Asked by: Richard Foster-Fletcher, via email
Although it’s theoretically possible to change the orbit of a planet, it’s probably completely impractical.
Moving Mars, for example, to an orbit closer to the Sun would require decreasing its kinetic energy enormously – perhaps by shunting large asteroids into close encounters with it. This would likely take many centuries or millennia to complete, and would require huge amounts of energy. If the aim is to aid in the terraforming of Mars, there would be far cheaper, quicker and more effective ways to do it.