Mars Exploration Rovers:
Transverse Impulse Rocket System (TIRS)
What is the Transverse Impulse Rocket System (TIRS)?
A rocket system called the transverse impulse rocket system (or TIRS, pronounced "tears") reduces the horizontal speed of the lander at touchdown. It consists of three small rockets mounted on the backshell. Using the backshell interial measurement unit (or IMU), when the rover's software detects that the backshell may be tilted too far off vertical, the software elects to fire one or two of these small TIRS rockets reduce the tilt effect. While not providing precision control, these little rockets can compensate for the swinging introduced by some of the worst winds predictable. Firing for a fraction of a second, these rockets provide just enough of a kick to the backshell to get it aligned with the vertical before the lander and rover are released some 15 m (49 ft) above the ground.