After decades in space, NASA's Voyager probes are preparing to leave the solar system.
Over 30 years after leaving Earth, the twin Voyager probes are now at the edge of the solar system and are still working.
These emissaries of Earth, which have traveled farther than any other objects ever created by humans, will probably leave the solar system and enter interstellar space some time around the year 2016.
They carry messages of peace from the people of Earth in 55 languages along with other records of our home planet assembled under guidance of the late scientist Carl Sagan to convey humans' greetings to any extraterrestrial civilizations they might meet.
The Voyagers' journey began in the late 1970s, when the probes took advantage of a rare alignment of outer planets to give them the boost needed to eventually take them into interstellar space.
Voyager 1 was launched on September 5, 1977 on a mission to swing by the gas giant planets Jupiter and Saturn.
On August 20 of that year, just a few weeks before Voyager 1's launch, NASA launched Voyager 2 on a grand tour of the solar system that allowed it to fly by Jupiter, Saturn, Uranus, and Neptune.
Voyager 2 was launched first but it is traveling slower and in a different direction than Voyager 1.
In 1980, Voyager 1 used the gravity of Saturn to fling itself slingshot-style out of the plane of the solar system. In 1989, Voyager 2 got a similar assist from Neptune.
"Voyager 1 and 2 have a knack for making discoveries," ScienceDaily quoted Ed Stone of the California Institute of Technology in Pasadena, a Voyager project scientist since 1972, as saying on Friday.
According to Stone, the list of achievements celebrated by the probes includes the discovery of volcanoes on Jupiter's moon Io, the presence of an ocean beneath the icy surface of Jupiter's moon Europa, methane rain on Saturn's moon Titan, the strangely-tipped magnetic poles of Uranus and Neptune, icy geysers on Neptune's moon Triton, and planetary winds that blow faster and faster with increasing distance from the sun.
"Each of these discoveries changed the way we thought of other worlds," said Stone.
The probes are now trying to get out of the solar system bubble, known as the heliosphere. Made of solar plasma and magnetic fields, the heliosphere is about three times wider than the orbit of Pluto and contains every planet, moon, asteroid, comet, and spacecraft, as well as all life, belonging to our solar system.
The messengers, which are energized by the radioactive decay of a plutonium 238 heat source, are expected to operate to at least the year 2020.