The ascent of a launch vehicle is a battle against gravity to gain the immense velocity required to stay in orbit — while re-entry is a struggle against the atmosphere to shed that same energy in a systematic way.
Initially, aerospace scientists believed that surviving atmospheric re-entry would be impossible because the massive kinetic energy of an orbiting space capsule would be converted into intense heat energy upon re-entry. The resulting temperatures would be so extreme that they would melt any known structural material. The breakthrough came with the blunt body theory, which proved that if the space capsule’s forebody is rounded with a large radius, it can deflect most of the re-entry heat into the surrounding air rather than being directed into the capsule.
More than 98% energy of a re-entering capsule is dissipated through the atmosphere and converted into heat. The capsule is shielded from this intense thermal environment by its heatshield, which has a robust thermal protection system: it dissipates the heat through either ablation — where the material sacrificially chars and erodes to carry heat away — or thermal insulation, which uses low-conductivity materials to prevent the heat from reaching the capsule’s primary structure.
What is a re-entry corridor?
To return to earth, a space capsule must break its orbit by reducing its velocity. It does this by performing a deorbit burn: turning 180 degrees and firing its engines in the opposite direction of its travel. Since forward speed is what maintains the orbit, losing that speed allows gravity to overcome the capsule’s centrifugal force. The capsule then drops out of its stable circular path and enters a shallow, downward elliptical curve, leading it into the upper atmosphere for re-entry.
The re-entry corridor is a precise atmospheric window that a spacecraft must hit to return safely, balanced between two extremes. If the entry angle is too shallow (the overshoot boundary), the capsule will act like a stone skipping across a pond, bouncing off the atmosphere back into space. Conversely, if the angle is too steep (the undershoot boundary), the capsule will hit the dense air too hard, generating lethal deceleration forces and frictional heat that exceed what the crew and capsule can survive.
What is a semi-ballistic body?
A ballistic body behaves like a falling stone: it cannot steer by itself and is slowed only by the air resistance (drag). In contrast, a semi-ballistic body flies at a specific angle, known as the angle of attack. This is achieved by intentionally offsetting its centre of gravity laterally, causing the body to fly at an angle relative to the oncoming air.
As the vehicle slams into the atmosphere at hypersonic speeds, this angle forces the air to flow asymmetrically over the body, creating an aerodynamic lift force apart from the drag force, which acts perpendicular to the direction of velocity. This lift force is strategically manipulated to allow the capsule to glide and bank through the atmosphere, providing the cross-range capability necessary to steer it precisely toward a targeted landing zone.
What is a communication blackout?
Another issue during re-entry is the communication blackout. The extreme heat generated during re-entry strips electrons from air molecules, turning it into a layer of ionised plasma. This plasma sheath acts like a metallic bubble around the capsule that reflects and blocks radio waves. Because signals cannot pass through this electrified layer, it causes a communication blackout, leaving the crew and ground control unable to speak to each other until the capsule slows down enough for the plasma to vanish.
To manage the dreaded communication blackout during re-entry, engineers utilise a combination of orbital relay networks and high-frequency signal physics. By transmitting data upward to relay satellites (such as NASA’s TDRSS) rather than downward to ground stations, the signal passes through the thinner, less dense regions of the plasma sheath in the rear of the capsule.
Why are parachutes deployed for landing?
When a capsule re-enters the atmosphere, it is decelerated by aerobraking, which is the process of using atmospheric drag to slow down a capsule. As the air density increases at lower altitudes, the capsule’s speed drops until it approaches its terminal velocity — the point where the upward force of air resistance balances the downward pull of gravity. Any further reduction in velocity has to be achieved by deploying additional aerodynamic surfaces like parachutes. For a capsule returning from space, the terminal velocity in the lower atmosphere is still hundreds of kilometres per hour, far too fast for a safe landing. Practically, even before reaching the terminal velocity, the parachute has to be deployed to reduce the velocity further to make a soft landing in the sea.
How will the Gaganyaan crew module re-enter?
ISRO pioneered its re-entry capabilities with the 2007 Space Capsule Recovery Experiment (SRE), proving it could safely return an orbiting craft to earth. This was further advanced by the 2014 Crew Module Atmospheric Re-entry Experiment (CARE), which validated the full-scale thermal protection and parachute systems essential for surviving the extreme heat of a sub-orbital re-entry.
The Gaganyaan orbital module has two parts: the crew module (CM) and the service module (SM). The orbital module will be de-orbited by the thrusters in the SM and after that, the SM will separate and be destroyed by the intense heat of re-entry. Upon atmospheric re-entry, the CM maintains its trajectory within the re-entry corridor, strictly avoiding undershoot and overshoot boundaries. Operating as a semi-ballistic body, the CM executes controlled manoeuvres to reach its targeted landing site by modulating its lift vector through bi-propellant thruster firings.
Once the module reaches lower altitudes, a three-stage redundant parachute system is deployed to ensure a safe and smooth splashdown in the Bay of Bengal, which is the primary landing zone for the mission.
Unnikrishnan Nair S. is Former Director, VSSC and IIST; Founding Director, HSFC; and an expert in launch vehicle systems, orbital re-entry and human spaceflight technologies. He is currently the Dr Sarabhai Professor at VSSC.
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