On June 27th 2011, a chunk of rock about the size of a school bus (though maybe as large as an 18-wheeler) whizzed by Earth at a distance of around 12,000 km (7,460 miles) from the ground. Discovered by LINEAR on June 22nd, it was designated 2011 MD by the MPC.
To put its close approach distance in perspective, consider a few facts:
2011 MD flies by the Earth in this schematic depiction with distances drawn to scale. Note the sudden change in its orbit due to Earth's gravitational attraction. Image courtesy of NASA/JPL-Caltech.
- Earth’s diameter is 12,750 km (7,925 miles).
- GPS satellites orbit 20,200 km (12,555 miles) above the Earth’s surface.
- Geostationary satellites such as Meteosat orbit at a height of 35,800 km (22,250 miles).
A modern jet airliner would take a little over 13 hours to cover the distance to 2011 MD at its closest approach, and it was so close to our planet that the asteroid’s orbit was violently changed by Earth’s gravity, as you can see in the picture to the right (and at the bottom of this article).
Despite this sudden change in direction, 2011 MD will not be thrown out of the Solar System but will continue to orbit the Sun as an Amor class of NEO and, barring any close encounters with another asteroid that would change its orbit, it shouldn’t come this close to Earth again.
Not that this close approach wasn’t exciting, certainly to astronomers the world over who trained their telescopes on this little asteroid to record its passage. Below I’ve included a video showing actual images of the asteroid as seen from Earth. You might be disappointed to see it’s only a speck of light, but a lot can be learnt from it; seeing how the brightness of 2011 MD changes over time is telling us that it’s rotating (or more likely tumbling), and that it’s not spherical in shape.
Pasquale Tricarico, at the Planetary Science Institute in Arizona produced these great animations of 2011 MD as it flies by. Make sure to scroll down the page, and be patient as there is some heavy image downloading going on.
Dedicated Austrian amateur astronomer Gerhard Dangl (MPC observatory code C47) has a rather technical page with lots of great info gleamed from his own observations of 2011 MD; I thoroughly recommend the visit to my geekier readers. Amongst other things, he provides a before and after picture of 2011 MD’s orbital inclination that shows how dramatic the encounter with Earth was for this little piece of rock.
Change in the inclination of 2011 MD's orbit after its encounter with Earth. Don't mess with Big Mummy Earth, little guy! Image courtesy of Gerhard Dangl.
As our telescopes improve, we’ll begin finding more and more of these close-flyby asteroids, though still only a few days prior to the encounter. What I’m left wondering is…how much longer until one of these enters the atmosphere? At a size similar to 2011 MD it will more than likely burn up or explode high in the atmosphere, but if it were 3 or 4 times larger…and metallic…then we could be in trouble.
Sweet dreams!