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`Oumuamua (I1 2017) was the first macroscopic ($l\sim100\,{\rm m}$) bodyobserved to traverse the inner solar system on an unbound hyperbolic orbit. Itslight curve displayed strong periodic variation, and it showed no hint of acoma or emission from molecular outgassing. Astrometric measurements indicatethat 'Oumuamua experienced non-gravitational acceleration on its outboundtrajectory, but energy balance arguments indicate this acceleration isinconsistent with a water ice sublimation-driven jet of the type exhibited bysolar system comets. We show that all of `Oumaumua's observed properties can beexplained if it contained a significant fraction of molecular hydrogen(H$_{2}$) ice. H$_{2}$ sublimation at a rate proportional to the incident solarflux generates a surface-covering jet that reproduces the observedacceleration. Mass wasting from sublimation leads to monotonic increase in thebody axis ratio, explaining `Oumuamua's shape. Back-tracing `Oumuamua'strajectory through the Solar System permits calculation of its mass and aspectratio prior to encountering the Sun. We show that H$_{2}$-rich bodies plausiblyform in the coldest dense cores of Giant Molecular Clouds, where numberdensities are of order $n\sim10^5$, and temperatures approach the $T=3\,{\rmK}$ background. Post-formation exposure to galactic cosmic rays implies a $\tau\sim 100$ Myr age, explaining the kinematics of `Oumuamua's inbound trajectory.

Evidence that 1I/2017 U1 (`Oumuamua) was composed of molecular hydrogen ice