Astronomy Blog

Asteroid 2005 YU55

Asteroid 2005 YU55 will pass the Earth and Moon over the next 24 hours or so. It is roughly 400 metres in diameter and the closest it gets to us is something like 300,000 km. The closest it will get to the Moon is roughly 240,000 km. That is a long, long way away from either despite what an excited person in my blog comments insists. It won't hit the Earth and it won't hit the Moon.

Although 2005 YU55 passes within the orbital distance of the Moon, it doesn't actually get between the Earth and Moon at any point. It goes past them both. Also, space has a third dimension and 2005 YU55 actually goes quite a bit above the Earth and Moon plane when you look at it from the side. I still don't understand why some people are insisting it could hit the Earth or the Moon. It couldn't. It isn't in the right place or heading in the right direction.

Most asteroids are too far away to be able to measure their sizes directly or see detail on their surfaces from Earth. 2005 YU55 is now close enough for some Earth-based telescopes to get a good look. The Goldstone radio telescope has already produced radar images of it and plenty of others will be ogling as it goes by. In a couple of days ESA's Herschel Space Observatory (~1.5 million km away from Earth in the opposite direction than the Sun) will also have a look. Herschel has to wait that long because until then Herschel would have to look in the direction of the Earth and Sun to see it and that wouldn't do its instruments much good at all.

Can you observe it?

The asteroid is moving pretty quickly and its apparent position varies depending on where you are on Earth due to parallax. As 2005 YU55 is only about 400m across, it'll be roughly 11th magnitude. That means it'll be about 100 times fainter than the limit of the unaided eye. Nevertheless, I think I'll look up and wave goodbye as it carries on its merry way.