MADDIE SOFIA, BYLINE: You're listening to SHORT WAVE...
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SOFIA: ...From NPR.
REBECCA RAMIREZ, HOST:
Hey, everyone. Rebecca Ramirez here. And right now, as you are listening to this, there are two spacecrafts sailing through the darkness of space, outside of our solar system, and each has on board a shiny Golden Record with greetings to aliens in many different languages.
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UNIDENTIFIED CHILD: Hello from the children of planet Earth.
UNIDENTIFIED PERSON #1: (Non-English language spoken).
UNIDENTIFIED PERSON #2: (Non-English language spoken).
UNIDENTIFIED PERSON #3: (Non-English language spoken).
UNIDENTIFIED PERSON #4: (Non-English language spoken).
RAMIREZ: Plus some distinctive sounds of Earth.
(SOUNDBITE OF TRAIN WHISTLING, CHIMPS HOOTING, BABY CRYING)
RAMIREZ: Now, I'm talking, of course, about NASA's twin Voyager probes, which were launched back in 1977 before I was born, but not before NPR science correspondent Nell Greenfieldboyce was born. Hi, Nell.
NELL GREENFIELDBOYCE, BYLINE: Hey, Rebecca. So doesn't it just boggle your mind that the Voyagers are still out there taking in data about their surroundings and reporting back to Earth after 44 years?
RAMIREZ: Completely. Forty-four years is a long time. And it's just kind of like, how long can they last? They must have some source of power. But, I mean, no source of power is infinite, right, Nell?
GREENFIELDBOYCE: So they're powered by plutonium. Plutonium is the power source. And researchers have just been trying to make it last as long as possible.
RALPH MCNUTT: We're looking at which instrument gets to have all their heaters turned off first because we're just flat running out of power.
GREENFIELDBOYCE: That's Ralph McNutt. He works at the Johns Hopkins University Applied Physics Laboratory.
MCNUTT: I'm still on the Voyager science team. I think I'm the youngest one on the Voyager science team.
GREENFIELDBOYCE: At least in terms of people who've been there since the Voyagers launched. He's 68 years old.
RAMIREZ: I mean, talk about loyalty. So how long does Ralph think that the Voyagers have left?
GREENFIELDBOYCE: He says, if we're lucky, maybe a decade at best, assuming that, you know, nothing breaks.
RAMIREZ: Well, everything has a life cycle, Nell. We're all going to die one day.
GREENFIELDBOYCE: Indeed, indeed. Don't I know it? But listen. NASA recently asked McNutt and a team of researchers to devise a new mission to pick up where the Voyagers left off, and they've come up with a spacecraft that could go out farther and faster. And that is what I wanted to talk to you about, the possibility of doing deliberately what the Voyagers have done by accident, sort of taking the next step out into the universe.
RAMIREZ: I mean, that sounds very promising. So today on our show, a possible successor to Voyager - what a practical, doable interstellar mission would look like for NASA and why the space agency might want to do it. This is SHORT WAVE, the daily science podcast from NPR.
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RAMIREZ: Now, Nell, Voyager 1 is the most distant human-made object in the universe. How far away is it exactly?
GREENFIELDBOYCE: Over 14 billion miles.
RAMIREZ: Oh, wow.
GREENFIELDBOYCE: And Voyager 2 is over 11 billion miles away.
RAMIREZ: So pretty far.
GREENFIELDBOYCE: Pretty far.
RAMIREZ: And why are scientists saying they're outside of the solar system? Did they, like, cross some unofficial boundary that I don't know about?
GREENFIELDBOYCE: So there's no sign out in space saying, you've now left the solar system. It's not like when you drive from New Jersey to Delaware or whatever.
RAMIREZ: Really?
GREENFIELDBOYCE: But imagine the solar system as this kind of giant bubble around the sun. And that bubble is created by a flow of charged particles that are constantly streaming out of the sun. So the Voyager's instruments show that they've left that realm of charged particles and crossed over into a different realm, the interstellar medium.
RAMIREZ: That is so cool. So was that the goal of the Voyager mission, to leave our solar system?
GREENFIELDBOYCE: No, no. This was a planetary mission. So...
RAMIREZ: Oh.
GREENFIELDBOYCE: ...The spacecraft were built to last around five years, and their main mission was to visit Jupiter and Saturn. That was the goal. NASA added on missions to Neptune and Uranus using gravity assists of other planets like slingshots. So, you know, the Voyagers were built to explore planets, but scientists knew that even if their instruments had all broken down after a while, the trajectories of these spacecraft would eventually get them out into the space between stars.
RAMIREZ: Oh, that's why they put those Golden Records, right?
GREENFIELDBOYCE: Yeah. Carl Sagan led a committee, and the idea was just to create a symbolic message in a bottle to whatever aliens might stumble across the spacecraft, if aliens are out there. But, you know, it'll be around 40,000 years before Voyager 1 will pass within a couple light-years of a star. And that's after traveling more than four decades. So, you know, the Voyager is maybe out of our sun's sphere of influence, but they are just, like, barely out into the interstellar space.
RAMIREZ: OK. Nell, you promised me that we were going to talk about another mission, though, to follow on the Voyagers, which sounds cool, but if there are no stars or planets or anything anywhere near where the Voyagers are, why try to send another probe out there? Isn't it just, you know, black empty space?
GREENFIELDBOYCE: I asked Stella Ocker that question exactly. So she's at Cornell University. She's working on her Ph.D. And her research relies on data sent back by the Voyagers. And she told me that, you know, admittedly, the density of matter out there is very, very low.
STELLA OCKER: Compared to our experiences here on this planet, yes, the interstellar medium seems like it's empty. But it's really not. It's full of gas and dust and cosmic rays, energetic particles.
RAMIREZ: Full seems relative here, but I get her point.
GREENFIELDBOYCE: Yeah. She says there is no substitute for having a spacecraft out there actually gathering data.
OCKER: There's still huge gaps in our knowledge about the interstellar medium that can really only be filled by direct sampling.
RAMIREZ: So I'm guessing that's why NASA's being so intense about planning ahead before the Voyagers go kaput.
GREENFIELDBOYCE: Precisely. A few years ago, they asked Ralph McNutt and some colleagues to dream up a new mission. That team's work is almost done. They're finishing up their final report now.
RAMIREZ: So what's the plan?
GREENFIELDBOYCE: It's a probe that's based on other successfully flown spacecraft. It basically uses tried-and-true technology or technology that's really far along in development. McNutt told me that this probe would have instruments specifically chosen to study the interstellar medium.
RAMIREZ: Fancy.
MCNUTT: A lot of things that we know now that it would've been nice to have had on Voyager, but the technology either didn't exist or we didn't have the mass and the power to accommodate them, or Voyager was a 4 1/2-year mission to Jupiter and Saturn, so why would you fly those things?
GREENFIELDBOYCE: He told me their probe would ultimately go twice as fast as Voyager 1, so more than 20 miles a second.
MCNUTT: And planned for lasting at least 50 years. And that'll get us out to about 375 astronomical units.
GREENFIELDBOYCE: Three hundred and seventy-five astronomical units is more than twice as far as Voyager 1 is right now, and that is the minimum.
RAMIREZ: Wow, the minimum. So what's the maximum?
GREENFIELDBOYCE: He thinks it's possible this probe could keep talking to Earth for a century and eventually go out more than five times farther than Voyager 1, so it would be, like, 74 billion miles away.
RAMIREZ: Wow, 74 billion miles away. I mean, that sounds like a really long distance. But, I mean, we're talking about space, which is huge. So how far away is the nearest star from Earth?
GREENFIELDBOYCE: Like, just for comparison?
RAMIREZ: Yeah.
GREENFIELDBOYCE: Like, what do you mean - the nearest star as in our beloved sun?
RAMIREZ: No, not the sun, Nell. You know what I mean.
GREENFIELDBOYCE: Like Proxima Centauri, that kind of nearby star?
RAMIREZ: Yeah.
GREENFIELDBOYCE: That is 25 trillion miles away. So 25 trillion with a T.
RAMIREZ: My brain can't even comprehend how far these scales are getting. But actually, aren't there proposed missions that would go out to a star like that?
GREENFIELDBOYCE: You might be thinking about the Breakthrough Starshot initiative. So that's a group that wants to send lots of teeny, tiny, little probes, like the size of a postage stamp, that would be put in space and then accelerated to astonishing speeds by ground-based lasers. Theoretically, that mission could reach a planet around Proxima Centauri in just a few decades.
RAMIREZ: Now that sounds exciting.
GREENFIELDBOYCE: OK, maybe a little too exciting.
RAMIREZ: Oh, OK.
GREENFIELDBOYCE: I talked to Michael Paul about this one. He's also at the Johns Hopkins University Applied Physics Laboratory. He told me he loves the vision of Breakthrough Starshot, but the technology is just not ready for prime time, and it would be hugely expensive.
MICHAEL PAUL: So you and I are probably never going to get to see them operate.
RAMIREZ: Oh.
GREENFIELDBOYCE: That's why he and Ralph McNutt and their colleagues wanted to devise an interstellar mission that was pragmatic.
PAUL: To make sure that we were able to hand NASA something that wasn't so pie in the sky that it just ended up on people's shelves.
GREENFIELDBOYCE: He says if NASA decides to build their proposed interstellar probe, it could realistically be built in time to launch in 2036. So that's like 15 years from now.
RAMIREZ: OK. And then when would it reach interstellar space?
GREENFIELDBOYCE: That would take another 15, 20 years after that.
RAMIREZ: And then you said it would collect data for decades longer, maybe even a century?
GREENFIELDBOYCE: Yeah. And to me, this is the most interesting thing about this proposed mission. So it would have the longest planned duration of any NASA mission ever by a long shot. Now, you know, there are some science missions at NASA that have lasted a long time sort of by accident, you know, like the Voyagers or like the Mars rover Opportunity that was built to last, like, 90 days and it lasted 15 years. But going out and deliberately planning for a century-long mission is just a different ballgame.
RAMIREZ: Yeah. I mean, no one lifetime is going to encompass all of that. Like, it sounds like it could be multiple generations of people's life work.
GREENFIELDBOYCE: One astronomer who's been helping to design the mission, Carey Lisse, told me he's done the math.
CAREY LISSE: I will be 75 in 2036 when we launch. That means that I know I'm not going to be on this mission probably for more than 10 years after launch. Just a fact. There is going to be handover. This isn't just theory or just talk. It's going to happen multiple times.
GREENFIELDBOYCE: They've actually been consulting with a sociologist on how to do that.
RAMIREZ: Oh, really?
GREENFIELDBOYCE: Yeah. Her name's Janet Vertesi. She's at Princeton University. And she says when they called her, they basically said, look; we're planning a mission to go to the interstellar medium, and we're probably all going to be dead when we get there.
RAMIREZ: Sure, sure.
GREENFIELDBOYCE: So, you know, obviously they knew that parts of the mission would have to get handed off to younger colleagues as time went on.
JANET VERTESI: I think what they hadn't really considered and where my expertise came in was how frequently that needs to happen in order for that to be an expected and normal part of the mission operations and not a major breach or a big problem.
GREENFIELDBOYCE: She says like in hospitals during shift changes, the doctors and nurses get good at it because, you know, there's checklists and procedures, and that's because the shift change is happening every single day.
RAMIREZ: And it's, like you said, a different ballgame to plan on a time scale of, say, a shift change every, like, 30 years.
GREENFIELDBOYCE: Right. Generally speaking, I think it's fair to say that humans aren't so great at making transition plans, especially if those plans involve, like, facing their own mortality. I was talking to Stella Ocker, that graduate student. She's 25 now. And she would be in her late career when this probe reaches interstellar space.
OCKER: Science fiction is almost inundated with this concept of, like, interstellar exploration by humans.
GREENFIELDBOYCE: But she said, you know, people don't stop to think about all the different incremental steps you need to take to actually do something like that.
OCKER: And we do need to learn how to conduct missions over these very long time scales if we're ever going to come close to achieving any of the kinds of, you know, aspirations of interstellar exploration that are so often posed in, you know, the popular media.
RAMIREZ: So obviously, Nell, the big question on my mind is, is this mission going to have a modern take on the Golden Record?
GREENFIELDBOYCE: Well, keep in mind this mission has not been given the green light at this point. It is still just a proposed mission for NASA. So we're going to have to wait and see what happens.
RAMIREZ: All right, Nell, well, keep us posted. We look forward to hearing more. And in the meantime, dear listeners, let's all ponder what sounds we hold dear here on Earth. Send us an email with what you'd want alien life to hear. We're at shortwave@npr.org.
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RAMIREZ: This story was edited by Gisele Grayson, who was 6 years old when the Voyagers took off. It was produced by Eva Tesfaye and fact-checked by Margaret Cirino, who were still just free-floating stardust at that time. And our audio engineer, Stu Rushfield, was but a tender 11 years old at launch. I'm Rebecca Ramirez. Thanks so much for listening to SHORT WAVE, the daily science podcast from NPR.
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