Why ISRO’s ‘docking’ mission today is critical for India’s space ambitions

The SpaDeX (Space Docking Experiment) mission will launch at 10 pm, as ISRO’s workhorse PSLV rocket lifts off from the first launch pad of the Sriharikota spaceport. The fourth stage of the launch vehicle will be used later as a platform to carry out 24 experiments, including several by startups.

What is space docking, and why is the demonstration of this capability by ISRO a big deal?

Docking is a process in which two fast-moving spacecraft are manoeuvred into the same orbit, then brought closer together, and finally ‘docked’, or joined to each other. Docking is essential for missions that require heavy spacecraft and equipment that cannot be launched in one go.

The International Space Station (ISS) for example, comprises various modules that were launched separately and then brought together in space. The ISS is kept running as modules carrying astronauts and supplies from Earth periodically dock with it; these modules also bring the older crew on the station back to Earth.

The docking capability is critical to the fulfilment of India’s vision to have its own space station by 2035. The planned Bharatiya Antarisksha Station will have five modules that will be brought together in space, the first of which is scheduled for launch in 2028.

ISRO will also use this capability for its next lunar mission, during which it plans to bring back samples. Chandrayaan-4 will require two separate launches, and dockings in space.

First, a propulsion module will carry most mission components to lunar orbit, after which a lander-ascender module will separate from it and land on the surface of the Moon.

After collecting samples, the ascender module will carry them back to lunar orbit, where it will dock with the transfer module.

The transfer module will then bring the samples back to Earth orbit, where it will dock with a separately launched re-entry module designed to bear the heat of entering the Earth’s atmosphere.

And what will happen during the SpaDeX mission?

Two small, identical satellites, SDX01 and SDX02, each weighing around 220 kg, will be launched in a 470-km circular orbit. Once the satellites have been injected into the intended orbit, the launch vehicle will provide a small relative velocity between them, allowing the satellites to drift away from each other.

After a distance of 10-20 km has been built up between the satellites within a day, the propulsion system on SDX02, the “Target” satellite, will be fired to counter the relative velocity. As a result, the satellites will stop drifting farther — and will now move with the same velocity in the same orbit, with a distance of 20 km between themselves.

SDX01, the “Chaser” satellite, will then begin to gradually reduce the distance between the two satellites — to 5 km, 1.5 km, 500 m, 225 m, 15 m, 3m, and will finally join with the Target satellite. As the docking takes place, a video camera tilt mechanism will be used to monitor the process. Once the docking is complete, the satellites will transfer electrical power between themselves.

The small size of these satellites — compared to the ones that will be used for the lunar mission or to build a space station — makes the docking process more challenging, requiring greater precision.

Afterwards, the satellites will undock and move to separate orbits to carry on experiments for the next two years. The Chaser (SDX01) has a high-resolution camera on board, which is like a miniature version of a surveillance camera. The Target satellite (SDX02) will carry a multispectral payload that will be used for monitoring natural resources and vegetation, along with a radiation monitor to study space radiation and create a database.

What new technologies are being used in the mission?

PSLV C60/ SpaDeX will be a mission of several firsts.

* This is the first time that India’s trusted launch vehicle has been put together in the new PSLV integration facility and transported to the launch pad on a moving platform. Launch vehicles have thus far been assembled on the launch pad itself, which has meant a longer lead-in period between missions.

* Second, the mission will use 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.

The mission will also use a new processor based on satellite navigation systems to determine the relative position and velocity of the other 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.

* Third, several special tests were developed by ISRO for the mission, such as Docking Mechanism Performance Test (to test the final phase of docking), Vertical Docking Experiment Laboratory (for testing the docking mechanisms under controlled conditions), and Rendezvous Simulation Lab for validating the algorithms with real-time simulation.

* Fourth and most importantly, the fourth stage of the launch vehicle will carry several innovative experiments, including a biological experiment for the very first time.

What are these experiments that will be carried out in the fourth stage?

The fourth stage of the launch vehicle will be used as POEM — or PS4 Orbital Experiment Module — to demonstrate 24 technologies, including 10 technologies from start-ups and educational institutions.

For the first time, an ISRO mission will carry a biological experiment. The CROPS (Compact Research Module for Orbital Plant Studies) experiment will see the germination of seed and sustenance of a plant up to a two-leaf stage.

Other experiments include a Debris capture Robotic Arm that will use a visual feed and object motion prediction to capture debris, and another moveable robotic arm that may be used in servicing satellites in space in the future.

Amity University will carry out a study of plant cells in microgravity and in earth gravity, and RV College of Engineering will study gut bacteria to generate their growth curve in space. A couple of synthetic aperture radars, and a green propulsion system will also be experimented with.