Asked by Martin Wilson
In general, the further a spacecraft gets from the Sun, the slower it will move – unless of course, the mission has been given enough velocity and rocket fuel to propel it to its destination. Other effects such as light pressure from the Sun, though, can speed up some rockets, as can drag from an atmosphere if they’re close to a planet.
To get a spacecraft to a planet such as Jupiter, you need to propel it fast enough in order to get it further from the Earth. If you simply shoot it into space, it will just slow down and fall back through the Earth’s atmosphere. Combined with this and the ‘drag’ mentioned above, there must be a balance in order to get the spacecraft to reach its destination.
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