Can you get Energy by Collecting Asteroids?

Correct me if I'm wrong but from my current understanding using gravity as energy wouldn't work because we need the same amount of energy collected from falling to get it back to its starting point. But what if we didn't have to bring the object back up instead just using a object that is in abundance. So my question is would this scenario work assuming we create technology to be able to harvest the energy. We use asteroids already free floating through space and shoot them into the gravity of a large planet and gather its energy as it falls without retrieving it instead we simply get another asteroid and send it getting more energy. So would this scenario work assuming we have the technology to gather the energy, or would you run into the same problem as stated at the beginning or another.
- Austin Baxter (age 20)
Fort Worth Texas U.S.

It's more common to worry about how to avoid getting too much energy from asteroids, which is what would happen if a big one accidentally smashed into the Earth. The technique that is most often discussed to avoid that is to detect the path of asteroids very early, when they are far from Earth, and then send up a rocket with some device to give them a little push to alter the path enough so it will miss the Earth. 

If somebody had a nice controlled way to capture and use the energy of an asteroid as it got close to Earth or another planet, then the same method could be used to steer one to approach the planet instead of miss it.

Jon Thaler, as told to Mike W.

(published on 05/14/2016)