Chapter 28 — Key Takeaways (Life Support and Human Spaceflight)

A one-page reference. Reread this before an exam, or before you human-rate any mission.

The one idea

On Earth, the planet supplies air, pressure, temperature, water, radiation shielding, and gravity for free. Take a human off the planet and every one of those must be manufactured by machinery, on a mass and power budget, with no repair shop. Life support is packing a survivable slice of Earth into a sealed can — the theme space is unforgiving, made literal.

Human requirements (per crew member per day)

Supply Mass Remove Mass
Oxygen $0.84\ \text{kg}$ Carbon dioxide $1.0\ \text{kg}$
Potable water $2.5\ \text{kg}$ Wastewater + humidity $3.5\ \text{kg}$
Food (as served) $1.8\ \text{kg}$ Solid waste $0.1\ \text{kg}$
Supplied total $\approx 5.1\ \text{kg}$
  • What the body responds to is $p_{\text{O}_2}$, not total pressure. Comfortable band $\approx 16$–$23\ \text{kPa}$. ISS: $101\ \text{kPa}$, $21\%$ O$_2$. Apollo: $34.5\ \text{kPa}$ pure O$_2$ (light, but a fire hazard — Apollo 1).
  • CO$_2$ must be held below $\sim 0.4$–$0.5\ \text{kPa}$ (Earth background $0.04\ \text{kPa}$).
  • Open-loop mission mass: $M = (\text{crew})\times(\text{days})\times 5.14\ \text{kg}$.

Open-loop vs closed-loop — the break-even

Open-loop Closed-loop
Model mass $\propto$ duration fixed recycler mass + small makeup
Best for short / cheap resupply (LEO, Apollo) long / far (Mars)
Mass vs time rising line flat offset + shallow slope

Break-even day $\;t^\* = \dfrac{M_{\text{hw}}}{(\text{crew})\,(r_{\text{open}} - r_{\text{closed}})}$. Example: $M_{\text{hw}}=1500\ \text{kg}$, crew $4$, $r_{\text{open}}=5.14$, $r_{\text{closed}}=2.47 \Rightarrow t^\*\approx 140\ \text{days}$.

Loop chemistry (know these three)

Reaction Equation Role
Electrolysis $2\text{H}_2\text{O} \to 2\text{H}_2 + \text{O}_2$ regenerate O$_2$ from water
Sabatier $\text{CO}_2 + 4\text{H}_2 \to \text{CH}_4 + 2\text{H}_2\text{O}$ recover water from CO$_2$ (methane vented)
Bosch $\text{CO}_2 + 2\text{H}_2 \to \text{C} + 2\text{H}_2\text{O}$ closes O$_2$ loop fully; carbon fouling
  • Why O$_2$ recovery caps near $50\%$: electrolysis gives ~2 H$_2$ per O$_2$; the crew makes ~1 CO$_2$ per O$_2$; Sabatier needs 4 H$_2$ per CO$_2$ ⇒ only half the CO$_2$ can be reduced. Hydrogen lost in vented CH$_4$ is the leak.
  • Water recovers $\sim 90\%+$; food does not close (the hard, unsolved loop).

Radiation

Environment Approx. dose Note
Earth background $2.4\ \text{mSv/yr}$ reference
ISS (LEO) $100$–$150\ \text{mSv}$ / 6 months below the Van Allen belts
Mars mission $0.6$–$1\ \text{Sv}$ total vs a $\sim 0.6\ \text{Sv}$ career limit
  • Units: absorbed dose in grays (J/kg); equivalent dose in sieverts (Gy × quality factor).
  • GCR: chronic, isotropic, nearly unshieldable (meters of material; secondaries). Best defenses: go fast; someday active shielding.
  • SPE: acute proton storm — shieldable. Retreat to a storm shelter walled with water, food, and waste (mass you already carry — "nearly free"). Historical near-miss: the August 1972 event.
  • Shielding is measured in areal density (g/cm²); hydrogen-rich beats aluminum per kg.

Microgravity physiology & countermeasures

Effect Scale
Headward fluid shift ~2 L; puffy face, plasma loss, orthostatic intolerance on return
Bone loss $\sim 1$–$1.5\%$ per month (weight-bearing bones)
Muscle atrophy significant in weeks without loading
Vision (SANS) may not fully reverse; a top long-mission concern

Countermeasure: $\sim 2$–$2.5\ \text{h/day}$ exercise, resistive most important for bone; plus diet and drugs. Slows, does not stop — hence interest in artificial gravity.

Artificial gravity — $a = \omega^2 r$

For $1\,g$ at… Spin rate Rim speed
$r = 224\ \text{m}$ $2.0\ \text{rpm}$ $47\ \text{m/s}$
$r = 100\ \text{m}$ $3.0\ \text{rpm}$ $31\ \text{m/s}$
$r = 50\ \text{m}$ $4.2\ \text{rpm}$ $22\ \text{m/s}$
  • Solve $\omega = \sqrt{a/r}$; $\text{rpm} = \omega\cdot 60/2\pi$; rim speed $v = \omega r$.
  • Coriolis effect: sideways $a_{\text{Cor}} = 2\omega v_{\text{rel}}$ on any relative motion — disorienting. Gravity gradient: head at $r-h$ feels $(r-h)/r$ of foot gravity.
  • Tradeoff: low spin = comfortable but huge/heavy; small radius = buildable but nauseating. Historic comfort limit $\sim 2\ \text{rpm}$ (⇒ hundreds of meters). Partial gravity or a tethered counterweight relaxes the trade. Never yet flown.

The human factor

  • ICE environment (Isolated, Confined, Extreme). Hazards: isolation, monotony, sleep disruption, crew friction, and the Earth–Mars comm delay ($3$–$22\ \text{min}$ one-way ⇒ no real-time help ⇒ crew autonomy required; cf. Ch. 26/27).
  • Countermeasures: selection, training, humane scheduling, private quarters, family contact, behavioral health, designed-in autonomy. The crew is a flight-critical subsystem.

Common pitfalls

Pitfall Reality
"Zero gravity, so the body rests." Weightlessness is a stressor; crews must exercise hard just to come home able to walk.
"Sabatier recovers all the O$_2$." Hydrogen-limited to ~50%; the rest of the CO$_2$ is vented.
"Shield the whole ship like the shelter." Shielding scales with area; only a small shelter is affordable. Whole-ship GCR shielding is impossibly heavy.
"A 5-m centrifuge gives comfortable 1 g." It needs ~13 rpm — brutal Coriolis and a 40% head-to-foot gradient.
"Life support is separate from power/thermal." It is a major power load (Ch. 25) and heat/humidity source (Ch. 24).

Mission / project additions this chapter

  • MDR: added the crew note — "N/A, uncrewed" (Tracks A–D) plus the human-rating impact statement, or a full consumables + loop-closure + shelter + artificial-gravity sizing for a crewed variant.
  • Chapter utility: crew_consumables(crew, days) and recycle_breakeven(hw, crew) — a sizing helper, distinct from the astrotools flight package. Feeds the Mars mission of Ch. 34.