2003 The SD220 is 1.6 km in diameter and will be only 2.9 million kilometers away – close to astronomical standards.
It rotates only once every 12 days, which is slow for an asteroid, and is oscillating.
"Known as" non-main axis spin, "it is unusual among near-Earth asteroids, most of which spin on their shortest axis," NASA said.
Images of the asteroid were taken by radar.
2003 SD220's latest fly-by Earth was on Christmas day (NZ time) in 2015, more than 10 million kilometers away.
According to Purdue University's impact calculator, if hippopotamus rock struck Earth at an angle of about 45 degrees, it could create a crater 22 km in diameter.