The Challenge of the Planets, Part Three: Gravity

Image credit: NASA.
It is strange that Lexell's Comet is not better remembered. Discovered by ace comet-hunter Charles Messier on the night of 14 June 1770, it passed Earth just two weeks later at a distance of only 2.3 million kilometers, closer than any other comet in recorded history. On the evening of 1 July 1770, its nucleus shown as brightly as Jupiter at its brightest, and its silvery coma was five times larger than the full moon.

Lexell's Comet then drew close to the Sun — that is, it reached perihelion — and was lost in the glare. Messier saw it next in the pre-dawn sky on 4 August. Having moved away from Earth and the Sun, it had become small and faint. Messier observed the comet with difficulty before dawn on 3 October 1770, then lost sight of it.

Comets are today named for their discoverer or discoverers, but in the 18th century it was the mathematicians who computed their orbits who got all the credit. Comet Halley is, for example, named for Edmond Halley, who computed its orbit and determined that what had seemed like a series of individual comets was in fact a single comet that returned again and again. Partly this was because in Comet Halley's case no one knows who discovered it; records of the comet's apparitions extend back at least to 240 BCE, but it almost certainly was noticed in Earth's skies much earlier.

Lexell's Comet was named for Anders Johan Lexell, who determined that it completed one elliptical orbit around the Sun in 5.6 years. This was for the time a remarkably short period for a comet, raising questions as to why it had not been observed before. Lexell hypothesized that the comet had previously had a large orbit with a perihelion close to Jupiter's orbit, but then had passed Jupiter at a distance of about 3.2 million kilometers in 1767. The giant planet had, he wrote, slowed it and deposited it into its new short-period orbit.

Lexell's Comet was due to reach perihelion again in 1776, but this occurred on the far side of the Sun as viewed from Earth and so was not observed. Astronomers eagerly awaited its next perihelion in 1781 or 1782, but nothing was seen. Again, Lexell offered an explanation: in 1779, as it neared the point in its new orbit where it was farthest from the Sun — its aphelion — the comet had again intersected Jupiter. This time, it had sped up and entered an unknown but probably long-period orbit. It might even have escaped the Sun's gravitational grip entirely. In any case, Lexell's Comet has not been seen since and is officially designated "lost."

The light-show of 1 July 1770 should have ensured that no one forgot Lexell's Comet, but both its close pass by Earth and its orbit changes soon faded from memory. If they had not, then Michael Minovitch's mathematical research in 1961-1964 might not have shaken the interplanetary mission planning world the way it did.

Minovitch, in 1961 a 25-year-old graduate student at the University of California-Los Angeles (UCLA), began his research while working a summer job at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He calculated that a flyby spacecraft which passed behind a planet would be seized by the planet's gravity and in effect towed as the planet orbited the Sun. This would increase the spacecraft's speed. As it departed the planet's vicinity, it would keep the speed it gained. Conversely, a flyby spacecraft that passed ahead of a planet would be slowed. Minovitch viewed this as a new form of propulsion; he called the effect the planet's motion had on the spacecraft "gravity thrust."

Minovitch determined that a spacecraft could use gravity thrust flybys to travel from world to world indefinitely without use of rocket propulsion. It could even return to the vicinity of Earth, enter a close solar orbit, or escape the Solar System entirely. In all, he calculated about 200 different planetary-flyby sequences using charts he devised and computers at JPL and UCLA.

Many engineers who learned of Minovitch's results assumed at first that they violated fundamental physical law. It seemed that the flyby spacecraft would get something for nothing. This was, of course, incorrect: when the spacecraft was slowed, the planet gained a very tiny amount of momentum; when the spacecraft was accelerated, the planet lost a very tiny amount of momentum. Nature thus balanced its books. Minovitch, for his part, was not very skilled at first at explaining his discoveries; he seems to have understood the clean elegance of numbers far better than he did the fuzzy vagaries of human beings.

Nevertheless, he had his champions. The most important was Maxwell Hunter, who met Minovitch at the American Astronautical Society's Symposium on the Exploration of Mars (6-7 June 1963) and quickly recognized the significance of his work. Before joining the professional staff of the National Aeronautics and Space Council (NASC) in January 1962, Hunter had worked at Douglas Aircraft for 18 years. He ended his career there as Chief Engineer for Space Systems. As part of the NASC, he was well placed to promote Minovitch's discoveries; the advisory body, chaired by Vice President Lyndon Baines Johnson, provided advice directly to President John F. Kennedy.

Hunter described Minovitch's "unconventional trajectories" in a report to NASC Executive Secretary Edward Welsh in September 1963. The report became the basis for a prominent article in the May 1964 issue of the trade publication Astronautics & Aeronautics. Hunter permitted Minovitch to review a draft before the article went to publication.

In June 1964, a month after Hunter's article made the spaceflight world aware of Minovitch's labors, JPL began planning what became Mariner Venus/Mercury 1973, the first planetary mission to employ one of the trajectories Minovitch had calculated. The MVM '73 spacecraft would fly past Venus to slow down and enter a Sun-centered orbit that would take it past Mercury. The flight past Venus was labelled a "gravity-assist flyby" — Minovitch's "gravity thrust" moniker never caught on.

At nearly the same time, high-energy propulsion systems, which had been deemed essential for travel to worlds beyond Venus and Mars, rapidly began to lose support. As described in the previous post in this "Challenge of the Planets" series, the leader among these systems was electric (ion) propulsion.

In 1962, JPL engineers had prepared a preliminary design for an automated 10-ton nuclear-electric "space cruiser" and proudly presented it at a conference attended by about 500 other electric-propulsion engineers. It was received with great enthusiasm. The system was still early in its development, but the JPL engineers expected that, with sufficient funding, they might develop it for interplanetary spaceflight in the 1970s.

By late 1964, however, such brute-force high-energy systems were increasingly seen as needlessly complex and costly (at least as far as the preliminary reconnaissance of the Solar System was concerned). NASA could instead use a relatively small booster rocket to place on an interplanetary trajectory a package comprising a small chemical-propellant propulsion system for course corrections, star-trackers for precise spacecraft position and trajectory determination, a cold-gas thruster system for turning the spacecraft, science instruments, a computer, an electricity-generating isotopic system or solar arrays, and a radio. By 1962 standards, such a package hardly qualified as a spacecraft, yet it remains the basic form of our proudest interplanetary flyby and orbiter spacecraft to this day.

Electric-propulsion supporters were loathe to give up their labors. In addition to developing small station-keeping electric-propulsion systems for Earth-orbiting satellites, they sought planetary exploration niches where electric propulsion could outshine gravity-assist trajectories.

Ironically, given the adventures of Lexell's Comet, the most significant niche they identified was comet rendezvous. Before the end of the 1960s, the 1985-1986 Comet Halley apparition became a particularly important target for electric-propulsion supporters. Their efforts to explore Comet Halley using electric propulsion will be described in forthcoming posts.

In the years that followed Mariner 10 (as MVM '73 came to be known), more of Minovitch's gravity-assist trajectories were put to use. Though often mistakenly attributed to JPL's Gary Flandro, among Minovitch's trajectories was the Jupiter-Saturn-Uranus-Neptune path of Voyager 2. (Flandro's oft-cited "grand tour" paper saw print in mid-1966, nearly five years after Minovitch began his research; in it Flandro gave credit where it was due by citing two of Minovitch's JPL internal reports.)

The Voyager 2 sequence of flybys has been touted as a once-in-176-years opportunity to visit all the outer Solar System planets during a single mission; Minovitch, however, was quick to point out that this claim is spurious. Jupiter, Saturn, Uranus, and Neptune are each massive enough to bend a passing spacecraft's path and accelerate it toward any other point in the Solar System at any time.

Voyager 2, with a mass at launch of about 726 kilograms, left Earth on 20 August 1977 atop a Titan IIIE rocket. It flew within 564,000 kilometers of Jupiter's trailing side on 9 July 1979; within 102,000 kilometers of Saturn's trailing side on 25 August 1981; about 82,000 kilometers from the trailing side of Uranus on 24 January 1986; and within 5000 kilometers of Neptune on 25 August 1989. In all, its primary mission spanned just over 12 years.

The intrepid spacecraft then began its Interstellar Mission, which continues to this day. At this writing, Voyager 2 is more than 19 billion kilometers from the Sun; unless humans catch up to it and reverently bring it home, it will in centuries to come depart the Solar System entirely and wander among the stars.

Minovitch calculated Venus-Earth gravity-assist trajectories; these came in handy beginning with the loss of the Space Shuttle Orbiter Challenger (28 January 1986) and subsequent cancellation of the Shuttle-launched Centaur G' upper stage. The accident and stage cancellation grounded the Galileo Jupiter Orbiter and Probe mission, which had been set to launch to Earth orbit in May 1986 in a Space Shuttle payload bay then boost directly to Jupiter on a Centaur G'.

The Space Shuttle resumed flights in September 1988. Galileo was launched in the payload bay of the Orbiter Atlantis (18 October 1989) and boosted from Earth orbit using a solid-propellant Inertial Upper Stage that was incapable of sending it directly to Jupiter.

Instead, Galileo flew by Venus (10 February 1990), Earth (8 December 1990), and Earth again (8 December 1992) before it built up enough speed to begin the trek to Jupiter. Galileo reached Jupiter on 7 December 1995. Over the course of 35 Jupiter-centered orbits, it explored the four largest Jovian moons using gravity-assist flybys to speed up and slow down. A final gravity-assist series caused it to orbit nearly 26 million kilometers from Jupiter and then perform a pre-planned death-dive into its atmosphere on 21 September 2003.

Missions that used gravity-assist flybys include (in no particular order) Voyager 1 (which flew by Jupiter and Saturn), the Cassini Saturn Orbiter (which carried out a Venus-Venus-Earth-Jupiter sequence of gravity-assist flybys and dozens of gravity-assist flybys of Saturn's lone large moon, Titan), the MESSENGER Mercury orbiter (Earth-Venus-Venus-Mercury-Mercury-Mercury), the Rosetta comet-rendezvous spacecraft and Philae lander (Earth-Mars-Earth-Earth), the Juno Jupiter orbiter (Earth), and the New Horizons Pluto flyby spacecraft (Jupiter). Even the Dawn Vesta/Ceres mission, which mostly relied on solar-electric propulsion, used a gravity-assist Mars flyby on 4 February 2009 to gain speed and reach the Asteroid Belt between Mars and Jupiter.


"Gravity Propulsion Research at UCLA and JPL, 1962-1964," R. Dowling, W. Kossmann, M. Minovitch, and T. Ridenmoure, History of Rocketry and Astronautics, AAS History Series Volume 20, J. Hunley, editor, 1997, pp. 27-106.

Comets: A Chronological History of Observation, Science, Myth, and Folklore, D. Yeomans, John Wiley & Sons, New York, 1991, pp. 157-160.

The Voyager Neptune Travel Guide, C. Kohlhase, editor, NASA JPL, June 1989, pp. 103-106.

"Fast Reconnaissance Missions to the Outer Solar System Utilizing Energy Derived from the Gravitational Field of Jupiter," G. Flandro, Astronautica Acta, Volume 12, Number 4, 1966, pp. 329-337.

"Utilizing Large Planetary Perturbations for the Design of Deep Space, Solar Probe, and Out-of-Ecliptic Trajectories," JPL Technical Report No. 32-849, M. Minovitch, December 1965.

"Future Unmanned Exploration of the Solar System," M. Hunter, Astronautics & Aeronautics, May 1964, pp. 16-26.

"Determination and Characteristics of Ballistic Interplanetary Trajectories Under the Influence of Multiple Planetary Attractions," JPL Technical Report No. 32-464, M. Minovitch, October 1963.

Future Unmanned Exploration of the Solar System, M. Hunter, Report to the Executive Secretary, National Aeronautics & Space Council, September 1963.

More Information

The Challenge of the Planets, Part One: Ports of Call

The Challenge of the Planets, Part Two: High Energy

The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)


  1. Excellent article. Of the planets in our solar system which would provide the fastest trajectory ?

  2. Jupiter offers the most opportunities for big velocity/direction changes, though any planet can be used for gravity-assist. Earth has been used more than any other planet.


  3. Lorraine Glynn11 August, 2017 14:00

    I was fortunate enough to meet and enjoy the conversation of this fascinating man at the Tennessee Valley Interstellar Workshop in Knoxville. I have pictures of him signing the supercomputer and enjoyed him as a dinner companion. Although slowed by the years his mind is still sharp and he gave a very interesting address and new advances.

    It is truly sad that so few people know that we owe so much of our solar system travel to his groundbreaking work.

    1. Hi Lorraine, over the years, there’s been a lot of misinformation about Minovitch introduced into the historical record - most of it by himself.

      Depending on where you look, you’ll run across things like the following:
      - Spacecraft were not capable of reaching the outer planets until Minovitch invented the concept of gravity assist.
      - Minovitch discovered what became the Voyager Grand Tour of the outer planets.
      - Minovitch’s groundbreaking efforts were ignored by his superiors because nobody initially believed that gravity assist could work.
      - JPL stole Minovitch’s ideas and refused to give him credit for his work.

      Welp, none of that is true.

      I published a book about Michael Minovitch. To write it, I interviewed him and communicated with him regularly. I reviewed his notebooks and papers. I interviewed his co-workers. I interviewed his supervisors. I interviewed space mission professionals who use the concepts of gravity assist in their everyday work. I also interviewed the real people who worked on Voyager. And I researched the historical record.

      I spent three years on this.

      One of the most common search results about Minovitch is an online article called “The maths that made Voyager possible” by Christopher Riley and Dallas Campbell.

      Riley knows that the thrust of his article is incorrect because I confronted him about it several years ago. (He stopped responding to me.) Riley never even called Minovitch himself, who is still alive last I checked. For his Minovitch source material, Riley used a series of biased conference presentations co-written by Minovitch himself. And that was it!

      Gravity assist is a concept that evolved over hundreds of years - going back to the time when astronomers first noticed that the path of a comet would be affected by its close passage of a planet.

      In the 1880s, Hubert Newton published a study of energy transfers between comets and Jupiter. He was already working on the math.

      In the 1920s, Walter Hohmann and Friedrich Tsander independently created sample gravity-assist missions with hypothetical spacecraft.

      In the early 1950s, Derek Lawden popularized gravity assist although conceding that many unsolved factors existed.

      In the late 1950s, Krafft Ehricke published “Instrumented Comets” and lectured at the university level about using gravity assist on space missions. He taught this at UCLA as early as 1959.

      In 1961, Richard Battin published multi-planet gravity assist sample missions based on his own 1958 research.

      Minovitch should be credited with developing a method of calculating gravity assist trajectories. But that’s about it. There were already plenty of other methods.

      The Riley/Campbell article does properly credit Gary Flandro for discovering the Grand Tour Opportunity. Flandro learned about gravity assist in college, FYI - long before Minovitch came along.

      I’m very happy to get into the minutiae of Minovitch’s story, if anyone wants to.

      Full disclosure: Minovitch sued me for libel & slander in 2013. He lost. He also sued Battin (listed above) and lost. He also sued Caltech & JPL (and lost). And then Minovitch has also sued “John Does #1-20,” who are yet-to-be-named co-conspirators in a supposed plot to steal his work and discredit him.

      Jay Gallentine
      Space Historian

    2. Jay: I encourage debate and discussion in the comments here on the blog. I generally don't participate much after a certain amount of time has passed because I'm working on new topics. For now I'll say that I have researched this matter as well and I stand by this and the other two posts that make up the "The Challenge of the Planets" series. The important thing here is context. Minovitch did his work in a specific environment at a time when it could have (and did have) great influence. You take pains to make Minovitch's work seen unoriginal and inconsequential, and that is obviously a flawed interpretation. dsfp

  4. Lorraine:

    I've not met Minovitch. I'm glad to hear that he's still well enough to travel.

    This is the third part of a three-part series. I think that several factors led JPL to "change history." For one thing, Minovitch wasn't a permanent JPL employee. For another, his behavior could be somewhat unusual: for example, he tells a story about how JPL wanted to give him an award but they scheduled the ceremony for 10 am, when he would still be asleep after spending the night using the JPL computers. So he didn't turn up. That single-mindedness and inappropriate response is not atypical of people on the autism spectrum. I'm not a psychologist, of course, but I think it's plausible that the same traits that made him a success denied him credit for that success.

    In addition, JPL had an "electric propulsion mafia" that suddenly found itself sidelined by gravity assist. It got worse when NASA transferred most electric propulsion work to NASA Lewis (now NASA Glenn). They were less tenacious than the "rover mafia" that started in the 1960s by trying to hitchhike automated rovers on Apollo missions, remained active throughout the 1970s and 1980s (mainly by insisting that all Mars Sample Return missions needed a rover), and eventually won through by turning MESUR Pathfinder, originally a test mission for MESUR (a network mission transferred from NASA Ames), into a minirover carrier. Talk about "singlemindedness"!

    I like JPL. It's a fascinating place. Like all big organizations, however, it has its share of baggage.


    1. Lorraine:

      I should add that often the people who work in an organization don't know its history so cannot be considered complicit in what we might see as "misdeeds" of the past. It's not their business to know its history, it's their business to do their present-day jobs. This is also one reason why history offices tend to go away or turn into public affairs branch offices. It's a misguided form of self-protection.


  5. For ports further afield, the nuclear ion-drive just requires time. The Solar Gravitational Lens Mission to 600 AU becomes doable in 20-25 year time ranges with a rendezvous at the far end. Essentially the same performance as the TAU mission study that concluded c. 35 years ago.


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