Many of us space enthusiasts dream of one day sending ships out to the distant stars. Unfortunately, the technological, sociological, and economic challenges involved in such an undertaking are immense, and it may very well be many generations before humanity is ready to engage in this type of project in any practical sense.

Even so, we are already taking our first steps toward exploring the outer universe, in the form of four spacecraft: Pioneers 10 and 11, and Voyagers 1 and 2. Each of these spacecraft was launched in the 1970s toward the outer solar system, with each one making a flyby of the giant planet Jupiter, and all but one going on to explore one or more of the other outer planets. Although each was only designed to function through the primary phase of their respective missions, these spacecraft were built to last, and even today they continue to return data about the environment of the far outer solar system. Due to the increase in speed that each received from passing near Jupiter (and whatever other planets they encountered), each one picked up enough velocity to escape the solar system altogether. Thus, all four of these objects will eventually leave the solar system, never to return, and thus they have become humanity's first emissaries to the distant stars. Each is leaving in a different direction, incidentally.

The information we receive from these four probes continues to tell us a lot about our solar system, and even about ourselves. In early 1990 Voyager 1,from a distance of over 3 1/2 billion miles, took one last look backward and snapped its famous "portrait of the solar system," wherein the Earth we inhabit appeared as nothing more than a "pale blue dot" nearly swallowed up by the sun's glare. At the same time, all four probes have participated in the search for the "heliopause," which can be considered as the boundary between the solar system and interstellar space. This is the spot where the solar wind, an energetic stream of charged particles blowing off the sun, encounters the radiation of deep space. As the sun travels through space, the point where this occurs resembles the bow of a ship plowing through the water as it moves. None of the four spacecraft have reached the heliopause yet, although just a couple of years ago some scientists announced that instruments aboard both Voyagers seemed to be detecting its presence. It was still pretty far away, though, and the heliopause crossing probably will not occur until well into the next century.

It was inevitable, of course, but one of the four spacecraft is now starting to run out of power. This is Pioneer 11, one of the two earlier missions launched in the early 1970s to pave the way for the more sophisticated Voyager probes launched later that decade. Pioneer 11 left Earth on April 6, 1973, and in December 1974 it zipped by within 27,000 miles of Jupiter. The giant planet's gravity deflected Pioneer 11's course, until in September 1979 it became the first spacecraft to fly by the planet Saturn. Among its findings as it flew by 13,000 miles from the ringed planet were the discoveries of a new outer ring (the so-called "F ring") and two small moons close in.

Until recently, NASA's Deep Space Network -- a worldwide series of large radio antennae designed for communicating with probes throughout the solar system -- had been communicating with Pioneer 11 on a daily basis. But because the power supply aboard the spacecraft has started to run down, the scientists in charge of Pioneer 11 reluctantly decided that these communications would cease on September 30. Nevertheless, they plan to continue listening for transmissions from Pioneer 11 every two to four weeks, until the spacecraft's transmitter loses all its power sometime in 1996. After that, Pioneer 11 will be a silent ship heading quietly out into the galaxy.

Right now, Pioneer 11 is located in the constellation Scutum -- currently visible in the southwestern sky after dusk -- although since it is now almost 4 billion miles away from us, it is far too dim to be visible with any telescope on Earth. Over the next few decades it will continue to track slowly eastward against the background stars in this part of the sky.

In a little over 34,000 years from now it will pass approximately 2 1/2 light years (15 trillion miles) from Ross 248, a dim star now located in the constellation Andromeda, and about 8,000 years after that it will pass by "only" 1 1/2 light years (10 trillion miles) from Gliese 445, another dim star that is currently located up near the bowl of the Little Dipper. (These calculations are taking into account the motions of the individual stars themselves.) In about 4 million years Pioneer 11 will pass fairly close to the star Lambda Aquilae, a naked-eye star near the "tail" of the constellation Aquila, the Eagle. And, after that . . .

Eventually, after a few hundred million years, Pioneer 11 will return to approximately this same spot in the galaxy, although our sun -- and Earth, and we -- will probably be nowhere near at that time. More than likely, the probe will look pretty much like it looks right now; the deep-freeze of interstellar space should keep it in a fairly pristine condition. While it is all but impossible to speculate as to what will happen to the human race by then, perhaps it can give us a sense of immortality to think that Pioneer 11 and its cousins will continue to travel through the galaxy for aeons to come, carrying the message that, yes indeed, "Humanity was here!"

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