  A planetesimal moves through the gas of its protoplanetary disc where it
experiences a head wind. Though the ambient pressure is low, this wind can
erode and ultimately destroy the planetesimal if the flow is strong enough. For
the first time, we observe wind erosion in ground based and microgravity
experiments at pressures relevant in protoplanetary discs, i.e. down to
$10^{-1}\, \rm mbar$. We find that the required shear stress for erosion
depends on the Knudsen number related to the grains at the surface. The
critical shear stress to initiate erosion increases as particles become
comparable to or larger than the mean free path of the gas molecules. This
makes pebble pile planetesimals more stable at lower pressure. However, it does
not save them as the experiments also show that the critical shear stress to
initiate erosion is very low for sub-millimetre sized grains.
