The robotic Cassini spacecraft that has been orbiting Saturn for the past 13 years began its final and most daring observation of the ringed planet by diving down through a small gap between the rings and the planet itself, a dangerous move never attempted by a spacecraft before. It proves once again that not only do these intrepid explorers probe the unknown, they go the extra mile, well beyond expectations.

When it comes to the best science return for the space buck, robots are the best investment. They cost many times less than sending humans to space, can travel for years, cover the vast distances between the planets and usually perform far longer than their original mission plan. Cassini was an ambitious, joint mission between NASA and the European Space Agency to spend four years exploring Saturn, its dazzling rings, a myriad of more than 50 moons, as well as having its attached Huygens probe perform a landing on Titan, Saturn's hazy moon, the first ever landing on a moon of another planet.

These spindly looking robots with antennae and scientific instruments sticking out in all directions are remarkably long-lived. They are also expensive, costing hundreds of millions or even billions of dollars to design, build, test and fly through the incredibly harsh environment of interplanetary space. Space scientists are very clever when they ask for funding. At first they design a mission to last a few months or a few years. But they over-build the machines, giving them the ability to last far longer than necessary. So once the mission is accomplished, they can go back to the administrators and say, "Hey the thing is still running, if you give us a little more money, we could keep going for another few years."

NASA's Cassini spacecraft captured images of Saturn's atmosphere never seen before in the first successful flight through the gap between the planet and its famous rings. (NASA/JPL-Caltech/Space Science Institute)

The strategy worked. Cassini was launched in 1997 and expected to run for almost 11 years. Then it was given an extension in 2008, another in 2010, and now it is in its final phase, called the Grand Finale, where it will end its long life by plunging into Saturn's atmosphere this September. Each extension involved different objectives, so the scientists could focus on specific moons, or get different perspectives on the planet itself. This last phase is the most daring and dramatic, passing extremely close to the planet's cloud tops and the rings, which is very dangerous, but there is nothing to lose, since the spacecraft is almost out of fuel.

These extensions make the whole Cassini program the equivalent of four missions rolled into one.

A history of success

This strategy of extending missions has seen spectacular results in the past. In 2004, twin rovers named Spirit and Opportunity began driving around on the dusty plains of Mars. Their mission was to run for three months. Spirit lasted more than six years and Opportunity is still running today. Too bad we can't make cars on Earth that run so long past warranty.

In 1977, the twin Voyager spacecraft were sent on what was to be a five-year journey past Jupiter and Saturn, which were conveniently lined up on the same side of the sun. After Voyager 2 passed Saturn, the mission was officially over, but the scientists knew that the two other giant planets, Uranus and Neptune, were also part of that alignment and Voyager could reach them. No spacecraft had ever visited those distant worlds and it was a rare opportunity that could not be passed up. They got an extension on funding to keep going.

This graphic illustrates how Cassini scientists think water interacts with rock at the bottom of the ocean of Saturn's icy moon Enceladus, producing hydrogen gas. (NASA/JPL-Caltech)

The scientists knew about that opportunity to visit all four planets before Voyager left Earth, so they gave the spacecraft an extra large fuel tank to travel the extra distance, and conveniently aimed Voyager at Saturn in just the right way so the gravity of the big planet would accelerate and aim the probe right towards Uranus and Neptune whether they got funding or not. They did get the funding and Voyager became the only mission to visit four planets in one grand tour.

They keep on going and going...

And it's not over: both Voyager 1 and 2 are still running 40 years later. They have been thrown right out of our solar system thanks to the gravity assists of the big planets and are now exploring the region between the stars.

Robots are the true explorers of the unknown. They have visited every planet in the solar system, visited asteroids, chased comets and discovered incredible worlds we didn't even know existed. They are our artificial senses that have discovered methane lakes on Titan, salty oceans under the icy surfaces of Europa and Enceladus, seen sulphur volcanoes erupting on Io and nitrogen glaciers flowing down ice mountains on Pluto.

Meanwhile, the robotic Hubble Space Telescope has been peering out to the edge of the universe since 1990 and is still sending back stunning images.

While there is a lot of talk about sending humans to Mars at tremendous expense and risk to the crew, no boot prints would ever be possible on the red planet without the early reconnaissance of a fleet of hardy robotic machines that boldly went there first.