An asteroid will fly over our heads today at a (completely safe) distance of 750,000 miles.
The asteroid (2004 BL86) is about the size of a skyscraper, being a third of a mile long. It will fly over the Earth on Monday (26 January) evening.
And yes - were it to hit us, it could probably destroy a very large city or create a pretty big wave in the ocean.
It will also be (just about) close enough to see with a pair of good binoculars.
But while it sounds terrifying there is no danger: the rock is nowhere near hitting us (it's three times farther from us than the Moon) and poses "no threat for the foreseeable future" according to NASA. It's also the closest any large asteroid will come to us until 1999 AN10 in 2027 -- which will also miss us.
BL86 was discovered using the Lincoln Near-Earth Asteroid Research telescope in New Mexico in 2004. It is one of about 10,000 near-Earth asteroids with the potential for causing widespread destruction that we already know about.
Unfortunately there are at least a million asteroids in the galaxy, and we only track less than 1% of them.
NASA's Near Earth Object program - sometimes known as 'Spaceguard' - has detected most of the threatening asteroids above a certain (large) size. Similar programs are run by ESA and others. But of the estimated million asteroids in our solar system that have the potential to destroy a city on Earth, we currently know about 10,000 of them -- or less than 1%.
In late 2014 more than 100 scientists and activists signed a pledge to support the first global Asteroid Day on June 30th 2015, in order to call for a 100-fold increase in the discovery and tracking of near-Earth asteroids, and for new technology to track those rocks.
"As scientists and citizens, we strive to solve humanity’s greatest challenges to safeguard our families and quality of life on Earth in the future.
"Asteroids impact Earth: such events, without intervention, will cause great harm to our societies, communities and families around the globe. Unlike other natural disasters, we know how to prevent asteroid impacts."
Unfortunately the tracking of very large asteroids can only make a practical difference if we're then able to do something about the rock - deflecting or destroying it, preferably. And unfortunately - again - NASA's best guess on how to do that so far is… prayer.