Later today, two asteroids will swing so close to the Earth that they pass between us and the orbit of the moon.
Anyone with a reasonable-sized amateur telescope should be able to spot the objects, which are both a few meters in diameter. However, because they are moving so fast, observers will need sharp eyes.
Neither has any chance of hitting Earth, says NASA.
The Catalina Sky Survey near Tucson, Arizona, discovered both objects on Sunday morning, during a routine scan of the skies. The Minor Planet Center in Cambridge, Massachussetts worked out the orbits.
Near-Earth asteroid 2010 RX30 is estimated to be 10 to 20 meters in size and will pass within 248,000 kilometers of the earth at 2:51am PDT today. The second object, 2010 RF12, is a little smaller at around 6 to 14 meters in size, but will come even closer, at 79,000 kilometers from Earth, at 2:12pm PDT.
What's unusual is for two asteroids to make such close passes at almost the same time. In fact, with around 50 million undiscovered asteroids out there, NASA reckons the odds are that one this size passes this close almost daily. One might strike Earth's atmosphere about every 10 years on average, it says.
More worrying is an asteroid predicted to arrive in 2182, which has a one-in-a-thousand chance of hitting Earth unless it is deflected.
Earlier this year, a new telescope in Hawaii started searching for potentially dangerous asteroids. However, the National Academy of Sciences recently warned that NASA's near-Earth asteroid tracking program may be in danger of running out of funds.