'City killer' asteroid to pass between Earth and moon's orbits this weekend

On hearing the words "city killer" and "asteroid" in the same sentence, you'd be forgiven for thinking something bad is about to happen.

But not to fear. Although an asteroid with the potential to cause significant damage if it hit a populated area is coming somewhat unusually close to our planet this weekend, it is on course to pass harmlessly between the orbits of Earth and the moon.

NOTE: The video in the media player is from a previous report.

On Saturday evening, the asteroid 2023 DZ2 will fly by at a distance of 105,633 miles (170,000 kilometers). The moon, by comparison, is around 238,855 miles (384,400 kilometers) away from Earth.

Referred to as "city killer," the asteroid - which measures between 40 and 100 meters (131 to 328 feet) in size - will do nothing of the sort. But the flyby is still notable, experts say.

"What's unusual about this object is that this is rather rare, that an object of this size passes so close to Earth. That happens about once every 10 years," Richard Moissl, ESA head of the Planetary Defence Office, told CNN Friday.

"But this is a once in a decade opportunity to get some nice close-up measurements in a relatively large body that is relatively easy to measure to that effect," he said.

Near-Earth objects are asteroids and comets with orbits that bring them to within 120 million miles of the sun, and this means they can "circulate through the Earth's orbital neighborhood," according to NASA.

Experts will light the 2023 DZ2 asteroid up with radar, and use this to get more precise measurements for the object, Moissl added.

The 'city killer' label

Moissl said that the phrase "city killer" gets bandied around by experts in reference to two known asteroid impacts.

During the Tunguska event of 1908, an asteroid "sent a shockwave down and flattened 2,000 square kilometers of forest" in Siberia, Moissl said. Also, some 50,000 years ago, an iron asteroid hit what is today Arizona, between Flagstaff and Winslow on the Colorado Plateau, creating a crater 0.75 miles (1.2 kilometers) across and about 600 feet (180 meters) deep.

When space rocks enter the Earth's orbit and hit land, "if they happen in uninhabited areas, then [it's] not much of a concern," Moissl said.

"If we ever find one of the sites where we see this will impact Earth, the first step is to find out where it will hit Earth because if it's in the middle of the ocean, in the middle of the desert, it's not a big deal. We just need to ensure that there is no air traffic or no people in the area," he added.

"That's where the [term] 'city killer' comes from. If such an object would be going down directly over a city, this would be a problem: The whole city would likely be heavily damaged, and should be evacuated.

"City killer is a nice slogan. It's not a bad description. That's why we didn't completely throw it out of the window. Because it says in two words: This is dangerous on the level of being capable of destroying the city," he explained.

However, that's not what's happening with 2023 DZ2. The asteroid, which is in a heliocentric orbit, meaning it is in an orbit and ellipse around the sun, will "continue to go round and round the sun," he said.

There are currently more than 1,450 near-Earth objects on the "risk list," Moissl said, and they are added whenever there is "the slightest possibility that it might impact [Earth] over the next 100 years."

"These objects are usually then observed quite a lot. And the measurements are refined," he added.

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