With the successful docking of the satellites of the SpaDEX mission, India became the fourth nation to ace the space docking technology, the Indian Space Research Organisation (ISRO) said on Thursday.

ISRO informed the merging of two small spacecraft -- SDX01, the Chaser, and SDX02, the Target -- weighing about 220 kg each.

The satellites were part of the Space Docking Experiment (SpaDeX) mission, which lifted off aboard the PSLV-C60 rocket, from Sriharikota on December 30.

India is now the fourth country, after the US, Russia, and China, to master the docking technology. “Docking Success Spacecraft docking successfully completed! A historic moment,” ISRO shared in a post on social media platform X.

“Docking successfully completed. India became the 4th country to achieve successful Space Docking. Congratulations to the entire team! Congratulations to India!” it added.

Dr. V. Narayanan, Secretary DOS, Chairman of Space Commission, and Chairman ISRO, congratulated the team ISRO.

On Sunday, the two satellites came as close as three metres in a trial attempt for space docking up to 15 m. It later moved back to a safe distance.

But now, “manoeuvre from 15m to 3m hold point completed,” ISRO said while noting that the docking was achieved “with precision, leading to successful spacecraft capture”.

“Retraction completed smoothly, followed by rigidisation for stability”.

The docking technology was indigenously developed and has been named the ‘Bharatiya Docking System’.

It includes a docking mechanism, a suite of four rendezvous and docking sensors, power transfer technology, indigenous novel autonomous rendezvous and docking strategy, and an inter-satellite communication link (ISL) for autonomous communication between spacecraft, incorporated with inbuilt intelligence to know the states of the other spacecraft, among others.

ISRO believes the SpaDeX mission will help establish India's capability in orbital docking -- a key technology for future human spaceflight and satellite servicing missions.

In addition to joining the elite club of spacefaring nations, docking technology is also key for India's impending space missions including the Moon mission, setting up the Indian space station, and lunar missions like Chandrayaan-4 without the support of GNSS from Earth.

According to ISRO, it will also demonstrate the transfer of electric power between the docked spacecraft, which is essential for future applications such as in-space robotics -- composite spacecraft control, and payload operations after undocking.