The space cannon was reportedly successfully tested recently here on Earth and is scheduled to be sent into space in 2014 aboard the Hayabusa-2 spacecraft. Upon meeting up with the asteroid in 2018 the cannon will separate from the main spacecraft and head to the asteroid surface where it will fire the metal bullet at speed to create a small crater. The spacecraft will then land itself on the asteroid surface where it will stay for about 18 months, collecting soil samples from the crater before blasting off again for the return journey to Earth, arriving back in 2020.
Samples taken from the newly formed crater should be less affected by the space environment and will have the potential to “revolutionize our understanding of pristine materials essential to understanding the conditions for planet formation and the emergence of life," according to the JAXA website.
Take a look at the animated simulation of the Hayabusa-2 voyage to see exactly how JAXA plan to complete the mission