Lunar EVA Walk-Back Limit Simulator (2D)

Mission Status

Nominal — within safe walk-back range

O2 Remaining

100.0%

CO2 Canister

0.0%

Distance from Base

5 m

Safe Walk-Back Radius

0 m

Mission Elapsed Time (EVA clock)

00:00:00

EVA Controls

Walking pace (outbound) 1.3 m/s

Metabolic workload 400 W

Terrain roughness 30%

Primary O2 tank 0.87 kg

LiOH scrubber canister 1.00 kg

Mission reserve margin 20%

How it works

A spacesuit's Primary Life Support System (PLSS) carries only two hard limits: the oxygen in its tank and the carbon dioxide capacity of its scrubber canister. Oxygen use tracks the astronaut's metabolic workload through the caloric equivalent of oxygen, then gets scaled by whatever terrain the boot is actually on:

O2 rate (kg/s) = (Metabolic power (W) / 14.6e6 J/kg) * terrain factor
CO2 rate (kg/s) = 1.15 * O2 rate     (respiratory quotient ~0.83, molar mass 44/32)

The canister removes CO2 by the LiOH reaction 2LiOH + CO2 → Li2CO3 + H2O, which absorbs about 0.9 kg of CO2 per kg of LiOH before breaking through. Whichever runs out first — O2 or scrubber capacity — ends the EVA.

Since Apollo 11–14 had no rover, NASA bounded every moonwalk by the "walk-back" rule: never go farther from the lander than you could walk back to it on your remaining O2 alone, holding a reserve margin for problems. That boundary shrinks continuously as consumables deplete, recomputed from the outbound-vs-return budget every tick:

usable O2 = O2 remaining - reserve
R_safe = 0.5 * usable O2 * (return speed / O2 rate)

Half the usable tank funds the walk out, half funds the walk back — that 50% split is the whole safety margin. Push the astronaut past R_safe on this map and the status flips to a hard warning: the outbound leg has already spent more than the return trip can afford. Apollo 15–17 carried the Lunar Roving Vehicle, which raised the return speed from a ~1.3 m/s walk to a ~5 m/s drive — toggle Rover Assist to see how much that single piece of hardware expanded the safe range, the actual technology jump between the early and late Apollo missions. The rougher the ground underfoot, the harder the suit works and the faster both budgets burn — watch the terrain patches on the map and the roughness slider push the consumption rate up and down in real time.

EVA CLOCK 00:00:00

Astronaut (PLSS backpack) Habitat / base Safe walk-back boundary