But there is one question we can answer, without spoilers, using only our knowledge of physics: How could the wreckage of a blown-up Death Star get onto this planet? Is it possible to “fall” from outer space? And if so, would there be anything left for space tourists to see?
Trust the Force
Let’s start with a simple case. Imagine a universe containing a single, stationary planet and a big hunk of space junk. Now, even if the thing is really remote, there’s still a gravitational tug on it. Any two objects with mass will be drawn toward each other by a gravitational force. (Including you and the screen you’re reading this on, but luckily it’s pretty tiny.)
As this equation shows, the magnitude of the gravitational force depends on the mass of each object (m1 and m2) and the distance (r ) between them.
Here, G is the universal gravitational constant, with a value of 6.67 x 10–11 N×m2/kg2. It’s "universal," so it should work even in a galaxy far far away. See the r2 in the bottom? This says the attractive force between two things declines with the square of the distance. Double the distance and the gravitational force is only a quarter as strong.
So a remote object has a very small force on it—but it’s never actually zero. There’s no cutoff line. And even a tiny force will change the momentum of the object. That inches it closer to the planet, where the gravitational force is a tiny bit stronger. As a result, the object gradually speeds up and eventually hits the surface. So far so good.
Sizing Up the Impact Velocity
But how hard does it hit? To answer that, it’s easier to think in terms of energy, rather than gravitational force. Think of the planet and space object as a system, where the total energy in the system is constant—there’s no external force acting on it. There’s two types of energy we need to consider here, kinetic energy and gravitational potential energy.