  Based on Gaia DR2 data and new CHIRON radial velocities, we have discovered
that two nearby stellar associations UPK 535 (318.08 $\pm$ 0.29 pc,
$25^{+15}_{-10}$ Myr, 174 stars) and Yep 3 (339.54 $\pm$ 0.25 pc ,
$45^{+55}_{-20}$ Myr, 297 stars) in the Gum Nebula have recently collided. We
project stars' current positions, motions, and measurement uncertainties
backward and forward through time in a 10,000-trial Monte Carlo simulation. On
average, the associations' centres of mass come within 18.89 $\pm$ 0.73 pc of
each other 0.84 $\pm$ 0.03 Myr ago. A mode of 54 $\pm$ 7 close ($<$1 pc)
stellar encounters occur during the collision. We cannot predict specific
star-star close encounters with our current $\sim$7.6 pc distance precision and
21.5-per-cent-complete radial velocity sample. Never the less, we find that two
stars in UPK 535 and two stars in Yep 3 undergo a nonspecific close encounter
in $>$70 per cent of trials and multiple close encounters in $\sim$30 per cent.
On average, the closest approach of any two stars is 0.13 $\pm$ 0.06 pc, or
27,000 $\pm$ 12,000 au. With impulse-tracing values up to $2.7^{+3.1}_{-1.1}$
M$_{\odot}$ pc$^{-2}$ km$^{-1}$ s, such close encounters could perturb stars'
Oort cloud comets (if present), cause heavy bombardment events for exoplanets
(if present), and reshape solar system architectures. Finally, an expansion of
our simulation suggests other associations in the region are also interacting.
Association collisions may be commonplace, at least in the Gum Nebula
straddling the Galactic plane, and may spur solar system evolution more than
previously recognized.
