That's about three times the size of its moon, which clocks in at a width of about 0.5 kilometers (.3 miles). NASA's asteroid trackers are braced for the space rock to shoot past our home planet around 12.05am United Kingdom time in the wee hours of Sunday, May 26.
Classified as a Near Earth Object, the 1.5-kilometer wide asteroid is expected to zip by Earth without causing any harm, though the Minor Planet Center has labelled it 'potentially hazardous.' Its companion moon is about half a kilometer wide. The larger half of the duo is almost a mile (1.5 kilometers) wide, while its smaller satellite is about a third of a mile wide (0.5 kilometers).
The two space rocks are traveling at a speed of about 48,000 miles per hour. The asteroid will travel by the Earth at a safe distance of 3.2 million miles, or roughly more than 13 times the distance from the Earth to the moon.
The 1999 KW4 completes an orbit around the Sun once every 6.18 months (188 days). Seventeen years from now, on May 25, 2036, the rocks will pass 55.2% closer to Earth, at a distance of just 1,443,511 miles (2,323,106 km) - again, posing no threat worth worrying about.
During its closest approach the asteroid will have more favorable viewing conditions in the southern hemisphere before it potentially becomes more visible in the northern by May 27.
And the asteroid's next close flybys falls on May 26, 2020, and May 31, 2021.
"But most of us who are inexperienced observers with unsophisticated equipment may not be able to spot it".
"Even at its very brightest, it will be around 12th magnitude or 250 times fainter than the faintest star that you can see with your unaided eye".
In fact, this is the fourth approach the binary asteroids have made toward Earth since they were discovered in 1999, and not the closest. This will be the closest that a binary system has ever approached Earth.