Reentry: The 25 Minutes of Plasma, Deceleration, and Fire That Bring a Spacecraft Home

Orbital velocity is approximately 7.8 kilometres per second. That number is not a curiosity, it is the engineering problem every reentry system exists to solve. A capsule returning from low Earth orbit carries kinetic energy equivalent to several tonnes of TNT. Physics demands that energy go somewhere. Reentry is the controlled argument between a spacecraft and the atmosphere about where.
The atmosphere does not compress politely out of the way. At those speeds, air molecules cannot move fast enough to clear the path, so they pile up in front of the vehicle and compress. Compression heats the gas, not friction, as the popular explanation goes, but compression. The air ahead of the heatshield reaches temperatures between 1,600 and 3,000 degrees Celsius depending on entry angle and vehicle mass. The heatshield's job is to absorb that heat and shed it as the ablative outer layer burns away in a controlled, deliberate sacrifice. On India's Gaganyaan capsule, ISRO has developed an indigenous ablative heatshield using a carbon-phenolic composite, tested across multiple unmanned missions before the crewed flight.
Between roughly 80 and 35 kilometres altitude, the ionised plasma sheath surrounding the capsule blocks all radio communication. Mission controllers lose contact. The crew cannot be reached and cannot reach anyone. This blackout typically lasts between four and eight minutes depending on entry trajectory. Apollo missions experienced it. Soyuz missions experience it. Gaganyaan will too.
During blackout, the capsule is not passive. Onboard computers fire reaction control thrusters to maintain the precise angle of attack, the angle at which the heatshield faces the oncoming airflow. Too steep, and deceleration forces exceed what a human body survives. The limit for trained crew in a reclined position is around 8 to 10g sustained. Too shallow, and the capsule skips off the upper atmosphere like a stone on water, back into space with a damaged heatshield and no second chance. The entry corridor for Gaganyaan is approximately 2 degrees wide. Two degrees.

Peak deceleration during a standard capsule reentry runs between 3g and 6g. Astronauts describe the sensation as a slow, building weight, not a jolt. The chest compresses. Breathing requires conscious effort. At 4g, lifting an arm demands the same muscular work as lifting four times its weight. Rakesh Sharma, who returned from the Salyut 7 space station in 1984 aboard a Soyuz capsule, described the transition from weightlessness to reentry deceleration as the body suddenly remembering it had bones.
The plasma phase and peak heating overlap with peak deceleration. The capsule is simultaneously at its hottest and pulling the hardest g-load. This is the designed crisis point, everything the heatshield and the structural frame were built to survive simultaneously.

Below 10 kilometres, the drogue parachutes deploy first. Small, high-drag chutes that stabilise the tumbling capsule and slow it enough for the main canopy system to open without tearing. The main parachutes, three of them on Gaganyaan, each roughly 36 metres in diameter, reduce terminal velocity from around 200 metres per second to approximately 8 metres per second. That is still faster than a sprinter. Retrorockets fire in the final seconds before splashdown, cutting velocity to under 2 metres per second at water contact.
Gaganyaan will splash down in the Arabian Sea, within recovery range of Indian Navy vessels positioned in advance. The recovery operation is its own precision exercise: the capsule must be located within minutes, the hatch opened in a specific sequence to prevent pressure differential injury, and the crew, whose vestibular systems are recalibrating from weeks of microgravity, helped out carefully. Astronauts who have just survived reentry often cannot walk unaided for hours.

The ablative heatshield is the least glamorous and most critical component of any atmospheric reentry system. It works by pyrolysis: the outer material chars, and that char layer insulates the layers beneath it. The charred material is then carried away by the airflow, a process called ablation, taking the heat with it. The heatshield does not reflect the heat. It absorbs it and leaves.
ISRO's Vikram Sarabhai Space Centre developed the carbon-phenolic ablative material for Gaganyaan through a series of ground tests at simulated reentry temperatures. The thickness of the heatshield is calculated to ensure enough material remains at the end of the plasma phase to protect the crew compartment. If the calculation is wrong by a meaningful margin, there is no redundancy. A heatshield has no backup.
The 25 minutes from first atmospheric contact to splashdown compress every physical extreme a spacecraft will encounter into a single continuous sequence. The plasma that makes communication impossible is the same phenomenon that decelerates the vehicle. The heatshield that burns away is the reason the crew does not. Each phase is the consequence of the one before it, which is why the entry corridor is 2 degrees wide, why the ablative material is calculated to the centimetre, and why Gaganyaan's recovery ships are pre-positioned before the capsule even begins its descent burn.