What was done and how – and why does it matter?

ISRO also demonstrated giving commands to the two satellites as one composite object.

The successful docking makes India the fourth country in the world — after the United States, Russia, and China — to have this capability.

“Congratulations to our scientists at @isro and the entire space fraternity for the successful demonstration of space docking of satellites,” said Prime Minister Narendra Modi on X.

Congratulations to our scientists at @isro and the entire space fraternity for the successful demonstration of space docking of satellites. It is a significant stepping stone for India’s ambitious space missions in the years to come.

— Narendra Modi (@narendramodi) January 16, 2025

Union science minister Dr Jitendra Singh posted: “Spadex has accomplished the unbelievable … and it is all (using the) indigenous Bharatiya Docking System. This paves the way for smooth conduct of ambitious future missions including the Bharatiya Antriksha Station, Chandrayaan 4 and will be like that.”

What is “docking”, and why is this achievement important?

Docking is a process by which two fast-moving spacecraft are brought to the same orbit, brought closer to each other manually or autonomously, and finally joined together.

This capability is necessary for carrying out missions that require heavy spacecraft that a single launch vehicle may not be capable of lifting off with.

The capability is needed not only for setting up a space station — for which separate modules are joined in space — but also for carrying crew and supplies to it.

When was the first docking in space achieved?

UNITED STATES: As the United States and the erstwhile Soviet Union competed fiercely during the space race of the Cold War decades, it was essential to demonstrate rendezvous and docking to achieve the objective of sending humans to the Moon.

In 1966, NASA’s Gemini VIII became the first spacecraft to dock with the target vehicle Agena. Gemini VIII was a crewed mission orbiting the Earth, commanded by Neil Armstrong, who in 1969 became the first human to set foot on the Moon.

USSR: While the US mission had astronauts on board to steer the spacecraft, the Soviet Union in 1967 demonstrated the first uncrewed, automated docking of the Kosmos 186 and Kosmos 188 spacecraft.

CHINA: China demonstrated its docking capability in 2011, when the unmanned Shenzhou 8 spacecraft docked with the Tiangong 1 space laboratory.

A year later, China demonstrated its first crewed space docking, when the astronauts manually joined its Shenzhou 9 spacecraft to the same space laboratory.

Why has India carried out its docking mission at this juncture?

ISRO has been working on key technologies to realise its vision of setting up a space station by 2035 and sending humans to the Moon by 2040.

Besides a new heavy-lift launch vehicle capable of carrying up to 30 tonnes to low earth orbit, the missions would require docking capability. The Bharatiya Antariksh Station, the space station that India envisages, will be built by bringing together five modules in space. The first of these robotic modules is slated to be launched in 2028.

Docking capability will also be required for the next lunar mission — Chandrayaan-4 — which aims to bring back samples from the Moon. This mission will see five key modules being sent into orbit in two separate launches.

The first launch will have four of the five modules — the propulsion module will carry the spacecraft from Earth orbit to the Moon orbit, from where the lander and ascender modules will go to the lunar surface and collect the samples. The ascender module will then hop up with the samples, and dock with the transfer module in the lunar orbit.

This transfer module will carry back the samples to the Earth orbit where it will dock with a re-entry module that will be launched separately. The module will be designed to withstand the heat of entering the Earth’s atmosphere.

In preparation for this mission, ISRO carried out a “hop experiment” towards the end of the Chandrayaan-3 mission.

A human mission to the Moon is likely to follow a similar plan.

What happened during the docking experiment?

ISRO carried out a series of manoeuvres to progressively bring the SDX01 or “Chaser” satellite close to SDX02, or the “Target” satellite. The satellites were allowed to drift close, and then their positions were held at around 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m, before finally being joined together.

The space agency has demonstrated giving command to the satellites as a single composite object. In the coming days, it will demonstrate sharing of electrical power between the two satellites.

Once that is complete, ISRO will demonstrate “undocking”, during which the satellites will separate and drift away to carry out their respective experiments over the two years of the mission’s life.

The docking experiment was initially slated for January 7, but was postponed after identifying an abort scenario. More simulations were carried out on the ground to improve the accuracy of docking.

The docking could not take place on January 9 either, with the satellites drifting more than anticipated during a manoeuvre to reduce the distance to 225 metres on the previous day.

After this, the satellites drifted nearly 5 km apart, and manoeuvres to bring them closer were restarted. Early on January 12 morning, the satellites were brought closer together, and this time they reached the hold point of 3 metres before being moved away to a safe distance.

ISRO said that the data were being analysed before the actual docking could be conducted.

What is the Bharatiya Docking System?

Several types of docking mechanisms have been used by space agencies over the years, including some interoperability. The spacecraft that go to the International Space Station follow the International Docking System Standard (IDSS), which was first baselined in 2010.

The docking mechanism being used by India is androgynous — meaning the systems on both the Chaser and Target satellites are identical. This is similar to the IDSS used by other agencies, but uses two motors as compared to the 24 used in IDSS.

The mission used several new sensors such as Laser Range Finder, Rendezvous Sensor, and Proximity and Docking Sensor to take precise measurements while bringing the two satellites closer and joining them.

It also used a new processor based on satellite navigation systems to determine the relative position and velocity of the spacecraft. This is a precursor to a completely autonomous system for future missions that would be able to achieve docking without satellite-based navigation data.