Wednesday marks a milestone for the Voyager 1 spacecraft. It will be 13.5 billion kilometres away from the sun, or 90 times farther away from the sun than the Earth.

But two groups of researchers disagree on whether Voyager 1 has left the solar system and entered interstellar space.

Voyager 1 was launched 26 years ago and has nearly reached – or has already penetrated – the edge of the bubble surrounding the solar system.

Artist's conception of bubble

The bubble is formed when highly charged particles from the sun, called the solar wind, collide with particles emanating from other stars in the galaxy.

The edge of the bubble, called the heliopause, is an ever-shifting boundary and it's unclear whether Voyager 1 has passed through it.

Scientists theorize that a barrier, called a termination shock, exists where the hot solar wind hits the cold, thin gas of interstellar space.

As particles of solar wind pile up on the barrier, they get hotter and skip back and forth across the boundary.

Researchers looked over data from Voyager 1's instruments for indirect evidence of the termination shock. An instrument that directly measures solar wind velocity stopped working years ago.

One team studying low energy particles concluded that the solar wind velocity around Voyager 1 slowed down in August 2002, indicating that the spacecraft had already gone through the barrier.

A second team, which studied higher energy particles, said Voyager 1 has encountered a higher density of particles recently, suggesting the spacecraft is approaching the boundary, but hasn't passed it yet.

Further observations from Voyager 1 and its sister craft Voyager 2 should resolve the dispute over the position of the edge of the solar system.

The nuclear power sources on the two spacecraft should last until 2020.

At its present distance, 90 astronomical units away from the sun, it takes about a half a day for signals from Voyager 1 to reach the Earth.