Interstellar Objects in the Solar System: 1. Isotropic Kinematics from
  the Gaia Early Data Release 3
  1I/'Oumuamua (or 1I) and 2I/Borisov (or 2I), the first InterStellar Objects
(ISOs) discovered passing through the solar system, have opened up entirely new
areas of exobody research. Finding additional ISOs and planning missions to
intercept or rendezvous with these bodies will greatly benefit from knowledge
of their likely orbits and arrival rates. Here, we use the local velocity
distribution of stars from the Gaia Early Data Release 3 Catalogue of Nearby
Stars and a standard gravitational focusing model to predict the velocity
dependent flux of ISOs entering the solar system. With an 1I-type ISO number
density of $\sim$0.1 AU$^{-3}$, we predict that a total of $\sim$6.9 such
objects per year should pass within 1 AU of the Sun. There will be a fairly
large high-velocity tail to this flux, with half of the incoming ISOs predicted
to have a velocity at infinity, v$_{\infty}$, $>$ 40 km s$^{-1}$. Our model
predicts that $\sim$92\% of incoming ISOs will be residents of the galactic
thin disk, $\sim$6\% ($\sim$4 per decade) will be from the thick disk, $\sim$1
per decade will be from the halo and at most $\sim$3 per century will be
unbound objects, ejected from our galaxy or entering the Milky Way from another
galaxy. The rate of ISOs with very low v$_{\infty}$ $\lesssim$ 1.5 km s$^{-1}$
is so low in our model that any incoming very low velocity ISOs are likely to
be previously lost solar system objects. Finally, we estimate a cometary ISO
number density of $\sim$7 $\times$ 10$^{-5}$ AU$^{-3}$ for 2I type ISOs,
leading to discovery rates for these objects possibly approaching once per
decade with future telescopic surveys.
