The US space agency placed the rock somewhere between 1.1 miles and 2.54 miles (1.8km and 4.1km) across.
But thanks to the Arecibo Observatory, we now know the asteroid only measures about 1.2 miles (2km) in diameter.
Astronomers have also learned the asteroid rotates once every 4.1 hours.
Mr Cox said: “The only thing we can actually measure it on, is the object’s apparent magnitude, it’s brightness.
“But as soon as it comes within the range of radar, as it’s done now, we can tell very, very specifically because we know what distance the object is and we can measure it very accurately.”