Voyager Data Suggest New Paradigm of Solar System's Edge

Thirty three years ago, when the twin Voyager spacecraft departed Earth and headed out into the solar system, it would have been difficult to imagine they would still be sending data back to Earth today, much less continuing to revolutionize scientific thinking.

Designed to explore the outer planets, the spacecraft have long since left them in the dust and transformed themselves into the vanguard of discovery in the field of heliophysics – the study of our Sun and its influence on Earth and the rest of the solar system.

As shown in illustrations available on NASA’s website, the Sun spews out particles that soar through the solar system – the solar wind. The Sun has a magnetic field, like Earth, and its magnetic lines form the solar wind into an elongated shape, the edge of which is called the heliopause – the end of the Sun’s influence. The region just inside that boundary is the heliosheath. That’s where the Voyagers are now.

What is it like, out there in the heliosheath? Until now, scientists thought it was a calm environment with all the magnetic lines connecting back to the Sun. Particles from outside the solar system – Galactic Cosmic Rays – would enter the solar system and follow the magnetic lines to the Sun, passing by Earth on the way. The Earth’s magnetic field largely deflects these cosmic rays, protecting both those of us on the surface and astronauts orbiting Earth from their deleterious radiation. Astronauts only need to worry about cosmic radiation when they leave Earth orbit and go to the Moon or elsewhere. Engineers do not know yet how to protect humans from cosmic rays. This has been one of the stumbling blocks in plans to return to the Moon for extended periods or travel further to asteroids or Mars.

Data from Voyager 1 and 2 paint a different picture of how cosmic rays enter the solar system, however. This new knowledge may or may not impact how human spacecraft are eventually designed, but scientists at a NASA media teleconference today were clearly excited about the new paradigm the data suggest. While couching the findings as a “possible” explanation of the data the spacecraft are reporting, Boston University’s Merav Opher said she was “pretty confident” of her team’s analysis.

The two spacecraft are racing out of the solar system in different directions. Voyager 1 first flew past Jupiter and Saturn. Voyager 2 also passed by Neptune and Uranus. The two spacecraft are now 60 degrees apart in latitude, with Voyager 1 in the solar system’s northern hemisphere and Voyager 2 in the southern hemisphere . The University of Maryland’s James Drake said that it was the difference in the readings sent back by the two spacecraft that forced them to reexamine the theory that the heliosheath was homogenous and quiet. Instead, Voyager 2, in particular, sent data showing that it was entering and leaving areas of relative quiet and relative turbulence in terms of how many energetic particles it was encountering.

Describing the region as roiling around “like the most bubbly part of your Jacuzzi,” Opher, Drake and renowned heliophysicist Gene Parker explained that it now seems that the edge of the solar system is a “sea of magnetic bubbles” that are unconnected to the Sun. Rather than calm, it is turbulent. Cosmic rays entering the solar system get trapped in the bubbles until they eventually find a magnetic line that is connected to the Sun, allowing them to continue their journey inward.

Parker, who developed the theory of the solar wind in 1958, emphasized that the finding would not make any difference with regard to human spaceflight beyond Earth orbit. To him, the most important point is that the data call into question what the density of cosmic rays is beyond the heliopause. It could be much greater than theorized since fewer of the cosmic rays may be able to break through the heliopause, which in a sense is the solar system’s line of defense. The turbulence of magnetic bubbles may throw some of them back into interstellar space.

The “warranty” on the Voyager spacecraft “has long since expired,” joked NASA’s program scientist Arik Posner, but he sees no reason they should not continue sending back data for at least five more years. The spacecraft use nuclear radioisotope thermal generators (RTGs) to provide power for spacecraft systems and instruments. The power level today is akin to a small light bulb, and NASA’s 70-meter Deep Space Network antennas are needed to receive the signals. Voyager 1 is 17.5 million kilometers from Earth, and Voyager 2 is 14.2 million kilometers away. It takes 32 hours and 26 hours for signals from Voyager 1 and 2 respectively to reach Earth.

NASA has another spacecraft, the Interstellar Boundary Explorer (IBEX), in Earth orbit that is studying the particles that get this far. Voyager 1 and 2 are the only spacecraft returning data directly from that region of space.