India’s space docking — a ‘fourth country’ feat for the fourth time 

Because it is very difficult to achieve. Imagine two objects moving in space at a speed of 8 km per second (Mumbai to Delhi in 3 minutes), separated by some distance (20 km in this case), and getting the one behind chase and latching on to the one ahead. It calls for extremely deft maneuvering, by remote control. 

How was it done? 

When the two spacecrafts – SDX01 and SDX02 – were injected into their orbits by the PSLV-C60 rocket, they were 20 km apart. Bringing them to an inter-satellite distance of 5 km was done by the usual, ground-based (orbit maintenance and attitude control) methods. Once they were 5 km apart, a suite of (four) sensors came into play.

The four sensors are built for some degree of overlapping operations. The first is the ‘laser range finder’, fixed on the chaser, which emits a laser beam when the chaser is 6 km from the target. Reflectors on the target (corner cube retro reflectors) reflect the beam right back to the LRF, regardless of the angle at which the beam hits them (angle of incidence). From the time taken for the beam’s return, the LRF calculates the distance between the two spacecrafts.  

When the two spacecrafts are about 2000 meters close, the ‘rendezvous sensor’ (RS) fires up. It works like the LRF, but its purpose is position determination. At this stage, both LRF and RS work complementarily, readying the space for the docking. When the two objects are 30 meters apart, the ‘proximity and docking sensor’ comes into action. Finally, when they are 40 centimeters apart, the ‘mechanism entry sensor’ gets to work. The chaser docks into the target, somewhat like a pen easing into the pen cap. 

• Also read: ISRO’s SpaDeX Historic moment: India becomes 4th country to achieve successful space docking

What is the point of docking? 

Learning space docking opens up a huge space, literally and figuratively, for the learner. When you learn docking there are many, many things you can do.  

For instance, you can refuel a satellite in space, thereby extending the life of the satellite and not have to send another one up to replace it. Satellites do not need fuel to orbit—they do that with gravity. However, an object in space is subject to many pulls are pressures, again due to gravity, and must be maneuvered back to its path, which is done by firing onboard engines. These engines need fuel—the quantity of fuel determines the life of the satellite, because when the fuel is exhausted, the satellite cannot be nudged back to its orbit—it drifts away. In-space refueling is therefore a life-extender.  

Secondly, India has ambitions to have its own space station—the Bharatiya Antariksh Station—by around 2035. This is not possible without docking, because shuttles will need to ferry astronaut-scientists and supplies from the earth to the station. 

Machines can be integrated in space and docking is core to the integration. For example, you can build a big solar plant in space that can keep receiving sunlight all the time and beam back the electricity in the form of microwaves.

Yet another use is clearing space debris. If satellites are built to accommodate docking, a satellite could latch-on to a dead one and bring it back to the earth’s atmosphere and burnt away . 

Learning docking is as important as learning to send rockets to space. 

When was docking done by any country first? 

The credit for the first docking goes to the US. This was achieved this back in March 1966, by none other than the first man on the moon, Neil Armstrong, who, along with David Scott successfully docked the Gemini 8 spacecraft with an uncrewed ‘target vehicle’ called Agena. Without docking, moon landing would not have been possible, because the astronauts would need to join back the mother spacecraft in orbit.   

Since then, Russia and China have also achieved docking many times. India, therefore, is the fourth country.

Why is this India’s ‘fourth fourth’ in ‘space’? 

This is the fourth occasion that India has turned out to be the fourth country to achieve something in space. On November 5, 2013, India became the fourth country to send a spacecraft to orbit Mars. On March 27, 2019, India successfully carried out an anti-satellite test (ASAT), shooting down a moving satellite (its own) -- a feat only three other countries had done so earlier. And on August 23, 2023, India famously became the fourth country to soft land a lander on the moon.