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accountably generating CO2 in the face of all that methane。
The surfaces of Miranda and Triton are unlike anything else we know。 There are vast chevron…shaped landforms and crisscrossing straight lines that even sober planetary geologists once mischievously described as 〃highways。〃 We think we （barely） understand these landforms in terms of faults and collisions； but of course we might be wrong。
The surface stains of organic matter—sometimes； as on Triton； delicately hued—are attributed to charged particles producing chemical reactions in simple hydrocarbon ices； generating more plex organic materials； and all this having nothing to do with the intermediation of life。 But of course we might be wrong。
The plex pattern of radio static； bursts； and whistles that we receive from all four Jovian planets seems； in a general way； explicable by plasma physics and thermal emission。 （Much of the detail is not yet well understood。） But of course we might be wrong。
We have found nothing on dozens of worlds so clear and striking as the signs of life found by the Galileo spacecraft in its passages by the Earth。 Life is a hypothesis of last resort。 You invoke it only when there's no other way to explain what you see。 If I had to judge； I would say that there's no life on any of the worlds we've studied； except of course our own。 But I might be wrong； and； right or wrong； my judgment is necessarily confined to this Solar System。 Perhaps on some new mission we'll find something different； something striking； something wholly inexplicable with the ordinary tools of planetary science—and tremulously； cautiously； we will inch toward a biological explanation。 However； for now nothing requires that we go down such a path。 So far； the only life in the Solar System is that which es from Earth。 In the Uranus and Neptune systems； the only sign of life has been Voyager itself。
As we identify the planets of other stars； as we find other worlds of roughly the size and mass of the Earth； we will scrutinize them for life。 A dense oxygen atmosphere may be detectable even on a world we've never imaged。 As for the Earth； that may by itself be a sign of life。 An oxygen atmosphere with appreciable quantities of methane would almost certainly be a sign of life； as would modulated radio emission。 Someday； from observations of our planetary system or another； the news of life elsewhere may be announced over the morning coffee。
THE VOYAGER SPACECRAFT are bound for the stars。 They are on escape trajectories from the Solar System； barreling along at almost a million miles a day。 The gravitational fields of Jupiter； Saturn； Uranus； and Neptune have flung them at such high speeds that they have broken the bonds that once tied them to the Sun。
Have they left the Solar System yet？ The answer depends very much on how you define the boundary of the Sun's realm。 If it's the orbit of the outermost good…sized planet； then the Voyager spacecraft are already long gone； there are probably no undiscovered Neptunes。 If you mean the outermost planet； it may be that there are other—perhaps Triton…like—planets far beyond Neptune and Pluto； if so； Voyager 1 and Voyager 2 are still within the Solar System。 If you define the outer limits of the Solar System as the heliopause—where the interplanetary particles and magnetic fields are replaced by their interstellar counterparts—then neither Voyager has yet left the Solar System； although they may do so in the next few decades。* But if your definition of the edge of the Solar System is the distance at which our star can no longer hold worlds in orbit about it； then the Voyagers will not leave the Solar System for hundreds of centuries。
* Radio signals that both Voyagers detected in 1992 are thought to arise from the collision of powerful gusts of solar wind with the thin gas that lies between the stars。 From the immense 。power of the signal （over 10 trillion watts）； the distance to the heliopause can be estimated： about 100 times the Earth's distance from the Sun。 At the speed it's leaving the Solar System； Voyager 1 might pierce the heliopause and enter interstellar space around the year 2010。 If its radioactive power source is still working； news of the crossing will be radioed back to the stay…at…homes on Earth。 The energy released by the collision of this shock wave with the heliopause makes it the most powerful source of radio emission in the Solar System。 It makes you wonder whether even stronger shocks in other planetary systems might he detectable by our radio telescope。
Weakly grasped by the Sun's gravity； in every direction in the sky； is that immense horde of a trillion ets or more； the port Cloud。 The two spacecraft will finish their passage through
the Oort cloud in another 20；000 years or so。 Then； at last； pleting their long good…bye to the Solar System； broken free of the gravitational shackles that once bound them to the Sun； the Voyagers will make for the open sea of interstellar space。 only then will Phase Two of their mission begin。
Their radio transmitters long dead； the spacecraft will wander for ages in the calm； cold interstellar blackness—where there is almost nothing to erode them。 Once out of the Solar System； they will remain intact for a billion years or more； as they circumnavigate the center of the Milky Way galaxy。
We do not know whether there are other space…faring civilizations in the Milky Way。 If they do exist； we do not know how abundant they are； much less where they are。 But there is at least a chance that sometime in the remote future one of the Voyagers will be intercepted and examined by an alien craft。
Accordingly； as each Voyager left Earth for the planets and the stars； it carried with it a golden phonograph record encased in a golden； mirrored jacket containing； among other things； greetings in 59 human languages and one whale language； a 12…minute sound essay including a kiss； a baby's cry； and an EEG record of the meditations of a young woman in love； 116 encoded pictures； on our science； our civilization； and ourselves； and 90 minutes of the Earth's greatest hits—Eastern and Western； classical and folk； including a Navajo night chant； a Japa