On November 8, 2011, asteroid 2005 YU55, a 400-meter-wide sphere of rock, slipped past Earth at a distance of 324,600 kilometers. That is 0.85 times the distance to the Moon. It was the closest approach by an object of that size and brightness in 35 years. The asteroid moved through the sky at about 13 kilometers per second, a speed that would have delivered catastrophic energy had it struck. Radar images from the Goldstone and Arecibo observatories revealed a dark, spherical, and slowly rotating body.
This event mattered because it was a drill. For the first time, astronomers had years of advance notice for a close-passing object of this scale. They used the opportunity to test planetary defense observation networks. The data refined predictions of its orbit for the next century, confirming it posed no threat for at least the next 100 years. The flyby demonstrated both our capability to track such objects and our vulnerability to those we have not yet found.
A common misunderstanding is that a "near-miss" in astronomical terms implies a narrow escape from doom. The trajectory of 2005 YU55 was known with high precision long before its arrival; its path was never on a collision course. The true significance was observational, not existential. It was a practice run for a real threat.
The lasting impact is procedural. The event underscored the necessity of programs like NASA's NEO Observations Program. It provided a benchmark for characterizing potentially hazardous asteroids using radar, a technique that reveals an object's size, shape, and surface features far more accurately than optical telescopes alone. The data from that night continues to inform models of asteroid composition and orbital dynamics, turning a celestial visitor into a reference point for planetary defense.