What Lies Beyond: Exploring Interstellar Space

Forget reaching the stars, first we need to explore the space between them

The scale of things is hard to grasp. The distance from Earth to the Moon - some four hundred thousand kilometers - is the shortest cosmic scale of any significance, but one that already stretches the imagination. The Apollo astronauts took three days to traverse that distance; a span that light, the fastest thing in the cosmos, takes a little over a second to cover.

Things quickly get worse. We can have little hope of really appreciating the distance to the Sun - one hundred and fifty million kilometers, eight light-minutes - or that to the orbit of Neptune - four point four billion kilometers. These are numbers and distances beyond human reckoning, relics of a cosmos so vast that our presence here quickly pales into an alarming insignificance.

Rather than kilometers or miles, astronomers often speak of astronomical-units - defined as the distance from the Sun to the Earth - or in terms of light-years, the span covered by a photon of light in one Earth year.  

Yet even here we can run into difficulties. Astronomical units, or AUs, are useful on the scale of the planets. Neptune, rather than being four billion kilometers distant, is a more relaxed thirty astronomical units away. Even Pluto, once the marker of the Solar System’s edge, never strays more than fifty astronomical units from Earth.

But what then? The Kuiper Belt, a ring of icy worlds encircling the solar system, may stretch as much as one hundred astronomical units from the Sun. The Oort cloud, the birthplace of comets, lies even further away, its inner edge sitting some two thousand AU from Earth. Its outer edge is still mysterious. Some speculate it extends halfway to Alpha Centauri, reaching as far as two hundred thousand AU.

Here, as our focus shifts from planets and comets to stars and galaxies, our measurement sticks must change once more. Alpha Centauri is a ridiculous three hundred thousand AU from Earth, a number so unwieldy that astronomers prefer to tell you it lies just four light years distant. Close, you might be tempted to think, especially when they tell you the galactic centre is twenty-six thousand light years further away.

Don’t be fooled. Alpha Centauri is so enormously far from Earth that we have little hope of ever travelling there. Even at the fastest speed we have ever reached - that of the Parker Solar Probe - it would take ten days to cover a single astronomical unit. Four light years, if we could somehow keep that speed up, would take seven thousand years to cross.

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I. Pioneer, Voyager and New Horizons

That may seem like bad news for the five spacecraft currently heading out of the solar system. Even the fastest - Voyager 1 - is moving at a mere sixty thousand kilometers per hour, a rate that will see it clock up its first light-year around the dawn of the twenty-first millennium. By then, you’ll be sorry to hear, Voyager 1 will long since have drained its batteries, leaving it far out of contact with whatever remains of the human race.

Yet although these intrepid explorers are unlikely to reach other star systems, they are still sending useful data back to Earth. The boundary they have traversed, that which marks the frontier between the Solar System and the rest of the galaxy, is a still mysterious place. When they crossed that border the data they sent back shocked researchers, forcing them to rethink models of the solar system.

Now that they have moved beyond, into a region known as the interstellar medium, they continue to surprise researchers. Little, indeed, is known for certain about the space surrounding our solar system. As Voyager flies ever further from Earth, its instruments are quite literally treading new ground.

Voyager 1, along with its partner Voyager 2, left Earth in 1977. Today, after close to half a century in space, their age is beginning to tell. Both are powered by nuclear batteries, a fuel source that allows them to keep running even as the Sun’s light dwindles to almost nothing. Yet those batteries are gradually powering down, subject to the irresistible laws of radioactive decay.

Within a few years, NASA thinks, their power levels will fall below the threshold needed to operate their antennas. When that happens the probes will fall silent, and their operators will be forced to abandon them. Two earlier spacecraft - Pioneers 10 and 11 - have already shut down, drifting on alone into the dark.

That will leave just one spacecraft still sending data from the edge of the solar system. New Horizons, a probe launched in 2006, passed Pluto in 2015. It is now some fifty-six AU from Earth, a distance that still leaves it far short of Voyager 1’s one hundred and sixty AU. It should last into the 2040s, a timespan that may allow it, too, to pass into the seldom seen interstellar medium. 

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II. What Lies Beyond: The Interstellar Realm

Sixty thousand years ago, at a time long before our ancestors settled down and became farmers, our solar system entered the Local Interstellar Cloud. The name is perhaps misleading. Though we call it a “cloud”, the concentration of matter within it is tenuous, far weaker than any cloud found on Earth. Yet compared to its surroundings, this cloud is an area of density: a region where dust and gas exist in slightly greater quantities.

Now, after sixty millennia, the solar system is once again reaching the edges of this cloud. In around two thousand years, researchers estimate, we’ll break free of it, falling instead into the neighbouring G-cloud. Exactly what will happen when we cross that boundary is unclear, though researchers suspect past such events may have affected the climate and atmosphere of our planet.

On a larger scale, however, both these clouds float within a volume of space known as the Local Bubble. Somehow we happen to sit close to the centre of this bubble, and so it extends for at least five hundred light years in every direction around us. Within it space has been swept clean - bar the occasional cloud - by shockwaves echoing out from a long ago supernova.

The evidence suggests it erupted three million years ago, almost exactly as our ancestors first began to sculpt stone tools. Quite how close the blast came to Earth is debatable, yet recent discoveries hint it might have been within a few dozen light years. Traces of Iron-60 - an element forged only in the heart of a supernova - have recently been found in Antarctic ice cores and oceanic sediments.

If a supernova had exploded close to Earth, it would have had a dramatic effect on the heliosphere, the protective shell of the Sun’s magnetic field. Normally this shell stretches far beyond the outer planets. Both Voyager spacecraft were more than 120 AU from Earth when they encountered it, putting its edge at over twice the distance of Pluto.

Yet as shockwaves from the supernova pummelled it, this shell would have contracted. Some studies reckon it could have shrunk to twenty-five AU in radius, leaving the planets beyond Saturn exposed to debris from the dying star. Earth would almost certainly have remained within the protective shell. The presence of Iron-60, though, shows the event did not leave us entirely unscathed.

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III. Target 1000AU

As the Voyager spacecraft fade, researchers are thinking about sending a new generation of explorers to the edge of the solar system. First up could be the Chinese, who have sketched out plans to send two probes to one hundred AU by mid-century. 

Though details on the mission are hard to come by, as is the case for many other Chinese activities in space, some have emerged. The two spacecraft would head outwards in different directions, giving us separate views of the heliosphere. One probe would target the “nose” of the heliosphere, the point that lies directly in front of the Sun’s direction of motion. The other would take a more lateral path, giving us a more side-on view of the heliosphere. When they will launch is anyone’s guess, though it could be as early as 2026.

An even more ambitious goal was sketched out by American researchers before last year’s decadal review of planetary science. NASA, they wrote in a five hundred page report, could send a probe to one thousand AU by the end of the next century. Doing so would be a challenge, they admit, requiring over a century of operations, but it would not be impossible.

The Interstellar Probe, as they dub it, could hitch a ride on one of the big rockets currently under development. Baseline plans called for it to use the Space Launch System, NASA’s moon rocket, to boost it towards the stars. But other options, like Starship, could soon be available, and offer a flight for a far lower cost. Either way, the probe would need a lot of speed. The plan calls for it to pass Jupiter seven months after launch, and to reach one hundred AU within fifteen years. 

From then on the craft would cruise towards one thousand AU, reaching that milestone some one hundred and forty years after lift-off. Designing a probe to last that long is tough, though Voyager has already made it close to fifty years in space. More difficult may be maintaining communications over such vast expanses. 

The ultimate challenge, however, will be to sustain operations over a century and a half. Voyager stretched over the entire careers of some engineers, a length that made it hard to maintain knowledge amidst a fast changing world. Planning for a multi-generational mission, as the Interstellar Probe calls for, will take new ways of thinking about technology and operations.

Still, perhaps we better get used to that. The scale of the cosmos makes us, and our lifespans, look puny. Reaching the edge of the solar system is a project measured in decades at best; reaching the stars - if we ever do - will take generations.