  Shell buckling is central in many biological structures and advanced
functional materials, even if, traditionally, this elastic instability has been
regarded as a catastrophic phenomenon to be avoided for engineering structures.
Either way, predicting critical buckling conditions remains a long-standing
challenge. The subcritical nature of shell buckling imparts extreme sensitivity
to material and geometric imperfections. Consequently, measured critical loads
are inevitably lower than classic theoretical predictions. Here, we present a
robust mechanism to dynamically tune the buckling strength of shells,
exploiting the coupling between mechanics and magnetism. Our experiments on
pressurized spherical shells made of a magnetorheological elastomer demonstrate
the tunability of their buckling pressure via magnetic actuation. We develop a
theoretical model for thin magnetic elastic shells, which rationalizes the
underlying mechanism, in excellent agreement with experiments. A dimensionless
magneto-elastic buckling number is recognized as the key governing parameter,
combining the geometrical, mechanical, and magnetic properties of the system.
