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April 28, 2011

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Voyager crafts at edge of Solar System

April 28, 2011
Courtesy of the NASA Jet Propulsion Laboratory
and World Science staff

More than 30 years af­ter they left Earth, NASA’s twin Voy­ag­er probes are at the edge of our so­lar sys­tem. And they’re still work­ing, eve­ry day beam­ing back what as­tro­no­mers de­scribe as a mes­sage both un­set­tling and thrill­ing: ex­pect the un­ex­pected.

“Voy­ag­er 1 and 2 have a knack for mak­ing dis­cov­er­ies,” said Ed Stone of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy in Pas­a­de­na, Calif., a Voy­ag­er proj­ect sci­ent­ist since 1972. NASA held a brief­ing on April 28 to re­flect on what the Voy­ag­er mis­sion has ac­com­plished and to pre­view what lies ahead as the probes pre­pare to en­ter the realm be­yond the So­lar Sys­tem.

A di­a­gram rep­re­sent­ing the cur­rent po­si­tion of the Voy­ag­er crafts, shown as two small yel­low blots at some dis­tance from the cen­tral Sun.  The in­ner blue bub­ble rep­re­sents the zone where the so­lar wind streams quick­ly out­ward from the Sun. The larg­er, darker blue ob­long "bub­ble," par­tially shown, is the he­lio­sheath, where the so­lar wind slows ab­rupt­ly, be­com­ing dens­er and hot­ter. (Cour­tesy NASA)
The ad­ven­ture be­gan in the late 1970s when the probes took ad­van­tage of a rare align­ment of out­er plan­ets for an un­prec­e­dent­ed Grand Tou­r. Voy­ag­er 1 vis­ited Ju­pi­ter and Sat­urn, while Voy­ag­er 2 flew past Ju­pi­ter, Sat­urn, Ura­nus and Nep­tune. (Voy­ag­er 2 is still the only probe to vis­it the lat­ter two worlds.) 

Pressed to name the top dis­cov­er­ies from those en­coun­ters, Stone paused, not for lack of ma­te­ri­al, he said, but due to an em­bar­rass­ment of rich­es. “It’s so hard to choose,” he de­clared. His par­tial list in­cludes the dis­cov­ery of vol­ca­noes on Ju­pi­ter’s moon Io; ev­i­dence for an ocean be­neath the icy sur­face of Eu­ro­pa; hints of meth­ane rain on Sat­urn’s moon Ti­tan; the crazily-tipped mag­net­ic poles of Ura­nus and Nep­tune; icy gey­sers on Nep­tune’s moon Tri­ton; and plan­e­tary winds that b­low faster and faster with in­creas­ing dis­tance from the Sun.

“Each of these dis­cov­er­ies changed the way we thought of oth­er worlds,” said Stone. In 1980, Voy­ag­er 1 used the gra­vity of Sat­urn to fling it­self slingshot-style out of the plane of the so­lar sys­tem. In 1989, Voy­ag­er 2 got a si­m­i­lar as­sist from Nep­tune. Both probes set sail in­to the void.

Soaring in­to the void sounds like a qui­et time, but the dis­cov­er­ies have con­tin­ued. Stone sets the stage by di­rect­ing our at­ten­tion to the kitch­en sink. “Turn on the faucet,” he in­structs. “Where the wa­ter hits the sink, that’s the Sun, and the thin sheet of wa­ter flow­ing [out­ward] from that point is the so­lar wind. Note how the Sun ‘b­lows a bub­ble’ around it­self.” There really is such a bub­ble, called the he­lio­sphere, and it is gar­gan­tu­an. Made of elec­tric­ally charged gas­es and mag­net­ic fields, it’s about three times wid­er than the or­bit of Plu­to.

The Voy­ag­ers are try­ing to get out, but they’re not there yet. To ex­plain, Stone re­calls our at­ten­tion to the sink: “As the wa­ter [or so­lar wind] ex­pands, it gets thin­ner and thin­ner, and it can’t push as hard. Ab­rupt­ly, a slug­gish, tur­bu­lent ring forms. That out­er ring is the he­lio­sheath—and that is where the Voy­ag­ers are now.” The he­lio­sheath is a very strange place, filled with a mag­net­ic froth no space­craft has ev­er en­coun­tered be­fore, ech­o­ing with low fre­quen­cy ra­di­o bursts heard only in the out­er reaches of the so­lar sys­tem, so far from home that the Sun is a mere pin­prick of light.

“In many ways, the he­lio­sheath is not like our mod­els pre­dict­ed,” said Stone. Last June, Voy­ag­er 1 beamed back a startling num­ber: ze­ro. That’s the out­ward ve­lo­city of the so­lar wind where the probe is now. No one thinks the so­lar wind has com­pletely stopped; it may have just turned a cor­ner. But which way? Voy­ag­er 1 is try­ing to fig­ure that out through a se­ries of “weather vane” ma­neu­vers, in which the space­craft turns it­self in a dif­fer­ent di­rec­tion to track the lo­cal breeze.

No one knows ex­actly how many more miles the Voy­ag­ers must trav­el be­fore they “pop free” of our system. But most re­search­ers be­lieve the end is near. “The he­liosheath is three to four bil­lion miles in thick­ness,” es­ti­mates Stone. “That means we’ll be out with­in five years or so.” There is plen­ty of pow­er for the rest of the jour­ney, he added. Both Voy­ag­ers are en­er­gized by the ra­di­oactive de­cay of a Plu­tonium 238 heat source. This should keep crit­i­cal subsys­tems run­ning through at least 2020.

Af­ter that, he said, “Voy­ag­er will be­come our si­lent am­bas­sa­dor to the stars.” Each probe is fa­mously equipped with a gold­en rec­ord, lit­er­al­ly, a gold-coated cop­per pho­no­graph rec­ord. It con­tains 118 pho­tographs of Earth; 90 min­utes of the world’s great­est mu­sic; an au­di­o es­say en­ti­tled Sounds of Earth (fea­tur­ing eve­rything from bur­bling mud pots to bark­ing dogs to a roar­ing Sat­urn 5 liftoff); greet­ings in 55 hu­man lan­guages and one whale lan­guage; the brain waves of a young wom­an in love; and saluta­t­ions from the sec­re­tary gen­er­al of the Un­ited Na­t­ions. A team led by Carl Sagan as­sem­bled the in­ter­stel­lar mes­sage-in-a-bottle as a sig­nal to pos­si­ble ex­tra­ter­res­tri­al civ­il­iz­a­tions that might en­coun­ter the space­craft, un­likely though that may be.

“A bil­lion years from now, when eve­rything on Earth we’ve ev­er made has crum­bled in­to dust, when the con­ti­nents have changed be­yond rec­og­ni­tion and our spe­cies is un­imag­inably al­tered or ex­tinct, the Voy­ag­er rec­ord will speak for us,” wrote Carl Sagan and Ann Druyan in an in­tro­duc­tion to a CD ver­sion of the rec­ord.

* * *

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More than 30 years after they left Earth, NASA’s twin Voyager probes are at the edge of the solar system. And they’re still working, every day beaming back what astronomers describe as a message both unsettling and thrilling: expect the unexpected. “Voyager 1 and 2 have a knack for making discoveries,” said Ed Stone of the California Institute of Technology in Pasadena, Calif., a Voyager project scientist since 1972. NASA held a briefing on April 28 to reflect on what the Voyager mission has accomplished and to preview what lies ahead as the probes prepare to enter the realm outside the Solar System, known as interstellar space. The adventure began in the late 1970s when the probes took advantage of a rare alignment of outer planets for an unprecedented Grand Tour. Voyager 1 visited Jupiter and Saturn, while Voyager 2 flew past Jupiter, Saturn, Uranus and Neptune. (Voyager 2 is still the only probe to visit the latter two worlds.) Pressed to name the top discoveries from those encounters, Stone pauses, not for lack of material, he said, but due to an embarrassment of riches. “It’s so hard to choose,” he declares. His partial list includes the discovery of volcanoes on Jupiter’s moon Io; evidence for an ocean beneath the icy surface of Europa; hints of methane rain on Saturn’s moon Titan; the crazily-tipped magnetic poles of Uranus and Neptune; icy geysers on Neptune’s moon Triton; planetary winds that blow faster and faster with increasing distance from the Sun. “Each of these discoveries changed the way we thought of other worlds,” said Stone. In 1980, Voyager 1 used the gravity of Saturn to fling itself slingshot-style out of the plane of the solar system. In 1989, Voyager 2 got a similar assist from Neptune. Both probes set sail into the void. Sailing into the void sounds like a quiet time, but the discoveries have continued. Stone sets the stage by directing our attention to the kitchen sink. “Turn on the faucet,” he instructs. “Where the water hits the sink, that’s the Sun, and the thin sheet of water flowing [outward] from that point is the solar wind. Note how the Sun ‘blows a bubble’ around itself.” There really is such a bubble, called the heliosphere, and it is gargantuan. Made of electrically charged gases and magnetic fields, it’s about three times wider than the orbit of Pluto. The Voyagers are trying to get out, but they’re not there yet. To explain, Stone recalls our attention to the sink: “As the water [or solar wind] expands, it gets thinner and thinner, and it can’t push as hard. Abruptly, a sluggish, turbulent ring forms. That outer ring is the heliosheath—and that is where the Voyagers are now.” The heliosheath is a very strange place, filled with a magnetic froth no spacecraft has ever encountered before, echoing with low frequency radio bursts heard only in the outer reaches of the solar system, so far from home that the Sun is a mere pinprick of light. “In many ways, the heliosheath is not like our models predicted,” said Stone. Last June, Voyager 1 beamed back a startling number: zero. That’s the outward velocity of the solar wind where the probe is now. No one thinks the solar wind has completely stopped; it may have just turned a corner. But which way? Voyager 1 is trying to figure that out through a series of “weather vane” maneuvers, in which the spacecraft turns itself in a different direction to track the local breeze. No one knows exactly how many more miles the Voyagers must travel before they “pop free” into interstellar space. But most researchers believe the end is near. “The heliosheath is 3 to 4 billion miles in thickness,” estimates Stone. “That means we’ll be out within five years or so.” There is plenty of power for the rest of the journey, he added. Both Voyagers are energized by the radioactive decay of a Plutonium 238 heat source. This should keep critical subsystems running through at least 2020. After that, he said, “Voyager will become our silent ambassador to the stars.” Each probe is famously equipped with a golden record, literally, a gold-coated copper phonograph record. It contains 118 photographs of Earth; 90 minutes of the world’s greatest music; an audio essay entitled Sounds of Earth (featuring everything from burbling mud pots to barking dogs to a roaring Saturn 5 liftoff); greetings in 55 human languages and one whale language; the brain waves of a young woman in love; and salutations from the secretary general of the United Nations. A team led by Carl Sagan assembled the interstellar message-in-a-bottle as a signal to possible extraterrestrial civilizations that might encounter the spacecraft, unlikely though that may be. “A billion years from now, when everything on Earth we’ve ever made has crumbled into dust, when the continents have changed beyond recognition and our species is unimaginably altered or extinct, the Voyager record will speak for us,” wrote Carl Sagan and Ann Druyan in an introduction to a CD version of the record.