How fast do interplanetary spacecraft travel?
You need to specify speed with reference to something–Earth, Sun, the targetted planet? In general to leave Earth and not go into orbit around Earth a spacecraft needs a minimum initial speed of 25,000 miles/hour with reference to Earth. In fact, most reach speeds in excess of 32,000 mph. However, taking the Mars Phoenix which will land on Mars next month, its speed right now with reference to Earth is 9,800 mph, with reference to the Sun is 16 miles per second, and with reference to Mars is 5300 mph, and due to speed up as it approaches, until the retrorockets are fired. For a spacecraft which flies by a planet or two before reaching its ultimate goal, such as the probe on its way to Pluto, there are even more possibilties. And ignore the idiot who said it takes 22 years to get to Jupiter. The probe to Pluto is only taking 15.
