• ISRO's upcoming SpaDeX mission will demonstrate in-space docking, a key technology for future lunar missions.
• The mission will launch two small spacecraft, SDX01 and SDX02, which will dock in a low-Earth circular orbit.
• The mission will also demonstrate the transfer of electric power between docked spacecraft, essential for future applications like in-space robotics.
• This mission is a significant step in India's space exploration journey, paving the way for future missions like Chandrayaan-4.

The Indian Space Research Organisation (ISRO) is on the brink of a significant milestone in its space exploration journey with the upcoming launch of the SpaDeX mission. This mission, set to launch aboard the PSLV-C60 rocket, will propel India into the elite club of nations with space docking capabilities. The launch window is open from December 30 to January 13, and the mission has completed all tests and clearances. The SpaDeX spacecraft has been integrated and moved to the Satish Dhawan Space Centre (SDSC) in Sriharikota, Andhra Pradesh, where it is currently undergoing preparations for launch.

The SpaDeX mission will demonstrate in-space docking using two small spacecraft, a groundbreaking technology that is key to future lunar missions and the construction of the Bharatiya Antariksh Station (BAS). The PSLV will lift off two small spacecraft, SDX01, the Chaser, and SDX02, the Target, each weighing about 220 kg. The mission will showcase the docking of the two spacecraft in a low-Earth circular orbit.

In-Space Docking: A Key to Future Missions

In-space docking technology is essential when multiple rocket launches are required to achieve common mission objectives. Through this mission, India is marching towards becoming the fourth country in the world to have space docking technology. The mission will also demonstrate the transfer of electric power between the docked spacecraft, which is essential for future applications such as in-space robotics, composite spacecraft control, and payload operations after undocking.

ISRO has developed indigenous technologies to enable this docking mission. This includes a docking mechanism, a suite of four rendezvous and docking sensors, power transfer technology, indigenous novel autonomous rendezvous and docking strategy, inter-satellite communication link (ISL) for autonomous communication between spacecraft, incorporated with inbuilt intelligence to know the states of the other spacecraft, among others. This mission will be a forerunner for autonomous docking needed for future lunar missions like Chandrayaan-4 without the support of GNSS from Earth.

The PSLV's Precision and the International Space Station

The PSLV's precision will create a slight relative velocity between the Chaser and Target spacecraft at the time of separation from the launch vehicle. This incremental velocity will allow the Target spacecraft to gradually move to a 10-20 km distance from the Chaser over a day. Using its propulsion system, the Target will compensate for the relative velocity, ensuring both spacecraft end up in the same orbit with identical velocities but separated by about 20 km - a phase known as Far Rendezvous. Following this, the Chaser will approach the Target in stages: first reducing the separation to 5 km, then 1.5 km, 500 m, 225 m, 15 m, and finally 3 m, before achieving a successful docking.

The International Space Station (ISS) is a large spacecraft that must be able to move about in space, maintain communications with controllers on the ground, and needs power to accomplish all of this. Eight large solar arrays provide electrical power from the sun. Each array is 240 feet (73 meters) long and - cumulatively-speaking - they cover an area of around 27,000 square feet (2,500 square meters). The arrays generate primary power - approximately 84 to 120 kilowatts of electricity, enough to keep the lights on at over 40 homes. NASA reports that while the ISS absorbs sunlight, around 60 percent of the electricity produced in this process goes to recharging batteries aboard the station.