Voyager 1: Earth's Farthest Spacecraft

Voyager 1 is the first spacecraft to reach interstellar space. It originally was launched (along with Voyager 2) in 1977 to explore the outer planets in our solar system. However, it has remained operational long past expectations and continues to send information about its journeys back to Earth.

Artist rendering of Voyager1

The spacecraft officially entered interstellar space in August 2012, almost 35 years after its voyage began. The discovery wasn't made official until 2013, however, when scientists had time to review the data sent back from Voyager 1.
Voyager 1 was actually the second of the twin spacecraft to launch, but it was the first to race by Jupiter and Saturn. The images it sent back have been used in schoolbooks and newspaper outlets for a generation. Also on board was a special record, carrying voices and music from Earth out into the cosmos.
Voyager 2 launched on Aug. 20, 1977, and Voyager 1 launched about two weeks later, on Sept. 5. Since then, the spacecraft have been traveling along different flight paths and at different speeds. The Voyager missions were intended to take advantage of a special alignment of the outer planets that happens every 176 years. It would allow a spacecraft to slingshot from one planet to the next, assisted by the first planet's gravity.
The spacecraft’s next big encounter will take place in 40,000 years, when Voyager 1 comes within 1.7 light-years of the star AC +79 3888. (The star itself is roughly 17.5 light-years from Earth.) However, Voyager 1's falling power supply means it will stop transmitting data by about 2025, meaning no data will flow back from that distant location.

Entering interstellar space

Voyager 1's official departure from the solar system occurred in August 2012; the discovery was made public in a study published in Science the following year.
The results came to light after a powerful solar eruption was recorded in Voyager 1's plasma wave instrument between April 9 and May 22, 2013. The eruption caused electrons near Voyager 1 to vibrate. From the oscillations, researchers discovered Voyager 1's surroundings had a higher density than what is found inside the heliosphere, or the region of space in which the sun's environment predominates.
It seems contradictory that electron density is higher in interstellar space than it is in the sun's neighborhood, but the researchers explained that at the edge of the heliosphere, electron density is dramatically low compared with locations nearby Earth.
Researchers then backtracked through Voyager 1's data and nailed down the official departure date to sometime in August 2012. The date was fixed not only by the electron oscillations, but also by its measurements of charged solar particles.
On Aug. 25, the probe saw a 1,000-fold drop in these particles and a 9-percent increase in galactic cosmic rays that come from outside of the solar system. At that point, it was 11.25 billion miles (18.11 billion km) from the sun, approximately 121 times the Earth-sun distance.

Voyager 1's interstellar adventures

As of February 2018, Voyager is roughly 141 astronomical units (sun-Earth distances) from Earth. That's roughly 13.2 billion miles, or 21.2 billion kilometers. You can look at its current distance on this NASA website.
Since flying past the solar system's boundary into interstellar space, Voyager 1 sent back valuable information about conditions in this zone of the universe. Its discoveries include showing that cosmic radiation is very intense, and demonstrating how charged particles from the sun interact with those of other stars, said project scientist Ed Stone in a September 2017 interview.
The spacecraft's capabilities continue to astound engineers. In December 2017, NASA announced that Voyager 1 successfully used its backup thrusters to orient itself to "talk" with Earth. These "trajectory correction maneuver" (TCM) thrusters hadn't been used since November 1980, during Voyager's last planetary flyby of Saturn. Since then, Voyager used its standard attitude-control thrusters to swing the spacecraft in the right orientation to talk with Earth.
As the performance of the attitude-control thrusters began to deteriorate, however, NASA decided to test using the TCMs to extend Voyager 1's lifespan. That test ultimately succeeded. "With these thrusters that are still functional after 37 years without use, we will be able to extend the life of the Voyager 1 spacecraft by two to three years," Voyager project manager Suzanne Dodd, also of JPL, said in a statement.
The Voyager spacecraft each celebrated 40 years in space in 2017, prompting celebrations from NASA and celebrities such as "Star Trek" star William Shatner. In September 2017, Shatner read out a message to the spacecraft originally crafted on Twitter, by Oliver Jenkins: "We offer friendship across the stars. You are not alone." Jet Propulsion Laboratory engineer Annabel Kennedy then transmitted the message to Voyager 1; it was projected to reach the spacecraft in about 19 hours.
"None of us knew, when we launched 40 years ago, that anything would still be working, and continuing on this pioneering journey," Stone said in a NASA statement from August 2017. "The most exciting thing they find in the next five years is likely to be something that we didn't know was out there to be discovered."