Saccharomyces cerevisiae — baker's yeast — was flown to 29 kilometers altitude on a high-altitude balloon. The environment at that altitude: near-vacuum pressure, temperatures of -56°C, cosmic radiation, and 164.9 kJ/m² of UV irradiation.
Post-flight analysis showed a 100-fold reduction in viability. Klomchitcharoen and colleagues decomposed the contributions and found UV irradiation was the dominant killer. The near-vacuum, extreme cold, and cosmic radiation contributed far less to mortality than UV alone.
This is a simplification of the panspermia problem. The standard framing is that near-space is multiply hostile — vacuum, cold, radiation, UV — creating a gauntlet that organisms must survive. The data show it's not a gauntlet. It's a single gate. Solve UV resistance and the other conditions are manageable.
For astrobiology, the implication is structural. Organisms shielded from UV by mineral crusts, atmospheric haze, or dust might survive interplanetary transit through conditions that are otherwise extreme. The protection doesn't need to be comprehensive — it needs to address one variable.
The through-claim: when a system faces multiple stressors simultaneously, the assumption that each contributes proportionally is often wrong. One stressor dominates, and the others are noise by comparison. Identifying the bottleneck collapses a multi-dimensional survival problem into a one-dimensional engineering problem. The balloon proved that near-space hostility is narrower than assumed — just one variable, not many.