  Within just two years, two interstellar objects (ISOs) - Oumuamuas and
Borisov - have been discovered. Large quantities of planetesimals form as a
by-product of planet formation. Therefore, it seems likely that ISOs are former
planetesimals that became unbound from their parent star. The discoveries raise
the question of the dominant ISO formation process. Here, we concentrate on
planetesimals released during another star's close flybys. We quantify the
amount of planetesimals released during close stellar flybys, their ejection
velocity and likely composition. We study the dependence of the effect of
parabolic flybys on the mass ratio between the perturber and parent star, the
periastron distance, inclination, and angle of periastron. Whenever ISOs are
produced, they leave their parent system typically with velocities of 0.5-2
km/s. This ejection velocity is distinctly different to that of ISOs produced
by planet scattering (4-8 km/s) and those shed during the stellar
post-main-sequence phase 0.1-0.2 km/s). Using the typical disc truncation
radius in various cluster environments, we find that clusters like the Orion
nebula cluster are likely to produce the equivalent of 0.85 Earth-masses of
ISOs per star. In contrast, clusters like NGC 3603 could produce up to 50
Earth-masses of ISOs per star. Our solar system probably produced the
equivalent of 2-3 Earth masses of ISOs, which left our solar system at a mean
ejection velocity of 0.7 km/s. Most ISOs produced by flybys should be
comet-like, similar to Borisov. ISOs originating from compact long-lived
clusters would often show a deficiency in CO. As soon as a statistically
significant sample of ISOs is discovered, the combined information of their
observed velocities and composition might help in constraining the dominant
production process (abridged).
