The space probe Voyager 1 has flown through the asteroid belt, past Jupiter and Saturn, past Neptune and Uranus, beyond Pluto, and out to the very edge of the solar system. Along the way it has taken many famous pictures:
Saturn and its Moons
Neptune. Notice the giant blue spot, bigger than the Earth.
Saturn's Moon Ariel
The solar system is as huge as it is beautiful. But, it is tiny compared to the distance of the nearest star system. If the orbit of Pluto around the sun was the size of a dime, the diameter of the entire solar system would be a hockey puck with the sun at the center. Voyager has now reached the edge of that hockey puck. Alpha Centauri, the nearest solar system to ours, is like another hockey puck, one that is sitting almost three hockey fields (nearly two football fields) away.
Voyager's mission was not just to take pictures, but also to collect scientific data from the vast outer reaches of the solar system
. This mission continues, as the probe still maintains continuous contact with the earth, from 11 billion miles away. 35 years in, most of the probe's instruments are still working, and only in the past few years have they begun to be intentionally shut down as the nuclear engine powering the craft slowly dies out. Some instruments will continue running all the way out to 2025, however.
Among these instruments are a magnetometer (which, as you might expect, measures magnetic fields), a cosmic ray detector, and a plasma wave measurement system (this essentially measures how many electrons are around). Astronomers use these instruments to learn about the area through which Voyager is now passing, the very furthest edge of the solar system. Astrophysicists have made theoretical predictions of what should be out there; now we get to find out firsthand.
Out beyond the planets, the main thing to see is the solar wind. This is a stream of charged particles (such as electrons) blasted out of the sun's atmosphere, into space and across the solar system. Howling past earth, it creates the auroras we see near the poles. The solar wind blows into the teeth of the interstellar wind, which is the general motion of charged particles throughout the space between stars. This has a different direction than the solar wind. The further from the sun Voyager travels, the weaker the sun's magnetic field and the solar wind should be. Soon, the spacecraft should reach a point where the two winds blow opposite each other to a standstill: the heliopause.
Voyager has begun to approach to this area over the past few years, and has been investigating it. Just this year however, things stopped going according to prediction.
Instead of fading out, the solar magnetic field has leveled off and even grown stronger. It seems to merge with the interstellar field, in a way such that particles from the sun can be grabbed by the interstellar wind and whipped out far into space, and interstellar particles can find their way into the solar system.
As Voyager sails further, beyond the heliopause, the only thing we expect it to find out there is the cold lonely interstellar wind. After successful encounters with all four of the gas giant planets and now its work determining the composition of the outer solar system, this is already one of the most successful space missions of all time. Who knows though, further surprises may still be in store!