A big asteroid with its own moon is passing by. Here's how to spot it

The largest asteroid to come near our cosmic neighborhood this year comes closest Saturday, and it has a traveling companion.

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2 min read

A rendering of a binary asteroid system

European Space Agency

A "potentially hazardous" asteroid and its smaller sidekick are zipping by Earth Saturday, and both amateur and professional astronomers are checking out the rare sight.

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. The larger half of the duo is nearly a mile (1.5 kilometers) wide, while its smaller satellite is about a third of a mile wide (0.5 kilometers). The asteroid was first discovered 20 years ago, and its latest close approach comes May 25 at 4:05 p.m. PT.

Two ninety-second exposures of asteroid 1999 KW4 on its way to a close approach. 

CNRS / ESA NEOCC / IRAP / Observatoire des Makes

The European Space Agency released a brief animation of the approaching system captured from an observatory on Reunion Island in the Indian Ocean on May 9.

If you have access to a telescope at least 8 inches in diameter, you might be able to catch it as it passes us and starts to make its way back out to deep space. EarthSky has charts and tips for backyard observers, and TheSkyLive is tracking the system's movements in near real time.

While its serious size has earned 1999 KW4 the title "potentially hazardous," 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 to get more data on both rocks and the separation between them.

The system is moving fast, 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. 

Originally published April 23.
Update, May 21: Adds details about Friday's flyby.  
Update, May 24: 
Adds details about how to spot the asteroid fly by.