India demonstrates space dockingby Ajey Lele
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| ISRO had been working on the concept of docking for many decades. However, it appears that the process of development was accelerated around 2016. |
ISRO appeared confident about the success of the mission and had originally planned a live broadcast of the docking experiment. However, they were required to make some last-minute adjustments to their plans for the execution of the experiment. ISRO maintained a highly professional approach, taking the necessary time and care without hurrying. There was a clear understanding that a collision between the satellites, instead of a successful docking, could lead to a significant risk of space debris. With this in mind, ISRO took extensive precautions, exercised patience, and ultimately achieved a successful outcome. It has been explained by the officials that the docking process was initiated with precision, resulting into successful spacecraft capture.
NASA defines docking as “mating operations where an active vehicle flies into the mating interface under its own power.” ISRO had done very systematic planning for the SpaDeX mission. Both satellites were into orbit at an altitude of 470 kilometers. The first step in this experiment was to ensure that as an initial condition, the satellites would stay apart at 20 kilometers. Gradually, from 20 kilometers, the inter-satellite distance was lowered to 5 kilometers, 1.5 kilometers, 500 meters, 225 meters, 15 meters, and 3 meters before finally the docking happened.
Initially, ISRO had announced that the docking would take place on January 7. However, the attempt was postponed. It was stated that the docking process required further validation through ground simulations based on an identified abort scenario, and as a result, the experimentation was rescheduled for January 9. Yet, ISRO was again required to postpone the process after discovering that during the maneuver to bring the satellites within 225 meters of each other, the drift was greater than expected.
There were indications that docking could occur on January 12, as ISRO had reduced the distance between the satellites to just three meters. However, ISRO exercised caution and refrained from attempting the docking immediately. It seems that ISRO took the opportunity to gather essential data while the satellites were three meters apart before increasing the inter-satellite distance once again. Throughout this period, ISRO dedicated considerable time to continuous data collection, assessment, and validation. Their approach appeared to be methodical, taking one step at a time to ensure that no potential glitches remained.
There are various types of docking mechanisms, and for this mission, ISRO employed an androgynous mechanism. In this system, the components on both the Chaser and Target satellites are identical. It is similar to the International Docking System Standard (IDSS), with the main difference being that IDSS uses 24 motors, while ISRO utilized only two motors. This is because ISRO’s mechanism is smaller (450 millimeters) with one degree of freedom for extension as compared to the IDSS (800 millimeters) on a hexapod. The process involved the Chaser satellite approaching the Target satellite. As the extended rings on both satellites made contact, they joined together, and eventually, the rings were retracted and locked in place. After the docking, ISRO also succeeded in controlling the joined satellites as a single unit. The mission will continue in the coming days with various planned maneuvers.
Now, the satellites are expected to share electrical power and function as a single unit for some time. According to the project design, ISRO also plans to perform an undocking, after which the satellites to drift apart and operate as independent systems once again. The expected lifetime of the satellites is around two years, and if all goes well, there could be additional docking and undocking attempts in the future.
According to the details provided by ISRO, SDX01, the Chaser, carries a High-Resolution Camera (HRC) with photo and video capabilities, while SDX02, the Target, carries a Multi-Spectral Payload (MMX) used for natural resource monitoring and vegetation studies. It also carries a Radiation Monitor, which will measure radiation doses encountered in space, which will help suitably plan and execute India’s proposed human mission Gaganyaan mission (expected to happen by 2026). In short, it could be said that one satellite is a scientific satellite and the other is a remote sensing satellite.
ISRO had been working on the concept of docking for many decades. However, it appears that the process of development was accelerated around 2016. There may have been a realization it is important to master this technology in view of some proposed projects like the establishment of a space station and crewed space missions. The Space Docking Experiment was approved by the government of India in 2017
ISRO is expected launch its next mission to the Moon, Chandrayaan-4, by 2027. Mastering the art of space docking is very crucial for this sample return mission. ISRO is planning to launch their fourth mission to Moon in two phases. According to the available information, the spacecraft will have five modules packed into two vehicles, which will be launched separately and the final spacecraft will get assembled into an integrated module by docking in Earth orbit before proceeding to the Moon.
| India needs to quickly master this robotic technology and progress toward human docking. |
Generally, docking processes are completed within a few hours after the spacecraft reaches the desired location. This is possible because spacecraft typically carry a significant amount of fuel, enabling them to fire their thrusters and maneuver to align with the target spacecraft. However, the SpaDeX mission involves both the chaser and target satellites weighing only 220 kilograms, limiting their fuel capacity. ISRO’s goal for the mission was to test indigenous docking technology, relying more on the natural drift of the two satellites and minimizing fuel usage.
For undertaking a space docking mission, ISRO followed the indigenous technologies development route and, along with the docking mechanism and a suite of rendezvous and docking sensors, it also had to develop various other technologies. That included inter-satellite communication links (ISL) for autonomous communication between spacecraft, a GNSS-based Novel Relative Orbit Determination and Propagation (RODP) processor to determine the relative position and velocity of the other spacecraft based on navigation satellite signals, and simulation testbeds for both hardware and software design validation and testing. ISRO patented its docking mechanism.
Space docking is crucial for various missions, such as on-orbit servicing, satellite refueling, debris removal, extravehicular activities, delivering supplies to space platforms, and deep space missions such as sample return missions. India recognizes that it is a latecomer in this field. The US demonstrated docking capability in 1966, the Soviets in 1967, and China in 2011. India needs to quickly master this robotic technology and progress toward human docking. With plans to undertake its human space mission by 2026, India is expected to include human space docking in subsequent missions.
There could be some strategic advantages, too, in possessing space docking technologies. India is also the fourth country to have an ASAT capability. While India identifies that concepts scuh as “space weaponization” and “space wars” have no place in the modern world, it understands the necessity of maintaining a comprehensive space deterrence architecture because of its unique security challenges. Technologies related to orbital rendezvous, docking, proximity operations, formation flying of spacecraft, and on-orbit satellite servicing can be considered dual-use technologies.
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