The image here is a 10-minute exposure centered on 8:00 p.m. EST (1:00 Universal Time on the 29th), during which 2007 TU24 appeared to cross a Moon’s diameter of sky. Estimated to be about 800 feet across, the asteroid was then about 370,000 miles from Earth and closing. Closest approach wasn’t for another 7½ hours, when the distance would narrow to 340,000 miles, about 1.4 times the Moon’s distance.
Visually the asteroid was a little brighter than 12th magnitude and brightening about 0.2 magnitude per hour. Observing with a 7-inch reflector about an hour after this photograph was taken, my S&T colleague Tony Flanders estimated 2007 TU24 to be about magnitude 11½ — fainter than he expected. In fact, Roger Sinnott, another S&T editor, was unable to spot it with a 5-inch scope.Did you look for this speedy interloper? If so, did you see it? Let me know via the Comments section below.