There are some strange stories floating around about MER-A's Entry Descent and Landing (EDL). Most people around here know that the transverse impulse rocket system (TIRS) was used to remove a 20-30 m/sec horizontal drift. Apparently this horizontal drift velocity was right on the hairy edge of what MER could tolerate (the TIRS worked perfectly to remove this threat). However apparently even odder things happened. People are saying the MER aeroshell was having angle-of-attack excursions of greater than 20 degrees at Mach 6. This is ***totally weird***. MER should have been rock stable at Mach 6. Also MER-A deployed its main parachute about a kilometer too low. Something funky was going on with the Martian atmosphere resulting in the density being lower that expected (or some weird thermal lapse occurred). MER-A's EDL software worked perfectly. Supposably MER's autopilot saw that something was amiss and did the computer equilvalent of saying: "Screw this, I'm popping my parachute now!". The word is that MER-B has been reprogrammed to deploy its parachute when the dynamic pressure drops to 750 pascals rather than 725 pascals. MER doesn't actually measure dynamic pressure but rather decelleration through its IMU (Inerital Measurement Unit). This means the parachute will deploy at an even higher Mach number than before (supposably there remains a safety margin but it's still scary). MER-B will probably succeed. The success of MER-A despite multiple surprises has demonstrated that the basic design is sound. However I'm a bit spooked that there was something very basic going on with the aerodynamics that we do not understand. This new surprise will be keeping the CFD and ballistic range guys busy for the next couple of years.

Gary