An asteroid flew pretty close to Earth last week. It missed us, you may have noticed. But we didn’t spot it until three days after it had flown past, which is a pretty terrifying reminder about the dangers asteroids pose to our world.

Asteroid 2017 001 was about 37 to 77 meters (121 to 252 feet) across. It came about a third as close as the Moon, 123,031 kilometers (76,448 miles) from our planet, which is a pretty safe distance. It flew past at some point in the night of July 20 at a speed of about 10.36 kilometers (6.4 miles) per second.

However, it was not until July 23 that we spotted the asteroid, after it had passed Earth, thanks to the ATLAS-MLO telescope in Hawaii. That’s, you know, not great.

“Asteroid 2017 OO1 is an important reminder that we need to do a better job at detecting even small-sized asteroids early,” Grigorij Richters, the founder of Asteroid Day that seeks to raise awareness about asteroids, told IFLScience. “Small asteroids can cause significant regional damage and we are the only species that can do something about this cosmic hazard. Just ask the dinosaurs!”

_{An animation of asteroid 2017 001 missing Earth. The Watchers}

For a bit of comparison, the Chelyabinsk meteor that exploded over Russia in 2013 was about 20 meters (65 feet feet) across. That injured hundreds, and caused considerable damage including shattering windows.

In 1908, the famous Tunguska meteor – measuring 50 to 100 meters (165 to 330 feet) across – exploded over Siberia. It flattened about 80 million trees over an area of 2,000 square kilometers (770 square miles), although fortunately no one was killed. That’s almost the area of Luxembourg.

We know of about 75 percent of asteroids larger than 1 kilometer (0.6 miles) in Earth’s vicinity, deemed as being able to cause extinction events. More alarmingly, we only know about 0.5 percent of asteroids smaller than 120 meters (390 feet). These could cause pretty major damage if they hit Earth.

Asteroid 2017 001 did not hit Earth. However, it should be a pretty good reminder that we need to do more to detect asteroids. Currently, most of our data comes from NASA’s WISE telescope in orbit, which has found thousands of asteroids since 2009 – but not nearly enough.

What would we do if one was coming our way? Well, there are a number of ideas, such as using an impactor to slightly change the trajectory of one. Sadly, ESA scrapped its involvement in a mission to test this out. Hopefully, the threat will be taken seriously in the future. The dinosaurs didn’t have a space program – we do.