Voyager’s Journey to the Edge of Interstellar Space

Edward C. Stone

California Institute of Technology

INTRODUCTION

The journey to interstellar space began 50 years ago with a discovery and a prediction. Launched during the International Geophysical Year, Explorers 1 and 3 revealed that belts of energetic protons and electrons trapped in the geomagnetic field encircle Earth. Soon to be named the Van Allen belts, they were the first major discovery of the Space Age that had begun only months earlier with the launch of Sputnik 1 by the Soviet Union.

During this time, Eugene N. Parker predicted that the solar atmosphere was expanding outward at supersonic speeds, filling interplanetary space with a dilute plasma of protons and other electrically charged ionized atoms of solar matter. Four years later during the first foray to another planet, Mariner 2 confirmed Parker’s prediction of a solar wind continuously streaming radially away from the Sun at more than a million kilometers per hour.

Although its density is much lower than the best laboratory vacuum, the solar wind is strong enough to create the heliosphere, a giant bubble around the Sun that envelops the planets (Figure 7.1). Outside the bubble lies interstellar space filled with matter from the explosions of nearby supernovas.

In the years ahead, the two Voyager spacecraft will be the first human-made objects to reach interstellar space, completing a journey that began with a discovery and a prediction during the International Geophysical Year nearly 60 years earlier.

THE HELIOSPHERE

The motion of the heliosphere relative to the local interstellar medium creates a wind that distorts the heliosphere into a long-tailed, comet-like shape. In front, a curved interstellar bow shock resembles the bow wave of a ship. Although there are no images of the heliosphere, there are Hubble images of bow shocks in front of astrospheres around other stars (Figure 7.2). Fortunately, the two Voyagers are headed toward the nose region of the heliosphere where the distance to interstellar space is shortest.

The interaction of the Sun with the interstellar wind is complex. Models show the supersonic solar wind expanding outward until it is balanced by the pressure of the local interstellar matter outside. Near the edge of the heliospheric bubble, a termination shock marks where the supersonic wind abruptly slows to subsonic speeds, forming a thick layer called the heliosheath. In this outermost layer of the heliosphere, the subsonic wind is deflected toward the tail of the heliosphere.

The outer boundary of the heliosheath is the heliopause, where the solar wind finally makes contact with interstellar matter that lies beyond. The two Voyager spacecraft have crossed the termination shock and are



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Voyager’s Journey to the Edge of Interstellar Space Edward C. Stone California Institute of Technology INTRODUCTION and a prediction during the International Geophysical Year nearly 60 years earlier. The journey to interstellar space began 50 years ago with a discovery and a prediction. Launched during THE HELIOSPHERE the International Geophysical Year, Explorers 1 and 3 revealed that belts of energetic protons and electrons The motion of the heliosphere relative to the local trapped in the geomagnetic field encircle Earth. Soon interstellar medium creates a wind that distorts the to be named the Van Allen belts, they were the first heliosphere into a long-tailed, comet-like shape. In major discovery of the Space Age that had begun only front, a curved interstellar bow shock resembles the months earlier with the launch of Sputnik 1 by the bow wave of a ship. Although there are no images of Soviet Union. the heliosphere, there are Hubble images of bow shocks During this time, Eugene N. Parker predicted in front of astrospheres around other stars (Figure 7.2). that the solar atmosphere was expanding outward at Fortunately, the two Voyagers are headed toward the supersonic speeds, filling interplanetary space with a nose region of the heliosphere where the distance to dilute plasma of protons and other electrically charged interstellar space is shortest. ionized atoms of solar matter. Four years later during The interaction of the Sun with the interstellar the first foray to another planet, Mariner 2 confirmed wind is complex. Models show the supersonic solar Parker’s prediction of a solar wind continuously stream- wind expanding outward until it is balanced by the ing radially away from the Sun at more than a million pressure of the local interstellar matter outside. Near kilometers per hour. the edge of the heliospheric bubble, a termination Although its density is much lower than the best shock marks where the supersonic wind abruptly slows laboratory vacuum, the solar wind is strong enough to to subsonic speeds, forming a thick layer called the create the heliosphere, a giant bubble around the Sun heliosheath. In this outermost layer of the heliosphere, that envelops the planets (Figure 7.1). Outside the the subsonic wind is deflected toward the tail of the bubble lies interstellar space filled with matter from the heliosphere. explosions of nearby supernovas. The outer boundary of the heliosheath is the helio- In the years ahead, the two Voyager spacecraft will pause, where the solar wind finally makes contact with be the first human-made objects to reach interstellar interstellar matter that lies beyond. The two Voyager space, completing a journey that began with a discovery spacecraft have crossed the termination shock and are 10

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110 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 7.1 The Heliosphere. The solar wind creates a bubble around the Sun, enveloping the planets. A bow shock forms as the interstellar wind from the left is deflected around the heliosphere, resulting in the formation of a comet-like heliospheric tail. SOURCE: Courtesy of Walt Feimer, NASA GSFC. exploring the heliosheath on their way to interstellar space. THE VOYAGER MISSION Voyager’s primary mission was to study the giant outer planets. It began in 1965 when the Jet Propulsion Laboratory was looking for times when the outer plan- ets were aligned so that swinging by one could be used as a gravitational slingshot to speed a spacecraft onto the next. Although there are frequent opportunities for gravity assists involving two or three planets, once every 175 years Jupiter, Saturn, Uranus, and Neptune are aligned so that a spacecraft can swing by all four, picking up speed at each flyby. This Grand Tour trajec- FIGURE 7.2 Bow shocks in the Orion Nebula. The large bow tory reduced the flight time to Neptune from 30 years shock in the center and the smaller one above it form in front of to only 12. invisible astrospheres surrounding two bright stars. SOURCE: Fortunately, an opportunity for launching on a Courtesy of NASA and the Hubble Heritage Team (STScI /AURA) Grand Tour trajectory came in the late 1970s. However, and C.R. O’Dell (Rice University).

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111 VOYAGER’S JOURNEY TO THE EDGE OF INTERSTELLAR SPACE a twelve-year flight proved to be too bold a step, so about the four planets. The broad icy rings of Saturn rather than losing such an opportunity, an ambitious were rippled with waves caused by the gravity of nearby project named Mariner Jupiter-Saturn ’77 was planned moons, and two moons were found to shepherd finer for a 4-year mission to Jupiter and Saturn as the first ring material into a narrow ring between them. Both leg of the Grand Tour. With 5 years to develop and test Uranus and Neptune have multiple narrow, dark rings, the first autonomous planetary spacecraft, the renamed while Jupiter’s faint broader rings are formed of dust Voyager 1 and 2 were launched on slightly different from nearby moons. trajectories that increased the likelihood of at least one Even the magnetic fields of the giant planets were successful encounter with Saturn and maintained the surprising. Jupiter’s magnetic field is the largest struc- option for a step-by-step completion of the Grand Tour ture in the solar system, inflated by the pressure of if both were successful. oxygen and sulfur escaping the surface of the moon Io. Voyager 1 led the way past Jupiter, which gave it a Equally unexpected was the orientation of the magnetic big enough boost to reach Saturn, with its rings and the fields of Uranus and Neptune, with their magnetic moon Titan as primary scientific objectives. They lie in poles nearer their equators than their rotational poles Saturn’s equatorial plane, which is inclined to the plane as on Earth and the other planets. of the planets. So, for an optimal study of Titan and the There were many such unexpected discoveries at rings, the Voyager 1 flyby trajectory was also inclined, each planet. sending the spacecraft northward out of the planetary plane with no further planetary encounters possible. Jupiter, Io, and Europa With the success of Voyager 1, Voyager 2 remained in the planetary plane during its Saturn flyby, continu- Jupiter’s Great Red Spot, a giant counterclockwise ing on to Uranus and Neptune. A flyby over Neptune’s rotating storm system nearly three Earth diameters north pole deflected Voyager 2 southward for a close across, is the largest of dozens of storm systems con- flyby of the moon Triton. As a result, Voyager 2 is head- tinuously forming and merging in the turbulent atmo- ing southward and Voyager 1 northward as they leave sphere (Figure 7.4). Clouds of ammonia ice crystals the solar system. mark bands of jet streams circling the globe at speeds of more than 300 kilometers per hour. Four large moons orbit Jupiter, each with distinc- THE GIANT PLANETS tive characteristics. The outer two, Ganymede and The Voyagers discovered unexpected diversity. Unlike Callisto, are half water ice and as large as the planet the rocky inner planets—Mercury, Venus, Earth, and Mercury. Impact craters scar the ancient icy surface of Mars—the outer planets are giant bodies of gas and Callisto, while faults and grooves from past geological liquid with no solid surfaces (Figure 7.3). Deep inside activity mark much of Ganymede’s surface. Jupiter and Saturn, hot, rocky cores lie buried under In contrast, Io and Europa, the inner two large a deep layer of mainly hydrogen and helium, with moons, are mainly rocky objects the size of Earth’s ammonia ice crystals forming visible clouds in their Moon. Io was the site of the astonishing discovery that atmospheres. Uranus and Neptune have similar cores set the stage for the mission’s subsequent revelations. but are smaller because there is much less hydrogen and There were eight active volcanoes, with plumes rising helium in their outer envelopes. Clouds of methane ice up to 300 kilometers above a surface pocked by hot, form at the top of their very cold atmospheres. dark lava-filled volcanic craters (Figure 7.5). Driven There are dozens of moons orbiting the giant by the tidal flexing of its crust as it orbits Jupiter, Io planets, several the size of the planet Mercury, but most has one hundred times the volcanic activity of Earth. much smaller than Earth’s Moon. Because it is so cold, It sheds a ton of sulfur and oxygen ions per second some moons are half water ice and half rock. Even so, that forms a doughnut-shaped torus around Jupiter, Voyager found each was distinctive, with icy surfaces inflating Jupiter’s giant magnetic field to twice the size often showing evidence of past geological activity. it would otherwise be. Voyager observed distinctly different ring systems Like Io, neighboring Europa is a rocky object, but is

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112 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 7.3 The planets. The rocky terrestrial planets, Mercury, Venus, Earth, and Mars, are at the top with the Moon. The lower four are the giant outer planets, Jupiter, Saturn, Uranus, and Neptune (not to scale). SOURCE: Courtesy of NASA/JPL.

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11 VOYAGER’S JOURNEY TO THE EDGE OF INTERSTELLAR SPACE F IGURE 7.4 J upiter, Io, and Europa. Jupiter’s Great Red Spot is a storm system nearly three times the diameter of Earth. There are orange deposits on volcanically active Io and a white icy crust on Europa. SOURCE: Courtesy of NASA/JPL. covered with water ice. Europa’s surface, the smoothest ocean of melted ice beneath the frozen crust. Evidence in the solar system, resembles a floating ice pack, with of such an ocean was found by the Galileo spacecraft faint streaks marking where tidal flexing has cracked in its close flybys of Europa during its mission in orbit the surface (Figure 7.6). Although Europa is further about Jupiter in the late 1990s. from Jupiter and the tidal effects are weaker than on Io, there is likely enough tidal heating to create an Saturn, Enceladus, and Titan Saturn is colder than Jupiter and the atmosphere is less turbulent and hazier. Unexpectedly, winds race at up to 1,800 kilometers per hour, more than five times faster than in Jupiter’s atmosphere Saturn’s rings make it the most beautiful of planets (Figure 7.7). These swaths of countless small icy frag- ments orbiting the planet also held surprises. Rippled by waves generated by the gravity of small nearby moons, the broad rings resemble an old phonograph record. At the outer edge of the broad rings, two small moons shepherded debris between themselves, creating a narrow, multi-stranded ring kinked by the gravita- tional effect of the moons. The shepherd moons and many other small moons appear to be irregularly shaped fragments of larger moons shattered by comet impacts. Such collisions left craters on the larger moons, some with diameters one- third that of the moon itself. Any larger impact would likely have shattered those moons as well. Enceladus, an icy moon only 500-km across, boasts a snowy white surface that is the brightest in the solar FIGURE 7.5 Jupiter’s moon Io. The large heart-shaped pat- system (Figure 7.8). Reflecting nearly all of the incident tern is the deposit of a volcanic plume erupting at its center. The sunlight, the average surface temperature is only 75 de- many black features are volcanic calderas. SOURCE: Courtesy grees above absolute zero. Some of the surface is ancient of NASA/JPL.

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11 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 7.6 Jupiter’s moon Europa. The icy crust is the smoothest surface in the solar system, with streaks formed by tidally induced cracks in an icy crust likely floating on an ocean. SOURCE: Courtesy of NASA/JPL. FIGURE 7.7 Saturn and its rings viewed from above and behind Saturn. The outermost A-ring, the brighter B-ring, and FIGURE 7.8 Saturn’s moon Enceladus. The grooves, faults, and the fainter inner C-ring are composed of countless icy fragments with ripples and gaps due to moons orbiting nearby. SOURCE: smooth regions are the result of extensive, continuing geological Courtesy of NASA/JPL. activity. SOURCE: Courtesy of NASA/JPL.

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11 VOYAGER’S JOURNEY TO THE EDGE OF INTERSTELLAR SPACE and heavily cratered as expected, but on such a small All smaller than Earth’s Moon, five icy moons cold body it was surprising to find tectonic features such orbiting Uranus were expected to have cooled off and as grooved terrain and long rift valleys. Other younger frozen quickly after forming. Unexpectedly, faults and areas are smooth, having few impact craters. other indications of past geological activity were found The Cassini spacecraft, launched in 1997 and on the surfaces of several. Miranda, the innermost, is now in orbit about Saturn, discovered why Enceladus less than 500 kilometers across, yet has one of the most boasts an unexpectedly bright and youthful surface. complex geological surfaces that Voyager observed Geyser-like plumes erupt from fractures in the south (Figure 7.9). Heavily cratered older regions surround polar region and deposit fresh ice on the surface, some patterns of parallel ridges and grooves, some 20 kilo- escaping to form the faint E-ring. meters deep, indicating that the surface of even such a Saturn’s largest moon Titan is slightly larger than small body was reshaped by an era of intense geological the planet Mercury. Unlike Mercury or any other activity likely caused by tidal heating. moon, Titan has a substantial nitrogen atmosphere as on Earth, but with a surface pressure one and a half Neptune and Triton times greater. There is no oxygen, which on Earth was originally produced by microbes. Instead, there is Neptune is 30 times as far from the Sun as Earth, so methane, or natural gas. Irradiation produces organic there is only one 900th as much solar energy to drive its compounds that rain onto the surface. More complex weather (Figure 7.10). Surprisingly, winds of 2,100 ki- molecules form opaque haze layers that optically ob- lometers per hour are the fastest in the solar system, and scure Titan’s surface. there was a Dark Spot that resembled Jupiter’s Greater Because of the opaque haze, Voyager could not Red Spot. However, a few years later the Hubble Space image Titan’s surface, but in 2005 the Huygens probe Telescope found that the Dark Spot had disappeared carried by Cassini parachuted down to Titan’s sur- and other spots have since appeared. face, revealing stream-like drainage systems and flat, dry lake beds. Subsequently, Cassini’s imaging radar peered through the haze layer, finding large lakes, likely formed from methane and other hydrocarbons that rain onto Titan’s surface. The complex organic chemistry in Titan’s atmosphere may resemble that which oc- curred in Earth’s early atmosphere before microbial life evolved. Future missions will study the chemistry of Titan’s atmosphere, the water beneath Europa’s icy crust, and the geysers erupting from Enceladus. They may tell us about the conditions necessary for the origin of life. Uranus and Miranda Uranus is tipped on its side, so its seasons differ from the other planets. At the time of the Voyager 2 encoun- ter, Uranus’ south pole was facing the Sun. Nearly twice as far from the Sun as Saturn and lacking a significant internal heat source, Uranus has the coldest planetary atmosphere in the solar system and much less energy FIGURE 7.9 Uranus’ moon Miranda. Extensive geological to drive atmospheric activity. Even so, a few faint cloud activity in the past has created a complex surface on this tiny features revealed wind speeds of nearly 900 kilometers moon with a diameter of less the 500 kilometers. SOURCE: per hour at mid-latitudes. Courtesy of NASA/JPL.

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11 FORGING THE FUTURE OF SPACE SCIENCE cantaloupe, along with ridges and valleys and smoother regions produced by icy flows. As a result, the surface is among the youngest in the solar system with most impact craters erased by the icy volcanism. Its icy surface reflecting much of the solar heat, Triton is the coldest object Voyager encountered. With a surface temperature only 38 degrees above absolute zero, most of the nitrogen freezes, forming a nitrogen ice polar cap. Surprisingly, even at this low temperature, geyser-like plumes erupt 8 kilometers upward into a thin atmosphere, depositing dark streaks of dust on Triton’s surface. THE OUTER HELIOSPHERE Because of our limited knowledge of the local interstel- lar medium, the size and shape of the heliosphere were FIGURE 7.10 Neptune. The Great Dark Spot is a large storm unknown. When the Voyager spacecraft finally crossed that has now disappeared. Clouds of methane ice revealed termination shock and began exploring the heliosheath, the fastest winds in the solar system. SOURCE: Courtesy of NASA/JPL. they revealed unexpected complexity (Figure 7.12). Voyager 1 is headed northward from the solar equator. In December 2004, it crossed the termination Triton, an icy Pluto-like body from the Kuiper shock at 94 astronomical units (14 billion kilometers; Belt, was captured by Neptune and orbits in the op- an astronomical unit (AU) is the distance from Earth posite direction to the planet’s rotation (Figure 7.11). to the Sun). As Voyager 1 moved deeper into the he- Initially, Triton was in an elliptical orbit, causing tidal liosheath over the next 3 years, the solar wind pressure flexing of its surface that heated and melted its interior. began decreasing, causing the heliosphere to shrink and This produced a uniquely textured surface resembling a the shock to move inward to 91 AU in the northern hemisphere. Voyager 2 is headed southward, and in August 2007 it found the shock at 84 AU, 1 billion kilometers closer to the Sun. So, the termination shock is pushed closer to the Sun in the southern than in the northern hemi- sphere, as could be caused by an interstellar magnetic field tilted so as to press inward more strongly in the south (Figure 7.12). As it crossed the shock, the solar wind plasma lost 75 percent of its kinetic energy as the speed dropped from 1 million kilometers per hour to half that speed in the heliosheath. It was predicted that the missing kinetic energy would heat the slow solar wind in the heliosheath to 1 million degrees celsius. However, the temperature reached only 100,000 degrees, indicating that most of the energy lost by the solar wind did not heat the wind itself, but heated interstellar ions that had FIGURE 7.11 Neptune’s moon Triton. The coldest body Voy- entered the heliosphere as neutral interstellar atoms and ager 2 observed has a polar cap of frozen nitrogen marked by dark streaks of dust deposited by plumes of geysers erupting were subsequently ionized. from its surface. SOURCE: Courtesy of NASA/JPL.

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11 VOYAGER’S JOURNEY TO THE EDGE OF INTERSTELLAR SPACE FIGURE 7.12 A side view showing the paths of Voy- ager 1 and 2 out of the heliosphere. In this model, the supersonic solar wind (green) streams radially outward from the Sun (gray arrows). The wind abruptly slows at the termination shock, forming the hot heliosheath (red) as it turns to flow tailward. The cooler interstellar wind from the left (blue with gray streamlines) carries the in- terstellar magnetic field (black arrows) with it. The wind is deflected around the heliosphere, pressing the mag- netic field strongly inward in the southern hemisphere and pushing the shock closer to the Sun at Voyager 2. SOURCE: Courtesy of Merav Opher, George Mason University Department of Physics and Astronomy. It was predicted that some of the interstellar ions stellar space by 2015. Surrounded for the first time by would bounce back and forth like cosmic ping pong matter from other stars, it will measure the direction balls between the magnetic fields on opposite sides of and strength of the local interstellar magnetic field the shock, becoming low energy cosmic rays as they draped around the heliosphere and the intensity of low slowly gained speeds up to half the speed of light. As energy cosmic rays from the galaxy that are blocked a result, it was expected that the intensity of these from entering the heliosphere. If the spacecraft re- cosmic rays would be highest at the shock. However, mains healthy, there is enough electrical power from neither Voyager found an intensity maximum at the its radioisotope thermoelectric generators to last well shock. Instead, the intensity is higher further out in beyond 2020 when Voyager 1 will be more than 150 the heliosheath away from the shock, indicating that AU from the Sun. the source of these low energy cosmic rays is not the shock regions crossed by the two spacecraft. Future A NEW VIEW OF THE SOLAR observations may reveal whether anomalous cosmic SYSTEM AND THE HELIOSPHERE rays originate at remote regions of the shock or in the outer regions of the heliosheath. The International Geophysical Year ushered in the As the two Voyagers continue to explore this new Space Age. The subsequent era of space science has region of the solar system, the Interstellar Boundary given us a new view of the solar system, revealing doz- Explorer (IBEX) launched in 2008 will make a two- ens of unexpectedly diverse worlds enveloped in a giant dimensional map of the heliosheath as viewed from heliospheric bubble created by the Sun. It has also given Earth orbit. Measuring the intensity of neutral atoms us our first journey to interstellar space, setting the stage streaming toward Earth from the heliosheath, IBEX for even more distant journeys. But most importantly, will provide a new estimate of the distance across the it has given us the both knowledge that there is much heliosheath to the edge of interstellar space. more to be discovered and the impetus to launch future Although uncertain, current indications are that journeys of exploration. Voyager 1 may cross the heliosheath and enter inter-