Japan's Hayabusa2 probe began descending for its final touchdown on a distant asteroid, hoping to collect samples that could shed light on the evolution of the solar system.
The simulation exercise is a regular event among the international community of 'planetary defence' experts.
Astronomers estimate a 1% risk an asteroid will collide with our planet on April 27, 2027.
Scientists say the story may not be quite that simple, and that massive volcanic eruptions over hundreds of thousands of years may have contributed to the dinosaurs' demise at the end of the Cretaceous period.
The rover mission marks the world's first moving, robotic observation of an asteroid surface.
The spacecraft, OSIRIS-REx, has snapped its first, blurry pic of the cosmic body.
The Hayabusa2 probe settled into an observation position 20 kilometres (12 miles) above the Ryugu asteroid, officials from the Japan Space Exploration Agency (JAXA) said Wednesday.
The asteroid, which originated in another star system, has been given the nickname "BZ".
The asteroid was named "2004 EW95".
Dubbed 2014-JO25, the asteroid came nearest at 12:20 GMT and is now hurtling away from the centre of our solar system, says astronomer Ian Carnelli.