Step-by-step explanation:
Here we will apply the law of conservation of momentum which is one of the powerful laws of physics. As this law states that the "if no external forces are acting on the system then the net momentum of the system before and after must remain conserved. As the astronaut has a hammer, if he throws it in the direction opposite of his space craft, he will automatically move towards the space craft to conserve the momentum. That's how he can reach the space craft easily by throwing away the hammer.