According to the Voyager Interstellar Mission Web site, on June 28th of this year, Voyager 2 completed 12,000 days of continuous operation since its launch on August 20th, 1977. Each of the Voyager spacecrafts has crossed the edge of the solar system, also known as the termination shock, and is headed towards uncharted territory. After its primary mission of planet exploration was completed in 1989, a new mission objective, the Interstellar Space Exploration, was launched.

"Our solar system exists inside a heliosphere, a bubble created by the outward flow of the solar wind. The region that separates our system from interstellar space is the heliopause. In between these is the termination shock, where the solar wind slows from supersonic to subsonic speeds." --Credit: JPL / NASA

The primary mission was the exploration of Jupiter and Saturn and then, after some successful snapshots, one of the space craft was sent to Uranus and Neptune. When both spacecraft made their closest approach to Jupiter in 1979, they took more than 33,000 photographs, revealing a number of things that were previously unknown. For example, when scientists examined the photographs of one of Jupiter's moons, "Io," this is the first time that they discovered active volcanism somewhere else in the Solar System.

Below is a time-lapse video recording Voyager 1's approach to Jupiter during a period of over 60 Jupiter days.

The spacecraft made their way to Saturn (about 9 months apart) in 1980-81. The fly-bys of Saturn produced a ton of scientific findings including the fact that its atmosphere is composed almost entirely of hydrogen and helium, and that it is the only planet less dense than water. The famed rings of Saturn were measured on a level never before seen, and we learned more about the structure of the rings, their thicknesses, ring-gaps and even variations in brightness due in some parts to the existence of clumps and in others to the near absence of material. New satellites were discovered as well. Previously there were thought to be 11 satellites surrounding Saturn, while now we know there are approximately 17. The two spacecraft discovered about six of those. More have been observed using ground-based observation since then.

Below is a picture of three Voyager 2 images of Saturn, taken through ultraviolet, violet and green filters—and then combined.

The spacecraft did a fly-by of Uranus in 1986, discovering 10 previously-unseen moons and two newly detected rings. We also discovered a Uranian magnetic field, giving us the first conclusive indication that the planet actually possesses a magnetosphere. A great deal of knowledge was gained about its surface and also its atmosphere, which is only comprised of 15% helium (studies based here on Earth previously put that figure around 40%).

Below is a photo of a crescent of Uranus that was taken by Voyager 2.

The summer of 1989 was the first time a spacecraft had ever observed the planet Neptune. Neptune is beyond the asteroid belt and is also known as a gas giant. It doesn't have any solid surfaces and is comprised of mainly hydrogen and helium. It's about 17 times as massive (heavy) as the Earth but not nearly as dense. Voyager 2 discovered that Neptune had a large dark area of swirling gases (similar to a hurricane) but the Hubble Telescope later discovered in 1994 that it had vanished.

Below is Voyager 2's view of Neptune (35 million miles away)

Click here to find the location of both Voyager Spacecrafts through 2015 (PDF Document: Credit: JPL / NASA)

The spacecraft are now over 10 billion miles from Earth and are still returning valuable scientific data. They have sufficient electrical power and thruster fuel to operate until around 2020. The equipment on the spacecraft are limited by that of time and it's "most likely" that they will pass silently into interstellar space. At some point, we will no longer receive signals from them and communications will be lost.

The Voyager space craft made history just a few months ago, as they approach the very boundary of the solar system, which is determined by the volume of space swept out by the solar wind and radiation. They continue to push the frontiers of science.