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Gravity Assist: Explorer 1 & Jim Green’s ‘Gravity Assist’ (1)
March 1, 2018


Gravity Assist producer Laurie Cantillo and NASA Planetary Science Director Jim Green at the NASA TV studios in Washington, D.C.   Credits: NASA

 The year was 1958; the space race was on. NASA’s first space satellite, Explorer 1, launched a Geiger counter and miniature tape recorder into space that registered astonishingly high radiation levels above Earth. This discovery gave humanity its first glimpse of the Van Allen radiation belts, named for James Van Allen. What you may not know about this famous scientist is that he mentored an aspiring student named Jim Green, now NASA’s Planetary Science Director. Jim discusses his Van Allen ‘gravity assist,’ Explorer 1, and what we’ve learned about these strange belts of energetic particles that surround Earth, other planets, and even a moon!   

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.

Laurie Cantillo: I’m Laurie Cantillo with the science division here at NASA headquarters and we’re going to do something a little different today to launch season two of Gravity Assist. We’re talking about mind-blowing science discoveries and one of them happened 60 years ago with our Explorer 1 mission, which revealed these intense radiation belts that we have around Earth; they’re called Van Allen belts.

It’s a cool story so to tell it we’re going to turn the tables on our host, Jim Green, and find out about his gravity assist. That’s the term we use to describe an event or a person in your life who helped you figure out what you really love to do and forever changed your path in life. So, Jim Green, what was your gravity assist?


James Van Allen of the University of Iowa poses with a rocket model. Van Allen designed one of three science instruments on Explorer 1. The cosmic ray experiment was essentially a Geiger counter that clicked as it detected cosmic rays. This experiment and a future one led Van Allen to conclude that a doughnut-shaped radiation belt wrapped around Earth. This became known as the Van Allen radiation belt.   Credits: Frederick W. Kent Collection, University of Iowa Archives

Jim Green:  Well, thanks, Laurie. You’ve done a fabulous job helping me with our Gravity Assist interviews over this first season, and I’m delighted to be able to kick off the second season and talk about some of my experiences. My gravity assists really started in high school.

I had a high school chemistry teacher who ended up with the keys of an observatory, a donated telescope. It turned out it was a 12-inch Alvan Clark refractor and for you amateur astronomers out there I’m sure your jaws have dropped.

Laurie Cantillo:  They still make those? (laughs)

Jim Green:  Alvan has gone out of business but I think this was made in probably in the ‘30s.

Laurie Cantillo:  Okay.

Jim Green:  Yeah, in the ‘30s, 1936 or so I think is when it was made. But this particular telescope, a fabulous telescope. I had it at my beck and call for a couple of years in high school and I made instruments for it.

I studied stars, I studied the planets, I took pictured that ended up in Sky & Telescope and I took images of the Sun every day for three months during one summer and three months during another summer. So, at the end of my high school I knew exactly what I wanted to do and that was get into college and get a degree in astronomy but that didn’t end there.

Many of our scientists, as you’ve heard in many of our other podcasts, they’ve had multiple gravity assists along the way and mine really started my first year at the University of Iowa. I was born and raised in Burlington, Iowa. It’s just a small town on the Mississippi and University of Iowa was one of the big universities in Iowa and I took astronomy 101 with about 400 other students.

Laurie Cantillo:  I know what that’s like.

Jim Green:  The place was packed. I walked in the first day and the only places available were up front.

Laurie Cantillo:  That’s what you get.

Jim Green:  Well, I always enjoyed sitting up front. I wear glasses and so my eyes aren’t really that good, so I would indeed sit up front. But I was taught by James Van Allen and --.

Laurie Cantillo:  Whoa, THE --?

Jim Green:  Yeah, the James Van Allen and we’ll learn more about him because, indeed, he was the first investigator of our first experiment ever in space on the spacecraft Explorer 1.


Credits: NASA

So, at the end of first semester of astronomy 101 I got an A, thank goodness.

Laurie Cantillo:  You’d better have. The Planetary Science Director getting less than an A; I don’t think so.

Jim Green:  Well, I wasn’t always great in school. The ups and downs happened, and we always have to remember you’ve got to stay the course. But second semester I had an opportunity to take a small two-credit hour course and so I took readings in astronomy and it was taught by his “staff.” You know, that’s what it says in the course catalog.

And so, when the course started at a designated time in Van Allen Hall room 701 I went to that room, opened the door and it looked like a warehouse. It was this huge room. Printouts all over tables, magnetic tapes and bookcases and a cluttered room from that perspective. These rooms were storerooms, and I’m standing in the doorway going through this course catalog figuring out I must have messed up.

Laurie Cantillo:  Wrong place. Wrong time.

Jim Green:  And so, from behind the bookcase Dr. Van Allen said “Jim, this is the place and you’re my only student.” So instead of being taught by a grad student, I had a full semester with James Van Allen! And when he found out that I had taken pictures within Alvan Clark refractor and in particular the solar set, it was during solar maximum in the late ‘60s, early ‘70s and he said “Well, let’s do some things. Let’s do some research with this data,” and I go, “Research? What’s research?” So, we talked about it. I showed him the data, he loved it. And the big suggestion that came from him is, “Why don’t you look at how fast the Sun spins? Let’s do that by looking at the sunspots and how long it takes for them to go around.” Indeed, I had plenty of data to make measurements of a solar rotation. I did the same research that had been done about 70 years earlier and wrote a science paper, just an introduction and discussions and abstracts and all of the stuff, components of a paper and he was the referee. And so, at the end of that semester I knew what research was all about and that gravity assist was absolutely unbelievable.

Laurie Cantillo:  It’s cool how many people will cite teachers as being that gravity assist—and good for you! I’m certainly glad you took that class.

Jim Green:  Yeah, me, too. But that connects really well with what this year is all about. This is the 60th anniversary of NASA. And it kicked off with our first successful spacecraft called Explorer 1.

Now you can get online and probably go to some of the images that the National Air and Space Museum post and you can see what the Explorer 1 rocket looks like. It’s about 2.5 stories tall and it’s a huge beautiful rocket that had on top of it an experiment, a Geiger-Mueller tube that looked at cosmic rays. That was Van Allen’s experiment and on the side of it are two letters, two huge letters painted on the side “U and E” and so everyone knows about it. You can see it’s very distinctive and that’s the way that goes but I finally found out the story behind UE.

Laurie Cantillo:  Yeah, why not U and S?

Jim Green:  Could have been. So, here’s how it goes. After I graduated from the University of Iowa I ended up at Marshall Space Flight Center at NASA, it’s one of the 10 NASA centers and it’s a fabulous place. They do all kinds of things.

This is where (Wernher) Von Braun was then they were building a series of rockets. So, Van Allen came down, I invited him to give a talk and then I had a big reception over at my house. And so, I had scientists and engineers from the center and I was walking around with a tray of hors d’oeuvres. So, I walk out up Van and many of us students called Dr. Van Allen Van, it was a very endearing term and he loved it and never batted an eye over that.

And so, I walked up and there Van was talking to one of our engineers, actually it was my next-door neighbor, Jack Waite, who worked with Von Braun and I’m standing there -- would you like an hors d’oeuvre? -- and Jack says, “Dr. Van Allen, do you know what U E means?” So, wow, I had to hear this, man.

Laurie Cantillo:  Yeah, no kidding. Great time to be there.

Jim Green:  I’d seen U E I don’t know how many times, I didn’t know what it meant and Dr. Van Allen said, ”No, I don’t think I ever knew what that meant.” And so, Jack says, “Well, here we are in Huntsville, Alabama.” This is where the Redstone Arsenal is -- and currently the Marshall Space Flight Center sits on that -- but Marshall hadn’t been put in place at that time. So Jack said, “There are nine unique letters in (the word) Huntsville: U is two and E is nine and so U E is a code for a number, 29.

And Van Allen said, “Why didn’t you just put 29 on the rocket? Just paint 29?” And Jack said, “Well, we didn’t want to do that, because if we put 29 on the rocket the press would ask us what happened to the other 28?”

Laurie Cantillo:  28, yeah, that’s a good point.

Jim Green:  And the problem with that is they all blew up and so Van looks at me and I look at him, and I’m thinking, “Thank God U E didn’t blow up,” but that’s the way it was early on.

We were struggling in this nation to be able to build rockets that work and get things into space. Sputnik had been orbiting the earth for months earlier. USSR was celebrating their launch of Sputnik, which occurred on October 4, and that was last year’s celebration, they had their 60th. So, in 1957 Sputnik went up and here we are in 1958 January 31, Explorer 1 takes off and gets into orbit.

What a fantastic time that was. I mean the press conference was something like 2:30 in the morning at the National Academy and that’s the very famous picture you see Dr. Van Allen, Pickering from Jet Propulsion Laboratory which helped build the experiment and integrate it into the rocket, and then of course Von Braun which got the rocket into space.

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Gravity Assist: Explorer 1 & Jim Green’s ‘Gravity Assist’ (2)

Laurie Cantillo:  Wow, and they’re holding it over their heads (in the famous photo). Just how heavy is that thing?


The three men responsible for the success of Explorer 1, America's first Earth satellite which was launched January 31, 1958. At left is Dr. William H. Pickering, former director of JPL, which built and operated the satellite. Dr. James A. van Allen, center, of the State University of Iowa, designed and built the instrument on Explorer that discovered the radiation belts which circle the Earth. At right is Dr. Wernher von Braun, leader of the Army's Redstone Arsenal team which built the first stage Redstone rocket that launched Explorer 1.  Credits: NASA

Jim Green:  That’s the third stage and the experiment is inside that. So you had several stages that are a couple of stories tall just to get that little bitty thing into space.

Laurie Cantillo:  That’s what really launched the space race and the whole era. It was in a sense the competition that really inspired and got people going but then the fascination that the American public had with space exploration really took off.

Jim Green:  Yeah, Explorer 1 was our first successful attempt and what we found out immediately is that it had things to teach us. The data came back, Van Allen also had several other Explorers over the next several months to get additional data and by May 1958 he was announcing the results of those several instruments, those Geiger-Mueller tubes, from Explorer 1 through Explorer 3 since two didn’t make it into orbit.

So, Explorer 1 and Explorer 3 told us that there was a set of very high energy particles racing around our magnetic field above our atmosphere and that was the radiation belt, we call that now the Van Allen radiation belt.

Laurie Cantillo:  So, was that discovery a total surprise or did we theorize ahead of time that there was something going on like that?

Jim Green:  Yeah, for most of the scientists I would say nearly everyone it was a huge surprise.

Laurie Cantillo:  Like so many things.

Jim Green:  You would think that there would be some basis for understanding that we would expect to see the particles trapped and there was some of the earlier ideas about how particles and magnetic fields interact. If you have a magnetic field, it can trap charged particles.

They have a tendency to spiral around the magnetic field and don’t leave the environment, don’t leave the area until they can hang around for long periods of time. And in fact, there’s hints from other planets that they have magnetic fields and that had been going on for many years prior to this.

Laurie Cantillo:  So, it’s a good thing that Earth has these magnetic fields, correct? These are what protect us from all of the space weather, from the radiation, from the solar wind…and did we understand that back then at that time that that’s what was the significance of this finding?

Jim Green:  No, we didn’t. We absolutely did not. It is important that this Earth has a magnetic field. The magnetic field does trap charged particles and as the Sun heats our atmosphere to the extent where the atmosphere can actually become disassociated into charged particles and actually evaporate into space, then the magnetic field traps those and so we have evaporated atmosphere that we call the plasmasphere.

Starts out in the ionosphere and goes out into space and it creates another region called the plasmasphere and that allows us to hang on to our atmosphere for long periods of time. So that’s just one effect that the Earth’s magnetic field has. And that includes all of the other planets that have magnetic fields too—that happens to them.

Mars, you know it lost its magnetic field and now we know (from NASA’s MAVEN mission) that the solar wind as it has continued to strip atmosphere out of Mars and in particular, the oxygen that is disassociated from water, from H2O and the O gets transported and then stripped away, has really changed that planet now into an arid and dry place.

Laurie Cantillo:  And then Jupiter, of course, the king of the solar system, has these intense radiation belts. How do those on Jupiter compare with the ones on earth and what did NASA have to do to protect our Juno spacecraft?

Jim Green:  So indeed, Jupiter we’ve known something was going on with Jupiter very early on, before Explorer 1. Bell Labs had taken radio dishes out to explore how we could use radiofrequency light for communication and as they were doing that they saw two bright things in the sky and they figured out one was the Sun and the other was Jupiter.


Explorer 1 was the first U.S. satellite and the first satellite to carry science instruments. The satellite was launched on Jan. 31, 1958, from Cape Canaveral, Fla.. Explorer 1 followed a looping flight path that orbited Earth once every 114 minutes. The satellite went as high as 2,565 kilometers (1,594 miles) and as low as 362 kilometers (225 miles) above Earth.  Credits: NASA

The radio emissions from Jupiter were coming from the trapped particles in the intense magnetic field and they were coming from the Jovian radiation belts. And that field is more than 20 times stronger than the Earth’s magnetic field. So, it’s pretty intense and it therefore holds very close to the planets really intense particles that spiral back and forth along the magnetic field lines and so that hurts our spacecraft.

These particles move so fast they actually will penetrate the ‘skin’ (of the spacecraft) and get into the circuits and really cause some problems if we don’t take that into account. And Juno has that. You know, Juno’s got --.

Laurie Cantillo:  Like a bank vault, sort of.

Jim Green:  It’s got this huge vault, that’s right. So, we built this bank vault if you will, put all of our sensitive electronics in that and that seems to be working well for us.

Laurie Cantillo:  You touched on something that since coming to NASA has really made an impression on me: and that’s how interconnected all of the science is that we do, that what we learn about Earth we can apply to what we’re studying on planets like Jupiter and Venus and vice versa. So, it really is important that we look at all of our planets and that by looking at other places in the solar system we can therefore learn about what might be Earth’s future.

Jim Green:  Absolutely. You know, the concept that the magnetic field was important to maintain our environment, in particular our atmosphere is really something that we really understand because of Mars and our study of Mars. And so, then when we look at Venus and we see well, Venus doesn’t have a magnetic field. Why doesn’t it have a very tenuous atmosphere like Mars? And so now we have to probe and look for other explanations and we do.

We find out that Venus is incredibly an active planet, probably longer than Earth has been in terms of its volcanic activity for whatever reason and we really don’t understand or know that and so it has dumped an enormous amount of CO2 gases and we believe it may still may be active today doing that.

Laurie Cantillo:  Now we have actually a moon in the solar system with a magnetic field. What’s going on with that?

Jim Green:  Yeah, well, that’s a really neat moon, it’s not our moon although we know enough about our Moon to realize that it probably had a magnetic field early on in its history but there’s a moon at Jupiter, it’s called Ganymede and it has its own magnetic field.

Now what’s really spectacular about Ganymede is we think of moons being smaller objects, Ganymede is huge, and Jupiter is the big planet in our solar system. But just to give you an idea how big Ganymede is, you know it’s bigger than the planet Mercury. It is the largest moon in our solar system and it generates its own magnetic field.

And so it also has another unique characteristic and that is it was made in the outer part of our solar system and it is an icy world and therefore because of its interaction with Jupiter’s gravity field with tides, we believe a lot of that ice has melted inside that beautiful moon Ganymede is probably an under ice crust ocean and we believe the ocean is pretty big, may have more water than we have here on Earth.

So, ESA has this fantastic mission called JUICE which is JUpiter ICy moons Explorer, and it is planned to be launched in 2022 and will arrive at Jupiter in 2029 and begin orbiting Ganymede soon after that.

Laurie Cantillo:  Well, let’s talk more about season two, we’re calling it “mind-blowing science discoveries”—things like exoplanets, frozen geysers on Europa and Enceladus and gravitational waves. You may have heard about that and recently we had this “Rip van Winkle” kind of spacecraft story.

It was called IMAGE. Jim, you were a scientist on that mission. It was thought to be lost and phoned home recently after 13 years. I can’t wait to hear that story.

Jim Green:  Well, I’ll say one thing about it because IMAGE was orbiting the Earth in the Van Allen radiation belts and one day it didn’t call home and that is, indeed, because we believe the radiation belts really clobbered it and the fact that it now came back to life is just remarkable.

Laurie Cantillo:  It’s a great story. We’d also like to ask for your ideas for future Gravity Assist podcasts, so how can we do that?

Jim Green:  Well, you can send us an email at gravityassist@nasa.gov. So, I really appreciate all of your help in pulling these together for me. I can’t do it myself and having you interview me as a person behind the scenes has just a delight.

Laurie Cantillo:  Thanks for being my gravity assist.

Source: Gravity Assist: Explorer 1 & Jim Green’s ‘Gravity Assist’