After 41 years on the road, NASA's Voyager 2 spacecraft has finally entered the interstellar space, exiting heliosphere - the protective bubble of particles and magnetic fields created by the Sun, NASA reported on Monday, December 10.
Comparing data from different instruments aboard the trailblazing spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on Nov. 5. This boundary, called the heliopause, is where the tenuous, hot solar wind meets the cold, dense interstellar medium. Its twin, Voyager 1, crossed this boundary in 2012, but Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space.
Voyager 2 now is slightly more than 11 billion miles (18 billion kilometers) from Earth. Mission operators still can communicate with Voyager 2 as it enters this new phase of its journey, but information - moving at the speed of light - takes about 16.5 hours to travel from the spacecraft to Earth. By comparison, light traveling from the Sun takes about eight minutes to reach Earth.
NASA announced back in October that it suspected this moment might happen soon. The spacecraft had been detecting increasing amounts of cosmic rays, something Voyager 1 experienced in 2012. But Voyager 2 has something its partner didn't when it left the heliosphere -- a functional Plasma Science Experiment. Voyager 1's stopped working in 1980, but Voyager 2's is still in working order, and it's able to take measurements of the solar wind. On November 5th, Voyager 2 detected a sharp drop in the speed of solar wind particles and since then, it hasn't measured any solar wind flow at all -- strong evidence that it has exited the sun's protective bubble.
Voyager 2 launched in 1977, 16 days before Voyager 1, and both have traveled well beyond their original destinations. The spacecraft were built to last five years and conduct close-up studies of Jupiter and Saturn. However, as the mission continued, additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible. As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left Earth. Their two-planet mission became a four-planet mission. Their five-year lifespans have stretched to 41 years, making Voyager 2 NASA's longest running mission.