Lunar EVA Walk-Back Limit Simulator

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

Metabolic workload 400 W

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:

O2 rate (kg/s) = Metabolic power (W) / 14.6e6 J/kg
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:

R_safe = (O2 remaining - reserve) / (2 * O2 rate / return speed)

This sim keeps the astronaut walking outward so you can watch that shrinking boundary catch up with the distance already covered — the moment the status flips to "return now" is exactly the moment a real crew would already need to be heading back. 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.

EVA CLOCK 00:00:00

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

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