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The largest asteroid in more than a century is set to pass by Earth at a relatively close distance of 4.4 million miles (7 million km), Nasa says. Florence measures 2.7 miles (4.4km) in diameter and will not pose a threat to Earth for centuries to come.
Amateur astronomers will also be tracking it, says Sky and Telescope Magazine. The rock is relatively easy to see with good observing equipment, not just because it is large but also because it reflects more than 20% of the sunlight that hits its surface. The Moon in contrast has an average reflectivity of just 12%.
Radar images of asteroid 3122 Florence obtained at the 70-meter antenna at NASA’s 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.