The 40-Meter Sensor Bug & Forensic Reconstruction
Investigating the December 3, 1999 descent cutoff over Planum Australe. Sourced from the NASA Casani Board Report.
Target Site
Planum Australe
-76.05° S, 164.7° E
Premature Descent Engine Shutdown Calculator
Adjust the engine cutoff altitude to calculate fall duration, terminal impact speed, and kinetic energy under Martian gravity (3.72 m/s²).
4.04 s
Free fall in Mars gravity
17.4 m/s
62.7 km/h (39.0 mph)
43,987 J
Chassis crush threshold: ~3,000 J
0%
Catastrophic structural failure
// Step 1: Leg deployment at 1,500m creates spring pulse
void on_leg_deployed() {
touchdown_flag = TRUE; ← LATCHED ON SPRING BOUNCE!
}
// Step 2: Terminal descent at 40m enables monitor
void terminal_descent_monitor() {
enable_touchdown_interrupt(); // Missing: reset_flag();
if (touchdown_flag == TRUE) {
command_engine_cutoff(); // Cut off at 40m!
}
}
Look deeper: mission background
Authentic NASA Review Board Context: Casani & Spear Report
In March 2000, NASA released the official Casani Board Report detailing the probable cause of the Mars Polar Lander loss. Testing revealed that landing gear leg deployment springs created magnetic sensor transients that set the touchdown bit in the computer at 1,500 meters altitude. When software began monitoring for touchdown at 40 meters altitude, it read the pre-existing TRUE bit and cut the engines in mid-air. The reforms resulting from this failure mandated the deep-space telemetry beacons and end-to-end hardware-in-the-loop testing that made Spirit, Opportunity, Curiosity, and Perseverance successful.