NASA's Voyager 1 officially is the first human-made object to venture into interstellar space. The 36-year-old probe is about 12 billion miles from our sun.
New and unexpected data indicate that the spacecraft has been traveling for about one year through the plasma, or ionized gas, present in the space between the stars. Voyager—some of whose team members are from Johns Hopkins' Applied Physics Laboratory—is in a transitional region immediately outside the solar bubble, where some effects from our 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 in September 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 APL's Stamatios "Tom" Krimigis, principal investigator for Voyager's Low-Energy Charged Particle instrument. "It's like the first time a satellite [Sputnik] went beyond Earth's atmosphere to an altitude of some 600 miles; Voyager is now leaving the solar bubble at an altitude of 11.3 billion miles. It's another historic milestone."
"Now that we have new, key data, we believe this is humankind's historic leap into interstellar space," adds Ed Stone, Voyager project scientist based at the California Institute of Technology, Pasadena. "The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we've all been asking: Are we there yet? Yes, we are."
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 the data analysis and interpretation could take months or years. Until mid-2010, the intensity of particles originating from inside our solar system had been holding steady. But in 2011 the intensity of those energetic particles (measured by the Low-Energy Charged Particle 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. "I remember saying then that if we had to decide based only on our LECP observations, we would say we crossed the heliopause last summer."
But without a plasma sensor that could regularly measure the density, temperature, and speed of plasma, Voyager scientists looked to the magnetic field, which didn't 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 they had encountered in the outer layer of the heliosphere. Density of this sort is to be expected in interstellar space.
The plasma wave science team went back through its recent data and found an earlier, fainter set of oscillations in October to November of 2012. Through extrapolation of measured plasma densities from both events, the team determined that Voyager 1 entered interstellar space in August 2012. This fitted with data from other instruments, such as LECP, showing the spacecraft had crossed into a new region.
"We literally jumped out of our seats when we saw these oscillations in our data; they showed us that the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," says University of Iowa's Gurnett. "Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma."
The new interstellar plasma measurements led investigators to reconsider that Voyager 1 might actually have crossed the heliopause back in August 2012. By combining data from Voyager in 2010 and energetic neutral atom images of the heliosphere taken by NASA's Cassini spacecraft, Krimigis says, the LECP team estimated the location of the heliopause as being at 121 astronomical units, very similar to the boundary measured by Voyager 1 last summer at 121.6 AU. (An AU is the distance between the sun and Earth, about 93 million miles.)
The scant evidence of solar influence that remains in Voyager's path just adds to the appeal of this mysterious region. "While our data has been consistent with a clear separation of solar and galactic plasmas, we are not in pristine interstellar space as long as the cosmic rays are not equally distributed around the sky," Krimigis says. "Perhaps we may arrive in the undisturbed galactic medium in the future, but we are not there yet. That's why Voyager observations are so exciting; we are in uncharted territory and we continue to be surprised every day."
Indeed, the Voyager mission isn't over. "The team's hard work to build durable spacecraft and carefully manage the Voyager spacecraft's limited resources paid off in another first for NASA and humanity," says Suzanne Dodd, Voyager project manager, who is based at NASA's Jet Propulsion Laboratory, in Pasadena, Calif. "We expect the fields and particles science instruments on Voyager will continue to send back data through at least 2020. We can't wait to see what the Voyager instruments show us next about deep space."