Travelling at 38,000 mph, the Voyager 1 probe was launched on 5 September 1977 with the intention of studying Jupiter, Saturn, Uranus and Neptune. Now, 33 years later and almost 11 billion miles from home, it is going where no Earth object has gone before.
Both Voyager probes are approaching the interstellar medium, with Voyager 1 further ahead than its sister ship Voyager 2 (which is travelling at 35,000 mph). It is here that the nature of the environment is expected to dramatically alter.
Dispatched towards deep space, the Voyager probes have functioned far beyond expectations, with their radioactive power packs continuing to work and send data back to Earth, although at such a distance away any radio message takes 16 hours to reach us.
Speaking to the BBC, Edward Stone, the Voyager project scientist, said: “When Voyager was launched, the space age itself was only 20 years old, so there was no basis to know that spacecraft could last so long. We had no idea how far we would have to travel to get outside the Solar System. We now know that in roughly five years, we should be outside for the first time.”
Our solar system is contained inside an area of space known as the heliosphere. Up to the heliosheath our Sun exerts a magnetic and energetic influence, with its solar wind extending to all corners of the system. Upon reaching the heliosheath, the solar wind slows considerably and begins to heat up as it reaches a shockwave known as the termination shock.
Voyager 1 crossed the termination shock in December 2004. Using data from its Low-Energy Charged Particle Instrument, the velocity of the surrounding solar wind has been constantly measured.
At the edge of the heliosphere is the heliopause, where the Sun’s influence is less apparent. It is here that Voyager 1 – and later Voyager 2 – will enter the interstellar medium, the matter between stars in our galaxy.
It is theorised that at this distance from the Sun, over 11 billion miles, the velocity of particles emitted by the Sun will slow to zero. Although it is not quite there yet, once Voyager 1 reaches interstellar space there should be a sudden drop in the density of hot particles and an increase in that of cold particles. The success of the Voyager probes so far has markedly increased our understanding inside our solar system, and its continuing journey will give us unprecedented information on the outside as well.