Image credit: NASA/ESA/G. Bacon/STScI

Even today the Voyager program ranks as one of the most exciting space missions ever undertaken. The twin Voyagers were launched within a few weeks of each other in 1977. Between them they studied all four giant planets, and 36 years later were still returning data as they approached the boundary of interstellar space, the space between the stars. On September 13, 2013 NASA officially announced that Voyager 1 had crossed that boundary.

Voyager Interstellar Mission
When Voyager 1 left Saturn in 1980, the primary mission was complete, and the interstellar mission began. Voyager 2 joined the mission after visiting Neptune. During the primary and interstellar missions, NASA has communicated with them through its Deep Space Network.

The Deep Space Network has antennae in three places: Goldstone, California; near Madrid, Spain; and near Canberra, Australia. They are located so that there is complete coverage to communicate with spacecraft in deep space. Hardware upgrades and systems improvements have made it possible to stay in contact with the Voyagers even though they're billions of miles from home.

The purpose of the extended mission was to explore the outer regions of the heliosphere and its boundary with interstellar space. In order to have enough power for basic data collection and essential communications, some of the instruments were deactivated. For example, they turned off Voyager 1's cameras in 1990. This was soon after the spacecraft took its famous “pale blue dot” picture, looking back at the planets of the Solar System.

Empty space isn't empty
We commonly refer to space as a vacuum. It's close to a vacuum, certainly compared to conditions on Earth. Yet it isn't completely empty. For example, the Sun emits particles all the time in what is known as the solar wind.

The solar wind particles form a plasma. A plasma is a gas made up of ions, which are charged particles. Atoms are normally neutral, with equal numbers of protons (+) and electrons (-). But in a plasma the electrons are stripped off, leaving a mixture of positive and negative ions.

The solar wind pushes outwards in all directions forming a bubble called the heliosphere. As the solar wind spreads out, its density decreases rapidly. Particles from the Sun dominate the space even beyond the orbit of Neptune. But somewhere out in the Kuiper Belt the solar wind starts to meet particles from interstellar space. The particles in the interstellar plasma come from supernova explosions that happened millions of years ago.

Into the heliosheath
The solar wind loses momentum when it pushes the interstellar particles away. At a certain point, it encounters the termination shock where the solar wind pressure and the interstellar plasma pressure are equal. The particles slow down noticeably. Both of the Voyagers crossed the termination shock into the heliosheath. The far edge of the heliosheath is the heliopause. Beyond this boundary lies interstellar space where the solar wind has a negligible influence.

The particle density we're talking about is much lower than Earth's best industrial vacuum. So these few particles in space may not sound very exciting. However they can tell us quite a lot. Theoretical models of the heliopause, for example, can be expressed in terms such as particle density, type of particle and details of the magnetic field. Scientists then use the model to make predictions, and Voyager data can test the predictions.

But this really is uncharted territory. If the data doesn't fit the model, what does that mean? Is the spacecraft not quite where they think it is? Or does the model need refining?

From the heliosphere to interstellar space
According to the model of the heliopause, there is a much higher particle density in interstellar space as well as a change in the magnetic field direction.

NASA announced that Voyager had crossed the heliopause when the data showed a plasma density forty times greater than before. This occurred when an ejection of material from the Sun known as a CME made the material around Voyager vibrate.

However there was no great change in the magnetic field, so some astronomers aren't convinced that Voyager 1 has crossed the boundary.

Where does the Solar System end?
Reports have been coming of Voyager leaving the Solar System ever since it went beyond the orbit of Pluto, then still considered the outermost planet.

In fact, since the Solar System is the Sun and everything that orbits it, its gravitational influence extends to the Oort Cloud. This is about two light years from the Sun, nearly half way to our nearest stellar neighbor Proxima Centauri. The Voyagers won't get there for another thirty thousand years or so.