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In the lower atmosphere, a returning capsule can still reach hundreds of kilometres per hour, far too fast for a safe landing, so the parachute has to be deployed to reduce the velocity for a soft landing in the sea
| Photo Credit: PTI
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.
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.