Apparently the transverse velocity for MER was right on the hairy edge (20 meters/sec). Somebody at JPL had the last minute bright idea of including a transverse impulse rocket system (TIRS) into MER that reduced the transverse velocity.

See: http://marsrovers.jpl.nasa.gov/mission/spacecraft_edl_tirs.html .

There was no TIRS in Pathfinder and it worked. However 20 meters/sec is pretty fast. The downward descent velocity on MER's parachute is about 50 meters/sec and that's lethal. Who ever insisted upon including the TIRS with MER deserves a nice cash bonus and a pretty NASA certificate. There were a lot of things wrong with Beagle 2's design. However it's quite possible the transverse velocity is what killed Beagle 2.

One of the people who spoke at the press interview expressed surprise that there was no radio blackout for MER during its hypersonic descent. For an EDL guy this was no big surprise. Viking had no black out and its entry velocity was 4.6 km/sec from Mars orbit. Viking's peak boundary layer edge temperature at the stagnation point was 2,900 deg.K. Mars Pathfinder did have radio black out and its entry velocity was 7.26 km/sec (remember that kinetic energy is velocity squared). Pathfinder's peak boundary layer edge temperature at the stag. point was 6,000 deg.K. MER-A's entry velocity was designed as 5.6 km/sec. Its peak boundary layer edge temp. at the stag. point was 3200 deg.K with a stagnation pressure of 5.95e+03 pascals. The free electron equilibrium mole fraction at peak heating for MER-A was 1.3305e-7 . However for Mars Pathfinder the free electron equilibrium mole fraction at peak heating was 3.3812e-2 . Five orders of magnitude difference in free electron mole fraction is the difference between having and not having radio blackout.

Gary