September seems to be quite the month for close encounters with asteroids. After the flyby of 2016 QA2 last week, three other objects will cross paths with our planet this month.

Today, we have had two close encounters. At 1.12pm EDT (6.12pm BST), asteroid 2016 RB1 cameas close as 40,000 kilometers (25,000 miles) to planet Earth, about 12 percent of the Earth-Moon distance. The space rock is 10 meters (32 feet) across, a pebble compared to 250458 (2004 BO41), another object that has crossed our sky.

250458 flew over our heads at 2.49am EDT (7.49am BST) and is estimated to be between 0.7 and 1.6 kilometers (0.4 and 1 miles) across. It passed at the very safe distance of 16 million kilometers (10 million miles), almost 39 times the distance to the Moon, as reported by NASA’s Near-Earth Objects (NEOs) Program.

If an asteroid this size were to hit Earth, it would spell certain doom for most life on the planet. According to a research paper from last year, an asteroid of 1 kilometer (0.6 miles) in diameter hitting a continental land mass would turn the planet into a dry, frigid, and dark world.

Later next week, another asteroid will pass nearby. The recently discovered 2016 QL44 will pass near Earth on September 17 at 2.48pm EDT (7.48pm BST), although due to some uncertainties the timing may vary. At its perigee (closest point), 2016 QL44 will be 1.5 million kilometers (923,000 miles) away and it's estimated to be between 27 and 60 meters (88 and 200 feet) across.

While these close passages shouldn’t be cause for alarm, we cannot put our heads in the sand and pretend there is no risk. Asteroids of all sizes are a clear and present danger for humanity, and many government agencies including NASA and ESA have projects to monitor NEOs.

The effort is there, but there is still much to do. We have found only about 1.5 percent out of the 1 million hazardous asteroids larger than 30 meters (100 feet) in size around Earth. We must keep our guard up.