  Passing stars can perturb the Oort Cloud, triggering comet showers and
potentially extinction events on Earth. We combine velocity measurements for
the recently discovered, nearby, low-mass binary system WISE
J072003.20-084651.2 ("Scholz's star") to calculate its past trajectory.
Integrating the Galactic orbits of this $\sim$0.15 M$_{\odot}$ binary system
and the Sun, we find that the binary passed within only 52$^{+23}_{-14}$ kAU
(0.25$^{+0.11}_{-0.07}$ parsec) of the Sun 70$^{+15}_{-10}$ kya (1$\sigma$
uncertainties), i.e. within the outer Oort Cloud. This is the closest known
encounter of a star to our solar system with a well-constrained distance and
velocity. Previous work suggests that flybys within 0.25 pc occur infrequently
($\sim$0.1 Myr$^{-1}$). We show that given the low mass and high velocity of
the binary system, the encounter was dynamically weak. Using the best available
astrometry, our simulations suggest that the probability that the star
penetrated the outer Oort Cloud is $\sim$98%, but the probability of
penetrating the dynamically active inner Oort Cloud ($<$20 kAU) is
$\sim$10$^{-4}$. While the flyby of this system likely caused negligible impact
on the flux of long-period comets, the recent discovery of this binary
highlights that dynamically important Oort Cloud perturbers may be lurking
among nearby stars.
