A big asteroid and its tiny companion moon will zoom harmlessly past Earth this weekend.

The so-called binary asteroid, dubbed 1999 KW4, will make its closest approach at 7:05 p.m. ET on Saturday, when the paired objects will be about 3 million miles from Earth and moving at a speed of about 50,000 miles per hour.

Though 1999 KW4 is designated a potentially hazardous asteroid, there's no chance it will hit Earth. Even so, astronomers will be watching the flyby using a combination of Earth- and space-based telescopes — part of an ongoing effort to improve our planetary defenses against catastrophic asteroid strikes of the sort seen in a disturbing simulation conducted recently in Washington.

"It's one of the closest binary flybys probably in recent history," Vishnu Reddy, a planetary scientist at the University of Arizona in Tucson, said. "That's what makes it a very interesting target."

Even at its closest approach, the asteroid will be too faint to be seen with the naked eye, according to Reddy.

Asteroid 1999 KW4 was discovered May 20, 1999, at the Lincoln Near-Earth Asteroid Research (LINEAR) facility near Socorro, New Mexico, and has been studied extensively since then.

Observations show that the primary, or Alpha, object is about 0.8 mile wide, shaped like a spinning top with a prominent ridge around its equator. The smaller, or Beta, object is roughly one-third the size of the Alpha object and orbits the latter at a distance of about 1.6 miles.

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