The Interstellar Age (31 page)

They also used images and other information from astronomers about the nature of similar “bubbles” seen around other nearby stars. Young stars embedded in so-called stellar nurseries—clouds of gas and dust such as the nearby Orion Nebula—are particularly useful because their darker stellar wind cocoons are easily visible against the brighter nebula gas and dust in the background. In places where the boundaries between the stellar and interstellar materials around other stars can be seen—for example, in Orion Nebula images from the Hubble Space Telescope—there appears to be a strong shock wave at the upstream sides of those boundaries (where those stellar winds are running into the background interstellar winds), almost like the shock wave near the nose of a supersonic airplane. Aeronautics experts, as well as astrophysicists, commonly refer to that leading shock wave as a bow shock (or just a bow wave, if
the difference in fluid speeds is not as high), an analogy to the bow of a ship plying its way through the water.

Indeed,
Voyager
and other spacecraft measurements (going back to
Mariner 2
in 1964) have shown that the solar wind is supersonic, meaning that a shock wave should exist close to its leading, or upstream, edge. The direction of the sun’s motion relative to nearby stars and to the overall motion of the Milky Way galaxy is well known, and so which direction is upstream is also well known. It just happens to be in the direction that both
Voyagers
are traveling. Ed and his colleagues predicted that a classic bow wave should exist at this upstream end of the heliosphere, since the relative velocity of the sun compared to the local interstellar medium was predicted to be about 15 miles per second. The fast-moving solar wind, plowing head-on into a fast interstellar wind moving in the opposite direction (like two rivers flowing into each other from opposing valleys), was expected to produce quite a strong wave front, perhaps even a shock wave (though they couldn’t be sure).
Voyager 1
was heading closest toward the actual predicted position of the bow wave itself (along the “nose” of the wave front) and thus was predicted to be able to get there first. Exactly when
Voyager 1
might cross that bow wave was anyone’s guess, however. Ten years after Neptune? Twenty? Thirty? No one knew where the edge of the heliosphere would be, but everyone knew that the
Voyagers’
plutonium power supplies wouldn’t last forever. If the spacecraft hadn’t crossed the line by the late 2010s or early 2020s, the power supplies might not last long enough to see it happen.

AN INTERSTELLAR MISSION

As both
Voyagers
sped on toward their interstellar destinies, the science and operations teams transitioned into a different kind of mission. The imaging team was essentially disbanded, now that there was nothing new left to photograph, and the cameras had been shut off. The same is true of the ultraviolet spectrometer team, although the instrument is still used to collect occasional “automated” astrophysical measurements of nearby interstellar hydrogen. Shutting off or curtailing instruments helps to save power, which is slowly dwindling on both spacecraft. While
Voyager
’s radioactive plutonium power supplies will take eighty-eight years to drop to half their power levels, more than forty years have now passed since that plutonium was produced, meaning that power is down to around 75 percent of maximum values. The most important thing this power is used for is to heat the computer, radio transmitter and receiver electronics, and the remaining instruments. If left to soak in the cold of deep space, the temperatures of those systems would quickly drop to just a few tens of degrees above absolute zero, causing solder joints to break, resistors to crack, or any of a number of other possible fatalities.

Shutting off instruments and scaling back mission operations also helps save money. NASA’s entire annual budget allocation from Congress has averaged around $17 billion per year lately, or about 0.4 percent of the entire federal budget. Of that, all the science done within NASA costs about $5 billion per year, and of that, all the solar-system science—robotic planetary missions and data
analysis, laboratory studies, technology development—has averaged about $1.5 billion per year. My colleague Casey Dreier at The Planetary Society has recently pointed out that that’s about the same as what Americans paid for dog toys last year. Don’t get me wrong—I love my dogs and I want them to have fun! But it’s important to put the costs to the taxpayers of this kind of grand exploration into perspective. And of that $1.5 billion per year, it costs about
$5 million
a year to keep the
Voyager
missions going. A significant amount of money, to be sure.
Voyager
scientists led by Ed Stone, along with JPL Project Manager Suzy Dodd and her mission operations team work hard to justify that $5 million request every year. “
Voyager
has been through something like eleven major reviews of its extended, extended mission,” says Suzy Dodd. Everyone involved takes it very seriously that they make sure to use these amazing far-flung laboratories as efficiently as possible, to push the frontier of human knowledge ever outward, and to learn as much as we can about the far reaches of our solar system. Spread out over the lifetime of the project, the total cost of
Voyager
has been about a dime per year for every American. That seems quite a bargain.

“Now that we’re in interstellar space,” Suzy Dodd says, “we’ve reached the rarefied air of being an untouchable spacecraft.” The
Voyagers
belong to all of us, they represent all of us, they will speak to the ages for all of us.

After Neptune, mission operations became known at JPL and NASA HQ as the
Voyager
Interstellar Mission. The goal of the
Voyager
Interstellar Mission, according to NASA, is “
to extend the NASA exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the sun’s sphere of influence,
and possibly beyond.” An important part of that, both officially and in the dreams of scientists like Ed Stone, has been to search for and find the heliopause, the boundary beyond the outer limits of the sun’s magnetic field and the outward flow of the solar wind, and to directly measure the interstellar fields, particles, and waves beyond the influence of the sun. In other words, to extend the reach of human senses into interstellar space. Quite a dream.

With no working cameras, the
Voyagers
may be blind, but they are nonetheless still capably feeling their way through the outer solar system. Five different science instruments are still being used, many almost daily since the start of the Interstellar Mission, to touch and smell and taste the distant heliosphere. These instruments measure the plasma ions in the solar wind (“plasma” is a physics term for an ionized gas consisting of positively charged ions and negatively charged electrons—a common example is the gas inside a fluorescent lamp); the compositions, directions, and energies of solar wind particles and interstellar cosmic rays; the strength and orientation of the solar or interstellar magnetic fields; and the strengths of natural radio waves that are thought to be originating from nearby interstellar space. One important part of one of
Voyager 1
’s instruments was not operating properly, however. The spacecraft’s instrument that was designed to measure the density of ionized hydrogen plasma in interplanetary space had stopped working shortly after the Saturn flyby in 1981. There were other ways to indirectly measure the amount of hydrogen
Voyager 1
was encountering, but its inability to make a direct density measurement would lead to some controversy later.

Although
Voyager 1
had a head start after completing its planetary mission at Saturn in 1981 and was already forty times farther
from the sun than at launch, the Interstellar Mission didn’t officially begin until
Voyager 2
passed Neptune in 1989, at a distance of about 31 AU. Even at those ranges,
Voyager
’s instruments were still measuring a steady stream of high-energy solar particles and magnetic fields heading out into space on somewhat “radial” trajectories—that is, radiating generally away from the sun, like air inside an expanding balloon. The fields and particles were not perfectly radial, however, but were instead deflected somewhat—as if responding to the looming edge of the heliosphere at some unknown distance ahead.

Since 1989, communications technicians at the NASA Deep Space Network facilities in California, Australia, and Spain have faithfully captured data from the spacecraft for six to eight hours almost every day, using the “smaller” 34-meter (111-foot-wide) DSN radio telescopes to capture the faint signals from the meager 23-watt transmitters on the
Voyagers.
By the time those radio signals travel for more than ten hours at the speed of light, across vast distances now more than 100 AU from Earth, that 23 watts has faded to
only 0.0000000000000001 watts, or barely a flea’s whisper. But the
Voyagers
do a good job of pointing their antennas right at the Earth, and the DSN does a good job of pointing its antennas right at the
Voyagers
, and the very narrow transmitter frequency is pretty far from Earthbound or celestial radio noise sources, and so—somewhat incredibly it seems—it all works. About every six months or so, the DSN points its even larger, more sensitive, 70-meter (230-foot-wide) radio telescopes at
Voyagers
, and the spacecraft are commanded to download a bunch of higher-quality twice-a-week plasma-wave-instrument “wide band” (higher resolution, more sensitive) data, which are used to infer plasma densities that have been stored on the 8-track tape recorder instead of transmitted in real
time. Once the data are confirmed to have arrived safely at Earth, the tape recorders are rewound and the process starts again . . . year after lonely year. . . .

Voyager
’s fields and particles scientists predicted that the spacecraft should pass through several distinct parts of the heliosphere before finally popping out of the bubble and reaching interstellar space. The first new and different place they figured they would encounter would be a boundary known as the termination shock. Ed Stone likes to give public talks about
Voyager
, and he is an enthusiastic and engaging speaker. One of his favorite slides is his “heliosphere in my kitchen sink” movie, where he tries to describe, using a kitchen-sink analogy, the kinds of places that the
Voyagers
will visit on their way out. Here’s how it works: Empty your sink, angle the faucet so that it points toward one side or the other of the drain (not in the middle, where I’m assuming your drain is), and turn the water on full-throttle. The water crashes into the bottom of the sink and fans out into a lovely circular disk of water maybe five or six inches across that is flowing
radially
away
from the impact point. That’s like the deep inside of the heliosphere’s bubble, except in the real solar system, the solar wind and the sun’s magnetic field are wound (by the sun’s twenty-five-day rotation period) into a huge Archimedean spiral with arms that move outward roughly radially away from the sun. But then look carefully at the water on the side of that bubble opposite the drain. That water is starting to slow down, to thin out, and to change direction because of the slight upward slope of the sink. There’s probably a turbulent little zone there where the water is bubbling and churning a bit. That place where the water stops being radial and changes speed and direction is the termination shock and it marks the transition to a new and turbulent part of the flow, where
the water is turned around and heads toward the drain. In the actual heliosphere, the termination shock occurs where the solar wind changes speed and direction because of the pressure of the interstellar wind coming from outside the heliosphere. The region just beyond the termination shock is called the
heliosheath
(the skin or covering on the heliosphere), and no one knew how large that region would turn out to be, because no one knew how much pressure was coming from the outside. The next stop beyond the heliosheath, though, should be the actual edge of the heliosphere—the heliopause.

Over the first decade of their Interstellar Mission, both
Voyagers
measured the density of the solar wind slowly decreasing, as it was spreading out at greater and greater distances. In December 2004, twenty-four years after passing Saturn and twenty-seven years after launch, at a distance of 94 AU from the sun,
Voyager 1
noticed a sudden drop in the speed of the solar wind (from million-mile-per-hour supersonic speeds to “just” a quarter-million-mile-per-hour subsonic speeds) and a jump in the density of the solar heliospheric particles, like a traffic jam on a busy freeway. At the same time, the instruments were able to sense an increase in the strength of the sun’s magnetic field.
Voyager 1
had crossed the termination shock. In August 2007, far to the south and at a distance of 84 AU from the sun,
Voyager 2
crossed the termination shock as well. Both spacecraft were now in the more turbulent heliosheath. Next stop: the heliopause. But when? A year? A decade? More? I figured there must have been a betting pool. But Ed Stone said, stoically, “No, no. There was no betting. Just the histograms that we kept track of with everyone’s estimates. I don’t even know if there’s a record of who voted for what.” Pity. I bet Ed would have won the kitty.

Despite the incredible speeds of both spacecraft, the missions
appear to be going in relative slow motion because of the enormous distance scale of the outer solar system. The spacecraft are traveling
10 miles every second
(imagine
Voyager
passing through your neighborhood at that speed) but still took nearly a decade and a half beyond their last planetary encounters to pass the termination shock and to enter the turbulent heliosheath. During that long cruise outbound, Ed Stone and other
Voyager
scientists moved on to work on other projects, to other missions, or even to retirement. “
I feel extremely fortunate to have become the project manager in 2010,” recalls Suzy Dodd. “During the twenty-year period that I was off the Project, we were just sort of sailing out towards interstellar space, not really having a good idea of how far away that was. I owe my job to the previous project managers who constantly had to fight against the mission being canceled during that time. It wasn’t easy to keep
Voyager
going this long, not just from a technical standpoint, but also from a financial standpoint. There were doubters out there in the early 2000s who wanted to cancel
Voyager.
Once we got through the termination shock, though, people thought—OK, we should be getting closer.” Indeed, once they crossed that boundary, many people began paying closer attention to the mission again, watching for changes in the densities, energies, and directions of the fields and particles measured by the
Voyagers
, searching for the telltale clue that the next—the ultimate—boundary had been passed.