The approximately 30-foot (10-meter) asteroid, dubbed 2010 RF12, will pass within about 50,000 miles (80,000 kilometers) of Earth at 5:12 p.m. ET.
About 12 hours earlier, at 5:51 a.m. ET, a 50-foot (15-meter) asteroid called 2010 RX30 passed within about 154,000 miles (248,000 kilometers) of our planet—roughly halfway between Earth and the moon.
"Even if they were to hit Earth, these two asteroids would disintegrate in the atmosphere," he said. "We'd get a few meteorites out of them, but there wouldn't be any damage to the ground."
Asteroids Found Only Sunday
Both asteroids were discovered on Sunday by the Catalina Sky Survey near Tucson, Arizona.
Astronomers estimate that a small asteroid like 2010 RF12 and 2010 RF30 passes between Earth and the moon every day. (Read about an asteroid that unexpectedly buzzed Earth last year.)
Most of these space rocks are never seen, because instruments scanning the skies for dangerous near-Earth asteroids aren't designed to notice rocks smaller than about a hundred feet (30 meters).
"The Catalina Sky Survey just happened to be looking at the right part of the sky at the right time to pick them up Sunday morning," Johnson said.
2010 RX30 was not visible to the naked eye when it made its closest approach to Earth, and 2010 RF12 isn't expected be discernible either.
"You'd need a fairly sophisticated amateur telescope, something on the order of 20 inches [50 centimeters] or so, to be able to see these asteroids," Johnson said.