A mountain-size space rock cruised past Earth this morning (Sept. 1) in a record-breaking encounter that has long had scientists drooling. 
 
 At 8:06 a.m. EDT (1206 GMT) today, the roughly 2.7-mile-wide (4.4 kilometers) asteroid 3122 Florence came within a mere 4.4 million miles (7 million km) of Earth — just 18 times the distance from our planet to the moon. 
 
 "Nothing this big has passed this close to Earth since we've been tracking," Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA's Jet Propulsion Laboratory in Pasadena, California, told Space.com. "This is a once-in-40-year-event kind of thing." [Famous Asteroid Rock Flybys and Close Calls (Infographic)] 
 
 The big asteroid 3122 Florence, as seen by the Virtual Telescope Project on Aug. 30, 2017 — two days before its closest approach to Earth. Credit: Gianluca Masi/Virtual Telescope Project 
 
 Only 10 or so asteroids as big as Florence exist in near-Earth space, Chodas said. There was never any chance of the space rock hitting us on this encounter, and astronomers have mapped out Florence's orbit well enough to know that it poses no threat to Earth for the next few centuries at least, he stressed. (But if something as big as Florence did hit us, it would be very bad: Scientists regard any impactor at least 0.6 miles, or 1 km, across as a potential civilization-ender.) 
 
 Florence was discovered by astronomer Schelte "Bobby" Bus in 1981 at Australia's Siding Spring Observatory. The space rock loops around the sun every 2.35 years on an elliptical path, getting as close to our star as 1 astronomical unit (AU) and as far away as 2.5 AU. (One AU is the average Earth-sun distance — about 93 million miles, or 150 million km.) 
 
 Astronomers have also determined that Florence is a superfast spinner, completing one rotation every 2.4 hours. 
 
 "If it were spinning any faster, it would fly apart," Chodas said. "What often happens is, asteroids that are spinning this quickly rearrange into the shape of a top, where they have kind of a bulge at the equator." 
 
 But that's just an inference in Florence's case; the asteroid's shape isn't known for certain. Indeed, not much about Florence is. For example, astronomers don't know if the space rock has a moon. And the 2.7-mile size estimate, which is based on infrared observations by NASA's Spitzer Space Telescope and NEOWISE mission, is just that — an estimate. It could change once researchers get some better observations, Chodas said. 
 
 "There's a lot to learn about this little world," he said. 
 
 On Sept. 1, asteroid Florence will fly by Earth, and come to within about 18 times the distance from the Earth to the moon. Credit: NASA 
 
 Which brings us back to today's flyby. Florence has come close enough on this encounter — eight times closer to Earth than it's ever been since its 1981 discovery, Chodas said — to be pinged by radar, and scientists are taking full advantage of the opportunity. They've been using powerful facilities such as NASA's Goldstone Solar System Radar in California and the Arecibo Observatory in Puerto Rico to get the first-ever good looks at Florence. 
 
 The radar images should reveal Florence's true size and shape. They may even achieve 10-foot (3 meters) resolution, allowing surface features such as boulders and craters to be spotted, Chodas said. Optical telescopes around the world have been trained on Florence as well, gathering other types of data that will further flesh out researchers' understanding of the asteroid, he added. [In Photos: Potentially Dangerous Asteroids] 
 
 This flyby is like a space mission to an asteroid, "but the asteroid is coming to us," Chodas said. 
 
 This sky map by Sky & Telescope magazine shows the location of the asteroid 3122 Florence as it moves across the night sky in late August and early September 2017. The asteroid will fly within 4.4 million miles (7.1 million km) of Earth on Sept. 1. Credit: Sky & Telescope Magazine diagram 
 
 Astronomers aren't the only ones who can get looks at Florence right now; the asteroid should be visible through backyard telescopes for the next few days. You can read about how to see asteroid Florence in telescopes here. 
 
 Editor's Note: If you capture a great image of asteroid Florence moving across the sky and would like to share it with Space.com and our news partners, send photos and comments to spacephotos@space.com. 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com. 

 Asteroid Florence, a large near-Earth asteroid, will pass safely by Earth on Sept. 1, 2017, at a distance of about 4.4 million miles, (7.0 million kilometers, or about 18 Earth-Moon distances). Florence is among 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. 
 
 "While many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller," said Paul Chodas, manager of NASA's Center for Near-Earth Object Studies (CNEOS) at the agency's Jet Propulsion Laboratory in Pasadena, California. "Florence is the largest asteroid to pass by our planet this close since the NASA program to detect and track near-Earth asteroids began." 
 
 This relatively close encounter provides an opportunity for scientists to study this asteroid up close. Florence is expected to be an excellent target for ground-based radar observations. 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. The resulting radar images will show the real size of Florence and also could reveal surface details as small as about 30 feet (10 meters). 
 
 Asteroid Florence was discovered by Schelte "Bobby" Bus at Siding Spring Observatory in Australia in March 1981. It is named in honor of Florence Nightingale (1820-1910), the founder of modern nursing. The 2017 encounter is the closest by this asteroid since 1890 and the closest it will ever be until after 2500. 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. 
 
 Radar has been used to observe hundreds of asteroids. When these small, natural remnants of the formation of the solar system pass relatively close to Earth, deep space radar is a powerful technique for studying their sizes, shapes, rotation, surface features and roughness, and for more precise determination of their orbital path. 
 
 JPL manages and operates NASA's Deep Space Network, including the Goldstone Solar System Radar, and hosts the Center for Near-Earth Object Studies for NASA's Near-Earth Object Observations Program, an element of the Planetary Defense Coordination Office within the agency's Science Mission Directorate. 
 
 More information about asteroids and near-Earth objects can be found at: 
 
 https://cneos.jpl.nasa.gov 
 
 https://www.jpl.nasa.gov/asteroidwatch 
 
 For more information about NASA's Planetary Defense Coordination Office, visit: 
 
 https://www.nasa.gov/planetarydefense 
 
 For asteroid and comet news and updates, follow AsteroidWatch on Twitter: 
 
 twitter.com/AsteroidWatch 
 
 News Media Contact 
 
 DC AgleJet Propulsion Laboratory, Pasadena, Calif.818-393-9011agle@jpl.nasa.govLaurie Cantillo / Dwayne BrownNASA Headquarters, Washington202-358-1077 / 202-358-1726laura.l.cantillo@nasa.gov / dwayne.c.brown@nasa.gov2017-223
– There was no reason to fret, but perhaps reason to be wowed: "Florence," an asteroid stretching an estimated 2.7 miles wide, became the largest known asteroid to come as close as it did to our planet when it whizzed by Friday at 8:06am EDT. But though it came relatively close to Earth by space standards, it was still 4.4 million miles away, or about 18 Earth-Moon distances, according to NASA. Astronomers who've mapped Florence's orbit were well aware the asteroid posed no danger to Earth—and it won't get any closer to us until after 2500—and were thrilled at the prospect of its flyby, which provided an opportunity to study the mass up close. Radar images could perhaps reveal details of Florence's surface as small as 10 feet wide, per Space.com. The images may also allow astronomers to gauge Florence's true size and shape for the first time. Until now, they've only been able to speculate that it may take "the shape of a top" with "a bulge at the equator," Paul Chodas of NASA's Jet Propulsion Laboratory tells Space.com. That's a common form for fast-spinning asteroids like Florence—named after Florence Nightingale and one of an estimated 10 large asteroids flying in near-Earth space. "If it were spinning any faster, it would fly apart," says Chodas, who says of the flyby, "Nothing this big has passed this close to Earth since we've been tracking." (The asteroid that wiped out the dinosaurs was twice as big.)