Florence doesn't pose a risk - but is being closely monitored in case of a catastrophic return in the future.
The monster rock is the largest spotted in close proximity to our planet since records began 20 years ago.
Florence, a Near-Earth Object (NEO), will pass by safely on September 1, but if she did smash into us one day, it would be game over.
Anything larger than 0.6 miles could extinguish life as we know it, scientists have calculated.
So at 2.7miles across, Florence could trigger an apocalyptic disaster.
Nasa has identified 16,000 NEOs since 1998.
None are on a collision course with Earth, but as the Chelyabinsk meteor incident proved, it's best to be prepared.
Buildings were flattened and hundreds were left injured after the 66ft space rock caused a powerful shock wave upon impact.
Paul Chodas, manager of Nasa's Center for Near-Earth Object Studies said: "While 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."
Named after the founder of modern nursing, Mrs Nightingale herself, it's hoped she'll be gentle if the time comes to change trajectory.
But that's unlikely in the next 500 years according to orbit predictions.
The enormous chunk will pass four million miles away from our planet on September 1 - which is about 18 times the distance between Earth and the moon.
Doomongerers have already warned that our days are numbered.
It's feared we have just 20 years to avert a mass extinction and one expert believes there's a BIG chance we'll kill ourselves off by the end of the year.
A previous asteroid impact caused this crater in Arizona
Our home planet will one day burst into a massive ball of fire when the sun swells to 100 times its size and munches everything in sight.
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