Hayabusa2's descent was delayed for about five hours for a safety check, but the unmanned craft is still due to touch down as scheduled on Friday morning, the Japan Aerospace Exploration Agency said.

During the touchdown, which will last just seconds, Hayabusa2 will extend a pipe and shoot a pinball-like bullet into the asteroid to blow up material from beneath the surface. If all goes successfully, the craft will then collect samples that would eventually be sent back to Earth. Friday's attempt is the first of three such touchdowns planned.

The brief landing will be challenging, because of the uneven and boulder-covered surface. Hayabusa2 is aiming for a 6-meter- (20-foot-) diameter circle to avoid obstacles. Space agency controllers will direct its approach until it is 500 meters (1,600 feet) above the asteroid's surface, after which it will be on its own because it takes 20 minutes for commands from Earth to reach the craft.