Earlier this week, the Jet Propulsion Laboratory (JPL) announced that readings taken in March indicated that an outburst of particles from the Sun (called a coronal mass ejection) had overtaken the Voyager 1 spacecraft. The readings made by the spacecraft recorded the local environment responding to the shock wave caused as the coronal mass ejection plowed into the denser plasma of interstellar space. According to the JPL, the coronal mass ejection took a full year to catch up with the distant probe.

What does the news mean for Voyager? There have been ongoing arguments over whether Voyager has "left the Solar System." That phrase is in quotes because it's not as simple a statement as it seems. Clearly, some bodies are gravitationally bound to the Sun even though they are further out than Voyager's current location—such as the Oort cloud that sporadically sends comets to the inner Solar System. But the Oort cloud is embedded within the plasma of interstellar space. Closer to the Sun, the magnetic field and solar wind help clear out a less-dense bubble within this plasma.

The new measurements of the shockwave confirm earlier indications that Voyager has moved into an area dominated by the denser interstellar material. Combined with similar measurements made earlier, it's clear that Voyager has (by at least one definition) reached interstellar space. Its slightly older twin, Voyager 2, is expected to do so in a few years.