When Voyager 2 sent back pictures of Neptune during its historic flyby of that planet in 1989, the spacecraft's radio signals traveled for 4 hours at...

When Voyager 2 sent back pictures of Neptune during its historic flyby of that planet in 1989, the spacecraft's radio signals traveled for 4 hours at the speed of light to reach Earth. How far away was the spacecraft? Give your answer in kilometers, using powers-of-ten notation.