The Indian Space Research Organization, or ISRO, on Thursday completed the unmanned docking of one satellite onto another, making India only the fourth country to achieve the feat successfully.

“Spacecraft docking successfully completed!” said ISRO in a social media post. “A historic moment.”

The United States, Russia and China are the only other countries to have completed unmanned docking, which is a critical milestone for future space exploration missions.

The Space Docking Experiment, or SpaDeX, mission consisting of two satellites – Target and Chaser – was launched from the Satish Dhawan Space Center in Andhra Pradesh on December 30, aboard an indigenously developed Polar Satellite Launch Vehicle rocket.

ISRO had missed two docking schedules, on January 7 and January 9, due to technical difficulties.

The space agency had previously planned to make the satellite docking a public event but switched to a “dock and inform” plan following the two postponements, The Indian Express reported quoting an unidentified senior official.

The postponements came amid a change of leadership at ISRO. On January 7, the Union government appointed V Narayanan as the agency’s new director. He assumed charge on January 14.

The successful mission on Thursday was a trial run. The first Indian mission requiring docking capability is likely to be the Chandrayaan-4, aimed at collecting lunar samples. The mission will include multiple docking and undocking exercises in both Earth and lunar orbits, The Indian Express reported.

The technology will also be important to achieve India’s goals of building a home-grown space station, Jitendra Singh, the minister of state (independent charge) for science and technology, had said in December.

Prime Minister Narendra Modi congratulated ISRO for the successful mission on Thursday, calling it a “significant stepping stone for India’s ambitious space missions in the years to come”.

Congress leader Rahul Gandhi also congratulated the space agency for the “historic success of SpaDeX”. “Your dedication and innovation continue to inspire the world and make every Indian proud,” he said.