  We present the first large scale numerical simulation of three-dimensional
Rayleigh-B\'enard convection near onset, under free-free boundary conditions
for a fluid of Prandtl number $\sigma=0.5$. We find that a spatiotemporally
chaotic state emerges immediately above onset, which we investigate as a
function of the reduced control parameter $\epsilon$. We conclude that the
transition from conduction to spatiotemporal chaos is second order and of
``mean field'' character. We also present a simple theory for the time-averaged
convective current. Finally, we show that the time-averaged structure factor
satisfies a scaling behavior with respect to the correlation length near onset.
