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(stock photo)
By Chuck McPartlin
Last Tuesday afternoon, May 15, 2018 Santa Barbara time, an asteroid estimated to be about 300 feet across made a close approach, passing Earth at about half the distance to the Moon, or roughly 126,000 miles. On its way in, about 12 hours before closest approach, it was visible from Santa Barbara at about 2 AM PDT Tuesday morning, bright enough to be visible in amateur telescopes, although still over a hundred times dimmer than stars detectable to the unaided eyeball. This is one of the closest observed approaches of an asteroid this big.
Here’s a compressed video speeded up 8X, showing three minutes of the asteroid’s motion in 22.5 seconds. It was high in the south, passing through the shoulder of Ophiuchus, the thirteenth constellation of the Zodiac, at almost 29,000 miles per hour. The video camera was integrating for 8.5 seconds, so the asteroid takes a little hop about once a second as it passes by the star HD 152277 near the center of the frame.
The asteroid is named 2010 WC9, but it is being called the Lost Asteroid. As the names imply, it was discovered in November of 2010, but was moving away and became too faint before enough observations had been made to determine its orbit accurately. On May 8 of this year it was rediscovered, and further observations showed that it was going to come close.
The size of 2010 WC9, estimated from its brightness and assumed reflectivity, make it about 5 times larger in diameter than the Chelyabinsk meteor that detonated in the upper atmosphere above Russia in February 2013. Radar observations during this pass should give us a better size value, but if WC9 is indeed 300 feet across, it would be in the same class as the Tunguska impactor of 1908.
WC9’s orbit makes it an Apollo asteroid, chunks of junk that cross the Earth’s orbit, spending their time in the area of the solar system between Mars and Mercury. The Chelyabinsk impactor was also an Apollo object. Watch out for falling rocks!
References for a Cloudy Evening
