of ejection, the final stage of the PSLV rocket gave a small relative velocity between the Target and Chaser spacecraft at the time of separation from the launch vehicle. Simply put, the target satellite will be travelling slightly faster than the chaser. This incremental velocity will allow the Target spacecraft to build a distance of 10-20 km with the Chaser.

Gradually, the satellites will autonomously reduce the distance between themselves by adjusting their respective velocities. From 20 km, the inter-satellite distance would be lowered to 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m, ultimately leading to the docking of the two spacecraft. After successful docking and unification, electrical power transfer between the two satellites will be demonstrated. Electrical power transfer is crucial for carrying out in-space robotics, control of the unified spacecraft, etc. 

ISRO Chairman Dr. S. Somanath told WION that the twin satellites could attempt more docking and undocking, depending on the mission requirement and the availability of fuel on both crafts. As small satellites weighing around 220 kg each, the SPADEX satellites carry very limited fuel when compared to their larger counterparts.