The Indian space agency ISRO's twin satellites completed space docking for the second time on Sunday, 20th April, at 8:20 pm Indian time. This second docking further validates the software, sensors, and techniques that ISRO has devised to perform a "space handshake" between two satellites that are travelling at over 28,000 kmph around the Earth. "For accomplishing the first docking in January this year, we had to make four to five attempts. This time it was done like child's play," ISRO Chief Dr. V. Narayanan told WION's Sidharth M.P. The twin SPADEX satellites were launched on December 30th, 2024, using India's PSLV rocket. 

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During the first docking in January 2025, electrical power transfer between the two docked satellites was not done, and this time we have done it successfully, said the ISRO Chief. "Power transfer from SDX 02 to SDX 01 satellite, as well as vice versa, was also exercised and accomplished on Monday, 21st April, 2025. The experiment involved operating a heater element in one of the satellites through power from the other satellite. The duration of power transfer was approximately 4 minutes, and the performance of the satellites was as expected," ISRO elaborated.

A few weeks back, we also demonstrated the circumnavigation process, where one satellite moves around and inspects the other. Therefore, all planned objectives of the Space Docking Experiment(SPADEX) mission have been accomplished successfully, Dr. Narayanan said. 

Queried about the further course of action with the twin SPADEX satellites, he said that both satellites have about 50 per cent fuel remaining and that multiple experiments can be performed in the coming times. Right now, the two satellites are docked, but we may decide to undock them in a couple of days, he added. 

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What is SPADEX? 

Space Docking is a technology that would enable India's future space missions. India hopes to build an independent space station by the year 2035, and the first module(component) of this space station is meant to be launched in 2028. Space stations are built by autonomously assembling various modules while they circle the Earth. SPADEX is India's maiden attempt at space docking, which is the process of coupling two satellites into a single unified entity. India's Chandrayaan-4 Lunar sample return mission will also involve highly sophisticated space docking. So far, only the US, Russia, and China have mastered space docking technology. 

Applications of Space Docking 

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Docking establishes a mechanical link between one orbiting spacecraft and another equally sized or larger orbiting body. When astronauts travel to the space station, the craft which carries them has to dock with the space station, following which the crew can move to the orbiting lab. Cargo is also delivered to the space station in a similar fashion. Likewise, docking can enable the transfer of fuel between two spacecraft, thereby extending the life of the recipient craft. 

Why is Space docking so complicated? 

When a chaser spacecraft is approaching the target spacecraft, the relative velocity between the two(the difference in velocity between the two) must be controlled very carefully. If the relative velocity is too high, the docking attempt can fail, causing damage or collisions. If it is too low, it may take too long to complete the approach, which could lead to fuel loss or mission delays.

When the spacecraft docks, their relative velocity must be near-zero. This means that the two spacecraft must be moving at the same velocity and in the same direction at the time of docking to ensure a smooth connection without any impact forces. This requires careful coordination and manoeuvring, all done fully autonomously.