The Kuiper Belt with Alan Stern
After making history by flying by Pluto in 2015, NASA’s New Horizons spacecraft is speeding toward a New Year’s Day 2019 flyby of a mysterious world in the outer realm of the solar system. In this episode of Gravity Assist, Jim Green talks with New Horizons Principal Investigator Alan Stern about what we’ve learned about Kuiper Belt object 2014 MU69 and the remarkable story of how -- against all odds -- the New Horizons team captured MU69’s fleeting shadow on Earth as the object passed in front of a distant star.
Jim Green: Our solar system is a wondrous place with a single star, our Sun, and everything that orbits around it, planets, moons, asteroids and comets. What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos with billions of other solar systems. Hi, I’m Jim Green, Director of Planetary Science at NASA, and this is Gravity Assist.
With me today is Alan Stern, the principal investigator of the New Horizons mission. New Horizons has made a fabulous historic flyby of Pluto just two years ago, and now it’s on its way into a region we call the Kuiper Belt. And, it’s gone a rendezvous with a very mysterious world, further than we’ve ever gone before, something we all need to pay attention to so we can see what this is all about. You know, Alan, welcome.
Alan Stern: Thank you, Jim. It’s great to be here.
Jim Green: You know, I think many people are surprised about the solar system, in fact, that it really doesn’t stop at Pluto. There’s something beyond it. When did the concept of the Kuiper Belt come about?
Alan Stern: Well, the concept actually came about just about the time that Pluto was discovered in 1930. And, a number of astronomers thought that perhaps Pluto was kind of the tip of the iceberg and if we could see deeper with future telescopes, we’d discover a lot more out there. The most famous argument made for that was by Gerard Kuiper, who was a giant in planetary science in the middle of the 20th century, and it sort of stuck with his name on it.
The discovery of the Kuiper Belt, though, had to wait for technology to develop, much better detectors, CCD (charge-coupled device) detectors, and fast computers to analyze mountains of data. And, so it was actually the 1990s before the first Kuiper Belt object other than Pluto was discovered. And, of course, that was much--a much smaller body, as most everything is. Pluto’s actually the largest thing in the Kuiper Belt.
But, it turns out, most importantly, the Kuiper Belt is dotted with other small planets like Pluto that no one really expected at all, that Pluto isn’t the misfit of the outer solar system. It was kind of the harbinger of things to come.
Jim Green: Yeah, it’s really been fascinating watching these objects being discovered. In fact, many of them are binary.
Alan Stern: A lot of them are binary. Pluto itself is a binary. In fact, even this tiny little one that we’re going after as our next flyby target may well be a binary. So, that’s telling us something about the original formation conditions, because we don’t see binary planets down in the inner solar system. Venus isn’t a binary, Mars isn’t a binary, Mercury isn’t a binary. Even the Earth/Moon system isn’t really a binary.
But, in the Kuiper Belt, they’re very common. So, something was very different back then in that place that made this binary formation mechanism routine.
Jim Green: Now, when you talk about binary, you mean that both the primary and secondary bodies are so massive, that they’re actually orbiting a barycenterthat exists between them.
Alan Stern: Right. It pretty much means that they’re like- sized objects orbiting one another.
Jim Green: What’s really fantastic about the next step that New Horizons is taking, going to MU69, is how that object was discovered. Can you give us a little background on that?
Alan Stern: You know, we knew from the beginning when we designed New Horizons that its mission was to go on exploring after Pluto, deeper in the Kuiper Belt. And, we put the fuel on board and the communications capability on board, and we designed the cameras to work even further from the Sun, so forth and so on. And, then after we got launched, we started using the biggest telescopes in the world to look for possible targets.
And, although we found many, none were within our fuel reach. And, as we started getting closer and closer to Pluto, I got kind of worried that we just weren’t going to be able to carry it out from the ground. And, so we asked for NASA’s help and the help came in the form of the Hubble Space Telescope, which spent a good bit of time in the summer of 2014, the year before we got to Pluto, scanning the region behind Pluto where New Horizons would be headed next, to find targets.
And, we found several and MU69 was the most easily reached of that group. And that’s what we’re bearing down on next.
Jim Green: What’s really fascinating that happened this summer was, of course, the opportunity that MU69 would pass between us and stars, at distant stars. And, so those occultations occurred, but to me, they were unbelievable in the way you put together your teams and deployed them, and actually made some unique observations.
Alan Stern: Yeah, well credit really goes to the people on those teams, and they were led by Marc Buie, who’s a part of New Horizons, in doing really state-of-the-art, more advanced calculations than had ever been done before, for where
the shadow of something so small, like MU69, would fly across the Earth, and getting telescopes in the right place at the right time, in the shadow, way down in South America, in Patagonia, in the middle of the winter, in one of the windiest places in Patagonia. And, it all worked out. Five telescopes saw MU69 make this particular nondescript star wink out.
But, each telescope saw it wink out for a different period of time, because it observed from a little bit different location. And, therefore, we could actually use those individual tracks to paint out the shape of MU69, get its size and its surface reflectivity. The most interesting result probably is that it looks like it could be a double itself, a binary, either a contact binary, where the two lobes are actually touching, or two objects in orbit around one another, like Pluto and Charon, but on a much tinier scale.
Jim Green: Is there another opportunity where we could catch it in another occultation?
Alan Stern: There is another one coming next August across some pretty rainy parts of South America, unfortunately, and some pretty dangerous parts of North Africa, with a lot of Atlantic Ocean in between. So, we’re looking at ways to go after that that would be about six months before our next flyby, the flyby of MU69, and whether we use ships or airplanes or perhaps find some places where the weather’s good enough and the local conditions are safe enough to put telescopes on the ground. We could learn a lot that would help give us some more advanced warning about what we’re going to find when we get there at the holidays of 2018.
Jim Green: You know, what also amazes me is how bright MU69 is. In fact, its magnitude 27. And, for those astronomers out there, it means there’s no way any Earth telescope would be able to see it. So, it’s really one that had to be found by Hubble.
Alan Stern: It’s true, only the Hubble, because it’s above the atmosphere, could do it. And, we even, after we found MU69, knew exactly where to look. We’d look with very large telescopes, like the Keckand the Gemini telescopes and others, Subaru, and they’ve never spotted it from the ground. In fact, the only time it’s been spotted except for Hubble was in those occultations.
Jim Green: Just a few photons.
Alan Stern: It’s extremely faint. Twenty-seventh magnitude means that it’s almost ten million times fainter than Pluto, which itself is a million times too faint for your eye to see. It’s just mind boggling.
Jim Green: [Laughter] Yeah, to me, this was clearly the greatest occultation captured that we’ve ever done on this planet.
Alan Stern: It was a major breakthrough. What Buie and his team pulled off was a masterstroke in not only prediction, but execution. And, they made it look easy. It was not. It was really beyond the state-of-the-art until they did it.
Jim Green: That really tells you what it takes to put together a team of talented people and let them do their work, you know?
Alan Stern: Yeah. And, I was on two of the occultation expeditions and, you know, folks were working 20 hours a day, in very harsh conditions, in the cold in the winter and the wind, and nights and weekends. And, they were all very highly motivated to do it, and they pulled it off.
Jim Green: You know, how that must have gone in terms of being able to put on an array of telescopes. I think you had something like 24 telescopes in a line?
Alan Stern: We did. We set them up like a fence line, perpendicular to the path, so we would catch our prey, even if it was a little north or south of where we were going to be. We got lots of help from the locals, the Argentinian locals, the national government, their space agency, CONAE, as well as the Governor and the Mayor of the town that we were in gave us all kinds of support, ranging from police escorts to blocking the highway so that the trucks wouldn’t come down with their bright headlights and ruin the observations, weather support, first aid.
They really bent over backwards to help, because, you know, they found out NASA was in town. And, to them, that was really something to be a part of. That NASA brand was what did the trick. That, and one very helpful translator named Adriana Ocampo.
Jim Green: Yeah, she’s a rock star down there, that’s for sure, a fabulous scientist that had worked on Chicxulub and the extinction of the dinosaurs. But, you know, out of the three occultations, two we caught on ground, but one we had to do something else with.
Alan Stern: One of them was over the ocean, and there are no observatories floating around in the ocean. So, we took NASA’s biggest airborne observatory called SOFIA and it was being based for the summer down in Christchurch, New Zealand to make observations of the southern sky. And, we were awarded time to go fly it up to the occultation path, several thousand miles north, near Tahiti.
I was on that mission as well. We flew five hours out of Christchurch north, did a U-turn and came back, and just as we turned south, the occultation event occurred, and then flew all the way back to Christchurch with the data. And, here’s this big lumbering 747 with a telescope the size of Hubble inside of it, looking out of a door, a hatch in the side, flying at 45,000 feet with a flight crew and a telescope crew and the science crew. And, you know, they were so on target that they were less than five wingspans off target at the exact moment of the occultation. It’s an amazing accomplishment.
Jim Green: Yeah, it is amazing, unbelievable. So, with all of the data in hand, you’re getting a picture of, perhaps, it being a binary?
Alan Stern: Uh-huh.
Jim Green: What about debris in the area? You know, we’re always worried about the safety of our spacecraft as we fly through.
Alan Stern: Yeah, and we’re worried, primarily, because we’re going so fast. You know, we’re going at ten miles per second. So even if you hit something small, it’s a very powerful wallop, and there’s basically no good place to hit New Horizons.
If you do computer modeling of that, anything, even a rice pellet that hit the spacecraft will cut a fuel line or take out a circuit board or destroy an instrument or what have you. So, we’re trying to certify the path as best we can. And, the occultations help us with that, because when--before and after you occult the hard body itself, you can look for dips due to rings, dips in the light due to rings or other debris structures that could be in orbit.
Now, fortunately, we didn’t find any. So, that tells us that some of the worst disaster scenarios that could be out there aren’t out there at MU69. They might be somewhere else, but they’re not at MU69.
But, still, we have to look even harder to certify the path on approach. We’ll do that with our own telescopes on board, and our own cameras on board, by sending that data back to Earth throughout the fall of 2018, and we’ll be scrutinizing those images as best we can. And, if we find anything that’s concerning, we’ve planned an entire backup flyby at a greater distance, which is presumably safer, to give ourselves some options for still getting good data, but avoiding danger if it’s in our path for the very closest approach.
Jim Green: I’m Jim Green and I’m here with Alan Stern, talking about the Kuiper Belt and what’s next for New Horizons. You know, one of the things that I always wondered about is our comets. We get comets from very far away from the Oort Cloud, but, you know, there’s got to be comets coming out of the Kuiper Belt, too. So, what do you think the relationship between what we have, Pluto like objects, and comets are?
Alan Stern: Well, that’s a really good question. We know that comets, the short period comets, come from the Kuiper Belt. And, we know that--we’ve seen quite a number of those up close with spacecraft missions now. Rosetta recently orbited one for two years, but we had various American and European spacecraft fly by comets, also some old Soviet spacecraft that flew by comets in the 80s.
They don’t look anything like planets. They’re small and lumpy and they don’t have the geological processes that big worlds, like Pluto, that, you know, would basically span the United States in diameter, have. And, also their composition turns out to be quite different.
We see on the surface of Pluto much more exotic ices than we see on the surfaces of comets, things like nitrogen ice that are very rare in comets. And, we see a bound atmosphere that we don’t see around comets. So, just like the asteroids and the Earth are different, the comets and the small planets of the Kuiper Belt are very different.
But, they’re all teaching us about the origin of the solar system and about the types of objects we can expect to find around other stars. And, so it’s all part of the basic exploration that we do, as we open up the solar system to space travel.
Jim Green: New Horizons, after it flies by MU69 on January 1st, 2019--and I know where I’ll be when that occurs.
Alan Stern: Me, too.
Jim Green: But, it’s heading out. It’s heading out of the solar system. Is it going in the same direction that the Voyagers are?
Alan Stern: It is going in roughly the same direction as the Voyagers. And, like the Voyagers, it will escape out into the galaxy.
Jim Green: Now, it has radioisotope power and that will last for a considerable length of time past MU69. Do you think it can make it to the heliopause?
Alan Stern: Well, that’s a good question. We have a lot of Kuiper Belt work left to do before we get to the heliopause. The heliopause is probably 100 astronomical units out. And, if you--you know, if you calculate the amount of power that we have on board, we could still be operating then. It’ll be in the mid to late 2030s.
The thing about the heliopause, and I know you know this, Jim, is that it breathes in and out with the solar cycle, and sometimes it’s further and sometimes it’s closer. And, although we can predict exactly where New Horizons will be in any given year, no one knows how to predict those solar cycles. So, it’s hard to know, in the ‘30s if the heliopause will be farther and we run out of power before we get there, or if it’s closer and we’ve crossed it out into interstellar space.
That’s part of the excitement of that part of the mission that’ll come, you know, after the planetary science is more or less done, which will probably be in the 2020s. And, that will be a very valuable mission scientifically because the instrumentation on board in New Horizons is a generation more sophisticated than Voyager could carry. So, we can learn new things about that whole region of the solar system with these much more sensitive instruments.
Jim Green: Now, one of the things that New Horizons is doing between now and when it flies by MU69, is really looking at other Kuiper Belt objects. What do you hope to achieve by doing that?
Alan Stern: What we’re trying to--to put MU69 in context. We’re going to swoop down on this one and study it with this spectacular battery of instruments and get all this detail, and the question is, how do the other ones look in comparison? What are their shapes like? What are the number of satellites there like? What are their surface properties like, compared to MU69?
So, we’re actually looking at dozens of others with our telescope camera, called LORRI, on board, and not just before MU69. There are a lot of them to look at after MU69, because the Kuiper Belt doesn’t run out. It actually turns into what’s called the Extended Belt, or the Scattered Belt, that goes out actually hundreds of astronomical units. So, it’s all about context and making sure that we understand this valuable data set at MU69 compared to all the other things out there in the Kuiper Belt.
Jim Green: You know, this science is just spectacular. We’re just pushing the limits of what we know about the solar system and making new discoveries every day. And, you know, I always ask what your gravity assist. What really got you excited about what we’re doing today?
Alan Stern: [Laughter] Well, I’ve had a number of gravity assists in my life, but I want to tell you about a very special one that occurred just a few weeks after the New Horizons flyby. I was in Vermont at a convention of amateur astronomers in August of 2015, and after my talk was over and most of the crowd had dissipated, there was someone there who said, “I’d just been waiting and waiting to just come up and tell you something.”
She said, “You know, people often say that our generation missed the boat on history, that we didn’t see a great world war--the great world war that triumphed over evil, and we didn’t have a chance to see the Moon landings or the birth of the computer revolution that we all live with now, and so many other things. And, we often hear the meme that we came too late for all those historic things.” And, she said, “I just want to tell you New Horizons is the best thing that’s ever happened in my lifetime.” And, wow, what a gravity assist for a scientist--
Jim Green: --Yeah.--
Alan Stern: --to hear something like that, when you’re a physicist and a planetary scientist who works on the research aspects and the technical aspects, and to hear that you could change people’s lives with the project that we did and that those of us on New Horizons could actually inspire someone that way, that was my gravity assist. And, it’s going to power me for the next 30 years to do more exploration.
Jim Green: Fantastic. Well, Alan, it’s just been a joy having you here, talking about the Kuiper Belt and what’s next for New Horizons.
Alan Stern: Thank you, Jim.
Jim Green: Thank you very much. You know, it’s just been fantastic having you here, talking about the Kuiper Belt and what will New Horizons do next. I really appreciate it. Thanks so much for coming down.
Alan Stern: Thank you, Jim. Any time.
I’m Jim Green, and this is your Gravity Assist.
Follow NASA’s New Horizons mission at: https://www.nasa.gov/mission_pages/newhorizons and http://pluto.jhuapl.edu/index.php and on Twitter: @NASANewHorizons and @NewHorizons2015
Love NASA science? Follow NASA’s Science Chief Thomas Zurbuchen on Twitter using @Dr_ThomasZ and check out #ScienceInSeconds for short videos.
And sure to listen to these NASA podcasts: “Houston: We Have a Podcast” from Johnson Space Center, Houston, and “NASA in Silicon Valley”from Ames Research Center in Moffett Field, California.
Last Updated: Jan. 31, 2018
Editor: Gary Daines
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