Why now? Nearly two months after ISRO successfully docked two satellites in space, it carried out an undocking procedure on Thursday. This makes India the fourth country — after the US, Russia, and China — to boast docking-undocking capabilities. But what is space docking? As the name suggests, space docking is essentially a process by which two fast-moving spacecraft are put into the same orbit, progressively brought closer, and finally joined together. It is an extremely complicated process. To demonstrate this capability, ISRO launched the experimental Spadex mission on December 30, 2024. This comprised launching two 220-kg satellites — SDX01 (Chaser) and SDX02 (Target) — in a 470 km circular orbit, with a small relative velocity between the two. From an initial distance of 20 km in space, these two were progressively brought together and then joined using their extended locking mechanism. For the mission, ISRO developed its own docking system on the lines of the one used at the International Space Station. Like the ISS system, ISRO’s is androgynous — meaning the systems on both the Chaser and Target satellites are identical. But it has only two motors, compared to the 24 used in the International Docking System Standard. After docking, ISRO scientists demonstrated power sharing capabilities between the two satellites, and were able to command it as a single composite structure. The undocking manoeuvre was carried out after two months following extensive on-ground simulations. It went off without a hitch. Why does this matter? This capability is essential for missions with heavy spacecraft that cannot be launched from Earth in one go. Weight is among the biggest limiting factors in what we can or cannot send to space. This is why things are often sent to space in parts, to be assembled while in space. This is how the ISS was built. The docking-undocking process is also essential to carry astronauts and supplies to a space station. With India setting its sights on extremely ambitious missions in the near future, including setting up its own space station by 2035 and sending a human to the Moon by 2040, the capability to rendezvous, dock, and undock will be critical. Chandrayaan-4, which is designed to bring back lunar soil and rock samples, will utilise this capability. When was the first space docking carried out? By whom? Amidst the Cold War era Space Race, the United States became the first country to demonstrate space docking capabilities. In 1966, NASA’s crewed mission Gemini VIII was manually docked with the target vehicle Agena. Interestingly, the spacecraft was commanded by astronaut Niel Armstrong, who would go on to become the first human to walk on the Moon in 1969. The first autonomous docking — where a spacecraft finds its target and attaches by itself without any astronauts or commands from the ground — was demonstrated by the erstwhile USSR, when it brought together Kosmos 186 and Kosmos 188 spacecraft in 1967. China, the only other country to have the capability, demonstrated unmanned docking in 2011, and a crewed docking capability a year later.
