The Interstellar Age (34 page)

In high school we learned about
Voyager
’s plutonium power supplies and the role that physicists had played in developing ways to power spacecraft far beyond the distances where solar panels would work. Plutonium is element 94, one of the “trans-Uranian” elements (heavier than uranium), radioactive, and one of a slew of very heavy elements on the bottom of the periodic table, only relatively recently discovered. In 1980 my best friend, Bob Thompson, asked our chemistry teacher how many elements were then known, and Dr. Manley replied that the best way to get the inside scoop would be to write a letter (remember, this is in the days before e-mail) to Dr. Glenn T. Seaborg at Berkeley and ask him about it. So he did. We all thought it was hilarious because Bobby’s “reward” for asking a question in class was more homework. A few weeks later, though, to our and Dr. Manley’s amazement, Seaborg wrote back to him. “
One-hundred-six elements are now known of which the last (element number 106) has not yet been given a name,” wrote Dr. Seaborg. It was a fairly short reply, but just the fact that a Nobel
Prize–winning physicist who had discovered
ten
elements on the periodic table took the time to write a personal letter was enough to get Bobby’s picture in the local paper—
VALLEY STUDENT
HAS NOBEL PEN PAL
read the caption. Bob has gone on to a career in astronomy studying giant stars and designing instruments for airborne and ground-based telescopes. In 1997, two years before Seaborg died, they named “element 106” Seaborgium, in honor of Bobby’s Nobel pen pal.

DELIVERY TIME

Even though the plutonium on the
Voyagers
has decayed by only about 25 percent of its starting amount, the spacecraft are already starting to feel the pinch of looming power limitations. Even if there were something useful to photograph, for example, the cameras can no longer be turned on because they would gobble up too much power to also be able to run their heaters. The five remaining instruments use less power, but the power needs of the radio transmitters and the heaters are relentless. In addition, the spacecraft need to use tiny amounts of thruster fuel to accurately point their antennas at the Earth, and that thruster fuel is a consumable, and dwindling, resource. However, amazingly, Suzy Dodd says that only just recently did they finally switch to
Voyager
’s backup thrusters, because the primary thrusters were getting close to their expected lifetime limits, “after nearly 350,000 thruster cycles and thirty-four years of flight!” she said. The backups, which had never been used, are working just fine.

Team members predict that the spacecraft will have enough
power and thruster fuel to stay in communication with the Earth and operate at least one instrument until sometime around 2025, when
Voyager 1
could be more than 160 AU from the sun (more than 15
billion
miles away), and
Voyager 2
could be out beyond 135 AU. By cycling off some of the remaining instruments and systems—those needing the highest power—when not in use after 2020 (or, at some point, off forever), mission controllers may be able to push the
spacecraft’s lifetimes beyond the mid-2020s. But eventually, the power levels will drop to critically low values where some of the heaters and other engineering subsystems will have to be shut off, and then the science instruments will fail or have to be shut off, one by one, with the lowest-power instruments like the magnetometer likely staying on the longest. Even then, though, according to Suzy Dodd, it might be possible to continue to operate the
Voyagers
“with just an engineering signal. We’ve been talking with the DSN about that possibility.” That is, it might be possible to just stay in occasional radio contact with them well into the 2030s.

Earth to Voyager . . . still there?

(long pause)

Still here . . .

Very well. Carry on. Talk to you soon.

There is some possible science that could come from simply monitoring the strength of that faint signal, beamed back over such vast distances. According to Ed Stone, “As long as we have a few watts left, we’ll try to measure something.” Randii Wessen says that no one really knows how long the spacecraft will keep going. “I started at JPL in 1980—at Saturn—as an intern of the
Voyager
science support team. I always thought that the mission would end sometime during my professional career. Now I’m not so sure.” Suzy Dodd has a specific goal: “We launched in 1977, and so if we can keep in contact, still doing science, until 2027, that would be fifty years. That’s my goal—to have
Voyager
operate for fifty years.” She and the
Voyager
team continue to have to fight to justify new NASA funding every few years. At some point, though, even the so-called engineering signal will cease from
Voyagers
, and they will embark on their final mission, to carry forth the Golden Record for all of humanity
.

I hope that we stay in touch with them in engineering signal mode for a long time after science measurements end. Even just the simple act of pinging them by radio and waiting the hours—and eventually days—that it will take for the ping to be acknowledged and sent back, can teach us something about where they are and what it’s like there. An interesting example of that comes from the precursor deep-space missions to
Voyager,
the
Pioneers.
After their encounters with Jupiter and Saturn, both
Pioneer 10
and
Pioneer 11
embarked on their own interstellar missions, heading out of the solar system in different directions from the
Voyagers: Pioneer 10
heading “downwind” in the heliosphere and close to the plane of the planets, and
Pioneer 11
heading “upwind,” like the
Voyagers,
but
only slightly above the plane of the planets. NASA’s Deep Space Network kept track of the
Pioneers
long after their planetary flybys, until their plutonium-based power supply systems ran out of enough juice to power the radio and other critical systems in 1995 (when contact was lost with
Pioneer 11
) and 2003 (
Pioneer 10
). Subsequent analysis of the
Pioneer
radio signals revealed something curious, however. The spacecraft were not as far away from us as they should have been—something was slowing them down, by a tiny amount,
year by year. It certainly wasn’t from the gravity of any known objects, as that was being properly accounted for, or from any other obvious known forces. Perhaps it was some kind of new physics that could only be discovered by a long, lonely trip through deep, nearly empty space? No one knew, and the discrepancy became known as the
Pioneer
Anomaly.

Over more than twenty years, astronomers, physicists, and spacecraft engineers tossed around hypotheses about the gravity of small bodies like KBOs, or dark matter, or some other cosmological effect, causing the
Pioneers
’ deceleration. Or maybe drag from particles in the heliosphere, or small helium gas leaks on the spacecraft that acted like mini thrusters, or some other spacecraft-related effect that hadn’t been properly accounted for. Eventually after much head scratching, physicists and spacecraft engineers finally solved the
Pioneer
Anomaly. With funding from The Planetary Society, they tirelessly sifted through nearly thirty years of
Pioneer
tracking data, some of it recovered from ancient magnetic tapes restored to modern digital data files with funding from Planetary Society members. They solved the mystery. The deceleration turned out to be from a tiny, almost insignificant force created by heat (“thermal photons”) leaking out of the plutonium power generation unit in a specific direction that happens to be opposite the sun based on the design of the spacecraft components. This tiny force directed away from the sun causes the spacecraft to recoil (Newton’s “equal and opposite reaction”) toward the sun, ever so slightly, slowing it down by the tiny amount observed. Pretty old physics, actually, but it took
modern-day spacecraft forensics work to track it down.

Eventually, as the
Voyagers
continue on in pursuit of the limitless expanse of interstellar space, they will leave not only the realm
of the solar wind but the realm of the sun’s gravity as well. Both are traveling faster than the escape velocity of the solar system, presently some 128 and 105 AU from the sun, respectively. The sun’s gravitational influence is predicted to extend one-third to halfway to the nearest stars, or maybe around 100,000 AU (or about 1.6
light-years
away).

Out there is a hypothesized spherical swarm of comets and asteroids that have been cast out of the inner solar system by encounters with the planets or the sun over their lifetimes. Every few years a new comet on a long, elliptical trajectory is discovered; some of them, like 1995’s Comet Hale-Bopp or 1996’s Comet Hyakutake, produce spectacular displays of gas and dust as their ices are boiled off by the sun’s heat into beautiful, gracefully arcing tails. Tracing the orbits of these and similar so-called long-period comets back to the outer solar system tells us that they come from enormous distances, and from any possible direction in the sky. This is what led Estonian astrophysicist Ernst Öpik and Dutch astronomer Jan Oort to hypothesize that the solar system is surrounded by a vast spherical shell of perhaps a trillion or more asteroids and comets, which we now call the Öpik-Oort Cloud (or usually, just the Oort Cloud), extending out to the edge of the sun’s gravitational influence.

At their current speeds—and at 10 miles per second, they are the fastest objects that humans have ever sent into space—the
Voyagers
will still take about 30,000 years to reach the outer edge of the Oort Cloud (they should reach the inner edge in “only” about 300 years). The distances between asteroids and comets out there are so vast that it is highly improbable that either spacecraft will pass anywhere near any of them. Another 10,000 years later
Voyager 1
will pass only about 100,000 AU past the red dwarf star Gliese 445,
which is now moving toward the sun and will by then be one of the closest stars to our solar system,
just under four light-years away. Around the same time,
Voyager 2
will pass only about 111,000 AU from another red dwarf star, Ross 248, which will actually by then be the closest star to the sun in the sky. If there are inhabited planets around those stars—unlikely, given their small size and very faint output of energy as compared to our own sun, but still, who knows?—I wonder if anyone will notice these little emissaries zipping past from the star next door?

Indeed, Carl Sagan and his Golden Record colleagues speculated about whether it might be possible to avoid “the near certainty that left to themselves, neither Voyager spacecraft would ever plummet into the planet-rich interior of another solar system.” Both they and I wonder if it might be possible to command one final “empty-the-tank” thruster firing, just before final communication with each
Voyager
is lost, to “
redirect the spacecraft as closely as possible so that they will make a true encounter [with these stars]. If such a maneuver can be affected then some 60,000 years from now one or two tiny hurtling messengers from the strange distant planet Earth may penetrate into their planetary systems.” If no one else does, I will try to remember to make this request to Suzy Dodd or whoever is running the
Voyager
Project in a decade or so, as the spacecraft power levels wind down. We have the fuel. Feel free to mention it to your congressperson.

It might also be interesting to see if we can
upload
images back onto
Voyager
’s tape recorders before we lose communication with them forever. While their Golden Records tell of their home world, there is nothing onboard each spacecraft that tells the stories of their magnificent adventures within their solar system. It’s the same
sentiment that motivates Jon Lomberg to pursue his “One Earth”
New Horizons
digital message project. “One thing I wish could have been on the
Voyager
Record,” he told me, “which we are going to remedy with the
New Horizons
digital message, is that I wish we could have had something of ‘here’s what
Voyager
was and here’s what Voyager found,’ because it’s one of the best things human beings have ever done. If they ever find
Voyager
they won’t know about its mission. They won’t know what it did, and that’s sad.” So I say let’s try to upload the Earth-Moon portrait; the historic first close-up photos of Io’s volcanoes and Europa and Ganymede’s cracked icy shells; the smoggy haze of Titan; the enormous cliffs of Miranda; the strange cantaloupe and geyser terrain of Triton; the swirling storms of Jupiter, Saturn, and Neptune; the elegant, intricate ring systems of all four giant planets; the family portrait of our solar system. Let’s arm our
Voyagers
with electronic postcards so that they can properly tell their tales, should any kind of intelligence ever find them.

INTELLIGENT BITS AROUND THE GALAXY

The idea of astronomers on other planets potentially noticing the
Voyagers
, or other signs of our civilization, is not as far-fetched as it used to be. In the past two decades, astronomers have discovered the first
evidence of planets around other nearby stars that are like our sun. The first such planets were found using sensitive instruments on ground-based telescopes to search for tiny wobbles in a star’s motion caused by the gravitational tug of planets orbiting around it. That technique, called the
radial velocity method
, is most
sensitive to really big planets that are really close to their parent stars, causing a big wobble. And indeed, astronomers began finding dozens and dozens of these so-called hot Jupiters (named because they’re Jupiter-sized but much hotter than our Jupiter because they are much closer to their parent stars) orbiting nearby stars. Some stars even have multiple close-in giant planets. Strange indeed, and nothing like our own solar system, where the giant planets are far away from their star. So is our solar system an oddball, and most stars have giant planets orbiting close-in? Or is our system typical, and the only reason we’re finding so many hot Jupiters is the streetlight effect: we’re finding what we’re seeking only because we’re looking where it’s easiest to find exactly those things (like looking near the streetlight at night for your lost car keys, because that’s where the light is). In this case, just because hot Jupiters are the easiest planets to find using the radial velocity method, that doesn’t mean that they are the most common kind of planet out there.