Are we there yet? Ed Stone, the project scientist for NASA’s two Voyager spacecraft, wants to know. Since their launch in 1977, the probes have ventured billions of kilo­metres beyond the outer planets. Now, Stone and his colleagues are looking for signs that Voyager 1 may finally be nearing the edge of the Solar System — where the heliosphere, the bubble of electrically charged particles blown outwards by the Sun, gives way to interstellar space (see ‘Edging into the unknown’).

Detecting and characterizing this threshold — called the heliopause — would be the ultimate bonus for a probe that logged its 35th year in space on 5 September. When Voyager 1 set out, says Stone, a physicist at the California Institute of Technology in Pasadena, who has coordinated the mission since the probes launched, “the space age was only 20 years old and there was no evidence that any spacecraft could travel this long and this far from the Sun”.

The extraordinarily long-lived Voyager 1 began detecting hints of a boundary region eight years ago. But exiting the Solar System is proving to be a longer and more complicated affair than Stone and his colleagues had anticipated. By the time Voyager 1 is well and truly out, it may have transformed researchers’ ideas about the Solar System’s invisible edge.

In the latest twist in the story, the craft seems to be traversing an unexpected ‘dead zone’. This week, Robert Decker, a space scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and his colleagues report1 in Nature that at Voyager 1’s current location, some 121.6 astronomical units (18.2 billion kilometres) from the Sun, the average velocity of solar particles has dropped to nearly zero. (Voyager 2, which is about 3 billion kilometres closer to the Sun and moving in a different direction, has yet to detect the same reduction in velocity.)

Decker’s team first reported2 the change last year, when it had measurements of the particles’ velocity only in the radial direction, outwards from the Sun. At the time, the team thought that the change was a sign that the craft was nearing the heliopause, where solar particles are expected to collide with powerful winds generated by supernovae that exploded some 5 million to 10 million years ago. The collision would force the solar particles to stop moving outwards and push them sideways, like a stream of water hitting a solid surface.

To test the idea, engineers commanded Voyager 1 to roll on its side seven times, so that its instruments could record particle velocities along a line perpendicular to its course. Given that sending a command to Voyager 1 now takes 17 hours, and that the spacecraft’s transmitter runs at 23 watts — about as powerful as a refrigerator light bulb — such communication is a feat in itself. The researchers were astonished to find that the particles had zero velocity in this polar direction, too — indicating that they were almost stationary rather than being buffeted by stellar winds. That cannot happen at the heliopause, says Decker. “We therefore conclude … that Voyager 1 is not at the present time close to the heliopause, at least in the form that it has been envisioned,” the team writes1.

Voyager 1 was launched in 1977. Four of its original instruments (labelled in yellow) are still returning data on conditions at the edge of the Solar System. Credit: NASA/JPL-Caltech

Decker and his colleagues now think that since 2010, when the craft first recorded a velocity drop, it has been in an antechamber to the heliopause, at least 1 billion kilometres thick. Why the particles are becalmed remains a mystery, says Stamatios Krimigis, a space scientist at Johns Hopkins and a co-author of the paper. This leaves theorists in a bind. “There no longer exists any guidance on what constitutes getting out of the Solar System and into the Galaxy,” says Krimigis.

Gary Zank, a theoretical physicist at the University of Alabama in Huntsville, disagrees. “I don’t regard the paper as forcing us to revise our models,” he says. His team and others theorize3 that a magnetic wall in the outer heliosphere, caused by a pile-up of magnetic field lines, could slow down the flow of charged particles and account for the near-zero velocities recorded by Voyager 1.

Although the craft has not yet made it to the helio­pause, the boundary may be within reach. This May, Voyager 1 recorded unprecedented bursts of cosmic rays — highly energized protons and atomic nuclei — coming from outside the Solar System. The spikes returned in July, this time along with a drop in the incidence of lower-energy cosmic rays thought to be accelerated in the Solar System. The changes suggest that Voyager 1 is nearing the fringe of the Solar System, and could cross the heliopause by the end of the year, says Krimigis. But, he adds, “nature seems to be much more imaginative than we are, so I could be quite wrong”.

Indeed, David McComas, a physicist at the Southwest Research Institute in San Antonio, Texas, and Nathan Schwadron, a plasma physicist at the University of New Hampshire in Durham, suggest an alternative explanation. In an article in press in The Astrophysical Journal, they propose that Voyager 1 is in a region where magnetic field lines running through the outer heliosphere link up with the magnetic field of the rest of the Galaxy. Here the field would create a conduit for galactic cosmic rays, causing the spikes in detection. Cosmic rays accelerated within the heliosphere would tend to move along other field lines and be less likely to get to Voyager. If this model is correct, say McComas and Schwadron, the heliopause may still be years away.

There no longer exists any guidance on what constitutes getting out of the Solar System.

When Voyager 1 does leave the Solar System, it may meet further surprises. Researchers have long assumed that a bow shock lies outside the heliopause. Similar to the shock wave around a supersonic aircraft, the bow shock is thought to form as the Solar System ploughs through the interstellar medium, forcing the local ionized gas to change density abruptly and discontinuously. But in May, McComas and his colleagues reported4 that data from NASA’s Interstellar Boundary Explorer (IBEX) mission cast doubt on this picture. From Earth orbit, IBEX probes the interstellar medium by detecting electrically neutral atoms that slip into the Solar System through the heliopause. Its measurements suggest that the Sun and planets are moving through the interstellar medium about 12% slower than previously calculated — too slow to generate a bow shock.

None of this uncertainty bothers Stone, who expects both Voyagers to cross the heliopause well before 2025, when the craft are due to go silent as the plutonium isotopes that supply their power run out. On the contrary, Stone adds, he is pleased that the one-way journey has taken so many unexpected turns. “One thing Voyager has taught us is to be prepared to be surprised.”

Credit: NASA/JPL-Caltech