Why the spacecrafts dont fall slowly to earth?

It is not practical for the shuttle to carry enough fuel to slow it down that much, for one thing. In orbit it is going at around 17,500mph, and it took both solid rocket boosters, all the fuel from the external tank and a little kick from its manoeuvring engines to get it to that speed. To slow it down again would require almost as much fuel. Either it has to carry it up there with it (which would make the shuttle heavier to begin with and would require even more fuel to get off the ground) or it has to rendezvous with another fuel tank and connect up (which requires another launch, is a very complex move, and which would require the main engines to be redesigned so they could be restarted in space).

In addition to the extra fuel requirements, there is the problem of gravity. Things only stay in orbit if they maintain their speed. The second it began to slow down it would start to fall towards Earth, so even if it were to carry all that extra fuel, it would still be going very fast when it hit the atmosphere. The only way to avoid it would be to use the engines to slow down and maintain altitude, which requires even more fuel.

It is far more economical to use the friction with the atmosphere to slow down on re-entry, as a suitable heat shield is easier to design and build than a system using all that extra fuel. Consider that of all manned space flights over the last 50 years, only one has ever failed due to a problem with the heat shield.

The space shuttle has to decelerate from orbital velocity to atmospheric flight in a relatively short period of time. But since energy doesn't just go away it's converted to heat... not from friction but from compression of all that air in front of it.

And one space shuttle DID sufffer a burn through during re-entry which caused the total breakup of the craft and the loss of all aboard. Re-entry is a tricky buisness... especially designing a spacecraft that's going to do it more than once.

It can't be done "slowly" because the shuttle is already rip-roaring fast coming out of orbit (around 17,000 MPH).

In theory, you could run a rocket engine as a brake to slow down, but the amount of fuel needed to do that would be highly impractical.

Instead, you use the friction of the earth's atmosphere to slow down. That has a lot of issues, but it's the only practical way to slow down.

Spacecrafts DO fall slowly to the Earth, because often times the atmopshere of low Earth orbit is still significant enough to cause some drag.

"why the NASA dont put some engine and start it when fall into earth??"

That sounds like exactly what they do.

That is exactly what NASA does. Any satellite in orbit requires occasional rocket boosting to stay in orbit. Otherwise atmospheric friction will bring the satelite back down.

Earth's gravity causes them to accelerate to Earth and that force is far greater then the drag of the atmosphere.

Sry but barly can under stand ur question use grammar when typing thx u