I know, it doesn't look scary. It's actually kind of beautiful which is why I'm using it as my current desktop background. But what *is* it? It's a map of orbits of every *known* Potentially Hazardous Asteroid (PHA). In other words, the map of every rock larger than 460 feet that has an orbit that passes uncomfortably close to the Earth. Close, because we're talking about space here, means within 5 million miles. To give you a sense of scale, the Moon is ~250,000 miles from the Earth. Five million seems like a lot of distance but when you consider that when it is at its closest, Venus is 38 million miles away. So we're talking very big numbers. The problem is that Earth has a serious gravitational pull and something within 5 million miles of the Earth is certainly going to be influenced by its gravity. How many objects are we talking about? Around 1,400! That's when it gets scary.
The reason why the size matters is that mass matters. Earth actually gets hit on occasion by fairly small pieces of rock but it's the large pieces that wreak havoc on the planet. That said, nothing in that graphic is in any real serious danger of hitting the planet in the next century. But longer out...? Something around 1,000 feet strikes the Earth approximately every 80,000 years. To give you sense of scale here, the asteroid that most likely blasted a chunk out of the Yucatan peninsula and likely delivered the coup de grace to the dinosaurs was somewhere in the range of 6 miles in diameter. Think Manhattan hitting the Earth.
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Cheers
Aj