Key points from article :
Double Asteroid Redirection Test (DART) is NASA's first planetary-defence mission.
Planetary defense is about largish asteroids that could theoretically collide with Earth.
DART's target isn't a threat to Earth, but project should allow us to do so if we were threatened.
DART will study the asteroid and then collide with it.
Will track how far the car-size spacecraft knocks the object off course.
Spacecraft will hit the space rock at nearly 13,500 mph (6 kilometres per second).