NASA scientists have come up with a surprisingly simple yet effective way to deflect an Earth-bound asteroid: park a large spacecraft close by and let gravity do the work, creating an invisible towline to tug the rock off its deadly course.

The strategy crucially relies on our ability to detect an asteroid threat about 20 years in advance. For larger asteroids this is realistic. But Erik Asphaug, a planetary scientist at the University of California at Santa Cruz, says many smaller asteroids, less than about 500 metres across, may go unnoticed until only a few years before impact. He suggests it may be better to invest in predicting when and where smaller asteroids could strike, than on massive hazard-averting spacecraft.