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Dozens of cosmic invaders blazed into Earth’s atmosphere 20 years ago tonight. Some formed bright fireballs that were visible across the western United States. Others weren’t as bright, but they were detected by satellites or special detectors on the ground.
The first fireball streaked across southeastern New Mexico and western Texas, leaving a trail of green sparks behind it. Its passage was detected by instruments that were designed to listen for evidence of covert nuclear explosions.
A second fireball appeared an hour and 44 minutes later over California. It exploded with a force equal to perhaps a few hundred tons of TNT. The blast was recorded by seismometers across the state.
At first, scientists thought the two fireballs were caused by a single space rock. They thought it skipped off the upper atmosphere over New Mexico and Texas, but was slowed down enough to circle around Earth and plunge toward the ground over California.
But a later analysis of videos, photos, eyewitness accounts, and the shockwaves painted a different picture. Scientists determined that Earth was hit by a barrage of debris — perhaps the remains of a fractured asteroid. The biggest chunks formed the bright fireballs. But about 60 other bits of debris also hit Earth. A few were seen across the United States over the following 24 hours or so. Others were recorded by satellites or the network of nuclear-bomb detectors — evidence of a cosmic invasion 20 years ago.