[meteorite-list] Radar Reveals Two Moons Orbiting Asteroid Florence

[Ron Baalke]

https://cneos.jpl.nasa.gov/news/news199.html

Radar Reveals Two Moons Orbiting Asteroid Florence
Lance Benner, Shantanu Naidu, Marina Brozovic, and Paul Chodas 
Center for NEO Studies (CNEOS)
September 1, 2017

Radar images of asteroid 3122 Florence obtained at the 70-meter antenna 
at NASA\u2019s Goldstone Deep Space Communications Complex between August 
29 and September 1 have revealed that the asteroid has two small moons, 
and also confirmed that main asteroid Florence is about 4.5 km (2.8 miles) 
in size. 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 two moons are not yet well known, but they are probably 
between 100 - 300 meters (300-1000 feet) across. The times required for 
each moon to revolve around Florence are also not yet known precisely 
but appear to be roughly 8 hours for the inner moon and 22 to 27 hours 
for the outer moon. 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 in extent and do not reveal 
any detail.
Animated sequence of radar images of asteroid Florence obtained on Sep. 
1, 2017 using the 70-m antenna at the Goldstone Deep Space Communications 
complex. The resolution of these images is about 75 meters. The images 
show two moons orbiting the much larger central body, which is about 4.5 
km in diameter. The inner moon briefly disappears as it moves behind the 
central body and is hidden from the radar. (NASA/JPL). Animated sequence 
of radar images of asteroid Florence obtained on Sep. 1, 2017 using the 
70-m antenna at the Goldstone Deep Space Communications complex. The resolution 
of these images is about 75 meters. The images show two moons orbiting 
the much larger central body, which is about 4.5 km in diameter. The inner 
moon briefly disappears as it moves behind the central body and is hidden 
from the radar. (NASA/JPL).

The radar images also provide our first close-up view of Florence itself. 
Although the asteroid is fairly round, it has a ridge along its equator, 
at least one large crater, two large flat regions, and numerous other 
small-scale topographic features. The images also confirm that Florence 
rotates once every 2.4 hours, a result that was determined previously 
from optical measurements of the asteroid\u2019s brightness variations.

[Animation]
The animated sequence to the left is built from a series of radar images 
of Florence. The sequence lasts several hours and shows more than two 
full rotations of the large, primary body. The moons can be clearly seen 
as they orbit the main body. Radar images are different from pictures 
taken with a digital camera but are similar to ultrasound images. The 
geometry in radar images is analogous to seeing an object from above its 
north pole with the illumination 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\u2019s Goldstone Solar System Radar in California 
and at the National Science Foundation\u2019s Arecibo Observatory in Puerto 
Rico through September 8. These observations should show more surface 
detail on Florence and provide more precise estimates of the orbital periods 
of the two moons. Those results are valuable to scientists because they 
can be used to estimate the total mass and density of the asteroid.