A rock about 10 meters in size will fly past the Earth Monday at
13:30 UTC (09:30 Eastern US time) 17:01 UTC (13:01 Eastern US time) [Note: the time of closest approach was updated this morning, June 27]. It'll be a particularly close shave -- passing just 12,400 km (7430 miles) from Earth's surface; a bit less than the diameter of the Earth itself -- but it'll miss for sure.
We're in no danger from the asteroid, named 2011 MD, since there's essentially zero chance it will hit us. Even if it did, it's too small to impact the surface, and would instead break apart and burn up in the atmosphere. That would be exciting, and make quite a show, but that's about it.
Here's a diagram of the asteroid's trajectory (note that the size of the Earth is not to scale!):
On this scale, the Earth is actually about half the size shown; it was enlarged on the diagram to make it clear. In this smaller diagram here the trajectory is shown from a different angle (edge-on to the Moon's orbit) with the Earth to scale, and you can see better that 2011 MD will miss us.
You can really see in those diagrams just how much the Earth's gravity bends the orbit. At this close an approach the Earth's gravity is significant, and the path of the object will be significantly altered. Just how much it's changed is difficult to know until observations are made after the event. However, I'll note the asteroid will be on the daylit side of Earth after it passes, making observations a bit tricky. I expect radar observations will be made using radio telescopes, which don't need darkness to work and can provide very accurate measurements of the orbital path.
Speaking of orbits, this one is interesting. 2011 MD's orbit is quite similar to Earth's around the Sun, taking 396 days to go around once. The orbit is more elliptical than Earth's, with a semi-major axis of 1.056 AU.
What does that mean? Well, the major axis is the length of an ellipse across its longest dimension, and the semi-major axis is half that length (that may seem like a funny unit but it's convenient to do the math). An AU is the average distance of the Earth to the Sun, and again is a convenient unit for astronomers to use. The Earth orbits the Sun in an ellipse that is pretty close to being a perfect circle, and by definition the semi-major axis of our orbit is 1.0.
So 2011 MD's orbit is a bit more oval and a bit bigger than ours, just barely crossing it. Asteroids like that are called Apollo asteroids, and there are quite a few in that category. As you can see in this diagram, the orbits are so similar it's hard to distinguish them on the left -- look to the right and you'll see them separate (also shown are the orbits and positions of Mercury, Venus, and Mars for June 27).
It orbits the Sun in 1.085 years, so every 13 years or so (12 of its orbits) we pass it. Some encounters are closer than others, and this is most likely the closest we've come in a long time, and most likely will be for some time in the future as well (especially given how much the orbit of 2011 MD will change). In fact, this will be the fifth closest approach of an asteroid on record... and one of those on the list, 2008 TC3, actually impacted us, harmlessly breaking up high over Africa in 2008.
That may be the fate of 2011 MD someday as well... but not today. For now, we get a very good view of this tiny interloper as it zooms past. And it'll get a good view of us: it'll pass over the South Atlantic at closest approach, and the Earth will fill half its sky. What a view that would be! Astronomer Pasquale Tricaric put together some very cool animations showing the asteroid's-eye-view, too.
And I do want to point out one more thing. Since the orbit is very Earth-like, the relative velocity of this rock to us is pretty small. Imagine two cars on a race track, one moving at 200 kph and the other at 210 kph. To someone standing by the track they both scream past at high speed, but to the slower car, the faster one overtakes it at 10 kph -- not very fast at all.
The same is true here. Both Earth and 2011 MD orbit the Sun at about 30 km/sec, but the relative speed between the two is small -- only about a kilometer/second (except right at closest approach, when the Earth's gravity speeds it up). That means if we were to launch a rocket at it, it wouldn't take very much fuel to "catch up" to it. That's why rocks like these are so very interesting: exploring them is easier in many ways than even getting to the Moon! It's too late for this particular asteroid this time around, of course... but there are plenty more out there, and their time will come.
And just to be clear once again, since I know some of my readers fret over these things: this rock will miss us for real and for sure.
Orbit diagram credits: NASA/JPL-Caltech