Since its 1977 launch, NASA's Voyager 1 has made discoveries and sent some of the most iconic images back to earth. It is now on the brink of leaving our solar system forever.
The team of experts that receives and interprets data from Voyager 1 has long hoped that the spacecraft will break through the outer limits of our solar system.
And the most recent data now has them convinced that Voyager 1 will indeed become the first manmade object to move beyond the sun's sphere of influence and into the Milky Way.
Speaking this week in Pasadena, California, one of the original scientists on the Voyager project, Ed Stone, confirmed that the 35 year old spacecraft was expected to leave the solar system.
"It's hard to imagine that it's going to be too much longer," said Stone, "but I can't tell you if it's days, months or years."
They may not know exactly when it will happen, but Stone and other Voyager project members at NASA’s Jet Propulsion Laboratory in Pasadena are eagerly awaiting the event.
"Crossing into interstellar space - that will be a historic moment when the first object launched from Earth finally leaves the bubble," said Stone.
Confident about the science
Voyager 1 has been travelling throughout the region that lies between the solar system and interstellar space - known as the heliosheath - for almost eight years.
Ulrich Köhler, a planetary geologist at the German Aerospace Center (DLR), says the space community believes the craft will cross the final barrier - known as the heliopause - within one to two years.
"It's not clear whether it really is already at the very edge of the solar system," Köhler told DW. "At this distance of about 18 billion kilometers (about 11 billion miles) away from the sun, Voyager is very close to hitting interstellar space and leaving the solar system forever."
Because scientists are unable to judge the true size of the heliosheath, they can only estimate how far Voyager still has to travel before it crosses the heliopause and reaches the Milky Way.
They have estimated Voyager 1's distance from the heliopause by relying on various measurements it sends to Earth on the environment in the heliosheath. For example, it has sent data on galactic cosmic rays entering the solar system from the Milky Way.
This summer, Voyager 1 measured a higher percentage of cosmic rays than ever before - jumping nine percent from May to June. By comparison, the number of cosmic rays between January 2009 and January 2012 rose by 25 percent over the whole three year period.
Scientists say the latest evidence suggests Voyager 1 could yet successfully cross into interstellar space, despite its age. They recently celebrated the 35th anniversary of its launch.
1970s technology still breaks boundaries
NASA equipped Voyager 1 with - what was at the time - the latest technology. Its onboard equipment includes cameras, sensors, detectors, and a computer with just 68 kilobytes of memory - that's far less than mobile devices offer today.
"It's unbelievable how long lasting these instruments are and how good the results have been even though the electronics at that time were very basic," says Köhler.
Voyager 1 is 35 years into what was intended to be a five-year mission. The spacecraft still runs on radioisotope thermoelectric generators (RTGs), powered by the radioactive decay of plutonium.
"It's so reliable and the results are yielding wonderful scientific ideas and visions of what is possible," Köhler says.
It was launched on September 5, 1977 from the Kennedy Space Flight Center in Cape Canaveral, Florida, several weeks after the launch of Voyager 2. The identical spacecrafts had similar missions of studying Jupiter and Saturn up close.
"It was simply impossible - ethically - to switch off such successful missions, so they've moved on and on [towards] the edge of the solar system," says Köhler.
Between 1977 and late 1980, Voyager 1 discovered two of Jupiter's moons, as well as many of Saturn's moons. It confirmed that Jupiter had rings and identified 16 active volcanoes on the moon, Io. The spacecraft also took iconic pictures, including the first spacecraft photo of the earth and the moon in 1977 and a portrait of the solar system in 1990.
NASA had planned the original Voyager Mission to coincide with a once-every-175-years alignment of Jupiter, Saturn, Uranus, and Neptune that would allow the two spacecrafts to use the planets' gravity to "swing" them from one planet to the next. This technique minimized the need for extra propulsion and travel time.
Today, both Voyagers run on about 350 watts and use few of their instruments.
"[What] Voyager has performed in the solar system is unparalleled and we will very likely never have it again in our lifetime," says Köhler.
Voyager keeps surprising
Both Voyagers have been exploring the edge of the solar system as part of NASA's Interstellar Mission.
"They have returned some very, very important data on a region of the universe about which we had no idea before," says Köhler.
In June 2010, Voyager 1 surprised the project team when it sent back data showing that solar winds had dropped to zero in an area of the heliosheath.
Measurements such as these have helped scientists revise their assumptions about the physical structure of the heliosheath and how objects can enter or exit this region between our solar system and the Milky Way.
"The younger generation of scientists is more eager to find out whether there's life on Mars," says Köhler. "But they need to know about the structure of the solar system because this will influence our image and our idea of our solar system as a whole."
If Voyager 1 successfully passes through the heliopause into the interstellar system, the scientists expect to continue receiving data until the spacecraft stops functioning in about 2025.