You might have seen a news alert that an asteroid the size of the Great Pyramid of Giza is going to zip past Earth today (July 24). It's true, but don't worry. If an asteroid were going to kill you, you probably wouldn't hear anything until it was about to knock you on the head.
The reality is that you almost never need to worry about an asteroid that you hear is headed toward Earth. This is mostly because big asteroids pass Earth all the time without incident — including asteroids significantly bigger than this one. These space rocks have to be very precisely aimed to hit our little planet. And even if they do make it into our atmosphere, most aren't big enough to cause significant destruction. And of the very rare ones that theoretically are, most don't come near population centers.
There's another reason not to worry: If you're hearing about an asteroid in the news before it approaches, that means NASA has already spotted it and has precisely tracked its path through space. The near-Earth objects (NEOs) that NASA spots and tracks like this aren't the concern; the space agency's Planetary Defense Coordination Office (PDCO) keeps an eye on them and predicts years in advance just how near-Earth they're going to get. [When Space Attacks: The 6 Craziest Meteor Impacts]
The real (though small) danger comes from the large volume of small to mid-sized asteroids that fly under NASA's radar and might in theory drop out of the sky at any minute — though such an event is very, very unlikely — and even more unlikely to threaten your life.
Still, as Jet Propulsion Laboratory astronomer Emily Kramer, an expert in NEOs, told Live Science in an unpublished interview, while 90% of NEOs larger than 0.6 miles (1 kilometer) have been discovered, researchers are only aware of a small fraction of those in the next size category down — those between 460 feet (140 meters) and 0.6 miles (0.4 km) in diameter. These mid-range space rocks pose a small but real threat to human life in the near-term, which is why NASA's PDCO is working to find and track them.
(That threat is tiny in your lifetime, though potentially dangerous events do happen. Over a long enough time horizon, however, the threat is significant enough that NASA keeps an eye on it. After all, something did take out the dinosaurs.)
This particular asteroid, 2019 OD, is between 167 and 360 feet (51 to 110 m) across — not even large enough to make that smaller category of concern the PDCO is looking at. And it won't come closer than 220,000 miles (356,000 km) from Earth. You can't even see it on a telescope. And it's not that unusual. So don't worry, whatever the tabloids say.
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Originally published on Live Science.