How ISRO raced against time to execute space docking

In the wee hours of January 16, Indian Space Research Organisation (ISRO) made history by successfully docking its two small satellites, SDX01 (Chaser) and SDX02 (Target), each weighing about 220 kg. This remarkable feat, achieved after overcoming nerve-racking hiccups three times in the past two weeks, opens doors for future human space flights, the establishment of an Indian space station, a sample return mission to the moon and a landing and rover mission to Mars.

Consider circus acrobats swinging high, gaining speed and then leaping fearlessly into the air, flying in a perfect arc while briefly extending their arms. The companion, perched high on a platform, leans forward, arms spread like a lifeline. In a wonderful moment of harmony, their hands firmly grip, sealing the daring tie. The task of space docking is no less bold and brave.

The two crafts have to rendezvous at a specific place at a particular time. Each craft has a circular portal (a doorway) surrounded by a ring-like handrail. Like the groves on the lid and jar, the latches on the portal of one craft must align with the partner’s portal clasp. Once the two crafts are close, a protruding ring from the Chaser craft enters the Target craft’s portal. The latches extend and securely buckle the partner craft in a tight clutch.

Unlike a video game with controllers in your hands, the rendezvous and docking of the two vessels must be done autonomously by the onboard computers employing artificial intelligence algorithms. Each ship is equipped with range finder sensors and cameras, which serve as the eyes and ears for the algorithm for the onboard computers, enabling it to manoeuvre the crafts gently to come closer while aligning their portals.

One craft, designated as the Target, remains relatively passive while the Chaser approaches and wiggles to align the portals and latch mechanism.

As the indigenously created algorithm was being tested for the first time, ISRO proceeded cautiously. In the first attempt on January 11, the two crafts approached with an inter-satellite distance of 230 metres. The docking process was intentionally arrested to evaluate the performance of the sensors. On the next day, a trial attempt was made to reduce the inter-satellite distance to 15 metres and then just three metres. Hoping to succeed, the attempt to dock was scheduled for January 13.

Ostensibly, there was a problem receiving data from a vital proximity and docking sensor, which tells the onboard computer how distant the partner ship is and what direction it must take to safely dock. This caused the algorithm to switch to the safe mode and abort the docking procedure. Even a minor directional deviation would have resulted in a collision rather than a docking. Due to this malfunction, the docking was halted at the last minute, and the spacecraft was moved to a safe distance.

Given the angle at which the vehicles were orbiting the earth, docking had to be completed before January 22, or else ISRO would have to wait till March. After January 22, the earth would have obscured the spacecraft for most of the period, decreasing power production for the onboard equipment.

Hard-pressed for time, ISRO reportedly reversed the roles of the two ships — the Chaser with defective sensors became the passive target, while the previous Target became the Chaser, and the docking was successful.

Successful docking was critical for validating ISRO’s indigenously built low-impact docking mechanism, a suite of sensors and rendezvous & docking algorithms. However, there is more to it than technical demonstrations.

Until now, ISRO has been like a goods transporter, building satellites, taking them to space and parking them in orbit. A container lorry needs no emergency escape door; however, if you are operating a bus carrying passengers, it is a must. ISRO has ambitions for human space flight, courtesy the Gaganyaan mission, and a docking mechanism is imperative for human-rated spacecraft.

Also, ISRO is planning to establish a space station akin to the International Space Station, enabling a few crew members to remain in space for weeks and months to carry out research. The docking mechanism is needed to transfer the crew from the craft to the station and back. Space stations are constructed by docking multiple uncrewed human-rated spacecraft in space.

Most importantly, ISRO has a radical jugaad to undertake interplanetary missions. Falcon Heavy, the American heavy-lift launch vehicle, can transport a payload of 63.8 tonnes in Low Earth Orbit (LEO), and the Chinese Long March 5-CZ-5B variant can place 25 tonnes in LEO. Even the mighty GSLV (Geosynchronous Satellite Launch Vehicle), known as ISRO’s Bahubali, can place only a measly eight tonnes in LEO.

However, ISRO has not been hampered in the past. It employed a frugal technique called ‘gravity assist’ to reach Mars with just a PSLV (Polar Satellite Launch Vehicle), which can only deposit four tonnes in LEO. In this manner, ISRO proposes to make its way to the moon and Mars even without heavy-lift vehicles.

Consider the Chandrayaan-4 scenario: Two launch vehicles will send two vessels into orbit, each carrying two modules. The spacecraft will dock and combine to form a single craft with four modules. Module 1 will serve as the propulsion module, guiding the vessel to the moon.

Once in the lunar orbit, Module 1 will be undocked and jettisoned. Modules 2 and 3 will undock and make a lunar landing, while Module 4 will circle the moon like an orbiter. The rover from Module 2 will go across the surface, collecting lunar soil and rock samples and depositing them in Module 3. Module 3 will lift off from the moon’s surface like a rocket and dock with Module 4, circling the moon. The samples will be transported to Module 4, returning to earth. Thus, docking and undocking are critical for the mission plan. Likewise, ISRO is designing a mission to Mars with multiple launches that will place components in space, where they will be assembled and taken to the Red Planet for a soft landing and rover mission.

Experts believe that space assembly will be the future of deep-space missions. It will be cost-effective and permit more ambitious missions. Perhaps ISRO will take the initial steps.