By Alexandra Witze Via Nature Headline Image Credit: Detlev van Ravensway/SPL A 340-metre-wide space rock named Apophis whizzed safely past Earth on 6 March. The next time it returns, in 2029, won't be so uneventful: Apophis will come within 40,000 kilometres of the planet, skimming just above the region where some high-flying satellites orbit. It will be the first time that astronomers will be able to watch such a big asteroid pass so close to us. Last week's fly-by gave scientists a chance to test the worldwide planetary defence system, in which astronomers quickly assess the chances of an asteroid hitting Earth as they follow its path across the night sky. "It's a fire drill with a real asteroid," says Vishnu Reddy, a planetary scientist at the University of Arizona in Tucson who coordinated the observing campaign. The Apophis fly-by highlights how much astronomers have learnt about near-Earth asteroids — and how much they still have to learn. Since 1998, when NASA kicked off the