Asteroid 2004 BL86 and its small moon

[Dotini]

http://www.nasa.gov/jpl/asteroid-that-flew-past-earth-today-has-moon/

Radar observations of small object apparently orbiting an asteroid near Earth.

Edit: a sharpened video.
http://www.space.com/28438-asteroid-2004-bl86-moon-sharp-video.html

http://www.kurzweilai.net/high-def-radar-images-of-near-earth-asteroid-captured

 

[Bystander]

"In the near-Earth population, about 16 percent of asteroids that are about 655 feet (200 meters) or larger are a binary (the primary asteroid with a smaller asteroid moon orbiting it) or even triple systems (two moons)." ---- 1st link
Does this imply an unusually well-sorted speed/velocity distribution? I'm having some difficulty evolving an asteroid belt from the old sci-fi picture of random rubble randomly colliding into something this organized.

 

[mfb]

Collisions can lead to moons, and distances between asteroids are huge so those systems can be stable over long timescales.

 

[Bystander]

stable over long timescales.

Once captured, yes. How do we go about dissipating energy to accomplish the captures? Tidal effects are going to be too small to be effective over the few billion years they've had to work. At which point, arises the question, "Delta V for captures is small enough to suggest an unusual sorting mechanism?"

 

[mfb]

Collisions dissipate a lot of energy. A study of the relative axes of rotation and orbits could be interesting, but that is hard to measure for most objects.

 

[TEFLing]

I recall that capture can occur with three bodies involved

And many asteroids look like dumbbells, being multilobed

Those separate lobes could shift around so as to act like dissipative multiple bodies

 

[TEFLing]

Maybe moons of planetoids and planets of stars and satellite galaxies etc are ubiquitous and universal at all size scales?

 

[Dotini]

http://www.universetoday.com/118650/awesome-new-radar-images-of-2004-bl86/
Awesome New Radar Images of Asteroid 2004 BL86

by Bob King on January 31, 2015

New video of 2004 BL86 and its moon
Newly processed images of asteroid 2004 BL86 made during its brush with Earth Monday night reveal fresh details of its lumpy surface and orbiting moon. We’ve learned from both optical and radar data that Alpha, the main body, spins once every 2.6 hours. Beta (the moon) spins more slowly.
The images were made by bouncing radio waves off the surface of the bodies using NASA’s 230-foot-wide (70-meter) Deep Space Network antenna at Goldstone, Calif. Radar “pinging” reveals information about the shape, velocity, rotation rate and surface features of close-approaching asteroids. But the resulting images can be confusing to interpret. Why? Because they’re not really photos as we know it.

Individual radar images of 2004 BL86 and its moon. The 1,100-foot-wide asteroid appears very lumpy, possibly from unresolved crater rims. The moon (230-feet) appears elongated but that may be an artifact of radar imaging and not its true shape. Credit: NASA
For one, the moon appears to be revolving perpendicular to the main body which would be very unusual. Most moons orbit their primary approximately in the plane of its equator like Earth’s moon and Jupiter’s four Galilean moons. That’s almost certainly the case with Beta. Radar imagery is assembled from echoes or radio signals returned from the asteroid after bouncing off its surface. Unlike an optical image, we see the asteroid by reflected pulses of radio energy beamed from the antenna. To interpret them, we’ll need to put on our radar glasses.
Bright areas don’t necessarily appear bright to the eye because radar sees the world differently. Metallic asteroids appear much brighter than stony types; rougher surfaces also look brighter than smooth ones. In a sense these aren’t pictures at all but graphs of the radar pulse’s time delay, Doppler shift and intensity that have been converted into an image.

Another set of images of 2004 BL86 and its moon. Credit: NAIC Observatory / Arecibo Observatory
In the images above, the left to right direction or x-axis in the photo plots the toward and away motion or Doppler shift of the asteroid. You’ll recall that light from an object approaching Earth gets bunched up into shorter wavelengths or blue-shifted compared to red-shifted light given off by an object moving away from Earth. A more rapidly rotating object will appear larger than one spinning slowly. The moon appears elongated probably because it’s rotating more slowly than the Alpha primary.
Meanwhile, the up and down direction or y-axis in the images shows the time delay in the reflected radar pulse on its return trip to the transmitter. Movement up and down indicates a change in 2004 BL86’s distance from the transmitter, and movement left to right indicates rotation. Brightness variations depend on the strength of the returned signal with more radar-reflective areas appearing brighter. The moon appears quite bright because – assuming it’s rotating more slowly – the total signal strength is concentrated in one small area compared to being spread out by the faster-spinning main body.
If that’s not enough to wrap your brain around, consider that any particular point in the image maps to multiple points on the real asteroid. That means no matter how oddly shaped 2004 BL86 is in real life, it appears round or oval in radar images. Only multiple observations over time can help us learn the true shape of the asteroid.
You’ll often notice that radar images of asteroids appear to be lighted from directly above or below. The brighter edge indicates the radar pulse is returning from the leading edge of the object, the region closest to the dish. The further down you go in the image, the farther away that part of the asteroid is from the radar and the darker it appears.
Imagine for a moment an asteroid that’s either not rotating or rotating with one of its poles pointed exactly toward Earth. In radar images it would appear as a vertical line!

Edit: The author of the above story says something potentially mixed up, IMO. He says, "The moon appears elongated probably because it’s rotating more slowly than the Alpha primary."

According to another source, Beta moving much faster than Alpha.
http://www.sciencedaily.com/releases/2015/01/150130142319.htm