The Voyager 2 spacecraft may have finally taken the plunge into interstellar space, according to a new NASA announcement. At a distance of a little over 11 billion miles (18 billion km) from Earth, the probe is now thought to have passed beyond the heliosphere – the protective bubble of particles and magnetic fields emanating from our star that envelopes the planets of our solar system.
Voyager 2 was launched on August 20, 1977. Counter-intuitively, the probe blasted off a full 16 days before its twin explorer, Voyager 1. Both spacecraft carried an identical scientific payload, along with a precious Golden Record designed to tell the story of our world through sounds and images for any space faring civilization that may one day recover the probes.
Having soldiered on for an impressive 41 years, Voyager 2 is NASA's longest running mission, but the original plan for the spacecraft envisioned a much shorter lifespan. Upon launch, the probe had an expected lifespan of just five years, and was tasked with making close observations of the gas giants Jupiter and Saturn.
With its initial challenges completed, the science team sent the probe further afield, directing it to visit Uranus and Neptune. Proving once more up to the task, the Voyagers were remotely upgraded, and given a new mission. They were to forge their way through our Sun's domain, and become the first human-made objects to break through into interstellar space.
In 2012, Voyager 1 would be the first to break through the outer edge of the heliosphere, known as the heliopause, and pass into interstellar space. However, despite coming second, Voyager 2 could provide scientists with the more valuable scientific account of the final stretch of this journey to the beyond.
This is because Voyager 2's Plasma Science Experiment (PSE) is still functional, whereas its twin explorer's instrument had malfunctioned in 1980, before it had entered the interstellar medium. The PSE analyses the electrical current of our Sun's solar wind to measure its speed, density, temperature and pressure.
On November 5, Voyager 2's PSE recorded a sudden drop-off in the speed of solar wind particles, and has detected no more in the time that has elapsed since. This is a major indicator that the probe has passed beyond the edge of the heliopause. Other instruments aboard the spacecraft also noted environmental changes around the same time that the solar wind dropped off, further supporting the science team's conclusion.
Voyager 2's new environment is strikingly different from anything it would have encountered before. The relative heat of the Sun's solar wind has been replaced by the cold, dense environment of the interstellar medium.
It's important to note that whilst Voyager 2 has left the Heliosphere, it isn't even close to leaving the solar system. The edge of the solar system is generally acknowledged to be located beyond the Oort Cloud.
This massive bubble made up of icy debris is thought to begin at a distance of 1,000 astronomical units from our Sun. For reference, an astronomical unit is the equivalent to the average distance between the Earth and the Sun, which is roughly 93 million miles.
At its current speed, Voyager 2 most likely wouldn't reach the inner edge of the Oort Cloud for another 300 years. Assuming it could survive the debris, it wouldn't pass through to the other side – a further 100,000 AU – for about 30,000 years.
Sadly, the Voyager spacecraft won't be sending back any images from interstellar space, not that there would be much to see if they did. Non-essential instruments and systems such as the cameras aboard the twin spacecraft have been turned off to preserve power.
The probes are far too distant from the Sun to rely on solar panels, and instead are powered by a radioisotope thermal generator (RTG), the output of which are diminishing at a rate of roughly four watts per year.
Reunited at last, the Voyager spacecraft can now gift astronomers with an unprecedented understanding of the interstellar medium. They will be assisted in this task by NASA's Interstellar Boundary Explorer (IBEX), alongside another planned addition to the agency's heliophysics fleet, the Interstellar Mapping and Acceleration Probe (IMAP), which is set to launch sometime in 2024.
Scroll down to watch a NASA video marking Voyager 2's historic achievement.