After 36 years of space exploration, NASA’s Voyager 1 spacecraft has officially traveled farther than any human-made object has gone before. Officials at the space agency announced last week that the spacecraft entered interstellar space—the area of space between stars made up of dust, gas, and plasma (ionized gas)—on or around August 25, 2012. It is now about 12 billion miles from Earth’s sun.
“Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the . . . history of science,” says John Grunsfeld of NASA. “Perhaps some future deep-space explorers will catch up with Voyager . . . and reflect on how this intrepid [adventurous] spacecraft helped enable their journey.”
VOYAGE INTO SPACE
Voyager 1 was launched on September 5, 1977, about two weeks after the launch of its twin spacecraft, Voyager 2. The pair flew past our outer planets, giving scientists some of their first up close looks at Jupiter, Saturn, Uranus, and Neptune. The two spacecraft continued flying deeper into the heliosphere—the bubble of charged particles surrounding our sun—toward interstellar space. Voyager 2 is now about 9.5 billion miles from our sun.
To determine whether Voyager 1 had entered interstellar space, scientists needed to know more about the plasma surrounding the spacecraft. Interstellar space is filled with cool, very dense plasma from stars that exploded millions of years ago. The plasma in the heliosphere, which radiates from our sun, is extremely hot and much less dense.
Voyager 1 has an instrument that can measure the temperature, density, and speed of plasma, but it stopped working in 1980. So scientists instead relied on the spacecraft’s working plasma-wave antennas, which detect the vibrations of excited plasma particles and convert them into a ringing sound. The density of the plasma affects the frequency of the ring—the denser the plasma, the higher the frequency. But something, such as a large solar flare, has to energize the plasma to get the antennas to make the sound. And when it does happen, it can take as long as a year for the shock wave to reach Voyager 1.
The antennas picked up two sets of vibrations in the past year, one from October to November 2012 and the other from April to May 2013. The frequency of the ringing sounds showed that the plasma was cold and dense—Voyager 1 was in interstellar space! Scientists measured the increase in frequency between the two sets of vibrations to determine August 25 as the date Voyager 1 left the heliosphere.
A LOT TO LEARN
Scientists say they still have a lot to learn from Voyager 1, which communicates with antennas on Earth. The spacecraft has enough electrical power to keep its science instruments working until at least 2020, when scientists will have to start turning off the instruments. It will continue to send data for a few more years after the last instrument is turned off, around 2025. Then it will move silently through space for the rest of time.
“Every day . . . we’re looking at data that no one has seen before and in a region where nothing has ever been before,” Voyager chief scientist Ed Stone, of the California Institute of Technology, told reporters. “I think we’re all looking forward to learning a lot in the years ahead.”