> But when we talk about sending say, a spaceship into space, it may fall on a different planet, with a different gravity (a function of the planet's mass), cha

> But when we talk about sending say, a spaceship into space, it may fall on a different planet, with a different gravity (a function of the planet's mass), changing the amount of energy yielded by the fall. I guess you imagine there "failing" as something we experience on the surface of the Earth when we drop an object. It's not what is happening when "sending a spaceship into space." It's actually very hard (as in, one really has to spend a lot of energy) to get a spaceship to some star or planet. One can't imagine it as "just let it fall". I suggest reading the following article for the start: https://www.forbes.com/sites/startswithabang/2016/10/01/ask-ethan-why-dont-we-shoot-earths-garbage-into-the-sun/ https://www.forbes.com/sites/startswithabang/2016/10/01/ask-... "Ask Ethan: Why Don't We Shoot Earth's Garbage Into The Sun?" Much less energy is needed to shoot something out of the Solar system than is needed to send something into the Sun, even if you want "just" to send it there to burn. Nothing from Earth can "just fall" there, huge amounts of energy from the outside are needed. What is actually happening is what already Newton figured out: when we're on Earth, we aren't aware of it but the whole Earth is constantly free failing towards the Sun, and yet we're not leaving the orbit around the Sun. > Where does the extra energy, if the ship falls into a heavier planet, come from? A lot of the energy needed for the ship to reach the planet has to be spent by the ship, for the current ships it is launched as the fuel with the ship and later "spent" by the ship (in reality "traded"). The ship can also "spend" the energy of other objects and that energy is really then "spent" (as in bookkeeping, debited to other account, that's what the conservation of energy and momentum is, a kind of bookkeeping) -- the object (e.g. a planet) is indeed slowing down a little, but the change is conveniently small enough: https://en.wikipedia.org/wiki/Gravity_assist https://en.wikipedia.org/wiki/Gravity_assist "This explanation might seem to violate the conservation of energy and momentum, apparently adding velocity to the spacecraft out of nothing, but the spacecraft's effects on the planet must also be taken into consideration to provide a complete picture of the mechanics involved. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, so the spacecraft gains velocity and the planet loses velocity. However, the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small even when strictly compared to the orbital perturbations planets undergo due to interactions with other celestial bodies on astronomically short timescales." In short, there's nothing "extra" happening -- all the energies and momentums are completely accounted for for everything that happens on the scales reachable to our spaceships. The only energy we can't account to anything else is the one we call "dark" but it is observable just in the movements of the whole galaxies, on the scales practically unreachable to our spaceships.