The Indian Space Research Organisation (ISRO) will close 2024 with a mission that marks a significant milestone towards achieving key future objectives, including bringing back a lunar sample, establishing a space station, and sending a human to the Moon.

On Monday, December 30, ISRO will showcase its groundbreaking capability to dock and merge two satellites in space for the first time.

The SpaDeX (Space Docking Experiment) mission is set to launch at 10 PM, with ISRO’s PSLV rocket lifting off from the first launch pad at the Sriharikota spaceport. The fourth stage of the launch vehicle will later be repurposed as a platform to conduct 24 experiments, including several from startups. 

What is space docking, and why is ISRO’s demonstration of this capability significant?

Docking is a process where two fast-moving spacecraft are maneuvered into the same orbit, brought closer together, and then joined, or “docked.” This technique is crucial for missions that involve large spacecraft or equipment that cannot be launched in a single trip.

For example, the International Space Station (ISS) is made up of various modules that were launched separately and then assembled in space. The ISS remains operational as modules carrying astronauts and supplies from Earth dock with it periodically. These modules also bring the older crew members back to Earth.

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The ability to dock in space is crucial to India’s goal of establishing its own space station by 2035. The planned Bharatiya Antarisksha Station will consist of five modules, which will be assembled in space, with the first module set to launch in 2028.

ISRO will also utilize this docking capability for its upcoming lunar mission, Chandrayaan-4, which aims to bring back lunar samples. The mission will involve two separate launches and dockings in space. First, a propulsion module will carry most of the mission’s components to lunar orbit. Then, a lander-ascender module will detach from the propulsion module and land on the Moon’s surface.

After collecting the lunar samples, the ascender module will transport them back to lunar orbit, where it will dock with the transfer module. The transfer module will then carry the samples to Earth orbit, where it will dock with a separately launched re-entry module, designed to withstand the intense heat of re-entering Earth’s atmosphere.

What to expect during the SpaDeX mission?

Two identical satellites, SDX01 and SDX02, each weighing around 220 kg, will be launched into a 470-km circular orbit. Once in orbit, the launch vehicle will create a slight relative velocity between the satellites, causing them to drift apart.

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After a day, the distance between the satellites will increase to 10-20 km. At this point, the propulsion system on SDX02, the “Target” satellite, will be activated to counter the relative velocity. This will halt further drifting, and the satellites will start moving together at the same velocity in the same orbit, maintaining a 20 km separation.

The “Chaser” satellite, SDX01, will gradually reduce the distance between itself and the “Target” satellite, SDX02, moving from 5 km to 1.5 km, then 500 m, 225 m, 15 m, and finally 3 m, before docking. A video camera tilt mechanism will monitor the docking process. Once the two satellites are docked, they will transfer electrical power between each other.

Due to their small size, especially in comparison to satellites used for lunar missions or space station construction, this docking process will be more complex and require greater precision.

After the docking, the satellites will undock and move to separate orbits to continue their experiments for the next two years. The Chaser satellite (SDX01) is equipped with a high-resolution camera, similar to a miniature surveillance camera. The Target satellite (SDX02) carries a multispectral payload for monitoring natural resources and vegetation, as well as a radiation monitor to study space radiation and compile a database.

What are the new technologies being used in this mission?

PSLV C60/SpaDeX will be a mission featuring several milestones.

  • For the first time, India’s reliable launch vehicle has been assembled in the new PSLV integration facility and transported to the launch pad on a moving platform. Previously, launch vehicles were assembled directly on the launch pad, which resulted in longer lead times between missions.
  • The mission will also utilize several new sensors, including a Laser Range Finder, Rendezvous Sensor, and Proximity and Docking Sensor, to take precise measurements while bringing the two satellites closer and docking them.

Additionally, a new processor based on satellite navigation systems will be used to determine the relative position and velocity of the spacecraft. This marks a step toward creating a fully autonomous system for future missions that can achieve docking without relying on satellite-based navigation data.

  • ISRO has developed several specialized tests for this mission, including the Docking Mechanism Performance Test (to assess the final phase of docking), the Vertical Docking Experiment Laboratory (to test docking mechanisms in controlled conditions), and the Rendezvous Simulation Lab (to validate algorithms through real-time simulations).
  • Fourth, and most notably, the fourth stage of the launch vehicle will host several innovative experiments, including a biological experiment for the first time.

What experiments will be conducted in the fourth stage of the mission?

The fourth stage of the launch vehicle will function as POEM (PS4 Orbital Experiment Module) to showcase 24 technologies, including 10 from startups and educational institutions.

This will be the first time an ISRO mission carries a biological experiment.The CROPS (Compact Research Module for Orbital Plant Studies) experiment will involve the germination of seeds and the growth of a plant up to the two-leaf stage.

Other experiments involve a Robotic Arm for capturing space debris, which will rely on visual feeds and motion prediction to track and capture debris, as well as a mobile robotic arm that may be used for satellite servicing in the future.

Amity University will conduct a study on plant cells in both microgravity and Earth gravity, while RV College of Engineering will research gut bacteria to develop their growth curve in space. Additionally, experiments will be carried out on a couple of synthetic aperture radars and a green propulsion system.