Today at 12.06 pm UTC - that's 8.06 EDT am - a fairly large asteroid came close to Earth. Asteroid 3122 Florence is about 4.4 km wide (2.7 mi). Scientists consider that if Earth were hit by an object of 1 km (0.6 mi) or more across, it would have the potential to end civilization as we know it.

Fortunately, when astronomers talk about "close" it's not quite what ordinary folk consider close. At its nearest, Florence was at a distance 18 times greater than our distance to the Moon. So nothing to worry about. In fact, the asteroid's orbit is well mapped and there was no chance that it could be a threat to Earth today or for the foreseeable future.

Mona, I was stunned by the size of Florence - we are so vulnerable here on our little blue speck in the stars!

Susan, the good thing is that the really big ones are easiest to find and monitor. But, of course, a downside is that there's yet no believable plan for dealing with one should they find it on a collision course with Earth. An impactor the size of Florence would leave us going the way of the dinosaurs. (Though we may well do it to ourselves before anything like that happens.)

Putting space rocks in perspective, her is an infographic about meteors from LiveScience.

Asteroid Florence has two moons.

Planetary astronomers probed the asteroid with big radar dishes, and NASA's Goldstone tracking station data showed a pair of moons. They think the moons are 100-300 m across and it looks like the inner moon orbits every 8 hours and the outer moon 22-27 hours.

This gives you a real sense of how fast the Martian moon Phobos moves, as it orbits Mars every 8 hours, and although Florence is a big space rock, it's much smaller than Mars.

It's amazing to me that an asteroid could have any moons!

It was a big story when the first moon was discovered orbiting an asteroid over twenty years ago. But I have to admit that I was surprised to learn that over two hundred asteroids are listed as having satellites.

Since asteroids are so small that they're difficult to see - unless they come close to Earth like Florence did - that teensy moons aren't going to be easily visible. Apparently, many of the satellites haven't been detected directly, but rely on glitches in the light curve. (Similar to detected of extrasolar planets)

The first confirmed asteroid and moon was asteroid Ida in 1993. This is a nice clear photo - much better than the radar detection of Florence's moon - because it was taken by the spacecraft Galileo. Galileo did a flyby of Ida while on its way to Jupiter.

Mona, thank you for posting the photo of asteroid Ida and its moon. Yes, the moon is so much smaller than the asteroid.