Voyager 2 became the second man-made object to enter interstellar space on December 5th, NASA announced. Analysis of data from the vehicle reveals the intrepid spacecraft has reached the heliopause, the outer limits of the heliosphere, a bubble of particles and magnetic fields generated by our Sun, as it passed into interstellar space.
It’s twin spacecraft, Voyager 1, reached the heliopause in 2012. Both vehicles were launched in 1977. Voyager 1, following a different trajectory to its twin, passed this boundary at a separate point six years ago, becoming the first object created by humans to leave the solar system. However, the most convincing evidence that Voyager 2 left the Solar System comes from the Plasma Science Experiment (PLS) aboard the craft. That instrument failed on Voyager 1 long before the vehicle reached the heliopause.
“Until recently, the space surrounding Voyager 2 was filled predominantly with plasma flowing out from our Sun… The PLS aboard Voyager 2 observed a steep decline in the speed of the solar wind particles on Nov. 5. Since that date, the plasma instrument has observed no solar wind flow in the environment around Voyager 2, which makes mission scientists confident the probe has left the heliosphere,” NASA officials reported on the agency website.
Other instruments on Voyager 2, including the cosmic ray subsystem, the low energy charged particle instrument and the magnetometer, confirm the idea that Voyager 2 is now beyond the reach of particles and energy from our parent star.
“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new. Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we’re still seeing things that no one has seen before,” said John Richardson of the Massachusetts Institute of Technology in Cambridge and principal investigator for the PLS instrument.
Voyager 2 is now so far from Earth – 18 billion kilometers (11 billion miles) – that data from the craft, traveling at nearly 300,000 kilometers (over 186,000 miles) per second, takes 16.5 hours to reach controllers on our home planet. To put this in perspective, light from the Sun takes just around eight minutes to reach the Earth.
Voyager 1 made observations of Jupiter and Saturn, while Voyager two visited both of those planets, as well as Uranus and Neptune. Both spacecraft are equipped with golden records, containing music and greetings from Earth, should they ever be encountered by an alien civilization.
Both vehicles are powered by radioactive decay of material within their radioisotope thermal generators (RTG), which are losing power as they age. This is forcing mission engineers to slowly turn off instruments on the spacecraft, including the cameras which provided such stunning images of the largest planets in our Solar System.
Although the spacecraft have passed the heliopause, they are still not completely out of the solar system. That outer boundary is generally considered to be the Oort Cloud, a collection of rocks and icy bodies surrounding the Solar System. Voyager 2 is not expected to reach that boundary for approximately 300 years, and will not pass through it for another 30,000 years. So, break out the champagne now – it’s going to be a long wait until the next passing.