  Asteroid 99942 Apophis will pass near the Earth in April 2029. Expected to
miss our planet by a safe margin, that could change if Apophis' path was
perturbed by a collision with another asteroid in the interim. Though the
statistical chance of such a collision is minuscule, the high risk associated
with Apophis motivates us to examine even this very unlikely scenario. In this
work, we identify encounters between known asteroids and Apophis up to April
2029. Here we show that Apophis will encounter the 1300 meter diameter asteroid
4544 Xanthus in December 2026. Their Minimum Orbit Intersection Distance (MOID)
is less than 10,000 km, with Xanthus passing that closest point just four hours
after Apophis. Though a direct collision is ruled out, the encounter is close
enough that material accompanying Xanthus (if any) could strike Apophis. We
also identify other asteroid encounters that deserve monitoring.
