Today, the Voyager 1 space probe, launched in 1979, is the farthest manmade object from Earth ever launched – and it is still operational. Although the Voyager probes are still known for the breathtaking pictures of the outer planets which they sent back in the 1980s, in several years’ time Voyager 1 will accomplish what may be an even greater feat: it will cross the heliopause and enter true interstellar space, becoming the first manmade spacecraft ever to do so. Unfortunately, this accomplishment comes too late in the long lifetimes of the nearly-exhausted Voyager probes for either to transmit back much more than a tantalizingly small amount of data about what they encounter there.
The Voyager probes were actually preceded into the outer solar system by a pair of earlier and equally ambitious probes, Pioneer 10 and Pioneer 11 (sometimes referred to as Pioneer G in an alternative counting system). Pioneer 10 was launched in 1972 and for quite some time was the most distant human object, until being passed by Voyager 1 in the late 1990s. Unfortunately, neither of the Pioneer spacecraft is still considered operational. No signals have been received from Pioneer 10 since 2003, or from Pioneer 11 since 1995. Nevertheless, both spacecraft are currently on trajectories which will eventually take them into interstellar space, and it was the tracking of the Pioneer probes which allowed scientists to discover that probes heading out of the solar system seem to decelerate more quickly than expected from the Sun’s gravity. The cause for this deceleration is unknown and is referred to as the Pioneer anomaly.
Voyager 1, a nearly 1600-pound probe, does remain operational, however, and is also travelling much more quickly than Pioneer 10 or Pioneer 11. After taking detailed photographs of Jupiter and Saturn and the two gas giants’ moon systems, it headed out of the solar system. It currently lies about 10.5 billion miles from Earth, or 113 times as far from the Sun as our own planet. Within several years (probably about 2015), Voyager 1 is expected to reach the heliopause, the point at which the solar wind radiating out from the Sun grows weaker than the pressure of the interstellar medium, which is pushing back from the other side. This is one of the standard markers of the edge of the solar system, and when it passes the heliopause Voyager 1 will have entered true interstellar space.
Unfortunately, Voyager 1’s onboard nuclear power system, a radioisotope generator (RTG), is already running low on fuel, so that Voyager 1 will not tell us nearly as much about interstellar space as we might hope to find out. By the time it reaches the heliopause in 2015, it is worth remembering, Voyager will have been operating in space for over 35 years. In contrast, the oldest spacecraft operational today is the 45-year-old Pioneer 6 spacecraft – and it is solar-powered. Voyager 1’s solar wind detector has already been switched off for nearly twenty years, making detection of the solar wind difficult as it is. By 2015, its data tape system will fail, followed by its gyroscope, and then by the remainder of the scientific instruments still operational. By about 2025, Voyager 1 will have lost power entirely.
Voyager 1 will pass by the AC+79 3888 star system in forty thousand years – a star which is travelling in the general direction of our own Sun at several times the speed of the Voyager probe itself. It carries a special gold phonograph record known as the Voyager Golden Record, which is intended to provide information about Earth and about human culture to any spacefaring alien civilization which should happen to discover the probe in the distant future.
Voyager 2 is trailing behind Voyager 1, having dallied in the solar system to visit Uranus and Neptune, but ultimately it will face a similar fate on a similar timetable, which means it will not travel as far out of bounds as Voyager 1 before exhausting its power reserves.