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Scientists have three key indicators for determining whether the Voyager spacecraft have reached interstellar space.
1)"Inside Particles"
A drop to about 2 particles per second of low-energy particles means that particles originate from inside the solar bubble. (Scientists consider a drop to the background rate of about 2 particles per second to mean there are no more inside particles.)
2) "Outside Particles"
An increase and leveling off of high-energy particles coming in from far outside our solar system.
3) A shift in the direction of the magnetic field or measurements of the plasma environment.
The gauge shows the data sent back from the Voyager spacecraft in average particles per second. If the outside particles show a sudden increase and the inside particles a sudden decrease, and these levels hold steady, the spacecraft are closing in on the edge of interstellar space. Scientists will only then have to verify the third key sign - a change in the direction of the magnetic field - before announcing that Voyager has entered interstellar space.