  The detections of 1I/Oumuamua and 2I/Borisov within just two years
demonstrate impressively that interstellar objects (ISOs) must be common in the
Milky Way. Once released from their parent system, these ISOs travel for Gyr
through interstellar space. While often imagined as empty, interstellar space
contains gas and dust most prominent in the form of molecular clouds.
Performing numerical simulations, we test how often ISOs cross such molecular
clouds. We find that the ISOs pass amazingly often through molecular clouds. In
the solar neighbourhood, ISOs typically spend 0.1-0.2% of their journey inside
molecular clouds, for relative slow ISOs ($<$ 5 km/s) this can increase to
1-2%, equivalent to 10 - 20 Myr per Gyr. Thus the dynamically youngest ISOs
spend the longest time in molecular clouds. In other words, molecular clouds
must mainly contain relatively young ISOs ($<$ 1-2 Gyr). Thus the half-life of
the seeding process by ISOs is substantially shorter than a stellar lifetime.
The actual amount of time spent in MCs decreases with distance to the Galactic
Centre. We find that ISOs pass so often through MCs that backtracing their path
to find their parent star beyond 250 Myr seems beyond the point. Besides, we
give a first estimate of the ISO density depending on the galactic centre
distance based on the stellar distribution.
