NASA's Pioneer 10 and 11 and Voyager 1 and 2 spacecraft are headed out of the solar system. Artist Don Davis portrays Pioneer lO, at present the most distant craft, moving outward from the Sun (the brightest star visible) and away from the heart of our Milky Way galaxy.
riASA Ames Research Center heliopause to contract. Solar scientists think that the heliopause lies at a distance of 100 to 200 astronomical units—100 to 200 times the Earth's distance from the Sun. Neptune orbits at a distance of 30 astronomical units.
Voyager 1 and Voyager 2, increasing their distance from the Sun by about 3.5 astronomical units per year, could arrive at the heliopause as early as about 2010. Both craft should continue to have enough electrical power from their nuclear generators for radio transmission and enough fuel for their thrusters to keep their antennae turned toward Earth until about 2010 or 2015. If the heliopause lies at about 100 astronomical units from the Sun, the Voyagers may tell us where. Beyond that point, the Voyagers will continue outward, but they will be mute and passive.
Yet even at the heliopause, the Voyagers' journey of departure from the solar system will scarcely have begun. Beyond them will lie the vast cloud of comets, no less members of the Sun's family than the planets The Oort Cloud may begin as close to the Sun as 50 astronomical units and extend as far as 135,000, halfway to the nearest star. Even though the Voyagers are separating themselves from the Sun at about 10 miles per second (16 kilometers per second), it will be 40,000 years until they are beyond the Oort Cloud and have truly crossed the boundary into in-
»liar space. Forty thousand years is about the period of time that grates us from Neanderthal Man.
0n its Grand Tour of the giant planets, Voyager 2 was traveling near the yovaQer 2 and . . 0f ¡he solar system. But at Neptune, Voyager 2 passed low over " ¡he planet's north pole, and its course was bent southward to encounter ■fliton So Voyager 2 will be headed out of our solar system on a southerly route. It>s pointed toward a rather drab region of the sky in the con-stelktion Pavo, the Peacock. To see that part of the heavens, we must be in the southernmost United States or farther south.
Yet, ironically, Voyager 2's first reasonably close stellar encounter will DOt be with any of the stars in the far southern sky but with a star now located in the northern constellation Andromeda, the Princess. That star, known only by its catalog designation of Ross 248, is a small cool red star with only about one-fifth the mass of our Sun. Find the Great Square of Pegasus and look north one length of the Square—and you will not see Ross 248. With a magnitude of +12.3, it is about 200 times too faint for human eye visibility.
Even though Voyager 2 will be traveling south and Ross 248 is located in the north, the two are moving toward one another. While Voyager 2 is consuming 40,000 years in its transit through the Oort Cloud, all the stars in the sky are moving in different directions. The constellations are gradually changing shape beyond recognition. Some of the stars are coming toward us faster than our spacecraft are going to meet them—almost as if they are coming to fetch the Voyagers.
Ross 248 is currently 10.3 light-years away, but while Voyager 2 rushes outward at 33,000 miles per hour (14.8 kilometers per second), Ross 248 is approaching our system at more than five times that speed. No sooner will Voyager 2 emerge from the Oort Cloud than, 40,176 years from now, it will encounter Ross 248, passing at a distance of 1.7 light-years, closer to Voyager than to any subsequent star known. Ross 248 will pass the outskirts of our solar system 3.25 light-years from our Sun, 25 percent closer than our nearest stellar neighbors, the three stars of Alpha Centauri, are to us now. Yet even when Ross 248 reaches that close range, it will be four times too faint for people on Earth to see without a telescope.
Still, its passage may eventually be seen and even felt indirectly as its gravity warps the orbits of millions of comets and redirects some of them inward toward the Sun where they will provide brilliant displays in the skies of Earth and perhaps even impacts on our planet.
Vnvxnrr 7 anH The Mme kind of stellar encounter awaits Voyager 1, even thoush vuyayci x aim traveling toward a very different part of the sky. It is pointed in th d-
tion of Rasalhague, the brightest star in the constellation Ophiuch
Serpent Bearer. But the star headed for a rendezvous with Voya^' ^
AC+ 79 3888.3 This star, with no name other than its catalog lisfoL' ?
currently to be found in the faint constellation CamelopardaBiii
Giraffe. This region of the sky is visible all night long to f north of the TVopic of Cancer. AC+79 3888 is just a short distance frro. Polaris, the North Star, and halfway between the bowl of the Big Di * and the W of Cassiopeia. AC + 79 3888 is slightly larger and brighter^ Ross 248, but it too is a small cool red star with only one-quarter the mass of our Sun. At its present distance of 16.6 light-years, AC+79 38jj is an 1 lth-magnitude star, nearly a hundred times too faint for the unaided eye to see.
While Voyager 1 is moving outward at 37,000 miles per hour iigg kilometers per second), AC+79 3888 will be traveling toward our sola-system at seven times that speed. In 40,272 years, at the same time that Voyager 2 will be scurrying by Ross 248 more than a quarter of the way around the sky, Voyager 1 will be only a little more than 1.6 light-years from AC + 79 3888, and AC + 79 3888 will be just 3 light-years from the Sun." Even so, AC+79 3888 will still be two times too faint for people on Earth to see without a telescope.
. __ Quite by accident, both of these first star encounters by the Voyagers are with single stars like our Sun—a minority in space, where most stars from Earth have one or more gravitationally bound companions. They may well have planetary systems, since we think the process that starts the formation of stars is identical to the process of planet formation.
Yet even with a family of planets, it is very unlikely that Ross 248 and AC + 79 3888 provide the right environment for life to exist. Both stars are much smaller than our Sun. They emit so little heat that a planet would have to be at precisely the correct distance with an almost perfectly circular orbit to stay in a habitable zone. Worse still, that planet would be so close to its star that it would be tidally coupled to it, like most moons are to their nearby planets, so that one side of the planet would fry in constant sunlight while the other side would freeze in constant night Most scientists do not expect life to exist in the solar system of a low-mass red dwarf star.
Even if both these stars illuminate planets populated by intelligent spacefaring beings, it would be extremely unlikely that they would detect a tiny silent spacecraft passing beyond the fringe of their comet clouds
Voyager 1 and Voyager 2. bound out of the solar system, carry sounds and pictures of Earth on a phonograph record to show a civilization that may find the spacecraft what life on our planet is like. Here, a technician is mounting the interstellar message on Voyager 2. HASA
And '.he stars are so widely separated that there is a vanishingly small chance that either of the Voyagers will hit or come very close to a star in the next billion years.
Still, just on the outside chance that some civilization deep in space may retrieve a Voyager, each craft is equipped with a special record that gives its finders pictures and sounds from the planet Earth.
The message was designed for NASA by Carl Sagan, Frank Drake, Ann Druyan, Timothy Ferris, Jon Lomberg, and Linda Salzman Sagan. Attached to the side of each Voyager is a gold-coated two-sided copper phonograph record, complete with enclosed stylus and cartridge and with instructions etched on its aluminum cover. The record should last a billion years On it are greetings in 55 different human languages and one whale language; the sounds of Earth—from thunder to frogs to a newborn baby; 90 minutes of music from around the world; and, encoded as vibrations, 118 pictures of our planet and ourselves.5
For the beings that find a Voyager along its endless journey, the spacecraft will have found a new and eloquent voice—no longer telling its home planet about other worlds but now telling other beings of its origin and the people who sent it outward.
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