During the morning hours of Thursday (Jan 16) the Indian space agency succeeded in mechanically attaching or docking two satellites flying at velocities over 28,000 kilometres per hour, almost 470 kms above Earth. India became the fourth nation - and the fifth space agency when we factor in European Space Agency - to achieve such space docking.

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Docking is an enabling technology crucial to transfer humans, cargo or fuel from one spacecraft to another. India's ambitions of returning samples from the moon (Chandrayaan-4) and building the country's own space station required mastery over docking technology.

So far, only the US, Russia, Europe, and China have performed space docking, and India joins that elite list with the successful execution of ISRO's SpaDeX (Space Docking Experiment) mission.

Over the course of this week, the satellites were gradually brought closer to one another from a distance of over eight km, in preparation for docking.

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Over the last two weeks, ISRO made two attempts at docking, but stopped short of the final process, coming as close as three metres on the attempt made on January 12. However, the process of bringing the satellites close to one another has helped in testing the sensors, cameras and software that enable the fully autonomous docking process. 

 

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What is the objective of Space Docking?

The primary objective of the SpaDeX mission is to develop and demonstrate the technology needed for rendezvous, docking, and undocking of two small spacecraft - SDX01, which is the Chaser, and SDX02, the Target. Each satellite weighs 220kg and is fitted with cameras, sensors and thrusters that will enable precise docking. 

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After the primary mission of docking is accomplished, the two satellites will then separate (undock) and start operating as two different satellites, operating their respective payloads for the expected mission life of up to two years. WION had earlier reported that the twin satellites could attempt more docking and undocking, depending on the mission requirement and the availability of fuel on both crafts. 

Why is Space Docking so complicated?

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When a chaser spacecraft is approaching the target spacecraft, the relative velocity between the two (the difference in velocity between the two) and the distance 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 inefficiencies or mission delays.

When the spacecraft dock, 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 mechanical connection without any impact forces. This requires careful coordination and manoeuvring, all done fully autonomously.

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Initially, the ISRO satellites were over 10 km apart from each other. Gradually, the satellites began to autonomously reduce the distance between themselves. The inter-satellite distance had been lowered to 5 km, 1.5 km, 500 metres and so on.

WION had earlier reported that SpaDeX is ISRO's most complicated mission yet in orbit around Earth. ISRO's twin SpaDeX satellites being of small size (and carrying commensurate fuel) also added to the complexity of the mission. It is widely regarded that docking larger satellites with larger quantities of fuel is relatively simpler.