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'Walnut' asteroid nearly a mile wide is flying by with a friend in May

The largest asteroid to come near our cosmic neighborhood this year has its own moon.


A rendering of a binary asteroid system

European Space Agency

An asteroid nearly a mile (1.5 kilometers) wide is flying in for a visit soon and it's bringing its sidekick along for the ride.

The unnamed space rock designated as Asteroid (66391) 1999 KW4 is a binary system, meaning it's composed of one large asteroid orbited by a smaller moon, which is about a third of a mile wide (0.5 kilometers). The asteroid was first discovered 20 years ago, and both backyard and professional astronomers are preparing to observe it as it makes its latest close approach May 25.

While its serious size has earned 1999 KW4 the title of a "potentially hazardous" asteroid, it'll clear Earth this year at a very safe distance of 3,219,955 miles (5,182,015 kilometers). This will be the second-closest approach it's made in the past two decades, and the nearest it'll come to our planet until 2036. It'll also be by far the largest object to come within about 20 lunar distances (4,647,790 miles or 7,479,894 kilometers) of us this year.

The Las Cumbres Observatory describes the main asteroid in the system as "slightly squashed at the poles and with a mountain ridge around the equator, which runs all the way around the asteroid. This ridge gives the primary an appearance similar to a walnut or a spinning top."

Some of the world's largest telescopes, including the massive Arecibo radio observatory on Puerto Rico, will observe 1999 KW4 as it flies by to get more data on both rocks and the separation between them.

The system will be moving fast when it visits, at 48,123 miles per hour (77,446 kilometers per hour), which may make it easier for amateur observers to get a look at it. If you want to try to spot it, NASA solar system ambassador Eddie Irizarry has some tips and helpful charts to get you started over at EarthSky.