About every 175 years the orbits of the outer planets — Jupiter, Saturn, Uranus and Neptune — are arranged so that a spacecraft on a particular flight path can swing from one planet to the next without the need for large onboard propulsion systems.
This arrangement allows a spacecraft to potentially travel to all four planets, reaching Neptune in just 12 years instead of the usual 30.
In the late 1970s through the 1980s, the outer planets were in this arrangement, and the United States launched two spacecraft, Voyager 1 and Voyager 2, to travel to Jupiter and Saturn.
The spacecraft were powered by radioisotope thermoelectric generators. These devices convert the heat produced from the natural radioactive decay of plutonium into electricity to power the spacecraft instruments, computers, radio and other systems.
Because of the decay of the radioactive material, these generators lose about 4 percent of their power production each year.
But with careful power management — turning off equipment no longer working or needed — the remaining instruments and radios could be powered for about 40 years.
Voyager 1 reached Jupiter in 1979 and Saturn in 1980. It traveled on a path for a close flyby of the large moon Titan and behind the rings of Saturn. This path sent Voyager 1 northward out of the orbital plane of the planets.
The slower flight path of Voyager 2 brought it to Jupiter later in 1979 and to Saturn in 1981. This completed the prime mission of the Voyager spacecraft, but the flight path chosen for Voyager 2 gave the option of having it complete the journey to Uranus and Neptune.
After the successful flyby of Saturn, since the spacecraft were still healthy, NASA provided additional funds to continue operating both of them, adding a flyby of Uranus for Voyager 2.
On Jan. 24, 1986, Voyager 2 encountered Uranus and obtained detailed photos and other data from the planet and its moons.
With still-healthy spacecraft, NASA then approved funds to continue operation of the Voyagers, adding a flyby of Neptune for Voyager 2.
On Aug. 25, 1989, Voyager 2 flew over Neptune’s north pole and was deflected southward below the orbital plane of the planets with a speed of 295 million miles per year.
After the planetary flybys, the Voyager mission was extended one more time. The flight path of both spacecraft would send them into interstellar space, beyond the planets and beyond the influence of the sun.
The sun emits a constant stream of particles into space. These particles radiate out from the sun as plasma, an ionized gas, known as the solar wind, which creates a bubble in interstellar space called the heliosphere.
The outer edge of the heliosphere, called the heliosheath, is composed of a slowdown region and an outer stagnation region. In the slowdown region, the solar wind slows to below the speed of sound and becomes turbulent.
In the stagnation region, the solar wind stops moving outward. This is the end of the heliosphere and the beginning of interstellar space.
The plasma of interstellar space is composed of different types of particles than those found in the solar wind, so one way of detecting the crossing of the boundary would be the change of particles detected by the spacecraft.
Although we still receive data from Voyager 1 every day, the instrument that can detect these particle types died in 1980.
However, NASA can still make these measurements indirectly with other instruments that are still working.
One instrument can measure the density of the plasma when an energy wave passes through it. In April a solar outburst produced such an energy wave, allowing us to measure the density of the plasma.
At the boundary, the interstellar plasma is denser than that of the heliosphere. The recent measurement was about 40 times denser than the plasma inside the heliosphere, which indicated that Voyager 1 is now in interstellar space.
From the analysis of Voyager’s data over the past year, NASA believes that Voyager 1 crossed the boundary between the heliosphere and interstellar space on Aug. 25, 2012, at a distance of more than 11 billion miles from the Earth — 121 times the distance between the Earth and the sun.
This makes Voyager 1 the first known spacecraft to enter interstellar space, and there is enough power to continue to collect data about this region for the next seven years.
Voyager 2 should follow into interstellar space in the next few years.
In a “Star Trek” paraphrase, Voyager is going where no probe has gone before.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at email@example.com.