Step Eight: Lander Separates
Ten seconds after the heat shield jettisons, the lander separates from the back shell and descends to the end of a "bridle", or tether. Spirit's altitude is about 20,000 feet at this point. The lander "rappels" down a Zylon tape on a centrifugal braking system built into one of the lander petals. The slow descent down the Zylon tape places the lander in position at the end of another bridle, which is made of a nearly 20-meter-long (65-foot-long) braided Zylon, an advanced fiber material similar to Kevlar. The Zylon bridle provides space for airbag deployment, distance from the solid rocket motor exhaust stream, and increased stability. The bridle incorporates an electrical harness that allows the firing of the solid rockets from the backshell, as well as provides data from the backshell inertial measurement unit (which measures rate and tilt of the spacecraft) to the flight computer in the rover.

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