Three near-Earth asteroid are worrying NASA engineers who are busy constructing a planetary defense system to stop killer asteroids.
NASA plans to land spaceships on asteroids and install robotic systems on the space rocks to turn them into basic spacecraft.
Then, these roboticized asteroids can be flown into other asteroids near the Earth to protect mankind from extinction.
The space agency plans to launch the first spacecraft next year and it will arrive at the asteroid Bennu by 2018; after collecting a sample from the space rock it will return to Earth by 2023.
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