Asteroid 2004 BL86 and its moon, observed by NASA's Deep Space Network antenna. Image credit: NASA/JPL-Caltech

Feb 11, 2015 A Near-Earth Asteroid And Its Surprise Moon

Sen—On January 26, 2015, a 325-meter-wide asteroid called 2004 BL86 slid past the Earth, approaching to within 1.2 million kilometers of our planet.

This was nothing to worry about; we knew the asteroid was coming and that it was giving us a comfortable amount of personal space as it passed—three times the distance to the Moon, in fact. A lot of telescopes were trained on the rock, since it’s rare to get one this big passing this close. No other known asteroid of this size will get anywhere near us for more than a decade.

It was big enough to be seen easily in amateur-sized telescopes; Spanish astronomer Dani Caxete used an 80mm refractor to capture the pass in a very cool time-lapse animation:

Each exposure was 15 seconds, and it captures over two hours of movement. At the end, sunrise over Madrid lights the frame.

The most dramatic footage taken, though, was using NASA’s Deep Space Network radio telescope at Goldstone, California:


This animated image shows asteroid 2004 BL86, as it flew past Earth on Jan. 26, 2015. Image Credit: NASA/JPL-Caltech

Look! BL86 has a moon! Statistically speaking this isn’t shocking; lots of asteroids are binary, and about 16% of near-Earth asteroids bigger than about 200 meters across have moons. It’s not well-understood how the moons form; a low-speed collision with another asteroid could have knocked a chunk loose, or they may have both been ejected from a collision on a larger body.

Another idea is that a single asteroid, over time, can have its rotation increased dramatically as it’s warmed by sunlight. It’s thought that many small asteroids are not solid, monolithic bodies, but instead loosely held rubble piles, more like bags of rocks held together by their own gravity. As they spin up, they can disrupt, and some of their material can be thrown off to form a small moon.

In the case of BL86, the moon is about 70 meters across, 1/5th or so the size of the main body.

So why does it look so much smaller? That’s because what you’re seeing isn’t an image like our eyes see, but is instead constructed using data from what’s called “delay-Doppler radar imaging”. The details are a bit complicated (though fascinating!), but it goes like this: The Goldstone antenna pinged the asteroid with radar pulses, similar to a police officer using radar on you as you drive by. Radar reflects off the asteroid, and two things happen: The pulses that bounce off the near side of the asteroid get back to Earth sooner, so a rough shape can be determined, and the other is that the wavelength (think of it as color) of the pulse changes, telling us the rotation rate of the asteroid.

The images created are actually more like a graph, with the x-axis (horizontal) showing you how rapidly the asteroid rotates, and the vertical y-axis showing pulse travel time, so parts closer to Earth appear higher up in the image. The faster the asteroid rotates, the more the wavelength gets smeared out, and the wider the object appears in the images. A rock that isn’t spinning at all would appear as a vertical line!

That’s why the moon looks so much smaller; it’s not that it’s physically tiny, it’s that it’s not spinning nearly as rapidly as the main body. In the images it’s closer to Earth at the start, but as it orbits the main body it appears to approach it. 

Observations like these are critical in understanding the physical nature of the asteroid. Other observations are important, too; for example, using an infrared telescope during the flyby, astronomers determined BL86 is similar in composition to the large main-belt asteroid Vesta, which was visited by the Dawn spacecraft a few years ago. Perhaps BL86 was formed by a collision on Vesta! The beauty of this is that by visiting Vesta and observing it carefully, we can learn more about other asteroids like BL86.


This mosaic synthesizes some of the best views Dawn had of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA

The more we know about near-Earth asteroids, the better. Although BL86 missed us, and by a wide margin, other asteroids get a lot closer, and some do in fact impact our planet. A loud reminder came last year when an asteroid impacted in the Chelyabinsk region of Russia. If we hope to be able to prevent a potential future impact, we need to understand exactly what it is hitting us. Is it a solid body made of metal, or rock, or is it spongy, or like a rubble pile? Any deflection strategy will depend on the asteroid itself, so we need to learn as much as we can about as many as we can.