On September 1, 2023, a large near-Earth asteroid named Florence will fly safely past our planet at a distance of about 4.4 million miles (7.0 million kilometers), or about 18 times the average Earth-Moon distance. This will be the closest approach by this asteroid since 1890 and the closest it will ever be until after 2500 .

Florence is one of the largest near-Earth asteroids that are several miles in size; measurements from NASA’s Spitzer Space Telescope and NEOWISE mission indicate it’s about 2.7 miles (4.4 kilometers) in size . It was discovered in 1981 by Schelte “Bobby” Bus at Siding Spring Observatory in Australia and named in honor of Florence Nightingale (1820-1910), the founder of modern nursing.

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) . Radar imaging is planned at NASA’s Goldstone Solar System Radar in California and at the National Science Foundation’s Arecibo Observatory in Puerto Rico.

Although many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller. “Florence is the largest asteroid to pass by our planet this close since the NASA program to detect and track near-Earth asteroids began,” said Paul Chodas, manager of NASA’s Center for Near-Earth Object Studies (CNEOS) at the agency’s Jet Propulsion Laboratory in Pasadena, California.

Asteroids are small, rocky bodies that orbit the Sun. Most of them are found in the main asteroid belt between Mars and Jupiter, but some have orbits that bring them close to Earth. These are called near-Earth asteroids or NEAs. NEAs are of interest to scientists because they offer clues to the origin and evolution of the solar system, as well as the potential threat they pose to our planet.

NASA has a program to detect and track NEAs using ground-based telescopes and spacecraft. The goal is to find and catalog at least 90 percent of the NEAs that are one kilometer or larger in size, as these are considered to pose the greatest threat to Earth. As of August 2023, more than 25,000 NEAs have been discovered, of which about 1,000 are larger than one kilometer.

The September 1 flyby of Florence will be a rare chance for both professional and amateur astronomers to observe a large NEA. Florence will brighten to ninth magnitude in late August and early September, when it will be visible in small telescopes for several nights as it moves through the constellations Piscis Austrinus, Capricornus, Aquarius and Delphinus .

If you are interested in observing Florence, you can find star charts and more information at the Sky and Telescope website: https://skyandtelescope.org/observing/large-asteroid-florence-fly-earth-september-1/

You can also visit NASA’s Asteroid Watch website for updates on Florence and other NEAs: https://www.jpl.nasa.gov/asteroidwatch

Florence is not a threat to Earth, but it is a reminder that we live in a dynamic solar system where asteroids and other celestial objects can come close to our planet. By studying these objects, we can learn more about our cosmic neighborhood and be prepared for any potential impacts in the future.