In uncharted territory

An artist’s concept shows the general locations of NASA’s two Voyager spacecraft. Voyager 1 (top) has sailed beyond the solar bubble into interstellar space, the space between stars. Its atmosphere still feels the solar influence. Voyager 2 (bottom) is still exploring the outer layer of the solar bubble.

NASA’s Voyager 1 spacecraft is officially the first human-made object to venture into interstellar space. The 36-year-old probe is about 19 billion kilometres from our sun.

New data indicate Voyager 1 has been travelling for about one year through the plasma, or ionised gas, present in the space between the stars. Voyager is in a transitional region immediately outside the solar bubble where some effects from the sun are still evident. A report on the analysis of this new data, an effort led by Don Gurnett and the plasma wave science team at The University of Iowa, Iowa City, was published recently in the journal Science.

“The crossing is like Voyager leaving the hot, million-degree atmosphere of the sun and entering into a region dominated by the ‘cold’, 5,000-degree atmosphere of the galaxy,” says Stamatios Krimigis of Johns Hopkins University Applied Physics Laboratory (APL), the principal investigator for Voyager’s Low-Energy Charged Particle (LECP) instrument. Voyager 1 first detected the pressure of interstellar space on the heliosphere, the bubble of charged particles surrounding the sun that reaches far beyond the outer planets, in 2004. Scientists then ramped up their search for evidence of the spacecraft’s interstellar arrival, knowing that data analysis and interpretation could take months or years. Until mid-2010, the intensity of particles originating from inside the solar system had been holding steady. But in 2011 the intensity of those energetic particles (measured by the LECP instrument) began declining, as though they were leaking into interstellar space, and the radial expansion velocity of the solar wind went to zero.

Readings over the past year showed that solar particles had essentially all left and galactic particle intensities increased dramatically, says Matthew Hill, an LECP team member and space physicist at APL.

But without a plasma sensor that could regularly measure the density, temperature and speed of plasma, Voyager scientists looked to the magnetic field, which did not change direction at all, seemingly indicating that the intrepid probe remained in the solar magnetic field. That changed when an unexpected gift from the sun allowed Voyager 1 to make measurements of its plasma environment.

A coronal mass ejection—or a massive burst of solar wind and magnetic fields—that erupted from the sun in March 2012 provided scientists with the data they needed. When the material eventually arrived at Voyager 1’s location 13 months later, in April 2013, the plasma around the spacecraft began to vibrate like a violin string. On April 9, Voyager 1’s plasma wave instrument detected the movement.

The particular oscillations meant the spacecraft was bathed in plasma more than 40 times denser than what it had encountered in the outer layer of the heliosphere. Density of this sort is to be expected in interstellar space.