The Indian Space agency has commenced the 25-hour countdown to the 22:00hrs, 30th December, Monday, launch of the PSLV-C60 mission that will be carrying the SPADEX (Space Docking Experiment) satellites to orbit. The countdown timer began ticking at 21:00hrs on Sunday, WION has learnt. Countdown is a critical pre-launch phase when comprehensive health checks of all rocket and satellite systems are carried out. Fuel filling in the liquid-propelled stages of the launch vehicle is also carried out during the countdown phase. In this case, the PSLV rocket's second and fourth (final) stages are liquid-propelled ones. 

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What is SPADEX? 

SPADEX stands for Space Docking Experiment, and it is a crucial technological feat 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-3 Lunar sample return mission will also involve highly sophisticated space docking. So far, only the US, Russia, Europe, and China have mastered space docking technology. 

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Applications of Space docking 

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 that carries them has to dock with the space station, following which the crew can move to the orbiting lab-cum-accommodation. Cargo is also delivered to the space station in a similar fashion. Likewise, docking can enable the transfer of fuel between two spacecrafts, thereby extending the life of the recipient craft. 

How will ISRO execute SPADEX?

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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 220 kg and will be hurled into a circular orbit that is 470 km above the earth's surface. 

At the time of ejection, the final stage of the PSLV rocket will be utilised to give a small relative velocity between the Target and Chaser spacecraft at the time of separation from the launch vehicle. Simply put, the target satellite will be travelling slightly faster than the chaser. This incremental velocity will allow the Target spacecraft to build a distance of 10-20 km with the Chaser.

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Gradually, the satellites will autonomously reduce the distance between themselves by adjusting their respective velocities. From 20 km, the inter-satellite distance would be lowered to 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m, ultimately leading to the docking of the two spacecraft. After successful docking and unification, electrical power transfer between the two satellites will be demonstrated. Electrical power transfer is crucial for carrying out in-space robotics, control of the unified spacecraft, etc. "Docking will be carried out between 6th January and 10th January," ISRO Chairman Dr. S.Somanath told WION. 

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.

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ISRO Chairman Dr. S. Somanath told WION that the satellites could attempt more docking and undocking, depending on the mission requirements and the availability of fuel on both crafts. As small satellites weighing around 220 kg each, the SPADEX satellites carry very limited fuel when compared to their larger counterparts. 

Payloads being carried by SPADEX satellites

SDX01, the Chaser, carries a High-Resolution Camera (HRC) with photo and video capabilities. This is a miniature version of the surveillance camera developed by ISRO's Space Applications Centre (SAC).

SDX02, the Target carries a Multi-Spectral Payload (MMX) used for natural resource monitoring and vegetation studies. It also carries a Radiation Monitor, which will measure radiation doses encountered in space, which will help suitably plan and execute India's homegrown Gaganyaan astronaut missions. 

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