Unmanned aerial systems, sometimes called drones, and other remote-controlled planes could pose a risk to larger passenger aircraft if theycollided or were sucked into an engine.
The 49ers secondary was so good that on two occasions, two of its defensive backs had such a clear shot at an interception that they actually collided in mid-air.
But they are individuals who collided heads on every play repetitively doing this, year after year, under levels that were tolerable for them to continue to play.
But it was poor defending for County's opening goal as Mignolet and team-mate Kieran Richardson collided as they tried to clear Westcarr's cross under pressure from Hughes.
In the closing moments of that match, Ferdinand collided with his goalkeeper Edwin van der Sar as they tried to clear a dangerous Cesc Fabregas free kick.
It was once thought that asteroids in the main belt occasionally collided with one another, sending fragments into orbital resonance bands where they could be affected by the gravity of Jupiter or Saturn.