The Interstellar Age (30 page)

The glut of Pluto-sized bodies being recently discovered beyond Neptune is what gave Pluto itself all that trouble, of course. Rather than accepting the fact that there are indeed many hundreds of newly discovered planets out there, and countless more still to be found, some astronomers chose to be “splitters” instead of “lumpers.” In 2006, after some contentious debate, the International Astronomical Union (IAU)—the world’s governing body tasked with giving planets and moons and asteroids and comets (as well as craters and mountains on those worlds) their names—decided to strip Pluto, and other places like it, of
their “planet” status. Instead, such worlds were demoted to “dwarf planet” status, and the number of true planets in the solar system was decreased to eight, throwing textbooks and elementary school science-fair projects into chaos and disarray.

I’m a card-carrying member of the IAU and generally proud and supportive of the work that my colleagues in that organization do on behalf of astronomy and planetary science worldwide. But this time, I think they got it wrong. Personally, I judge a planet (like a person) on what’s on the inside, rather than what it looks like or where it’s been. Mercury is a planet because it has had a complex geologic history, including formation of a core, mantle, and crust, and the eruption of volcanoes on its surface, all fueled by substantial internal heat. It happens to be in orbit around the sun. Io, comparable in size, has had a similarly complex surface and interior geologic history. It happens to be in orbit around Jupiter, but it is still
the same kind of object
. So I call Io a planet. As well as Europa, and Ganymede, and Callisto. Plus Titan, Triton, Enceladus, Dione, Rhea, Tethys, Ariel, Ceres, Vesta, Eris, our own moon, and lots more. And Pluto—for God’s sake, it’s got an atmosphere and
five moons of its own.
If that’s not a planet, I don’t know what is. By my reckoning (and I’m a bit of a weirdo among my astronomy friends for this), our solar system has about thirty-five known planets so far, and it’s likely that dozens more will be discovered over the coming decades. Let’s celebrate those numbers and the diversity of planetary characteristics within our cosmic neighborhood rather than splitting them up into categories implying substandard status, such as “moon” or “dwarf planet.” I’m a lumper rather than a splitter.

SOLAR WIND

Although astronomers and planetary scientists don’t yet know exactly how far toward the nearest stars the sun’s
gravitational
influence extends (it’s probably somewhere near a half to two-thirds of the way), they have been expecting over the past decade or so that far-flung spacecraft like
Voyager
should soon be able to find the edge of the sun’s
nongravitational
influence on the solar system. The sun produces energy by the conversion of four hydrogen atoms into one helium atom deep in its interior, at super-high pressures and at temperatures of millions of degrees. The conversion releases a tiny bit of energy, in the form of photons and other subatomic particles like protons and electrons, that bounce around inside the sun and eventually make their way out. The sunlight—photons—that warms our faces on a sunny afternoon was created, on average, deep inside the sun,
maybe 50,000 years ago or more. The stream of protons and electrons coming off the sun every second creates a flow of charged particles called the
solar wind
. The solar wind creates a giant spherical “bubble” around the sun in interstellar space, known as the
heliosphere
. The heliosphere extends far beyond the orbit of Neptune, until it becomes so diffuse and weak that it merges into the background of rarefied hydrogen and helium gas that permeates the space between the stars—the
interstellar medium
. The sun and every other star reside inside their own such cocoons, blowing bubbles in the interstellar medium from their own solar, or stellar, winds. Like all bubbles, there must be an edge, a boundary between inside and outside of that bubble. Inside the bubble is the solar wind, outside the bubble is the
interstellar wind.
Finding that edge, then, and going beyond it, provides a way to explore truly
interstellar
space.

In some grand and philosophical way, as
Voyager
plowed on toward the boundary of this bubble, it became important to be able to determine the precise moment in time when we could say without
question that we had left the confines of our solar system and we were now “outside,” in interstellar space, the space between the stars.

While the vast distances traversed by
Voyager
to date are nearly incomprehensible on any human scale, it is perhaps even more difficult to grasp the lonely future of our spacecraft as it travels through interstellar space, heading off into infinity. Yet despite the cosmic emptiness that we are facing, the dream of the Golden Record remains alive in our hearts and minds. We can imagine a time in this incomprehensible future when some vastly superior beings, traveling these distances with the ease of today’s intercontinental airline flights, would receive an earnest message from an Earth long gone but preserved in small part aboard our timeless
Voyager
emissaries.

But before it could be declared that Voyager had crossed into interstellar space, particles and fields emanating from our sun as well as from outside of our solar bubble would have to be tracked so we could witness the changes directly. Solar astronomers have discovered that, like the winds of our own planet, solar wind streams are in constant motion, acting out their own solar weather systems. Sometimes the solar wind is gentle and flows smoothly, like a breezy day. The slow solar wind (where “slow” is only 900,000 miles per hour) is an extension of particles that were accelerated through the sun’s upper atmosphere—the expanding “corona” of the sun. And the fast solar wind (at more than 1.7 million miles per hour) appears to stream off the sun’s visible surface (known as the
photosphere
). A few billion pounds of material streams off the sun every second, but the mass lost over time has still been only a minuscule fraction of the overall mass of the sun. Although invisible to our eyes, we can see evidence for the solar wind in the beautiful ion tails of comets,
which always point downstream in the solar wind, away from the sun. These somewhat steady breezes are interrupted by occasional gale-force storms of particles called
coronal mass ejections
—the giant, looping arcs of hot plasma gas that launch off the surface of the sun and send electromagnetic shock waves and sprays of ionizing radiation outward toward the planets. Sometimes these waves and radiation produce glorious auroral displays in Earth’s polar regions, and sometimes they also wreak havoc with electronics in orbiting satellites and surface power grids. The sun has weather, and it’s important for our modern, electronic civilization to pay attention to it.

That’s where space physicists like Ed Stone come in. Ed has spent his career working to understand high-energy particles from the sun and other cosmic sources, how they interact with the magnetic fields of the sun and the planets, and what they can tell us about how the planets, the sun, and other stars work. Ed’s Cosmic Ray Subsystem (CRS) instrument is designed specifically to measure the energies and intensities of high-energy particles from the sun and other sources in the galaxy, in order to map out the sun’s magnetic field as a function of distance, and to understand the effects of that field on the planets. It’s an example of “squiggly line science” (you know, like the medical devices they use to monitor your vital functions, or the seismometer plots you can see monitoring for earthquakes in some science museums). Ed’s instrument generates streams of data that most often appear on plots and graphs, rather than images. But scientists like Ed read them with ease. If you want to study the geological diversity of a planet’s surface, pictures are the way to go. But if your aim is to understand the energies and densities of subatomic particles in space, a picture simply won’t do. Other sorts of measurements are necessary. And it’s a little-known fact that some of the most important
discoveries from missions like
Voyager
, the Mars rovers, and many other missions come not necessarily from the pictures but from the investigations that produce squiggly line science.

After the Neptune flyby, and the success of the
Pale Blue Dot
photograph,
Voyager
’s focus was shifted almost entirely toward the fields and particles investigations that helped to characterize the interactions of the solar wind with the magnetic fields of the giant planets. With our beloved planets fading into the distance, these became the only instruments with something left to measure. And not just s
omething
, but something profound. Where does the solar wind stop blowing? Where does the sun’s influence give way to the different kinds of fields and particles that infuse the spaces between the stars? Finding this edge, the edge of the heliosphere’s bubble known as the
heliopause
—and studying for the first time the nature of interstellar space—became
Voyager
’s prime directive.

Ed Stone’s CRS instrument can tell the difference between high-energy particles (nuclei) that have come from the sun (solar energetic particles) and those that have come from outside the solar system—from elsewhere in the galaxy (cosmic rays). Some high-energy cosmic rays do make it across the heliopause boundary, and
Voyager
has been characterizing them for decades. However, many of the lower-energy protons and electrons that make up cosmic ray particles from elsewhere in the galaxy can’t actually pierce the bubble of the sun’s heliosphere, and instead they are diverted around the solar system, like water diverts around an island in a river. Indeed, trying to measure the full range of energetic particles that are characteristic of the interstellar wind, not just the solar wind, was a goal for Ed’s investigation way back when the instrument was being designed and built in the early 1970s.

I asked Ed whether deep in the recesses of his wildest dreams he actually harbored any hope back then, back when he was dreaming up his cosmic ray experiment in the ’60s or when he became project scientist in 1972, that at least one of the
Voyagers
would survive to make it to interstellar space, to measure that interstellar, galactic wind. “Well, we hoped. When we started this mission, we had, as one of the objectives, to get to 20 AU,” he told me. (“AU” is the astronomical abbreviation for “astronomical unit,” where 1 AU equals the average distance between the Earth and the sun, or about 93 million miles.) “No one knew where the boundary would be. We had to propose for a new mission extension once
Voyager 2
completed Saturn, and we called it the
Voyager
Uranus-Interstellar mission. It was one leg at a time. Then the next leg after Uranus was the ‘
Voyager
Neptune-Interstellar’ mission. So ‘Interstellar’ has always been there, it’s just that none of us knew how big the bubble really is! And none of us knew how long the spacecraft could last.”

The “Interstellar” focus of
Voyager
’s postplanetary mission was not an accident. Ed believes that his own cosmic ray instrument, for example, was put on specifically for this possible, hoped-for extended phase of the mission, where interstellar particles could be measured in their native habitat. “Which is really kind of remarkable, when you think about it,” he confesses. There is a modern penchant for cut-rate mission budgets and strict adherence to carefully crafted specific mission goals. Ed Stone adds, almost to himself, “I’m not sure that today that would happen.”

Both
Voyagers
had already been accelerated to high-enough speeds, by launch and by their subsequent giant-planet gravity assists, to be on escape trajectories from the sun’s gravity.
Voyager 1
, which was diverted northward during its flyby of Saturn in 1980, is
traveling fastest, at about 10 miles per second (more than 38,000 miles per hour).
Voyager 2
, which arced over the north pole of Neptune in 1989 and was then diverted southward, is not too far behind, traveling at about 9.5 miles per second (more than 34,000 miles per hour). Three other spacecraft—
Pioneer 10
and
Pioneer 11
launched in 1972 and 1973
,
respectively, and
New Horizons
, launched in 2006—
are also on escape trajectories out of the solar system, but they are all traveling slower than the
Voyagers
, which are leading the race to find the edge of interstellar space.

While we’re no longer in contact with the
Pioneers
,
Pioneer 11
could leave the heliosphere within the next decade or so, but
Pioneer 10
will take much longer (perhaps thirty to fifty years or more) because it is traveling “downstream,” along the extended “tail” of the sun’s magnetic field. How far does that solar magnetotail extend? Ed says, “I’m not sure. Hundreds, hundreds of AU?” At first that may seem surprisingly long, but consider that the sun’s magnetic bubble extends more than 100 AU in the “upstream” direction, where it’s flowing against the current of the interstellar wind. Maybe it’s not so surprising, then, that it could extend two or three times as far in the other direction, where it’s flowing downstream in the same direction as the flow outside the bubble. “The edge of the sun’s magnetotail is undoubtedly a ragged or filamentary kind of thing based on the shapes of the magnetotails of the giant planets, and it probably just eventually merges with the interstellar background,” Ed adds.
New Horizons
is traveling upstream into the interstellar wind, like the
Voyagers,
but it, too, is probably twenty to thirty years away from leaving the heliosphere, based on its later start and slower speed compared to the
Voyagers.

Ed Stone and other space physicists worked up computer
models and predictions for when these spacecraft might reach the heliopause. The models use information on changes in the strength and shape of the solar wind measured over many decades by the
Voyagers
and
Pioneers.
“During the ’90s, we would have a meeting every few years,” Ed says, “and there would be a histogram showing the range of predictions. And that histogram just kept moving out in time as
Pioneer
kept moving out in space, not yet crossing the boundary. It seemed like the edge of the bubble was always 20 AU beyond where we were! Things were changing slowly, but really, we didn’t know. By the early 2000s, however, all the different ways of estimating the size of the heliosphere were converging on the first major boundary—called the
termination shock
—occurring at about 90, plus or minus 10, AU. So if we saw that in just a few years’ more travel, we’d know that we were getting closer to exiting the bubble.”