New Radar Images Show Florence Is A Triple Asteroid

This radar image of Florence shows its newly discovered moons. They appear as streaks because of radar captures images of such small objects. Credit: NASA/JPL

Florence has company! Radar observations of this near-Earth asteroid currently cruising across our evening sky discovered not one but two tiny moons orbiting the 2.8-mile-wide (4.5 km) object. Florence is only the third triple asteroid known in the near-Earth population out of more than 16,400 that have been discovered to date. All three near-Earth asteroid triples have been discovered with radar observations and Florence is the first seen since two moons were discovered around asteroid 1994 CC in June 2009.

The sizes of the moons aren’t precisely known, but they’re probably between 300 – 1,000 feet (100 – 300 meters) across. The inner moon orbits mama in roughly 8 hours for the inner moon and the outer in 22 to 27 hours. The inner moon of the Florence system has the shortest orbital period of any of the moons of the 60 near-Earth asteroids known to have moons. In the Goldstone radar images, which have a resolution of 75 meters, the moons are only a few pixels wide and show no detail.


Radar Captures Two Moons Around Florence. For the best view, choose the full-screen option at lower right. Credit: Arecibo Observatory

Take a close look at the animation, and you’ll see that although Florence is fairly round, it has a higher ridge along its equator, at least one large crater, two large flat regions and lots of other textures. The images show that Florence rotates once every 2.4 hours, which confirms an earlier estimate made by astronomers with an optical telescope who recorded repeating cycles of brightness variations as the asteroid rotated.


Near-Earth flyby of 3122 Florence — individual photos combined into an animation by John Chumack

The animation is built from a series of radar images of Florence made over several hours and records more than two full spins. Radar images are different from pictures taken with a digital camera but are similar to ultrasound images. In radar images, we’re getting a view of the object as if hovering above its north pole with “light” coming from the top. Projection effects can make the positions of Florence and its moons appear to overlap even though they are not touching.

Florence reached its closest approach to Earth early on September 1 and is now slowly receding from our planet. Additional radar observations are scheduled at NASA’s Goldstone Solar System Radar in California and at the National Science Foundation’s Arecibo Observatory in Puerto Rico through September 8. You can still see it through a small telescope as it moves northward through Delphinus into the Northern Cross in the coming nights.