How Close Will 2005 Yu55 get?

2005 Yu55, assembled from radio data received by Arecibo in 2010
By NASA/Cornell/Arecibo, via Wikimedia Commons
On Tuesday, an asteroid bearing the name 2005 Yu55 will pass (in cosmic terms) within a hair's breadth of Earth: the 400 metre wide asteroid will pass within 0.85 lunar distances of us. There a lot of blog posts and news items of the "oh my [insert deity of choice], we're all nearly going to die!" variety. But these numbers don't mean a lot to most people, so in terms we can get our heads around, how close will it get?

To start with, "0.85 lunar distances" means "85% of the distance to the Moon". The Moon is about 385,000 km away, so that means that the asteroid will, at its closest approach, be around 327,250 km away from us.

That's a bit better, but it's still hard to imagine.

The Earth itself is about 12,735 km in diameter. This helps to put 2005 Yu55's closest approach into perspective: 327,250 km is more than 25 times larger than 12,735. This means that, at closest approach, you could fit 25 more Earths in the gap between Earth and 2005 Yu55.

That's better still, but how about this to get things into perspective:

As a rough guess, my head is about 20 cm in diameter. Yours will be similar. This means that if we could shrink the Earth down to the size of my (or your) head, 2005 Yu55 would be about the size of a grain of dust, but lets imagine it as a pea (because you can see one of those easily). When it passes the Earth its closest approach would be like someone throwing that pea at your head and missing by five metres.

This is, however, the closest approach of such an object that we've had advance warning of, and as such makes for interesting science - every such incident improves our understanding of close approaches, and it'll give us an opportunity to study an asteroid up-close (relatively speaking*). Also, it's worth mentioning that whilst the head-pea analogy above works well for comparing sizes, it falls down when we think about what would happen if the pea-thrower had a better aim: for the analogy to work in this case, the pea would have to leave a crater in your head the size of your eye-socket. If we considered 2005 Yu55's true relative grain-of-dust size, then the crater that would be left would be getting on for the size of a nostril - still pretty big and fairly noticeable on your face.

I'll leave you with that thought...

* Unfortunately, budding home-astronomers won't be able to see it unless they've got a fairly powerful telescope.


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