Indian space agency ISRO has said that the twin SPADEX satellites are now moving closer to each other, in preparation for initialisation of docking. “The drift has been arrested and spacecraft put in a slow drift course to move closer to each other. By tomorrow, it is expected to reach initialisation conditions,” ISRO said. As per Switzerland-based firm S2a systems, the satellites are at a distance of 4.6 km from each other, as of evening, Thursday, 9th January, and have grown closer than they were this morning. 

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Docking attempts postponed twice


The SPADEX satellites were meant to complete autonomous docking in the morning hours of 9th Jan, Thursday. However, on Wednesday night, as one of the twin satellites initiated the drift to move closer from 500 m to 225 m, the drift between satellites was found to be more than desired, therefore leading to temporary postponement. Early on Thursday, private firms Digantara and s2a systems tracked the satellites when they were almost 6.5 km apart.

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This is the second postponement of the docking. Earlier this week, the docking was postponed as ISRO wanted to perform a software test and on-ground simulation.

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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 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 m.

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

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As this is India’s maiden docking mission, ISRO is predominantly relying upon the natural drift of the satellites to bring them closer, which is why it is a time consuming process. It must be noted that SPADEX was launched on 30th December and docking was expected at least a week later. In contrast, large cargo-carrying or human-carrying spacecrafts dock with the International space station in eight hours after being launched into space. This is done by firing the on-board engines of the spacecraft and syncing up in close proximity with the space station, leading to docking.