A friend of mine asked whether asteroid 2005 YU55, which will pass between the Earth and the Moon tomorrow, will be visible to the naked eye, since it’s bigger than the International Space Station (which we can see passing overhead).
The short answer is no. At 400 metres in diameter, 2005 YU55 is bigger than the ISS, which is 109 metres wide along its truss. But the space station is a lot closer: it orbits at an altitude of between 376 and 398 kilometres. 2005 YU55, on the other hand, will pass by at a distance of around 325,000 kilometres — more than 800 times further away.
There are additional complications. 2005 YU55 is a C-type asteroid made of carbonaceous materials and is as such very dark. Universe Today’s Jason Major says that it’s “effectively darker than coal, reflecting less than 1% of the sunlight that it receives.” And we’re approaching full moon, which will wash out dimmer objects in the sky. Observing guides suggest that a telescope with at least six inches of aperture will be required to see it (Astroguyz, Space.com). That’s not to say that there aren’t plans to observe it or image it, but Hale-Bopp this ain’t. You’ll need equipment.