Sentence:  It is tempting to speculate about the incentives or compulsions that might explain why anyone would take to the skies in [the] basket [of a balloon]: [...]; perhaps to moralise on the oneness or <WSD>fragility</WSD> of the planet, or to see humanity for the small and circumscribed thing that it is; [...]. 
Meaning of the word marked with <WSD> tags:  The condition or quality of being fragile; brittleness; frangibility.