Interspace is empty on a level that is hard to imagine.
There are 2.652×10^25 molecules in one m^3 of air.
That is 26520000000000000000000000.
In intellar space?
The is 1.
IE: the probe would hit more atoms in one second on earth moving at 1 m/s than it would travelling the entire age of the universe so far through interstellar space.
Even the space between the planets is thick with matter by comparison.