By this date in 1989, the little-space-probe-that-could had already accomplished its mission goals. It had gathered a lot of information about Jupiter and Saturn—and their moons and rings—and it had continued the Grand Tour by flying by Uranus and Neptune. Now Voyager 2 was about to exit the solar system, collecting and transmitting information as it went, and following in its identical twin's footsteps. (Voyager 1 was launched a few weeks after Voyager 2 but only visited Jupiter and Saturn on its way out of the solar system.) Scientists wanted Voyager 2 to check out one more world before it left:
Triton is Neptune's largest moon. It's only half the size of Earth's moon, but it is far colder (of course, being so very far away from the sun), and it has a thin atmosphere. Triton is unusual in that it is the only large satellite with a retrograde orbit. What does that mean? Well, all of the moons in the solar system orbit their planet in the same direction as the planet rotates. (Scientists explain that this is due to the way that planets and moons form.) That is, all but one: Triton orbits Neptune in the opposite direction of Neptune's rotation on its axis.
Voyager 2 sent back to Earth some pretty great photos of Triton. Check out NASA's photojournal.
By the way, both Voyager 1 and 2 are still operating, still sending us information about deep space! It's been almost 34 years—far longer than the expected lifespan of the space probes—and they are still working for us!