Asteroids whizz past Earth on a regular basis, and thankfully they're usually only a few meters wide. But next month we're due for a close encounter with a monster space rock measuring 4.4 km (2.7 mi) across, making it the largest near-Earth object (NEO) to come this close to the planet since NASA began tracking them almost 20 years ago.

NASA and other agencies began tracking potentially hazardous objects in 1998, in order to give us enough warning to send up Bruce Willis (or a pair of orbit-altering probes). Since then, over 16,000 NEOs have been spotted, and although none have been found to be on a collision course, it's best to be prepared: as the 20-m (66-ft) Chelyabinsk meteor demonstrated in 2013, even a relatively small rock can wreak havoc.

But it's the really big ones that we need to keep a close eye on. Anything larger than 1 km (0.6 mi) could trigger planet-wide, life-extinguishing chaos, and by NASA's most recent count, there are over 880 of these monsters zipping around our neighborhood. At 4.4 km in size, asteroid Florence is one of the biggest, and if it were ever to crash into Earth it would be a swift Game Over for humanity.

Luckily, although it's about to make its closest pass on record, NASA says there's no chance of a collision this time, or at any point in at least the next 500 years. Florence will whip past on September 1 at a distance of 7 million km (4.4 million mi), or about 18 times the distance between Earth and the Moon.

Florence will fly past Earth at a distance of 7 million km (4.4 million miles), or 18 times the distance between Earth and the Moon NASA/JPL-Caltech

"While many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller," says Paul Chodas, manager of NASA's Center for Near-Earth Object Studies (CNEOS). "Florence is the largest asteroid to pass by our planet this close since the NASA program to detect and track near-Earth asteroids began."

Florence will be visible to small telescopes for a few nights in late August and early September, and astronomers are planning to use the opportunity to observe the asteroid more closely. The Goldstone Solar System Radar in California and the Arecibo Observatory in Puerto Rico will both be used to capture radar images of Florence to get a more accurate reading of its size and surface details, down to a scale of about 10 m (30 ft).

Earth is in for a far closer shave in October, as asteroid 2012 TC4 brushes past within 50,100 km (31,130 mi) or just 0.13 lunar distances. But this rock is much smaller, measuring only 15 m (49 ft) across.

Source: NASA