At the closest point to the Earth, asteroid 2004 BL86 will be at a distance of 1.2 million kilometers which – approximately three times the distance from the Earth to the moon. Estimated to be 0.5 km in diameter, it is classified by scientists as potentially dangerous.
A space object is considered “potentially dangerous” if it crosses the Earth's orbit at a distance of less than 0.05 AU (approximately 19.5 distances from the Earth to the Moon), and if its diameter exceeds 100-150 meters. Objects of this size are large enough to cause unprecedented destruction, or generate a tsunami in case they fall into the ocean.
You can also watch 2004 BL86's flyby in visible light (as opposed to radar observations) live online Monday via the Virtual Telescope Project in Italy.