The Indian space agency ISRO has postponed the 9th January, Thursday morning's planned docking of its twin SPADEX satellites, owing to a drift or growing distance between the two satellites that are meant to gradually get closer to each other and mechanically attach themselves, all fully autonomously. 

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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 before proceeding with the docking. 

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While ISRO has assured that the satellites are safe, it will take a short while before ground-based antennas will be able to "see and speak" to the satellites. Typically, satellites that are closer to earth (SPADEX satellites are 470 km above earth) are only visible for a couple of minutes from each ground-based station, as the satellites move at speeds of over 28,000 kmph.

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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) 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.

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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 connection without any impact forces. This requires careful coordination and manoeuvring, all done fully autonomously.

Initially, the satellites were 20 km apart from each other, and gradually, the satellites began to autonomously reduce the distance between themselves. 

From 20 km, the inter-satellite distance had been gradually lowered to 5 km, 1.5 km, 500 m. 

On Wednesday night, one of the twin satellites had initiated the drift to move closer from 500 m to 225 m, and that's when the drift between satellites was found to be more than expected, therefore leading to temporary postponement. 

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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 adds to the complexity of the mission. It is widely regarded that docking larger satellites with larger quantities of fuel is relatively simpler.