More than 18 billion kilometers from home, Voyager 2 has become the second probe to break through the heliopause, a tenuous boundary between the solar system and the rest of the Galaxy.

Illustration showing position of Voyager 1 and Voyager 2
This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto.
NASA / JPL-Caltech

Our Sun’s influence extends well beyond the planets. A steady breeze of charged particles known as the solar wind blows far past Neptune until it gives way to the interstellar plasma that fills the Galaxy. To some researchers, that boundary, known as the heliopause, marks the dividing line between the Sun’s territory and interstellar space.

Now, the Voyager 2 spacecraft has become the second ambassador from Earth to cross that boundary, mission scientists announced December 10th at a meeting of the American Geophysical Union in Washington, DC.

“Voyager 2 is now getting a first-hand view of the local galactic neighborhood,” said Georgia Denolfo (NASA).

Voyager 2 passed the heliopause on November 5th, when a plasma detector onboard the spacecraft recorded a sharp decline in the speed of the solar wind. “We’re not seeing the solar wind anymore,” said John Richardson (MIT), principal investigator for Voyager’s plasma experiment. “That means we must be in the interstellar medium.”

Around the same time that the solar wind vanished, Voyager 2 also saw a sharp uptick in cosmic rays — high-speed atomic particles that whiz around the Galaxy — as well as an increase in the ambient magnetic field. This confluence of events gave mission scientists confidence that the probe had finally broken out of the heliosphere, a bubble of space surrounding the Sun in which the solar wind reigns supreme.

Animation of Voyager 2 data
This animation shows the changes in galactic cosmic rays and solar wind particles detected by Voyager 2 as it crossed the heliopause.
NASA / JPL-Caltech

This marks the second time that a spacecraft has crossed this threshold. In 2012, Voyager 1 crossed the heliopause, though its plasma detector had stopped working back in 1980. With Voyager 2 now joining its twin in interstellar space, scientists will be able to obtain the first direct measurements of the ionized gas that drifts between the stars and will have a second set of eyes on the flux of cosmic rays impinging on the solar system — measurements that can only be obtained from outside the heliosphere.

“The heliosphere acts as a force field,” Denolfo explained. It blocks many (though not all) of the galactic cosmic rays from reaching Earth. These zippy particles, hurtling through space at nearly the speed of light, are thought to blast out of supernovas, stellar explosions that have helped shape the region of space surrounding the solar system. “Galactic cosmic rays act as tiny messengers from our local galactic neighborhood,” said Denolfo.

This latest milestone is just one of many in the storied four-decade-plus journeys of these two spacecraft. Voyagers 1 and 2 launched 16 days apart in 1977 and embarked on a grand tour of the outer solar system. Both probes briefly visited Jupiter and Saturn. Voyager 1 continued on toward interstellar space while Voyager 2 veered off to Uranus and Neptune, becoming the first and only spacecraft to fly by the two ice giants.

Now more than 18 billion kilometers (11 billion miles) from home, Voyager 2 puts roughly another 1.3 million kilometers (820,000 miles) between itself and Earth every day. At that distance, radio signals traveling at the speed of light require over 16 hours to get back to Earth. Voyager 1 is farther out still, nearly 22 billion kilometers (13 billion miles) away. In about 300 years, the Voyagers should encounter the inner edge of the Oort Cloud — a shell of icy debris enveloping the solar system that every so often sends a comet our way — and could pass beyond it in another 30,000 years.

The probes will run out of power well before then, but they should still have a few good years left in them. “Both spacecraft are very healthy, if you consider them as senior citizens,” said Voyager project manger Suzanne Dodd (JPL). The biggest concerns right now are electrical power and heat, both of which are needed to keep the instruments working. In particular, the temperature on Voyager 2 is just shy of 4 degrees Celsius, she said, which is close to the freezing point of hydrazine needed to operate the thrusters.

In the coming years, scientists will have to decide which instruments to switch off and when. “We anticipate operating for another 5 to 10 years,” said Dodd. “My own personal goal would be to get these spacecraft to last 50 years.”

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Comments


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Robert-LaPorta

December 10, 2018 at 5:35 pm

Don’t forget Pioneer 10 and 11. I believe Pioneer 10 is still more distant than Voyager 2, but Voyager 2 has a faster trajectory and will overtake Pioneer 10 sometime in the future.

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Anthony Barreiro

December 11, 2018 at 7:24 pm

Could you cite a reference? When I searched for information about Pioneer 10 and 11, the most definitive information I got was that Pioneer 11 sent its last transmission in 1995 (1) and Pioneer 10 in 2003 (2). I couldn't find anything about their current distances.

(1) https://en.wikipedia.org/wiki/Pioneer_11

(2) https://www.nasa.gov/centers/ames/news/releases/2003/03_13AR.html

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Robert-LaPorta

December 12, 2018 at 9:50 am

I found this

"On January 1, 2019, Pioneer 10 is predicted to be 121.69 au from the Earth (about 11.3 billion miles); and traveling at 12.04 km/s (26,900 mph) (relative to the Sun) and traveling outward at about 2.54 au per year.[63] Voyager 2 was projected to have passed Pioneer 10 by April 2019."

https://en.wikipedia.org/wiki/Pioneer_10

Rob

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Anthony Barreiro

December 16, 2018 at 12:26 am

Thanks Rob. I wonder what the error bar is on that predicted distance? We lose track of Earth-orbiting satellites and inner solar system probes when they stop transmitting, The Pioneers and Voyagers are traversing a less well understood environment. I wonder how much variance there is from ideal ballistics.

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Robert-LaPorta

December 24, 2018 at 10:10 am

Except for very minor gravitational influences, I doubt the trajectories would change much at all. The environment may not be well understood, but it is mainly a vacuum much better than any we can produce here on Earth.

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fif52

December 20, 2018 at 6:33 pm

what's their current direction in coordinates, please. can't someone help.

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Robert-LaPorta

December 24, 2018 at 10:06 am

Various wikipedia areas may answer that question. I would also check out NASA deep space sights. I am certain the trajectories are well understood. Don't forget Deep Horizon also

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