Whatever you call it, Asteroid 2015 TB145 was discovered only three weeks ago – on October 10, 2015 by the University of Hawaii’s Pan-STARRS-1.
It is just over half a kilometer in diameter (600 meters) and made its closest approach to Earth today at 1 p.m. EDT. It’s distance to Earth at its closest point was 486,000 km – or about 1.3 times distance from the Earth to the Moon.
Fittingly, since today is Halloween, if you rotate the images just right the comet/asteroid does sort of look like a skull. Spooky.
The above images were created by NASA using radar data from the 305 meter Arecibo Radio Observatory in Puerto Rico. The images were captured October 30, 2015.
Astronomers have determined, primarily by examining the amount of light the object reflects, that it is likely a dead comet. That is to say it’s a comet, but over the eons it has lost its volatile materials, and so is now reasonably dark and doesn’t produce the typical sign of a comet: a tail. This is why it was initially thought to be (and named) an Asteroid.
In any event, observatories around the world are pointed at it to learn everything we can about it’s composition and orbit. It also underscores the need to keep an eye on the sky, since this is a big piece of rock, reasonably nearby in the grand scheme, and we only found it three weeks ago.