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In April 2011, NASA announced that the two Voyager spacecraft would soon be the first human-made objects to leave our solar system. While on their journey, each day the Voyagers send back this message, "Expect the unexpedted."
At this point the power left in Voyager 1 from the radioactive decay of a Plutonium 238 heat source was expected to keep it running until 2020.
Probably the Voyager image that stunned scientists the most was that of an active volcano on Jupiter's moon Io. The Pioneer probes had detected volcanoes on Io but none were believe to be alive.
Here's a closer look at an inactive volcano on Io. The moon has about 100 times the volcanic activity as Earth.
Scientists made another major discovery on Io, called the Io torus, a ring of ionized sulfur and oxygen which comes from the moon and adds to Jupiter's magnetic field.