April 23 2009: asteroid 2009 HJ21 flies past the Earth today at a distance only 30% greater than that of the Moon

It's kind of sad at the same time though since as far as we know this is the last really close encounter we're going to have all this year. JPL's list of NEO close approaches here doesn't give anything closer than 6.6 LD (Lunar Distance = 384,400 km) for the rest of the year after this. It's still likely though that we'll encounter a few other asteroids this year that we still haven't discovered and won't know about until they are right on top of us, as was the case a few times last month. It's nothing really to worry about though since the ones we aren't able to discover until they are that close are generally pretty small, only a few dozen metres in diameter, and even if one turns out to be bigger than that it's still only likely that it would land in the ocean or an uninhabited part of northern Russia or Canada.

This asteroid 2009 HJ21 is pretty small too, no more than 20 m in diameter:

Asteroid	Date(UT)	Miss Distance	Mag.	Size
2009 HJ21	Apr. 23	1.3 LD	18	14 m

(the other page says 8.3 - 19 m)

Since I decided a few days ago that approximate surface area should be given with asteroid measurements along with diameter, here's the approximate surface area of this asteroid: 615 m². That's only one tenth of a football field, so very tiny. Why, it's so small I could even whip up a quick image showing what it would look like to stand on an object that small. Here's your average 1.8 m man standing on an object of about this size:


Compare this to a 100 m size asteroid NASA eventually plans to land people on:


Now that's an asteroid worth exploring. And when you look at it like that it's easy to visualize how the surface area of the second body encompasses some six football fields where's the first is only a tenth of one.