In 1609 Galileo turned his telescope to the heavens.
He looked at Jupiter and was astonished; he saw four moons orbiting the planet.
As the centuries passed, people invented larger and more powerful telescopes.
But these improvements could not satisfy their curiosity about the other planets that orbit our sun.
Instead of improving telescopes on Earth, scientists thought, why not send telescopes to the planets?
In 1977 the U.S. National Aeronautics and Space Administration (NASA) sent two probes,
Voyager 1 and Voyager 2, to explore the mysterious planets beyond Mars.
These four planets are called gas giants, and they are about 60 to 1,300 times the size of Earth.
Both Voyager 1 and 2 explored the closest two, Jupiter and Saturn.
Voyager 1 also studied Saturn's moon Titan, while Voyager 2 traveled on to study the planets Uranus and Neptune.
From January to April of 1979, Voyager 1 photographed and studied Jupiter and its many moons (scientists keep discovering new ones).
Beautiful pictures were sent to Earth, allowing scientists to research the complicated weather systems that wrap around the planet.
Jupiter's atmosphere to core temperatures can range from -145 to +24,000 degrees Celsius.
Jupiter has lightning and its own auroral phenomenon, just like Earth's northern lights.
When images of one of Jupiter's moons, Io, were sent back to Earth, scientists were astonished.
Moons, it was believed, could not have volcanoes because they were too small and cold.
But Io has volcanoes more violent than any on Earth, with eruptions that send material three hundred kilometers high.
Later, scientists realized that because Jupiter is so big,
its gravity pulls the side of the moon closest to Jupiter more strongly than the opposite side.
As Io rotates, its different parts are constantly stretched and pulled, and this creates heat.
For example, if you take a metal spoon and bend it back and forth, the place where it bends becomes hot.
Jupiter's gravity is so powerful that it bends the interior of its moons. Scientists call this process tidal heating.
On Jupiter's moon Io, tidal heating is powerful enough to cause eruptions hundreds of times larger than any volcano on Earth.
The Voyager probes also discovered that Jupiter's moon Europa has a surface of ice.
If tidal heating warms Europa's interior enough to melt the ice, is it possible that life exists in vast oceans underneath its icy surface?
Scientists are now designing probes to land on Europa to answer this question.
From Earth, Saturn and its rings appear peaceful and beautiful, but the Voyager probes discovered something different.
Saturn has violent windstorms over 1,400 kilometers per hour, and its rings are not the perfectly smooth objects we see through telescopes.
Some rings have gaps, like a necklace missing a few pearls.
But strangest of all, the rings have patterns in them that look like braided hair.
Scientists believe that the uneven orbits of Saturn's small moons cause these strange patterns, and the gaps as well.
Voyager 1 also studied Titan, a large moon that orbits Saturn.
Titan is the only moon in the solar system to have an atmosphere.
Voyager 1 found that Titan's atmosphere is made mostly of nitrogen, while the surface is a mixture of ice and rock.
Before there was life on Earth, its atmosphere was also mostly nitrogen.
Some scientists believe that when we look at Titan, we are seeing what Earth looked like far in the past.
In 1986 Voyager 2 arrived at Uranus and discovered a shocking mystery that is unexplained to this day.
Most planets, like Earth, spin on their axes vertically.
Their north and south poles point up and down compared to the rest of the solar system.
Uranus is the opposite. Its axis lines up horizontally, so that one pole faces toward the sun and the other away from the sun.
This strange fact had long been known, but Voyager 2 discovered a stranger contradiction.
Even though the equator of Uranus receives almost no sunlight,
its equator is always as hot as, or hotter than, the polar region that directly faces the sun.
Voyager 2 arrived at Neptune in 1989. It discovered six moons orbiting Neptune, as well as a faint ring system around the planet.
Neptune is a beautiful world, a deep blue color with giant white clouds of methane rising and falling, just as water does in Earth's atmosphere.
After its mission to Neptune, Voyager 2 began to travel out of the solar system.
The success of the Voyager missions encouraged scientists to send more probes to explore our solar system.
The Cassini-Huygens spaceship, after traveling for seven years, is now orbiting and studying Saturn.
In 2004 NASA landed two rovers on Mars.
These small, six-wheeled, solar-powered vehicles are sending back pictures and doing tests on Martian rocks.
Since the Voyager probes were launched in 1977,
NASA, the European Space Agency, and the Japanese Aerospace Exploration Agency
have sent more than 35 new missions to explore the inner and outer planets, the moon, asteroids, comets, and the sun.
Today both Voyager 1 and 2 are on their way out of the solar system.
Voyager 1 is over 16 billion kilometers from Earth and is the most distant man-made object in the universe.
But the missions of these pioneering probes continue.
They are now studying the complex area of space where our solar system ends and the rest of the universe begins.
In addition to their scientific missions, Voyager 1 and 2 may play a much more important role in humanity's exploration of space.
Both probes carry a gold-plated disc containing messages from Earth.
If aliens ever find a Voyager probe,
they will be able to hear the natural sounds of our planet, a selection of music, and greetings in over fifty languages.
But perhaps the most important message is not on the discs at all.
Both spaceships are themselves a statement about one of humanity's best qualities: curiosity.
Like Galileo, many of us on Earth want to see farther and understand what we see.