  In recent years, collisional charging has been proposed to promote the growth
of pebbles in early phases of planet formation. Ambient pressure in
protoplanetary disks spans a wide range from below $10^{-9}$ mbar up to way
beyond mbar. Yet, experiments on collisional charging of same material surfaces
have only been conducted under Earth atmospheric pressure, Martian pressure and
more generally down to $10^{-2}$ mbar thus far. This work presents first
pressure dependent charge measurements of same material collisions between
$10^{-8}$ and $10^3$ mbar. Strong charging occurs down to the lowest pressure.
In detail, our observations show a strong similarity to the pressure dependence
of the breakdown voltage between two electrodes and we suggest that breakdown
also determines the maximum charge on colliding grains in protoplanetary disks.
We conclude that collisional charging can occur in all parts of protoplanetary
disks relevant for planet formation.
