Indian Space Agency ISRO's twin SpaDeX satellites which were 500 meters apart and closing in on Wednesday night, have now drifted apart and are at a 13-fold distance, and are approximately 6.5 km away, as per private firms tracking the mission. Sharing their tracking data from the morning hours of January 9, Thursday, Indian private firm Digantara said that they tracked the SpaDeX A and B satellites as being 6.5 km apart. Likewise, Switzerland-based s2a systems tracked the twin satellites as they were 6.7 km apart.

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Also Read: ISRO's SPADEX docking mission postponed for second time; satellites safe

While ISRO mentioned that the satellites' drift was "more than expected" and that they are safe, the distance between them has not been specified at the time of writing. 

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Both private firms that shared the SpaDeX distance data are specialists in providing Space Situational Awareness(SSA)- which includes understanding and monitoring of the space environment, the identification, tracking, and characterisation of objects in space, such as satellites, debris, and others. SSA is aimed at ensuring the safe, sustainable, and secure use of space.

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Two postponed docking attempts

The SpaDeX satellites were meant to complete autonomous docking in the morning hours of Jan 9, Thursday. However, on Wednesday night as one of the twin satellites initiated the drift to move closer from 500m to 225m, the drift between satellites was found to be more than expected, therefore leading to temporary postponement. 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.  

Also Read: SpaDeX: Docking date shifts to January 9 - A developing story


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.

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.

Initially the satellites were over 10kms 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, 500m. 

On Wednesday night, one of the twin satellites had initiated the drift to move closer from 500m to 225m, and that's when the drift between satellites was found to be more than expected, therefore leading to temporary postponement with no new docking date announced so far.

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

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 spacecraft dock with the International Space Station 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.