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Our knowledge of the solar system is expanding. Here’s how scientists define interstellar space.

Pluto is an example of how our definition of what’s in our solar system continues to evolve

2012: The Voyager 1 spacecraft, originally launched in September 1977, enters interstellar space, becoming the first man-made object to do so. During its mission, the probe became the first to provide detailed images of Jupiter and Saturn and their moons. Voyager 1, along with its sister craft Voyager 2, remains on its extended mission today, tasked with locating and studying the boundaries of the Solar System.
2012: The Voyager 1 spacecraft, originally launched in September 1977, enters interstellar space, becoming the first man-made object to do so. During its mission, the probe became the first to provide detailed images of Jupiter and Saturn and their moons. Voyager 1, along with its sister craft Voyager 2, remains on its extended mission today, tasked with locating and studying the boundaries of the Solar System. (NASA/JPL-Caltech)

Earth’s neighborhood as we know it is changing as new technology can help reveal more about our little spot in the Milky Way, including what defines interstellar space.

NASA’s two Voyager spacecraft, launched in August and September 1977, went on the first-ever tour of the planets of our solar system. They sent back some of the first detailed images of worlds we had only seen through telescopes before, but after Neptune, the robotic explorers kept going.

According to NASA, on Aug. 25, 2012, Voyager 1 flew beyond the heliopause and entered interstellar space, leaving our “solar bubble.” At the time, it was about 11 billion miles from the sun. Then, in November 2018, Voyager 2, traveling in a different direction, also crossed the heliopause into interstellar space.

The spacecraft are considered the first interstellar missions even though they still haven’t surpassed the farthest known area of our solar system known as the Oort cloud. They continue to send back data today.

This week, on “Space Curious,” listener David Mostardi, of Berkley, California, wanted to know how big is the solar system -- and if the Voyager spacecraft didn’t reach the Oort Cloud, did it really reach interstellar space?

“Back when I was a boy in the 1960s ... the popular idea was that the solar system ended at Pluto ... you put up a nice white picket fence, that was the end of it,” Mostardi said. “But all of a sudden, the print media and online media started reporting that Voyager 1 had left the solar system. Well, no, no, hang on. We just, we just had these discoveries of these cool big objects like (dwarf planet) Eris that was farther away. ... How could the Voyagers have left the solar system?”

The Oort cloud is the most distant region of our solar system, according to NASA. And it’s many times farther away from the sun than even the Kuiper Belt, which is beyond Neptune. Its size is so massive that its outer edge is estimated to be billions upon billions of miles away from the sun.

So, if Voyager hasn’t reached the Oort cloud, it’s still in the solar system, right?

Not quite.

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In this episode of “Space Curious,” experts help explain what it means to go “interstellar,” how we define the vastness of space and who’s making these decisions.

NASA’s New Horizons missions operations manager and a Cornell University planetary scientist joined the show to break down this fascinating topic. Original music from Rhein was used for this episode.

“Technically, when we use the words ‘the solar system,’ what we’re referring to is the sun, all of the planets and beyond to the planets, the Kuiper Belt and the Oort cloud -- and the Oort cloud is kind of the further most edge of the solar system, and it’s this huge cloud of kind of rocks and debris and dust that encases all of the planets and everything inside the solar system,” Cornell University planetary scientist Stell Ocker said. “And the Oort cloud is actually very far away.”

Ocker was the lead author on a recent paper based on Voyager spacecraft data focusing on interstellar gas.

Ocker and her team discovered that the Voyager 1 spacecraft was experiencing something new within this interstellar space in radio wavelengths.

“Until this study, Voyager 1 has been seeing these bright, kind of, thunderstorms ... which are essentially these events that happen when the sun emits coronal mass ejections and these coronal mass ejections send shockwaves coursing out through the solar system into the interstellar medium,” she said. “And when those shockwaves enter the interstellar medium, they cause the gas to vibrate very strongly in response. And so Voyager 1 can see those really large, brief vibrations. And those are kind of like the thunderstorms of the lightning storms that Voyager 1 has seen.”

This “gentle rain” is in an area called the heliosphere, which is essentially the area of space that is encased by the solar wind, or the most powerful influence from our solar system’s star, the sun.

And that’s why the Voyager missions are considered interstellar, because they are no longer within the bubble of influence that surrounds Earth and the other planets.

Big planetary science definitions are decided by the International Astronomical Union, or the IAU.

“Now, when it comes to defining the area of interstellar space that Voyager is traveling through, that’s still an area of open research. And so it’s a little less straightforward,” Ocker said. “And you’ll see in the literature, people sometimes have different ways of talking about this, this region of space, and we’re still not sure exactly how far the sun’s influence extends into the interstellar medium. And so that’s one of the open questions that we’re trying to answer using Voyager.”

Sometimes IAU decisions make waves. For example: Pluto.

This image made available by NASA on Friday, July 24, 2015 shows a combination of images captured by the New Horizons spacecraft with enhanced colors to show differences in the composition and texture of Pluto's surface.

Pluto is also the perfect example of how our definition of what’s in our solar system continues to evolve.

“When Pluto was just discovered, it was thought to, you know, be that missing planet. That was theorized, but it didn’t quite fit the mathematical prediction of what that ninth planet would look like. And now we know that it was that it is actually the first Kuiper belt object that was discovered,” said Alice Bowman, New Horizons spacecraft mission operations manager at the Johns Hopkins Applied Physics Laboratory.

In 1992, Jane Luu, an astronomer at MIT, and her colleague David Jewitt, discovered the very first Kuiper Belt object beyond Pluto using a telescope on Mauna Kea in Hawaii.

“So that was, I guess, the beginning of us realizing that no, the solar system doesn’t stop at Pluto,” Bowman said.

Then in 2006, educators, astronomers and the general public were outraged and confused when Pluto was declassified as a planet and declared a dwarf planet. How could the IAU just decide our ninth planet wasn’t a planet anymore?

That debate intensified when NASA sent the New Horizons spacecraft on a flyby of Pluto and it sent back the most detailed images yet showing a complicated, fascinating world.

Pluto is the largest object within the Kuiper Belt, which is an asteroid belt that surrounds our planets -- yet another layer to go through before reaching interstellar space long before the Oort cloud.

But for a long time, it was thought Pluto was our ninth and final planet.

According to Bowman and her team, it doesn’t matter if Pluto is a dwarf planet or considered our ninth planet.

“It’s just an amazing place. And it really has brought to light what these objects so very far from the sun still have in their bag of tricks,” Bowman said. “I mean, liquid water ocean? How would you ever believe that something so far from the sun could have a liquid water ocean or ice volcanoes? And it sort of implies that, you know, these types of objects are very interesting. They’re not just gray balls of rock.”

New Horizons didn’t stop at Pluto though. It’s still going, looking for other Kuiper Belt objects to fly by and study. The New Horizons team is planning to update the spacecraft’s software in July to allow scientists to continue to use the cameras onboard to search for more Kuiper Belt objects.

It takes seven hours for a signal from Earth to reach the spacecraft via the Deep Space Network, a series of antennas around the world communicating with many NASA spacecraft and rovers on Mars.

New Horizons will actually follow the Voyager 2 spacecraft into the realm of interstellar space sometime in the mid-2040s, but unlike both Voyager missions, it won’t be able to send back any data.

“The universe is a continual place,” Bowman said. “And I think it just tells us that there’s so much more to discover -- things that we can’t even imagine.”


“Space Curious” is a podcast from WKMG and Graham Media Group that answers your intergalactic questions. Hosted by space reporter Emilee Speck, each episode is designed to inspire everyone, from the space curious to the space fanatics. Questions for the podcast can be submitted here.

Subscribe or follow wherever you listen to your favorite podcasts, including Spotify, Apple Podcasts, Stitcher and Google Play. New episodes drop every other Wednesday.


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