In my opinion, the most interesting comment about Asteroid YU 55 passing within approximately 202,000 miles of Earth is not that
"...(it)... will be visible from the planet's northern hemisphere," or that, "The best time to observe it would be in the early evening on November 8 from the East Coast of the United States," but that, "It will be too dim to be seen with the naked eye, however, and it will be moving too fast for viewing by the Hubble Space Telescope."
Senior research scientist Don Yeomans, with NASA's Jet Propulsion Laboratory in Pasadena, California told the Reuters news agency, "It is going to be very faint, even at its closest approach. You will need a decent-sized telescope to be able to actually see the object as it flies by."
That interests me (as an author of alien romances) because it means I can plausibly go on using spaceships disguised as asteroids and comets. (And for the grammatical purists, I deliberately echoed a "boldly go" phrase.)
Of course, my own alien spacecraft are larger that this large asteroid. YU 55 is described as the size of a battleship, and is estimated at a quarter-mile wide. Mine, influenced by a variety of How To manuals was concealed under a mile of water ice. (The "water" part of "water ice" was considered important at the time, because one can have ice that is not water-based.) That would make a battleship sized alien space craft at least two-and-a-quarter miles wide.
Which might be visible to the naked human eye. However, my original spacecraft, which I drafted in 1994 about the time that Steven Spielberg was polishing his game Dig (involving humans landing on what they think is an asteroid, and discovering that it is a spaceship), is set -- as it were -- in ice. I cannot suddenly turn it into something cooler, like YU 55, which is blacker than charcoal,and is thought to be made of carbon-based materials and some silicate rock.
It's a C-type. Sounds rather Mercedes, doesn't it? And it gets better. There is an S-type!
The best resource I've seen on Asteroids is http://nineplanets.org/asteroids.html
It will tell you the size of the largest known asteroid ( 1 Ceres. 974 km in diameter.) And explain the classifications of asteroids into types according to their spectra (and hence their chemical composition) and albedo (to do with reflection of starlight/sunlight).
C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;
S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;