After successfully bringing together two satellites in space, scientists at the Indian Space Research Organisation (ISRO) are drawing up plans for a second SpaDeX mission — this time to dock two satellites in an elliptical orbit.
The ability to join two satellites in space, known as docking, is critical for India’s upcoming Chandrayaan-4 mission and the proposed Bharatiya Antariksh Station, the country’s planned space station.
In the first SpaDeX mission, two 220-kg satellites were launched into a 470-km circular orbit. With a small relative velocity introduced between them, the satellites were allowed to drift apart before being progressively brought closer. On January 16, they successfully docked. The experiment also demonstrated power sharing between the satellites and their ability to receive commands as a single composite unit.
With this, India became only the fourth country, after the US, Russia, and China, to demonstrate in-space docking capabilities. However, the feat was carried out in a relatively easier circular orbit.
“Docking in a circular orbit is much easier than docking in an elliptical orbit. This is because the trajectory and velocity of the satellites remain constant in a circular orbit, whereas they keep changing on an elliptical orbit. What this essentially means is that calculations done for one point will not be relevant after a few minutes,” said a scientist in the know of the matter. “This is, however, what the SpaDeX 2 experiment will attempt.”
This capability is likely to play a vital role in future missions, such as Chandrayaan-4, where multiple modules may be launched separately, and docking and undocking will be required in both Earth and lunar orbits.
For moon missions, ISRO typically launches spacecraft into an elliptical Earth orbit, gradually raising the apogee (farthest point) through engine burns at perigee (closest point) to use minimal fuel. This process sets up a slingshot trajectory toward the Moon, making docking in elliptical orbits a practical requirement for complex missions.
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The first docking operation took a considerable amount of time, as the agency approached it with extreme caution.
“This was the first time ISRO was attempting docking and undocking, so everything had to be meticulously planned and tested. In fact, several of the sensors being used were developed for this mission itself and had to be calibrated to readings in space. Once that was done, the readings were used to conduct several simulations on Earth before the actual docking was attempted. And, even then, the satellites were brought closer very slowly. With all the knowledge gathered during the first docking, the second became easier. It was quicker, without the satellites needing to stop and go as many times as the first time,” the scientist said.
During the initial attempt, the SpaDeX satellites were brought progressively closer, halting at designated checkpoints – 5 km, 1.5 km, 500 m, 225 m, 15 m, and 3 m – before finally docking. In the second attempt, post-separation, the process was smoother and faster, with fewer halts en route to redocking.