A huge, Amor-class asteroid named 3122 Florence (1981 ET3) will flyby Earth at 18.38 LD (~7 million km / 4.4 million miles / 0.047 AU) at 12:06 UTC on September 1, 2017. This is the largest asteroid to pass this close to our planet since the first near-Earth asteroid was discovered over a century ago. Its diameter is between 4 and 9 km (2.5 and 5.6 miles).
At its closest point, Florence will be 7 million km (4.4 million miles) from Earth, or about 18 times the average Earth-Moon distance. This will be its closest approach since 1890.
"Although many known asteroids have passed by closer than this, all of them were smaller asteroids. Florence is the largest asteroid to pass this close to our planet since the first near-Earth asteroid was discovered over a century ago," according to Paul Chodas of Center for NEO Studies.
Florence is also the brightest known near-Earth asteroid, brighter even than 5.1 km (3.1 miles) wide 3200 Phaeton, thought to be the source of the Geminid meteor shower.
At magnitude +9 in late August, Florence will be a relatively easy target for experienced observers at sites with dark skies. It won't be visible to the unaided eye but will become visible in small telescopes by late August.
The September 1 flyby of Florence will provide astronomers with an excellent opportunity to make detailed measurements of a large near-Earth asteroid. In particular, radar scientists expect to obtain high-resolution images of Florence that could reveal surface features as small as about 10 meters (30 feet).
Although its orbit is well established, comparatively little is known about this large asteroid’s physical properties. Infrared measurements from NASA’s Spitzer Space Telescope and the NEOWISE spacecraft indicate that Florence is roughly 4.3 km (2.7 miles) in size, and measurements of its brightness variations indicate that it rotates once every 2.36 hours.
Asteroid Florence was discovered in 1981 and named in honor of Florence Nightingale (1820-1910), the founder of modern nursing. Tracking observations of asteroid Florence span nearly 40 years, and its orbit is already well known.
The orbital calculations indicate that it poses no risk of colliding with Earth for many centuries to come.
The next time 3122 Florence will come nearly as close as this will occur around 00:41 UTC on September 2, 2057. All the close encounters within 0.1 AU occur from late August to early September when the asteroid is near its ascending node. The intervals between successive approaches < 0.1 AU are often 40, 73, and 87 years.
Featured image credit: ESA (representative image)