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(Images: Tom Gauld)

NASA says our furthest emissary has at last broken though into interstellar space – but Voyager’s dispatches reveal that nothing is simple at the outer limits

HEARD the joke about déjà vu? Several times. Then you’ve probably heard the news about NASA’s Voyager 1 spacecraft leaving the solar system. Its departure has hit the headlines many, many times – only for NASA to change its mind. Over the past decade, the veteran space probe has been in, out and even shaken all about.

Last September, though, it looked like this game of space hokey-cokey was finally over. Voyager 1’s normally cautious project scientist Ed Stone declared that, after 35 years, the probe had left for real. “This is humankind’s historic leap into interstellar space.”

So why has it been so hard to tell if Voyager 1 has crossed the border from the solar system to interstellar space? And can we be sure that it has really made it this time? If the latest results are anything to go by, the story of Voyager 1 is far from over.

Mission control isn’t what you might imagine. Forget the cavernous darkened room with rows of anxious engineers and scientists intently watching giant screens on the walls. What I’m ushered into is a single-storey building next to a busy traffic junction just north of Pasadena in California. Inside, there’s a small open-plan office. It is empty apart from two pairs of computer screens aglow with tables of figures, one set green, the other blue. From the ceiling hangs a sign: “Mission Controller”.

It hasn’t always been so low-key. When in 1977, mission control was at the Jet Propulsion Laboratory in Pasadena. But times change: “We’ve been here since 2003, when NASA needed the control room for the Spirit and Opportunity rovers on Mars,” says Suzy Dodd, the Voyagers’ project manager.

What haven’t changed are the giant radio ears around the globe picking up faint whispers from the distant probes. They beam data to the control room for sending on to the science teams. The two blue screens are displaying messages from Voyager 1; the green screens are full of posts from Voyager 2.

For all the similarity in the displays, there’s one huge difference. Voyager 2 is telling us about conditions in the heliosphere – the giant magnetic bubble surrounding the sun where we and all the planets reside. But Voyager 1 is flying higher, through a region of the universe where no probe has ever been before: the space between the stars.

Or is it? The hinterland of the solar system is a confusing place. For a start, where does it end? Voyager 1 has passed all the planets – it crossed Neptune’s orbit back in 1988 – and is now 127 times further out from the sun than Earth. Yet it’s still well within the sun’s gravitational embrace, and won’t break free for thousands of years when it passes the remote Oort cloud, homeland of the comets (see diagram).

To settle the matter, the Voyager 1 team would like to find out whether the probe has burst through the heliosphere. This is formed from the hot wind of particles, laced with magnetic fields, pumped out by our star. The solar wind sweeps past Earth and travels beyond Neptune until it eventually loses its oomph when it hits interstellar space – marking the edge of our solar system.

Although nothing has visited interstellar space before now, at the University of Chicago has been able to characterise it by looking at its imprint on light from nearby stars. Far from being a featureless void, interstellar space contains clouds of gas and dust that are being blown about by the force of ancient supernova explosions. Frisch has shown that our solar system lies inside a cloud about 40 light years across, called the . Here a teaspoon of space contains about half a dozen atoms, which is far denser than the solar wind when it reaches the edge of the solar system.

This difference helps us tell if Voyager 1 has left the heliosphere and crossed into interstellar space. Amazingly, most of its scientific instruments are still working, though to save power NASA controllers long ago switched off the cameras that took close-up images of Jupiter and Saturn.

In August 2012, Voyager 1’s instruments measured a dramatic drop in the number of solar-wind particles. At the same time, they detected a much higher rate of arrival of cosmic rays – high-energy particles from interstellar space that struggle to penetrate the heliosphere’s magnetic shield. Things seemed pretty clear-cut: Voyager 1 had left the solar system.

However, a third indication refused to fall into place. Voyager 1’s on-board compass should have picked up an abrupt change in the magnetic field at the edge of the heliosphere. “Yet the field direction was just really incredibly flat,” says Alan Cummings, who was the last person to see Voyager 1 before lift-off.

Faced with this contradiction, early last year NASA scientists were sitting on the fence. Then came what seemed to be the clincher, from an unexpected direction. In April 2013, Voyager 1 felt the gas around it shaking violently, as a giant eruption from the sun 400 days earlier finally reached the spacecraft. The strength of this pounding showed that Voyager 1 was in a region far denser than the interior of the heliosphere. After pondering the finding for a few months, Stone – as NASA’s spokesman – made the historic declaration: “Voyager 1 is now bathed in matter from other stars.”

Ribbon in the sky

So far, the interstellar medium is what we’d bargained for, says Stone. It is 40 times denser than the gases in the heliosphere, just as we would expect for a wall of gas piling up at a largely impenetrable boundary. Yet for all the hype surrounding Voyager 1, the doughty old spacecraft isn’t the only show in town. The remarkable satellite is mapping the edge of the solar system too – without leaving Earth-orbit.

IBEX is on the lookout for high-speed atoms travelling towards Earth from beyond the planets. These atoms started out as electrically charged ions ejected from the sun. When they reached the edge of the heliosphere, its magnetic field whirled them round in all directions. Some of the ions picked up electrons from the interstellar gas and became neutral atoms. At that instant, they stopped feeling the magnetic force and shot off in all directions, like a whirling conker flying away when the string breaks (or think of the hammer throw). Some of these energetic atoms head back home and are scooped up by IBEX, allowing the satellite to map where the gas is found and hence the edge of the solar system.

“The Voyager and IBEX observations are complementary,” says Dave McComas of the in San Antonio, Texas, and head of the IBEX team. “A good analogy is from medicine, where the global imaging (IBEX) is done by a CT scan, while a few precise local measurements (Voyager) of a tumour require sampling by a biopsy.”

One surprising feature stands out. IBEX’s map shows a , indicating a region at the edge of the solar system that’s sending far more atoms back towards Earth. “The IBEX ribbon was a shocking and fascinating discovery,” says McComas.

It’s most likely that the ribbon is caused by the magnetic field found outside the solar system, and so the orientation of the IBEX ribbon reveals the direction of the magnetic field in interstellar space. According to these results, it aligns roughly north-south. If we could see it in the sky, one magnetic pole would lie in the northern sky near the bright star Arcturus, and the other in the south in the dim constellation Eridanus.

Further confirmation comes from Frisch. Her team has been plotting the direction of the magnetic field in the Local Interstellar Cloud by observing the light from nearby stars. The cloud’s magnetism regiments its microscopic grains of dust, so they act like the filters in a pair of Polaroid sunglasses. “We’re finding the interstellar magnetic field has a direction that nearly agrees with the magnetic field direction from the IBEX ribbon,” she says.

But what of that pesky reading from Voyager 1? The compass needle on board is still resolutely pointing east-west – the direction of the magnetic field within the heliosphere. What gives? Nathan Schwadron of the University of New Hampshire in Durham has a contentious new idea: Voyager 1 hasn’t left the solar system after all.

That clearly chimes with the magnetism readings. But Voyager 1 is indisputably in a region of dense interstellar matter. So Schwadron suggests that that has penetrated into the heliosphere.

Working with McComas, Schwadron points to an analogy with Earth. “Portals” sporadically open in Earth’s magnetic bubble, letting in particles from the solar wind that cause spectacular auroras. Scale this up to the sun’s magnetic bubble, and similar portals could allow interstellar material to enter the heliosphere without changing the magnetic field direction (see diagram).

The jury is still out on the idea, but it does help to explain why Voyager 1 encountered the edge of the heliosphere much sooner than predicted, at 127 astronomical units from Earth rather than 150 AU. If Voyager 1 is in a finger of interstellar matter, the true edge of the heliosphere could still be much further out. “It will certainly be exciting if future boundaries are observed,” says Stone, conceding his announcement of Voyager 1’s departure from the solar system may have been premature.

“The announcement of Voyager 1’s departure may have been premature”

That means it’s critical to keep in touch with Voyager 1 for as long as possible. The craft is slowly dying, as the power from its radioactive generators fades away. At mission control, I’m amazed that the feeble transmitters on board are still communicating with Earth – though at a measly 160 bits per second.

The science instruments on board don’t eat up the electricity. “The main drain on power is the heaters which keep the electronics at 18 to 20 °C,” says Steve Howard of the Voyager team. Out in the cosmic deep freeze, that means a lot of power. NASA expects to keep all the instruments running until 2020. “We haven’t talked a lot about what we do then,” admits Cummings, “whether to switch them off in sequence, or share power.” One option, says Howard, is to run the experiments without the heaters, to see if any of them work in the chill of space.

While Voyager 1 has been grabbing all the headlines, its sister craft may provide some of the answers to “Voyager’s little puzzles”, as Cummings puts it. Voyager 2 is heading out in a different direction and is still “only” 104 AU from the sun. I ask Stone when it’s likely to reach the same intriguing region of space as Voyager 1. He laughs: “Nature will tell us!”

Send in the back-up

Voyager 2 passed a boundary within the heliosphere known as the termination shock, where the solar wind speed drops significantly, three years after Voyager 1 did. So Stone thinks it may encounter interstellar matter three years later, too. But the prediction is complicated by the gustiness of the wind rushing out from the sun. As Cummings puts it, “the whole heliosphere is breathing in and out”.

More clues might come from a third NASA probe that is reaching for the stars. New Horizons will pass Pluto in 2015, and should break out of the heliosphere about 30 years from now. “New Horizons is heading nearly directly between the Voyagers, and into the middle of the IBEX ribbon,” says McComas, “so it would be very exciting data.” At the moment, though, it looks like we will lose touch with the craft in the late 2030s, when it’s not quite 100 AU from the sun.

The only certain way to get to the bottom of the Voyager 1 mystery is with new technology. Stone is looking forward to a purpose-built interstellar mission, with a much more modern set of instruments that will be tuned to investigate the space it’s swimming through. “The key thing is to get there much faster – as much as four times faster,” says Stone. “With Voyager 1 it took us 35 years, and that’s a very long time.” The Interstellar Probe, now on the drawing board, would be shot into space by NASA’s powerful new rocket, the inelegantly named Space Launch System. Accelerated through space either by an electrical propulsion system or a solar sail, it could reach 200 AU – almost twice as far as Voyager 1 lies now – in only 15 years.

“The Interstellar Probe could reach twice as far as Voyager 1 in half the time”

McComas favours a different approach. He thinks a follow-up to IBEX would yield more science for the buck. A recent NASA report singled out another proposed craft, the Interstellar Mapping and Acceleration Probe – a larger and more precise version of IBEX. “IMAP is the most important next mission, and the global information it provides is far more important than another direct interstellar sample,” says McComas.

It’s not just about the buzz of exploring a new realm: researchers are concerned about the effect that interstellar weather could have on Earth. “It’s absolutely certain that the cloud around the heliosphere affects the environment where Earth lives,” says Frisch.

Already, we are starting to draw up interstellar meteorological charts. By comparing the IBEX data with more primitive measurements from early probes and satellites, Frisch has discovered that the gas beyond the solar system is changeable: the interstellar wind is gusty. “It’s highly likely that the direction of the interstellar wind has changed over the past 40 years,” she says. That’s not surprising, she adds, as the solar system is moving through the edge of the Local Interstellar Cloud, where the gas is likely to be turbulent.

As the wind sweeps past the solar system, it changes the way that the heliosphere deflects the energetic cosmic rays from deep space. The strength of the heliosphere’s magnetic shield determines just how many cosmic rays get through and bombard Earth, triggering cloud formation in the atmosphere and so affecting our long-term climate. Cosmic rays also break up DNA in living cells, causing mutations that can lead to evolutionary changes.

So as well as exploring deep space, Voyager 1 and its fellow explorers are also opening a window on the story of life on Earth, unravelling how the history of our planet has been moulded by the capricious storms of the galaxy. Expect plenty more news reports from the edge of the solar system.

Find out more about Voyager in our gallery: “What Voyager’s golden record tells ET about Earth“

This article appeared in print under the headline “Are we nearly there yet?”