More than four decades ago a space probe was launched into the clear blue skies over Cape Canaveral in Florida.
Scientists hoped the probe – Voyager 2 – would visit the outer reaches of the solar system and photograph the giant planets.
It has somewhat exceeded expectations.
Late year the probe punched through the protective bubble that surrounds the solar system.
It is now in interstellar space, the vast region between the stars.
On Tuesday, scientists revealed the first data from that journey beyond the bubble – revealing two interesting surprises.
“This has really been a wonderful journey,” said Professor Edward Stone, who has supervised Voyager 2 since its launch in 1977.
The heliosphere, as it is known, is a protective bubble around the solar system formed by the solar wind, a stream of extremely hot and fast electrons and protons – known as plasma – that comes roaring out of the sun’s nuclear heart and sprays out into space.
The bubble provides invisible protection for life on Earth.
The Milky Way has its own wind, made of super-cold plasma, generated by vastly powerful events elsewhere in the galaxy such as black holes and exploding supernovas.
Our bubble protects the solar system from this 'interstellar wind', and from cosmic radiation which could wreak havoc on our DNA.
The bubble’s edge lies about three times the distance of the sun to Pluto.
It has taken Voyager 2 more than four decades to reach it.
Because the probe took a meandering path through the solar system, visiting Jupiter, Saturn, Uranus and Neptune, it took seven years longer to reach the edge of the bubble than its twin, Voyager 1.
Voyager 1 passed through the heliosphere in 2012 but its crucial plasma instrument had been damaged during a fly-by with Saturn.
That left scientists with valuable but tantalisingly incomplete data about what was found beyond the reaches of our solar system.
Voyager 2 and its working plasma detector fill that gap. In five papers published in Nature Astronomy on Tuesday, scientists revealed two unexpected findings.
First, Voyager 2 detected “shockwaves” from the sun beyond our bubble, said the University of Iowa’s Professor Donald Gurnett.
These explosions of plasma periodically shoot from the sun’s surface. But the had been expected to peter out well before the bubble's boundary.
Voyager 2 was able to detect those shockwaves well past the bubble’s edge.
"We were outside," said Dr Stone, "but we were seeing particles from the inside."
Second, the probe detected an unexpected thickening of plasma outside the bubble.
Professor Gurnett believes this is made by the interstellar wind pushing up against the edges of the bubble, creating what he described as a “bow wave”, like a ship moving quickly through water.
The twin spacecraft will now continue through the space between the stars, with scientists hopeful of making more measurements of interstellar space.
But there may not be much left to add.
Both spacecraft run on nuclear power, and their generators are near the end of their lives. In as little as five years, the mission team suspect, they may no longer have enough power to call home, and the twins will go silent.