Voyager 2 has Officially Entered Interstellar Space

NASA has just announced that the time has come to say goodbye to one of the most famous explorers of our age: Voyager 2. The spacecraft has entered interstellar space.

The announcement comes before a news conference at the annual meeting of the American Geophysical Union, which you can watch live beginning at 11 a.m. EST (1600 GMT), thanks to NASA.

Voyager 2 was launched in 1977 and has spent more than 40 years exploring our solar system. It famously became the only probe ever to study Neptune and Uranus during planetary flybys.

Now, it has joined its predecessor Voyager 1 in interstellar space, a milestone scientists couldn’t precisely predict. What’s even more interesting is that humanity’s second crossing doesn’t look just like data from the first journey out.

Ed Stone, a physicist at the California Institute of Technology and project scientist for the Voyager mission, said during a scientific talk before the announcement that it is a very different time and a very different place but similar in characteristics. The next months ahead could be very revealing!

Voyager 2 is the only spacecraft to have visited all four gas giant planets — Jupiter, Saturn, Uranus and Neptune — and find 16 moons, in addition to phenomena such as Neptune’s mysteriously transient Great Dark Spot, the cracks in Europa’s ice shell, and ring features at every planet.

Since late August, scientists have been watching for Voyager 2’s final departure, when data sent back by the probe suggested it was nearing what scientists call the heliopause, a bubble created by the solar wind of charged particles flowing out from our sun and influencing the environment within our solar system.

The heliopause is used by scientists to mark where interstellar space starts, but depending on how you define our solar system it can stretch all the way to the Oort Cloud, which begins 1,000 times farther away from the sun than Earth’s orbit.

Aside from that bubble, spacecraft fly through many more cosmic rays — much higher-energy particles — than the lower-energy particles of our solar system. Two instruments that the Voyager 2 probe is equipped with are used to track these particles as they crash into the spacecraft. The transition from mostly lower-energy particles to nearly none of these and a sudden surge of cosmic rays lets scientists know the probe has crossed the heliopause.

It’s a tough transition to predict because Voyager 2 is only the second spacecraft to cross that bubble with those its instruments functioning. Voyager 1 crossed it in 2012. However, the heliopause is not a perfect sphere, and it shrinks and expands with the ebb and flow of the solar wind. Thus, scientists had to wait and see what the data said. Starting in August, the data began to foreshadow the cosmic departure, with a general increase in cosmic rays and decrease in local particles. However, the grand exit took time.

Rob Decker, who works with one of Voyagers particle detectors, said during the scientific talk that scientists are especially excited for this second crossing since Voyager 2 still has an instrument that had stopped working on Voyager 1 long before the heliopause — the Plasma Science Experiment. That means Voyager 2’s journey will not just generate new data, but a new type of data, according to NASA officials in a statement.

Nevertheless, where the two trips can be compared, the instruments have shown how uneven that journey can be. Voyager 1 ran into flux tubes that caused dynamic data, whereas Voyager 2 has not flown through it. However, the second probe has seen a bizarre bump in some of the data.

Stone explained that they haven’t sorted out what these features are yet. What’s more, Voyager 2 may not have as straightforward an exit as Voyager 1 since the current solar cycle phase means that the sun’s bubble is growing a little.

Although there is lots of excitement surrounding this milestone, not much will change for Voyager 2 itself. It will continue sending back updates to its scientists here on Earth for as long as it can. Eventually, the spacecraft plutonium supply that powers it will run out, and the probe will shut down its instruments. Sometime after 2025, the team expects the probe will turn off entirely, without enough power to cross the ever-wider abyss to Earth.

Jessica Zeitz