Charley Kohlhase has been our ambassador to the planets

James Blinn (far left) and Charles Kohlhase working at JPL on the revolutionary computer animations of the planned Voyager encounters with the outer planets. Credit: Charles Kohlhase
James Blinn (far left) and Charles Kohlhase working at JPL on the revolutionary computer animations of the planned Voyager encounters with the outer planets. Credit: Charles Kohlhase

Charley Kohlhase officially ‘retired’ from NASA’s Jet Propulsion Laboratory in 1998. At his retirement party, a recording was played of Carl Sagan, who had died two years previously. ‘…. His voice suddenly, pleasingly rang through the room. From the afterlife, Sagan implored, “Charley Kohlhase’s name should be as well known as Michael Jordan’s!”’ (from ‘Ambassadors from Earth’ – Jay Gallentine, Nebraska University Press, 2009).

Sporting heroes today are celebrated with much media pomp and circumstance. On the other hand those who contribute to the advancement of science and technology generally remain unknown outside their field. Rocket scientists are among these real, unsung heroes of human progress. As rocket scientists go, Sagan was certainly right about Charley!

Charles E. ‘Charley’ Kohlhase, Jr. is one of the most accomplished and revered figures in unmanned spaceflight and solar system exploration. He worked for forty years at NASA’s Jet Propulsion Laboratory (JPL), leading the mission design and management of many of their greatest successes, from the early days of Ranger and Mariner missions, through Viking, the Voyager ‘Grand Tour’ and Cassini. Since ‘retiring’ in 1998 he has continued to advise JPL until 2014, serving as a technical consultant for the Cassini, Mars Sample Return and other future Mars missions, Genesis, Kepler and the planned Jupiter Europa Orbiter, as well as participating in the choice of landing sites for the twin Mars Exploration Rovers and working on risk assessments for a variety of future programmes. He is also on the Advisory Council of the Planetary Society, an accomplished writer, speaker, digital artist, photographer and a committed environmentalist – a true Renaissance Man.

Charley spoke on the Voyager Panel at this year’s Spacefest, and I had the opportunity to speak with him afterwards about his long and distinguished career at JPL.

RocketSTEM: What initially motivated you to work in the space programme?

Charley KOHLHASE: “When I was young my grandmother used to tell me adventure stories, so when I started reading I read such stories, like the ‘Count of Monte Cristo’ and ‘The Three Musketeers’. She also introduced me to the beauty of nature, art and architecture. Then later I got into science fiction adventures too. Sometimes, I found myself wishing to escape from regimented family life and the private military academy to which I had been sent by my father, a strict disciplinarian. People like my school friends said they would see me lying on the lawn at night looking up at the stars. I also had an inspirational teacher at school who taught me to enjoy math and to reason. I was a dreamer, but I was also curious, and must have had an internal desire to know what was out there.

“However, my father did not encourage me in that direction. In fact, he insisted the only thing worth studying was mechanical engineering, and so he paid for my tuition to go to Georgia Tech, but only as long as I studied in the mechanical engineering department. At the end of my sophomore year, I guess the call to science was very strong, so I changed to physics. It bothered him so much that he said ‘You’re making a great mistake.’ Then he cut off all support to me from that point on. And so, for the last two years I basically had to pay my own college expenses and I did so by being responsible for one of the large dormitories, to make sure the students behaved. I also taught physics and math classes at the freshmen and sophomore levels and received a small stipend for that.

“So I always had a great desire for adventure and I did well in physics and math, and I went down that pathway, even at the expense of being rejected by my father. I’m happy that I found the courage to follow this dream. You must always be honest with yourself.”

Charley at work in his home in 2010, designing a futuristic space scene.
Charley at work in his home in 2010, designing a futuristic space scene.

RS: Were you reconciled with your father afterwards, once your career at NASA developed?

KOHLHASE: “It’s funny you should ask that. My father was a heavy smoker, smoking two or three packs a day, and at the age of 59 he died of lung cancer. I knew he was dying, and, when I was working at JPL, I flew back to see him in Georgia a month or two before his death. Surprisingly he said to me ‘Son, I’m proud of what you’ve done.’ He had followed my work at JPL in newspapers, including the early work on the Ranger and the Mariners, but that was the only time he ever said that. I must have been 34 then, in 1969, and was just getting aboard the Mariner Mars 1969 missions – Mariners 6 and 7 – so he must have known that I had been on Mariner Venus which was in 1962. And I’d moved on to Mariner 69, under Bud Schurmeier, who later became the Voyager Project Manager, so I guess my father must have felt he had enough information at that point to say he was proud of me. But he only said it this one time.”

RS: You started at JPL when you were 24. How did you come to work there?

KOHLHASE: “At first, when I graduated from Georgia Tech I had a commitment to put in time with the Navy. I was a Naval Reserve Officer Training Candidate (NROTC) student at Georgia Tech. When you went through college, by taking courses through the navy, you got your degree from college while also receiving a commission. I became an Ensign in the Navy and reported to duty on an aircraft carrier called the U.S.S. Essex with the 6th fleet in the Atlantic. It’s funny, the way I ended up there.

“I was playing golf one day and the lady I had been assigned to play with was special assistant to a Congressman, which I didn’t know at the time. During the round she asked me what I wanted to do after I graduated. I told her I had to put two years into the Navy and said I’d like any assignment related to physics, so she got me an assignment that first sent me to nuclear weapons school at Albuquerque, New Mexico. I was then assigned to an aircraft carrier because it had the ability to carry nuclear weapons. And, further, the Congressman had me assigned to a carrier operating out of the Atlantic, because I was vulnerable to sea-sickness and the Pacific was much rougher! So I put in my two years with the navy, starting in June 1957, first on the Essex, then the carrier U.S.S. Independence. However, after that, I knew I didn’t want to make the Navy a career.

“So, I started looking around and applying for jobs. I remember that the largest salary offer was from General Dynamics Convair, in Fort Worth, Texas, to do military work. It was $750 a month! I’d also applied to JPL. Their offer was only $610 a month, but JPL excited me because I knew they were going to get into space exploration. And the Chamber of Commerce sent me these beautiful pictures of California with orange trees, blue skies, no pollution! So, I had a telephone interview with Vic Clarke, the supervisor of the lunar and planetary trajectory group at JPL. I had to talk to him from a broken, battered pay-phone in the Brooklyn Naval Shipyard. I didn’t know it at the time, but Vic was the kind of person who never said very much, so most of the interview was me trying to think of something to say and him just grunting at the other end of the line. So, I thought I had failed the interview terribly, but not long after that I got an offer to come and work at JPL in May 1959 when I finished with the Navy.”

RS: How long had JPL been operating at that stage?

KOHLHASE: “JPL had been established in 1936. It had done early work for the Army, above all the Corporal and Sergeant missile programmes, which had started in the mid-1940s, as well as other classified rocket projects. NASA as the civilian space agency wasn’t created until 1958, the year before I started at JPL, so for me the timing was just right, at the dawn of the space age.”

RS: These must have been very exciting times?

KOHLHASE: “They were! It was a unique experience, getting in on the early stages. We lived and breathed each mission, often at the expense of “getting a life,” routinely rechecking our work to ensure success. The early JPL folks didn’t watch clocks or work from 8am to 4:30pm, then race home. We rarely took coffee breaks, and usually talked “shop” during abbreviated lunch breaks. We pondered and worked as needed to get the job done. We were self-motivated and highly responsible.

“Most of all, I think we were a breed apart. We were dreamers and always excited about what we were doing. The years flew by. We were having fun.”

RS: What was your first role when you began at JPL?

KOHLHASE: “I was only a lowly employee with the title of associate engineer then in Vic Clarke’s group, but it gave me the first opportunity I had to do something meaningful. It was interesting. I had the theory – I knew math and physics, and now I could apply it. I’d go to meetings where someone would say ‘We want a trajectory from the Earth to the Moon that gets there efficiently and doesn’t exceed the capability of our launch vehicle,’ so I could apply the academic principles which I’d learned to do that job.”

RS: I understand that you were thrown in at the deep end at one early morning meeting when you encountered two legends of spaceflight at the time.

KOHLHASE: “Yes, it was early 1960 and I’d been at JPL for about eight months, when my supervisor asked me to design a trajectory to go from Earth to Mars at the next opportunity in the fall of 1962. He wanted to know the most favourable launch and arrival dates, and asked me to describe what Mars would look like if we planned a safe fly-by on the generally sunlit side of the red planet. By “safe,” he meant not too close to risk impact, given our navigation uncertainties.

“So, I walked into this small conference room in 1960, mentally rehearsed what I planned to say and draw on the blackboard concerning a Hohmann transfer ellipse to Mars, then waited for the remainder of the dozen or so invitees to arrive. Not more than three minutes passed before the door opened and in walked Dr. William Pickering (Director of JPL) and Dr. Wernher von Braun. They sat down next to me, Pickering on my left, von Braun on my right. Surrounded by such Titans, I would surely have had a heart attack at that moment were it not for my young age of 24. That is all that saved me from a sudden and early death!

“We waited a few minutes longer before my supervisor called the meeting to order and introduced me. I struggled on wobbly legs to reach the blackboard which, by now, was blurry and indistinct. I can still remember raising my hand to draw the Sun surrounded by the relevant orbits of Earth and Mars. I can usually make pretty good free hand circles and ellipses, but I suspect these were erratic. Pickering and von Braun followed with serious faces, eyes and ears trained on my modest delivery. I rambled on for some twenty minutes, a few questions were asked, and the two super-brains finally excused themselves. I have no idea how I came across, but I will never forget the experience.”

RS: Before the Mars missions came about, you were part of the team designing missions aimed at going to the Moon.

The Ranger Manoeuvre Model, designed by Kohlhase and Curkendall, used to check mid-course corrections on early unmanned flights to the Moon. Credit: Charles Kohlhase
The Ranger Manoeuvre Model, designed by Kohlhase and Curkendall, used to check mid-course corrections on early unmanned flights to the Moon.
Credit: Charles Kohlhase

KOHLHASE: “Yes. There were nine Ranger missions to the Moon (between August 1961 and March 1965). Several of the early Rangers failed and the Lab was greatly criticised for that. (Note: Congress launched an investigation into “problems of management” at NASA-JPL at the time) Although many of those were mistakes by the launch vehicle contractor, and the later Rangers were generally successful, it was felt that the Lab had too academic an attitude, and that its people weren’t disciplined enough to make sure that mistakes weren’t made. Soon this resulted in Deputy Directors coming in from the military – retired generals or admirals brought in to shape us up and give us more rigour, and to get us away from a perceived ‘loose’ Caltech-JPL attitude.

“In that regard, a young engineer was doing the manoeuvre analysis for Ranger. This involved tracking the Ranger spacecraft after launch to determine its orbit and, if it was not on target for the Moon, making a mid-course correction by rotating the spacecraft and firing its engines to correct the trajectory. The young engineer gave the spacecraft team a series of turns and a burn to execute. However, these instructions took Ranger further from the Moon rather than closer because there had not been a firm agreement on the sign convention (minus or plus) to use in the calculations between the navigation team and the spacecraft team. It hadn’t been carefully defined. That shook up the people at the Lab and they didn’t ever want that to happen again.

“For that reason, they asked me and Dave Curkendall, who has since passed away, to design an analogue device that could check all the Ranger manoeuvres from then on and make sure they didn’t turn in the wrong direction. It’s a funny-looking thing, with a little spacecraft at the top (see picture). We would take all the Ranger orbit determination estimates from then on and actually rotate our device, like a slide-rule, to check them with the computer programme to see if they agreed. That made everybody feel better, and that was probably my main contribution to Ranger, making sure we never made another mid-course correction error.”

RS: You mention the computer programme. What was the computer capability back then in the early 60s? Could you pre-programme much of what happened on board the spacecraft?

KOHLHASE: (Laughs). “Well, even much later on, in the Voyager spacecraft, the onboard computers have a total memory of only 8 KB. (Note: To put that in perspective, a cell-phone today has more than 100,000 times as much memory as a Voyager spacecraft!). For Ranger you could programme its simple ‘computer’ so that at a certain time in the future the spacecraft would turn or change course, you could turn the camera on, simple commands like that, but it wasn’t nearly as sophisticated as Voyager. It didn’t have the ability to do onboard fault protection and to correct itself, and so you had to know pretty well in advance what you wanted to do as a function of time.”

RS: Computing power was only one of the challenges you faced working in this totally new field of robotic planetary exploration.

KOHLHASE: “There were many new challenges. But the secret was that once I did a job on any particular programme, whether it was Ranger, Mariner, looking at trajectories, navigation or targeting, as soon as I’d developed that particular job skill, and the supervisor said ‘That’s right, that’s what we want’, then I added a new tool to my experience kit. What helped me the most was that I had the ability to simplify almost anything. No matter how complicated a problem was, I could simplify it to two or three essential parameters. Then I could use those “rules of thumb” to bootstrap myself into each new challenge.

“It’s basically about developing what we call an ‘expert system’. A good example can be seen with the big oil companies at that time. They had the resources to know if they got a drill stuck when out in the field, whether they could free it without breaking it, whether to press ahead or not. They had teams of expert consultants that would advise them on what to do if certain situations arose. The small companies, though, couldn’t afford the pricey consultants, so someone had the idea of interviewing them all, then creating a computer programme based on their answers called ‘PROSPECTOR’. It consisted of statements such as ‘If such and such happens, then do this’. It was what we called an ‘If…then…else’ programme. They discovered that the expert consultants operated on no more than about 200 to 300 rules of thumb.

“Then, as this principle was taken into other areas it was found that the same was true for brain surgeons or rocket scientists. The human mind can only deal with a certain number of principles, between 200 and 300. I would say that I developed that capability, that I knew what those principles were for rocket science and space mission design. So, with the ability to simplify, I was sort of like an ‘if … then … else’ computer programme, which helped a lot.”

RS: It must have been much more difficult when you moved from Ranger onto the Mariner Venus programme in 1962, aiming for another planet, rather than the Moon close by?

KOHLHASE: “Well not really, because the principles carry across, whatever the target. Venus had a different mass and the approach speeds were different, but it was easy to translate the targeting concept from the Moon to a planet, taking into account gravity, celestial mechanics, Kepler’s laws and so forth. Also, as the technology improved and we gained more experience, we felt more comfortable in taking on missions of greater complexity. Although we had fewer people to do the testing in those days, it has always been my nature and that of most of my colleagues to check everything so carefully that nothing could possibly be overlooked, so most of the missions thereafter were successful, say from the mid-1960s, when my own role carried more responsibility, through to the successful Cassini launch and Mars Pathfinder missions in 1997.

“However, we really wanted the Mariner-Venus mission to succeed at JPL, after the Ranger failures. Mariner 1 failed at launch, but Mariner 2 was the first successful planetary mission and it passed Venus more or less where we intended it to (within 22,000 miles of the planet). That was my first planetary mission, although I did yet not have a management role on that.”

Mariner 7, following Mariner 6’s flyby on July 31, had its closest approach at a distance of 3,524 kilometers, in what was the first dual mission to a planet. By chance, both craft flew over cratered regions and missed both the giant northern volcanoes and the equatorial grand canyon that were discovered by Mariner 9. Their approach pictures did, however, show that the dark surface features long seen from Earth were not canals, as once thought in the 1800s.
Mariner 7, following Mariner 6’s flyby on July 31, had its closest approach at a distance of 3,524 kilometers, in what was the first dual mission to a planet. By chance, both craft flew over cratered regions and missed both the giant northern volcanoes and the equatorial grand canyon that were discovered by Mariner 9. Their approach pictures did, however, show that the dark surface features long seen from Earth were not canals, as once thought in the 1800s.

RS: Was the mood of excitement comparable with, say, that of the recent landing of Curiosity on Mars?

KOHLHASE: “We were excited that the spacecraft worked long enough to achieve the first planetary fly-by, but there were so many fewer people then following the mission. The press did turn up, but not in big numbers. It took time to establish confidence in the fact that we could successfully capture scientific results from far-away planets.”

RS: And Mariner 2 did deliver good scientific observations from both Venus and the interplanetary environment during the cruise.

KOHLHASE: “It did and from then, as we began to succeed and build up this confidence in people with each follow-on mission, the press began to take more interest, especially on Mariner 6 and 7, the first dual mission to Mars in 1969. First, though, after the Venus mission, Mariner 4 went to Mars in 1964 and was successful. Mariner 3 had failed during the launch sequence when the protective shield over the spacecraft failed to open, but Mariner 4, its twin, made it all the way to Mars and was successful, capturing the first images of another planet. (Note: Mariner 4 flew past Mars at a distance of 9,846 km on 15th July 1965, returning 22 images and detailed information on its atmosphere).

“For me also there was a steady progression and I was able to bootstrap myself up through my navigation work on Ranger, then Mariner-Venus, Mars in 1964, and then Mariners 6 and 7 in 1969. At this point I was beginning to have more to do with things, to really be in control over the aiming points for the trajectories, the navigation, and the mission-spacecraft interactions, writing the reports and putting the requirements on the launch vehicles.”

RS: From the science point of view, what were the reactions to the first images sent back from Mars in 1964, which showed a cratered world?

KOHLHASE: “Well, firstly Mariner 4, then even Mariners 6 and 7, seemed to show Mars as a cratered, somewhat uninteresting place. Mariner 7’s instruments did image the edge of the southern polar cap, showing frost or ice, but Mars did not emerge as a really interesting world until the first successful orbiter, Mariner-71 (Mariner 9), which arrived in November 1971. We had to ride out a dust storm for a couple of months, but then the complexity of the surface and its atmosphere were revealed. The Soviets were there just after us but their lander missions (Mars 2 and 3) failed.

“Now, I myself didn’t work on Mariner-71. I jumped to Viking after the Mariner-69 missions.

“I was the Navigation Development Team Leader for Viking, but now also got more into the overall mission design, so I was now writing mission requirements and documents on all the systems. In fact, I co-authored a high-level document referred to as the “Mrs. D”, or MRSD, which stood for Mission Requirements on System Design”.

RS: When was the decision made to mount the Viking mission and how did it come about? Did what Mariners had shown you have a bearing on this?

Mariner Mars ’69 flyby model used in displaying the Mariner 6 and 7 encounters with the red planet. Credit: Charles Kohlhase
Mariner Mars ’69 flyby model used in displaying the Mariner 6 and 7 encounters with the red planet. Credit: Charles Kohlhase

KOHLHASE: “There was a desire to go to Mars on a project initially called Voyager, believe it or not. It was planned between 1966 and 1968 as part of the Apollo Applications Program, and was scheduled for launch in 1974–75. It was very risky, launching two orbiter-lander payloads piggybacked on a huge Saturn C5. We were going to use the biggest launch vehicle in the US and put two complete orbiter-lander missions on a single launch. They were firstly conceived as precursors for a manned landing on Mars in the 1980s. Then NASA realised that we would have all our eggs in one basket, which would be a mistake, so that was cancelled.  And Viking arose in its place in 1968, led by Project Manager James Martin, former assistant manager for the Lunar Orbiter Project. It separated the missions on different launch vehicles and not stacked on top of each other.

“Incidentally, later, when I got onto what would become Voyager and we were trying to pick a better name than Mariner Jupiter-Saturn 1977, the name ‘Voyager’ was on a list of names along with ‘Pilgrim’, ‘Nomad’ and others. Because there had already been a programme called Voyager which had been cancelled, several people said it would be bad luck to use the Voyager name for this new mission. Then we said, ‘Wait a minute, we’re scientists, we’re not superstitious,’ and we liked the name so much we went with it.

“But let’s get back to Viking. It was very interesting work. It was an expensive, challenging mission which was run out of Langley Research Centre. Martin Marietta of Denver was building the lander and JPL was building the orbiter.

“Now, I remember having a hard decision to make in 1974. I could stay with Viking and become the Navigation Team Leader for operations, or I could get on this great Voyager mission, where the planets only line up every 176 years. This would give me the chance to work with Bud Schurmeier again, with whom I’d worked on Mars ‘69 (Mariners 6 and 7). I didn’t think I’d get the job, however, as several other people wanted it, but I applied for it. I’ll never know why Bud finally chose me, but something had happened when I was on Viking which may have played a role in his decision.

“There was a lot of competition on Viking. The Langley people were running the program. We at JPL were the deep-space navigators, but the people at Martin Marietta in Denver wanted to take the navigation away from us.  And the Johnson Space Centre also wanted to get this plum role. Now, we managed to hang on to it with a big effort from myself, Lou Kingsland, and the experienced navigators at JPL.  But during that time Gentry Lee was playing a lead role on Viking at Martin Marietta, and they invited me to take a job with them and to take over Gentry’s department (Note: Lee worked in a number of positions on the Viking project from 1968-76, and was Director of Science Analysis and Mission Planning during the Viking operations). I was happy at JPL and not looking to change jobs, but I got a call from them saying they really wanted me for this position. So I took a Friday off – an unpaid vacation day from the Lab – to fly to Denver to be interviewed by some high-up managers. Now the morning interviews were all great; everyone treated me like a little god, ‘We all want you…’ and so on. Then we had lunch in the Executive Dining Room, and Gentry took me aside after lunch and said to me ‘Listen. This afternoon, for the rest of the interview, they’re all going to seem like they’re not interested. Things have changed. It’s not you, but we got a call from Pickering, the Director of JPL, telling us to lay off.’ What I didn’t know was that Martin Marietta had stolen two of JPL’s key people, and I just happened to be the third case that set Pickering into action. Pickering was so respected that, when he called and said ‘Lay off Kohlhase,’ I never got an offer from them. Afterwards, my section manager said to me ‘We know we shouldn’t have done that. You had your free right to go and take a job anywhere you wanted, but since you have chosen to stay with us, we won’t forget this.’

“Now, whether all that gave me a leg-up later, whether they felt they owed me something, or whether I just earned it on my own merits, I will never know, but Schurmeier gave me the job over the other candidates. So, I left Viking in late 1974, even though it was an actively-funded project, and moved to Voyager and I loved every minute of it! I did worry at first about whether I was capable enough to do the job, and I had some sleepless nights. But I had developed enough of the fundamentals of mission design, navigation and spacecraft performance to handle it.”

A boulder-strewn field of red rocks stretches across the horizon in this self-portrait of Viking 2 on Mars’ Utopian Plain. Credit: NASA/JPL
A boulder-strewn field of red rocks stretches across the horizon in this self-portrait of Viking 2 on Mars’ Utopian Plain. Credit: NASA/JPL

RS: At that stage, you were a bit like Gene Kranz and others working for Chris Kraft on the early manned missions, writing the rule books and procedures as the missions evolved.

KOHLHASE: “Yes, with Schurmeier looking over my shoulder initially, but he began to trust me and soon I was off on my own, generating the requirements and procedures on the project and mission systems for Ed Stone (Voyager Project Scientist) and the science team from then on.”

RS: For how long were you involved with Voyager?

KOHLHASE: “Most of my intense work was from 1974 until launch in 1977, designing all the options for the missions. For example, Jupiter-Saturn-Titan, then if we didn’t get Titan first time with Voyager 1, how to plan for Voyager 2 to repeat it, and so forth.”

RS: You had to balance the interests of engineers and scientists. That must have been a difficult job, trying to satisfy the requirements of both groups.

KOHLHASE: “There were certain engineering constraints we felt pretty strongly about. We could lay those out in the launch date-arrival date space and make sure we avoided, say, flying behind one of the planets and looking back at the Sun if the Earth was in the same direction. There were many things to consider.

“I think the most interesting issue we had to deal with was the sheer number of trajectory possibilities. There were some 10,000 of these, taking into account the orbital periods of Jupiter and Saturn and those of their dozens of satellites. The trick was to find the best encounter dates at each planet that gave you the largest number of close encounters with bodies of interest. We also had three different launch opportunities in 1976, ‘77 and ’78. 1977 was the ‘Goldilocks’ opportunity. In ’76, since Jupiter is moving faster around the Sun than Saturn, it ‘trails’ Saturn, so to speak. You can still do a gravity assist on the trailing side of Jupiter, but you have to fly very close to get enough deflection to get to Saturn, which is further ahead. In ’77 the arrangement was perfect for flying through the region of the Galilean satellites. In ’78, Jupiter being further ahead, you don’t want too much bending, so you have to fly further away from it and beyond those satellites to reduce the amount of deflection.

Kohlhase meets legendary astronomer, Clyde Tombaugh, discoverer of Pluto, at the Voyager Neptune encounter in August 1989. Credit: Charles Kohlhase
Kohlhase meets legendary astronomer, Clyde Tombaugh, discoverer of Pluto, at the Voyager Neptune encounter in August 1989. Credit: Charles Kohlhase

“Then we wanted to fly by Io, which orbits Jupiter every 1.7 days, so we started dividing the encounter dates at Jupiter into 1.7-day increments. Titan goes around Saturn every 16 days, and we divided up the encounter at Saturn in this way too, but we also wanted to encounter Titan before we crossed the ring plane, rather than afterwards – we called these ‘Titan-before’ encounters. So, basically, my job was to go through the thousands of opportunities over the launch date-arrival date space and choose the best. In the olden days we used to have to integrate the equations of motion in small steps to run trajectories, but we just could not afford the time needed to integrate thousands of trajectories. However, we knew we could use “patched conics” – ellipses, parabolas, hyperbolas and so forth – to approximate the integrated trajectories, so we basically developed software that could approximate and run hundreds of cases overnight on a computer. We picked those which satisfied all engineering constraints, then asked the science teams which ones best suited their needs, for example close flybys of certain satellites.

“Finally, through this process, out of the 10,000 or so possible trajectories, we targeted 110 with the Titan-Centaur launch vehicle, and finally launched on two of them. By the time we had launched both Voyagers we had pulled it off, coming in within our development phase budget, about $387 million, as we’d promised NASA we would do.” (Note: This is out of a total mission cost of $865 million up to the end of the Neptune encounter in 1989).

RS: There was a lot more press coverage of Voyager than the early missions you worked on.

KOHLHASE: “Voyager 2 launched first, on August 20th, and this confused the press for a while. (laughs). We said they’d just have to deal with this for a few months until Voyager 1 overtook Voyager 2 in December of 1977!

“By the second Saturn encounter of Voyager there were huge numbers of press at JPL, perhaps 200 or so. I remember also, Clyde Tombaugh came to JPL for that encounter. I used to have sideburns then and I wanted to be in a picture with Clyde Tombaugh. I got his autograph.”

RS: I remember that coverage of the missions was also strong in the UK, both in the press and through TV documentaries. The public perception of the Voyager encounters was also enhanced by your novel computer animations which visualised how they would unfold. This was ground-breaking digital work at that time.

KOHLHASE: “Yes, Jim Blinn and I created the first computer-animated flybys for each of the six Voyager encounters. Basically, he created the software and I used it to create the accurate flyby movies as might be seen by a viewer near the spacecraft as it passed through each planetary system.

“Now, as we released the animations before the flybys, we didn’t know exactly what the moons looked like close-up, so well-known space artists Don Davis and Rick Sternbach helped us to visualise and render the surfaces of these, as yet, unknown worlds. Then, after the Voyager 1 encounters, when we had real photos of the moons, we substituted these into the animations.”

Image from the computer animation of the Voyager Saturn encounter, created by Blinn and Kohlhase, which was so important for public outreach for the missions. Credit: Charles Kohlhase
Image from the computer animation of the Voyager Saturn encounter, created by Blinn and Kohlhase, which was so important for public outreach for the missions. Credit: Charles Kohlhase

RS: Yes, those first images sent back by Voyager. The two spacecrafts’ cameras returned so many stunning and iconic photographs of these beautiful and strange worlds. Do you have a favourite image?

KOHLHASE: “It’s impossible to pick one favourite, but one image has a special meaning for me. It’s one taken by Voyager 1 several days out from Jupiter, through its narrow-angle camera, and which shows Io and Europa in front of the Jovian disk. We were beginning to see detail on the surface of these Galilean satellites and they were beginning to look like their own fascinating worlds – we were seeing new worlds up close for the first time in human history.” (a selection of Charley’s favourite Voyager images can be foundin Paul Hoversten’s ‘Air & Space Magazine’ article – see references).

RS: How did your role on Voyager change after launch?

KOHLHASE: “In theory, and I’m always grateful to Casani for this (Note: John Casani, Voyager Project Manager, Prelaunch Phase), once the Voyagers were launched I probably could have and should have gone to another project, but John said ‘We’d like you to stay aboard, not as the Mission Analysis and Engineering Manager’, which is what I’d been for nearly 4 years, ‘but as the Mission Planning Manager to decide how much we could afford to do at each planet’. Here we have these two spacecraft with limited memory capacity. To programme them to do scientific observations is going to require manpower. There are constraints – we don’t want to put demands on the Deep Space Network which they can’t meet, we want to make sure we’re not trying to send commands during occultations, and all those sorts of things. He said, ‘And further, we want you to limit the size of the computer-controlled scientific observation sequences, so that the sequence team has the time to build and carefully check each sequence load.’ Now, during the mission you send a load up and the spacecraft executes for several days, then you send another load up, and so on. You have to build those in advance, which takes manpower. We had said we’d do the job for a certain cost, and the sequencing team that had to translate all these commands into instructions for the computer needed time to do that. So what my office produced was mission planning guidelines and constraints for the various cruise and encounter mission phases.

Voyager 1 took this photo of Jupiter and two of its satellites (Io, left, and Europa) on Feb. 13, 1979. Io is about 350,000 kilometers (220,000 miles) above Jupiter’s Great Red Spot; Europa is about 600,000 kilometers (375,000 miles) above Jupiter’s clouds. Credit: NASA/JPL
Voyager 1 took this photo of Jupiter and two of its satellites (Io, left, and Europa) on Feb. 13, 1979. Io is about 350,000 kilometers (220,000 miles) above Jupiter’s Great Red Spot; Europa is about 600,000 kilometers (375,000 miles) above Jupiter’s clouds. Credit: NASA/JPL

“For example: the encounter period for Jupiter was going to start at minus 80 days and end at plus 30 days. It’s going to be divided into three basic phases, such as far encounter, near encounter, and post encounter. Each phase will have a defined duration and prescribed number of sequence loads; this is the number of activities we’ll have; here’s how much we can afford to sequence. In this way, we basically took each encounter and produced the first thing that came out, the mission planning guidelines and constraints that would be planned for and executed by the sequence team and the spacecraft team with the assurance that they would not place demands on the other teams that they could not meet, but, at the same time, assuring that the spacecraft would be active enough to meet the scientists’ desires.

“So, once we launched in ‘77 my job as Mission Planning Office Manager was balancing all this. I had a small office and group of people, but all well-chosen. I had a chance at various times to get off the project. I could have left after Saturn, but it was too exciting, the spacecraft were working, we were making constant and amazing discoveries, and we were going to Uranus and Neptune!”

RS: How much time were you putting into Voyager then?

KOHLHASE: “In the most active time, during the encounters, I was spending about 50 hours a week on the project. I did drop to half time on Voyager between the Uranus (1986) and Neptune (1989) encounters, and started work half time on Cassini, but didn’t leave the mission until after Neptune.”

RS: After the success of the previous three encounters, there were concerns in 1988-89 about the final encounter, and about how close Voyager 2 needed to get to Neptune. You wanted to do two occultations behind the planet (note: both of the Earth and the Sun) and also plunge down to reach Triton. This was your closest approach to a giant planet and there were worries that you might lose attitude in the atmosphere. How was the encounter planned?

KOHLHASE: “The timing had to be very good. When you’re deciding where to aim for a gravity assist you use a B-plane, which is like a picture you can draw to show different aiming points at the target, such that if you pass through one of these, then you will later fly by Triton, or go through an occultation zone, or any other chosen point beyond the planet. (Note: The B-plane is a planar coordinate system used for targeting during a gravity assist. It can be thought of as a target-plane attached to the assisting body, and is perpendicular to the line of the approaching trajectory). In the delivery of Voyager 2 to this targeting plane relative to Neptune, we only missed the two B-plane coordinates by about 6.5 kilometres. And the timing accuracy was better than one second. Now, as you passed over the pole of Neptune, there were some amplifications of those errors, so the difference was greater when we got to Triton, but the accuracy was still phenomenal.

During August 16 and 17, 1989, the Voyager 2 narrow-angle camera was used to photograph Neptune almost continuously, recording approximately two and one-half rotations of the planet. Credit: NASA/JPL
During August 16 and 17, 1989, the Voyager 2 narrow-angle camera was used to photograph Neptune almost continuously, recording approximately two and one-half rotations of the planet. Credit: NASA/JPL

“Now, Neptune is 30 AU (astronomical units) from the Sun, and one AU (= one mean Earth-Sun distance) is 150 million kilometres, so that’s about 4.5 billion kilometres – let’s round that off to 5 billion. So we had an error of about 5 kilometres out of 5 billion – that’s one part in a billion! If you take the radius of a cup on a golfing green as a couple of inches, then that’s the equivalent of sinking a golf putt from a distance of over 30,000 miles! And the target is moving.”

RS: You talked earlier about having a few sleepless nights. Was the need for this degree of accuracy a cause for concern?

KOHLHASE: “There was one thing that worried me a great deal on Voyager which would awake me in the middle of the night. I was responsible for the navigation and I would calculate analytically, when we flew through the Jupiter system and experienced the gravity assist and deflection of the trajectory by roughly 90 degrees, how much it would cost us on the outbound leg leaving Jupiter to correct any navigation errors made when inbound, because these errors were going to be amplified by Jupiter. But, more than that, if we flew by one of the large Galilean satellites, the dispersions would be even more increased and we only had a limited amount of propellant onboard to make corrections. Well, I did the analysis and the results showed that it wasn’t going to be that bad – only 10 or 15 m/s – and I thought ‘That can’t be right’ and kept worrying about it. But it turned out OK after all, and the rest is history.”

RS: And then you moved onto another historic mission, Cassini.

KOHLHASE: “Yes, then I went to a large Saturn orbiter, and an international cooperative venture with the Europeans carrying their Huygens probe to Titan. I had the best job anybody could ever have.”

RS: What was the situation at that early stage in planning Cassini?

KOHLHASE: “When I first joined Cassini, it wasn’t yet called that, but Mariner Mark II. It was initially designed as a pair of missions, one an orbiter mission to Saturn with a Titan probe, the other CRAF – Comet Rendezvous Asteroid Flyby. The comet mission was subsequently cancelled in 1992 because of reduced funding. (Note: These were part of a planned JPL programme to explore the outer Solar System between 1990 and 2010, designed to be more affordable than the so-called multi-billion dollar ‘flagship’ missions of the 1970s. Mariner Mark II was replaced by the very successful lower-cost Discovery Programme ). I had people working on the comet mission who were heartbroken and had to leave and find other jobs.

“In fact, NASA Administrator Dan Goldin threatened to also cancel Cassini in ’94. He was big on ‘better, faster, cheaper’ and Cassini as a Flagship Mission would cost NASA over $2 billion. But when word got out about this, the Director General of ESA, Jean-Marie Luton, wrote a special letter of appeal that went to Vice-President Gore. It more or less said ‘We are partners with NASA on this project. We want you to know that if you cancel Cassini, don’t look for any further international cooperation.’ That’s a classic letter that I’ve saved in a file. Our relationship with the Europeans would have been damaged. In my mind that letter saved Cassini and I stayed with the program from that point.”

RS: And since then, Cassini, like Voyager, has been a major achievement in solar system exploration.

KOHLHASE: “It has been an enormous success, orbiting Saturn for the last ten years, studying the planet and its rings, finding the hydrocarbon lakes on Titan, the geysers on Enceladus, the intricate detail in the Saturn rings, and variety among the smaller icy satellites. A remarkable success.”

In the Payload Hazardous Servicing Facility at the Kennedy Space Centre Kohlhase (left) and Cassini Programme Manager, Richard Spehalski, hold the DVD containing 616,400 digitized signatures of people from 81 countries which was sent to Saturn on board Cassini, seen behind them. Charley designed the disk with the flags of 28 nations and symbolic golden eagle wing feathers. Credit: NASA/JPL
In the Payload Hazardous Servicing Facility at the Kennedy Space Centre Kohlhase (left) and Cassini Programme Manager, Richard Spehalski, hold the DVD containing 616,400 digitized signatures of people from 81 countries which was sent to Saturn on board Cassini, seen behind them. Charley designed the disk with the flags of 28 nations and symbolic golden eagle wing feathers. Credit: NASA/JPL

RS: You were Science and Mission Design Manager for this very complex mission. How did you and your team plan for the orbital part of the mission after it arrived at Saturn?

KOHLHASE: “At the time of launch (on October 15th 1997) the way Cassini worked was that, after gravity-assist flybys of Venus, Earth and Jupiter, we planned to go into orbit around Saturn then make an initial tour based on 45 flybys of Titan. As the largest moon, Titan has enough gravity so that, if you fly by at pretty close range, it can reshape the trajectory of the spacecraft. Every flyby designed before launch was done for a purpose. We’d be raising the inclination of the orbit, or dropping down, altering the orbit to look at Saturn, its rings or icy satellites, but the whole time Titan was the shaper, using the laws of gravity assist. And of course we looked at Titan itself, every time we passed by. There were a few changes to this schedule after the launch based on scientific requests, but the concept remained the same. So, my team designed the complete first tour, which ran from July 2004 until July 2008.”

RS: Cassini obviously had a much bigger payload of experiments and instruments than the earlier missions you worked on. How much harder was the planning process for you and your team, given this increasing complexity of the spacecraft?

KOHLHASE: “It was more complicated because there were more interfaces. The instruments interfaced with the spacecraft and with each other, so if you turned on one instrument, you might not be able to turn on another because it used too much power. The complexity goes up, not linearly, but maybe as some power of the number of scientific instruments comprising the payload.

“However, no matter how complex it may be, there are four secrets to working in this job. You have to be well-educated. That is, you have to understand the physics, the math, the engineering, and of course the astronomy. You also need to be able to simplify, a skill not many people have. Out of a hundred people at JPL, maybe ten will have that ability, and those tend to be the core group that run each project. So, if you have a problem you have to be able to see your way through to the two or three elements which really matter. You do that by using first principles that you’ve learned over time. You must also have absolute integrity. You must never hide a problem. You share it immediately with the team and you solve it. And finally, you must have a sense of humour! You might have a terrible problem, but you don’t just wring your hands, it’s good to laugh about it to break the ice, then get on with and solve it.

“So – education, ability to simplify, integrity and humour – those are the four secrets to success.”

RS: Talking of success, you have had a hand in many missions to the Moon and other planets during your time at JPL, and they’ve all been successful. That’s quite a record!

KOHLHASE: “Let’s see – Ranger, Mariners 2, 4, 6 and 7, Viking, Voyager, Cassini, and some of the recent Mars missions (Mars Exploration Rovers) – the order of 10 missions. As for success, one of the Ranger missions might have been marginal.”

RS: Have you played a role in the Equinox or Solstice Mission extensions to the Cassini programme since 2008?

KOHLHASE: “No, I came off the programme soon after launch. I retired then, although I was pretty young – 62. I was a workaholic and I’d built up something like 4 or 5 months of paid vacation, so I arranged it so that I could take some of the vacation and officially retire in May 1998.”

RS: You didn’t really retire, though, did you?

KOHLHASE: “I’d been retired a few months when I got a call from somebody at the Lab asking me if I would come back and help them by doing some consulting. I said ‘No, I think I really want to stay retired.’ Well, I had a bad taste in my mouth from a disappointing experience. What you don’t know is that John Casani had recommended that I get a NASA Distinguished Service Medal for my 40 years of contributions at JPL. However, a deputy director by the name of Larry Dumas turned it down, denying me something I had earned over 40 years, using the argument that it was only given to Project Managers or higher. Nowhere in the NASA qualifications for this medal was there a caveat like that, there was no condition that said you had to be at a certain level or higher. The best janitor in the NASA service could get the Distinguished Service Medal if he or she had earned it by distinguished service. So, I’d had this wonderful career, I loved JPL, but after this distasteful experience I said ‘No’ and Gentry Lee got the job I’d been offered instead.

“Then another year passed and Frank Jordan at JPL (Manager of JPL’s Mars Program Planning and Architecture Office) said ‘Just come back and help us a little’, so I came back and started consulting part-time, even though I still wasn’t happy about the medal incident. While working there a couple of days a week I ran into Tom Gavin, a friend from Voyager and Cassini days – he’d been Spacecraft System Manager for Cassini – who asked why I hadn’t come back the previous year and I told him about Larry Dumas and the medal. Tom hadn’t heard about this. He thought it was very unfair and went to the existing Director of JPL, Charles Elachi, and said ‘Let’s resubmit the medal recommendation.’ It was done and NASA approved it, so I ultimately did receive the Distinguished Service Medal, not in 1997, when it was first proposed, but in May 2003, 6 years later. I was very fortunate that Tom Gavin, a friend of mine, had risen to a place on Elachi’s Executive Committee, which met regularly, and had Elachi’s ear.

“Anyway, I am grateful to Elachi for rectifying the injustice, so I continued to consult up until about 6 months ago. My latest work in 2013 was continuing to support the Mars Sample Return Campaign, as special senior advisor, by completing a major risk assessment study for the three missions comprising the campaign. And I’m sure I’ll get another invitation soon to go in and give an opinion on something, so I’m not quite dead … just fading away slowly!” (Laughs)

Charley’s analogy of the gravity assist flyby of Voyager as it passed Jupiter. Credit: Charles Kohlhase with artwork by Gary Hovland.
Charley’s analogy of the gravity assist flyby of Voyager as it passed Jupiter. Credit: Charles Kohlhase with artwork by Gary Hovland.

RS: Fifty-five years and still at the cutting edge of planetary exploration – that’s a wonderful career, Charley!

KOHLHASE: “It is, but, you know today, the environment is what I really care about the most. There’s simply no greater problem.

“Humans are destroying the Earth. They’re overfishing the oceans, destroying habitat, clearing rainforests, polluting our seas and atmosphere. We have climate warming. All of these issues are really important. One day there will be no more polar bears or Bengal tigers or elephants. I do not understand why people do not elevate their concern for protecting the planet we live on to a level above everything else. Above space exploration, and certainly above defence. Did you know that the US annual defence budget is nearly $700 billion. (Note: It was $673 billion for 2013, nearly 18% of the total US Federal budget. NASA’s proposed budget for FY 2014 is $17.7 billion, or 0.5% of the total). For a one-time cost of $50 billion, a mere fraction of the US Defence budget for only one year, you could permanently protect the last of all the biodiversity hotspots on the Earth. A group called Conservation International has identified these unique areas. The fact that this isn’t being done is a travesty.

“I truly love the wilderness. I believe the world’s great wilderness areas must never be destroyed for whatever reason – not for SUV fuel, nor for ores to be mined, not for expensive vacation homes, not to solve overpopulation, not for trade advantages or increased market share, not for political compromise – not for any reason. We should all be stewards of the Earth, not its destroyers.”

RS: How can space exploration help us in this respect?

KOHLHASE: “We have a lot of Earth-orbiting satellites tracking the loss of these natural resources and climate changes, but I don’t see politicians using that data to make any difference. We have the information we need, but we don’t have anybody with the courage to use it. We are consuming natural resources at five times the rate at which the Earth can replace them. It would now take five Earths to support the population and economic activity of the Earth. Honestly, the human population of Earth needs to be under one billion.”

RS: Do you see people turning this situation around? Are you optimistic about the future of our global environment?

KOHLHASE: “I try to be, but I am constantly discouraged. As I get older, one of the few things that I can take some joy in is that I live in one of the last great eras for the Earth. For my grandchildren and their children, though, I think it’ll be a severely reduced Earth. I can still go to Alaska and see the grizzlies, I can still enjoy a lot of the beauty, but by the turn of the next century much of it will be gone. I don’t know what the Earth will be like at that point. We’ll no doubt still be trying to mass-produce stuff, only to throw it away a few months later, if all we want is new TV sets and transportation to the city, and if we don’t care about songbirds or whether there are any salmon still spawning in our rivers. But that’s not the world I want.

“I’m basically an optimistic person. But will we act in time to save the Earth? I think that we won’t.”

RS: Do you have any final advice for how each of us should live our lives? You once penned a 75-word piece on this, I believe.

KOHLHASE: “Rise early and seize each day. Learn much and use this knowledge well. Spend time with those you love. Never abuse your pets. Use logic to fight the irrational, for it is everywhere. Defend the environment and its wildlife. Meld mind and heart for greatest creativity. Follow your dreams, and become all that you can be.”

You can find out more about Charley’s diverse interests, his work, science, art, the environment and his world at

Other links about Charley’s work at JPL, his art and philosophy can be found at:

This article appeared in the 9th issue of RocketSTEM magazine.

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