There is more to controlling the descent than the parachute itself.
Actuation usually means the ignition of a mortar charge which launches a folded pilot chute through a break-off patch in the aft cover, such that the chute can inflate well clear of the turbulent wake of the probe. The pilot chute serves to stabilize the probe at transonic speeds. It may also act to pull off the back cover and/or the main chute.
Actuation of the parachute may be triggered by the deceleration profile - since the overriding concern is to ensure deployment in a dynamic pressure regime that will ensure safe inflation and/or to avoid transonic Mach numbers at which the entry configuration may be unstable. Usually (following extensive modelling) the deployment is triggered at a time after some downgoing deceleration threshold has been crossed, measured by g-switches or accelerometers. In other circumstances, altitude as determined by a radar altimeter or barometer (or even time) may be an appropriate trigger, e.g. for staging a parachute.
The riser may incorporate a swivel, in order to decouple the spin of the parachute from the probe itself. This swivel (which may pose lubrication challenges) is usually necessary in order to permit a controlled rotation of the probe via spin vanes for scientific reasons (e.g. to pan instruments). Another feature may be 'reefing' the chute, whereby the opening of the canopy is initially restricted by additional lines to reduce the total drag of the parachute until some later time. This allows, for example, for a more rapid initial descent to minimize wind drift, or to reduce the initial deceleration loads. Reefing, or staging of one parachute to another, may require various pyrotechnic systems.
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