physic ( question need to explain )

The Magdeburg hemispheres were a pair of large copper hemispheres with mating rims. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart by teams of horses. The Magdeburg hemispheres were designed by German scientist Otto von Guericke in 1650 to demonstrate the air pump he had invented and the concept of air pressure. When the air was sucked out from inside them, they were held firmly together by the air pressure of the surrounding atmosphere.

Theory:
Air molecules collide with the surface of an object that's exposed to the air. Many billions of molecules are striking such an object every minute. Even though atoms are really, really small (really), you'd think that so many of them crashing into the object together would exert some kind of force on it. Indeed they do... such a force is called air pressure. The typical pressure of the atmosphere on your skin is 1.01 Newtons for every square centimeter of your body.

Magdeburg hemispheres are two half-spheres of equal size. Placing them together traps air between them. This air is merely trapped, and not compressed, so the pressure inside is the same as the pressure of the atmosphere outside the spheres. That is to say, the air is pushing equally strongly inside as it is outside. The spheres thus pull apart with nearly no resistance.

Now what if all the air were removed from the inside? Then no air remains to push on the inside of the sphere, but the air outside is pushing at atmospheric pressure. The push from the outside isn't balanced by another push on the inside, so the two hemispheres become very hard to separate.

I hope this can help your understanding. :)

Separation will be much easier in the outer space.