  We derived 90% confidence limits (CL) on the interstellar number density
($\rho_{IS}^{CL}$) of interstellar objects (ISO; comets and asteroids) as a
function of the slope of their size-frequency distribution and limiting
absolute magnitude. To account for gravitational focusing, we first generated a
quasi-realistic ISO population to ~750 au from the Sun and propagated it
forward in time to generate a steady state population of ISOs with heliocentric
distance <50 au. We then simulated the detection of the synthetic ISOs using
pointing data for each image and average detection efficiencies for each of
three contemporary solar system surveys --- PS1, the Mt. Lemmon Survey, and the
Catalina Sky Survey. These simulations allowed us to determine the surveys'
combined ISO detection efficiency under several different but realistic modes
of identifying ISOs in the survey data. Some of the synthetic detected ISOs had
eccentricities as small as 1.01 --- in the range of the largest eccentricities
of several known comets. Our best CL of $\rho_{IS}^{CL} = 1.4 \times 10^{-4}$
au$^{-3}$ implies that the expectation that extra-solar systems form like our
solar system, eject planetesimals in the same way, and then distribute them
throughout the galaxy, is too simplistic, or that the SFD or behavior of ISOs
as they pass through our solar system is far from expectations.
