Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)

The authors of the dual-stopover study did not design a spacecraft. The 6.4-year cycle of mission opportunities they identified repeats endlessly, however, so the NASA image above, which shows a present-day design for a piloted Mars spacecraft, can be pressed into service to illustrate this post. With relatively minor changes, this spacecraft might orbit both Mars and Venus during a single mission. 
The piloted flyby missions NASA studied in the 1960s often included close encounters with both Mars and Venus. The October 1966 NASA Planetary Joint Action Group report Planetary Exploration Utilizing a Manned Flight System, for example, emphasized a piloted Mars flyby mission departing Earth orbit during the September 1975 free-return opportunity, but also noted an opportunity to launch a Earth-Venus-Mars-Venus-Earth flyby in February 1977 and an Earth-Venus-Mars-Earth flyby in December 1978.

Piloted flybys in the 1970s were intended to clear a path to piloted "stopover" missions in the 1980s. Stopovers — a category which included Mars and Venus orbiters and Mars landings — almost always emphasized a single objective. That is, each mission would travel to a single world, then return to Earth. The closest stopovers came to visiting more than a one planet was when a Mars stopover mission performed a Venus "swingby" to bend its course, slow its approach to Earth to enable a safe direct Earth-atmosphere reentry, or accelerate toward Mars.

During a Venus swingby, a Mars stopover spacecraft might explore the cloudy planet much as piloted Venus flybys were meant to do. That is, it might drop off probes insulated and armored against Venusian temperatures and pressures and scan the hidden Venusian surface with radar.

That a piloted spacecraft might stop at both Mars and Venus during a single mission was unthinkable. It was widely accepted that such a mission would demand enormous quantities of propellants, all of which would need to be launched into Earth orbit atop costly heavy-lift rockets.

In a brief September 1969 NASA Technical Memorandum, E. Willis and J. Padrutt, mathematicians at NASA Lewis Research Center (LeRC) in Cleveland, Ohio, sought to overturn the prevailing view of what would be possible during stopover missions. Lead author Willis was no stranger to NASA piloted Mars mission planning: he had designed interplanetary trajectories at LeRC at least since early 1963.

Willis and Padrutt's mission design would see a piloted spacecraft depart a circular low-Earth orbit and capture into an loosely bound high-apoapsis (that is, orbit high point) elliptical orbit around Mars or Venus. It would then transfer to a loosely bound high-apoapsis elliptical orbit around Mars (if the first stopover were at Venus) or Venus (if the first stopover were at Mars). From there, the spacecraft would transfer back to Earth, where the crew would reenter the atmosphere directly in a small capsule. Its usefulness ended, the dual-stopover spacecraft would, meanwhile, swing past Earth into a disposal orbit around the Sun.

The mission plan was designed to reduce the amount of energy required to move between worlds, thus conserving propellant. The piloted dual-stopover spacecraft would travel between planets only when an opportunity for a minimum-energy transfer occurred; that is, only when the planets moved into positions relative to each other necessary for a minimum-energy transfer. Loosely bound orbits would reduce energy needed to capture into and escape from orbit. Direct reentry into Earth's atmosphere would ideally require only enough energy to deflect the capsule's course so that it would intercept Earth after it separated from the dual-stopover spacecraft.

The LeRC mathematicians calculated the total "propulsive effort" necessary to carry out the seven dual-stopover missions in the 1979-1986 cycle. They measured propulsive effort in terms of the total velocity change firing the dual-stopover spacecraft's rocket motor or motors would produce. Propulsive effort would expend precious propellants, so most of the time small velocity changes were to be preferred over large ones.

They explained that they had discovered a repeating 6.4-year cycle of seven potentially useful dual-stopover mission opportunities. The seven opportunities varied only slightly from one 6.4-year cycle to the next. The first, fourth, and sixth opportunities would begin with an Earth-Mars transfer, while the second, third, fifth, and seventh would begin with an Earth-Venus transfer. In most cases, the minimum propulsive effort needed to perform Earth-Venus-Mars-Earth dual-stopovers would be less than that needed for Earth-Mars-Venus-Earth dual-stopovers. In their paper, Willis and Padrutt emphasized the 6.4-year cycle that would begin in late 1979.

A hand-drawn illustration from Willis and Padrutt's NASA Technical Memorandum outlines the dual-stopover mission beginning in late 1979. 1 = departure from circular low-Earth orbit on a minimum-energy path to Mars. 2 = Arrival in high-apoapsis elliptical Mars orbit at the beginning of a 78-day stopover during which Mars's position changes as it orbits the Sun. 3 = Mars departure on a minimum-energy path to Venus. 4 = Arrival in high-apoapsis elliptical Venus orbit at the beginning of a 177-day stopover during which Venus's position changes as it orbits the Sun. 5 = Venus departure on a minimum-energy path to Earth. 6 = Arrival at Earth. Image credit: NASA.
A spacecraft launched during the late 1979 dual-stopover mission opportunity would spend 78 days at Mars and 177 days at Venus. During each stopover, the planet would orbit the Sun, eventually reaching the correct position to enable the spacecraft to make a minimum-energy transfer to its next destination planet. The Earth-Mars, Mars-Venus, and Venus-Earth tranfer legs of its voyage would together require 638 days. Adding the time spent at Mars and Venus to the time spent between worlds would yield a mission duration of 894 days — that is, slightly less than two and a half years. Total propulsive effort would amount to 9.382 kilometers per second (kps).

The second opportunity of the 6.4-year cycle would occur in the first half of 1980. The dual-stopover spacecraft would depart Earth on a minimum-energy path to Venus. It would spend 180 days at Venus, 10 days at Mars, and 669 days between worlds, for a total mission duration of 860 days (two and a third years). This made it the shortest dual-stopover mission of the seven-mission cycle.

Because short missions limited the time available for hardware breakdowns and the crew medical problems, they were to be preferred to long ones. Willis and Padrutt acknowledged, however, that the opportunity's short stopover at Mars would provide little time for exploration. Total propulsive effort would amount to 8.738 kps.

The third opportunity would occur in late 1981. The dual-stopover spacecraft would leave Earth for Venus, where it would spend about 265 days. It would stop over for 133 days at Mars, and spend 629 days between worlds, yielding a total mission duration of 1027 days (nearly three years). Total effort would equal 8.7 kps.

The fourth opportunity would occur at the end of 1981. The dual-stopover spacecraft would leave Earth for Mars. It would spend about 274 days at Mars, 340 days at Venus, and 680 days between planets, for a total duration of about 1294 days (a little more than three and a half years). Total propulsive effort would equal 9.252 kps.

The fifth opportunity would occur in the first half of 1983. The dual-stopover spacecraft would leave Earth for elliptical Venus orbit, where it would spend just 10 days. It would spend 601 days at Mars and 619 days between worlds, yielding a mission duration of 1230 days (a little less than three and a half years). Total propulsive effort would total 8.896 kps. The short stopover at Venus might make the opportunity undesirable; on the other hand, the mission's Mars stopover would be the lengthiest in the 6.4-year cycle, enabling a long period of exploration.

The sixth opportunity would see the dual-stopover spacecraft depart Earth for elliptical Mars orbit in early 1984. The spacecraft would spend 200 days at Mars, 250 days at Venus, and 639 days between planets, for a total mission duration of 1089 days (a little less than three years). Total propulsive effort would amount to 9.339 kps.

The seventh and last opportunity of the 6.4-year cycle would occur in mid-1985. The dual-stopover spacecraft would spend 767 days in elliptical Venus orbit before voyaging to Mars for a 78-day stopover. It would spend a total of 599 days between worlds — the shortest travel time of the seven opportunities. The long Venus stopover would, however, result in a mission duration of 1444 days (about four years), making it the lengthiest of the seven dual-stopover missions. Total propulsive effort would amount to 9.321 kps.

The 6.4-year-cycle Willis and Padrutt studied in detail would end just before the first dual-stopover opportunity of the next 6.4-year cycle. That opportunity, very similar to the late 1979 Earth-Mars-Venus-Earth opportunity, would occur in the first half of 1986.

Willis and Padrutt compared the total propulsive effort necessary to accomplish four of the dual-stopover missions in the 1979-1986 period with that needed to carry out four Mars stopover/Venus swingby missions. They sought to reduce dual-stopover mission duration, however, so permitted increased propulsive effort. This would enable shorter stays at planets and shorter transfers between planets. The Mars stopover/Venus swingby missions — all of which would include a 30-day Mars stopover — were assumed to leave Earth on approximately the same dates as the dual-stopover missions.

They found that the first dual-stopover mission, the December 1979 Earth-Mars-Venus-Earth mission, would need a total propulsive effort of about 13 kps to reduce its duration to 700 days. A Mars stopover/Venus swingby mission launched at about the same time could be performed in 700 days with a total propulsive effort of only eight kps. The same missions could be carried out in 575 days with propulsive efforts of 20 kps and a little less than 11 kps, respectively. These numbers indicated that the first opportunity in the 6.4-year dual-stopover cycle was not a favorable one for dual-stopover missions of reduced duration.

Dual-stopover missions launched in the other three opportunities compared more favorably with Mars stopover/Venus swingby missions. The fourth mission of the 1980s dual-stopover cycle — another Earth-Mars-Venus-Earth mission — could be shortened to 700 days if a total propulsive effort of about 12 kps were permitted, while a 700-day Mars stopover/Venus swingby mission departing Earth at about the same time would need a propulsive effort of about 10 kps.

The sixth dual-stopover mission (Earth-Mars-Venus-Earth) could be accomplished in just 625 days with a total propulsive effort of a little more than 10 kps. Willis and Padrutt calculated that a 625-day Mars stopover/Venus swingby mission launched at the same time would actually need a greater total propulsive effort: a little less than 12 kps.

The seventh dual-stopover mission in the cycle — an Earth-Venus-Mars-Earth mission — could be shortened to 675 days with a total velocity change of about 10 kps. A 675-day Venus swingby/Mars stopover mission launched at the same time would need a velocity change of eight kps.

Willis and Padrutt conceded that the minimum propulsive effort required to carry out a dual-stopover mission would almost always exceed that of a single mission that traveled from Earth to either Venus or Mars and back to Earth. They noted, however, that the minimum propulsive effort of a separately launched Earth-Venus-Earth stopover mission and a separately launched Earth-Mars-Earth stopover mission combined would always exceed that of a single dual-stopover mission. The two separate missions would together need a minimum propulsive effort of at least 17 kps; that is, nearly double the minimum propulsive effort of a typical dual-stopover mission.


Round Trip Trajectories With Stopovers At Both Mars and Venus, NASA TM X-52680, E. Willis and J. Padrutt, NASA Lewis Research Center, September 1969.

Planetary Exploration Utilizing a Manned Flight System, NASA Office of Manned Space Flight, 3 October 1966.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Humans on Mars in 1995! (1980-1981)


  1. Wow! I had no idea there were plans to include stopovers at Mars and Venus on the same mission.

    The old book Manned Spacecraft to Mars and Venus; How They Work by Walter Hendrickson has a discussion about how a Venus flyby mission could incorporate a landing on Mercury. There isn't much detail at this link, but here you go:

    I don't know how seriously this idea was studied.

    I haven't watched the video at this link for a while, but it's the only other proposal I've seen to send people to Mercury. I don't have time to watch the video again right now, but as I recall it involved mobile base on the planet, slowly changing its position as the planet rotates to keep the base from getting too cold or too hot.

    1. Thank you, Phillip, for posting about Walter Hendrickson's book (I'd been looking for it, but not remembering its exact title or the author's name stymied online searches--I recalled a landing illustration from it whose caption mentioned "exploring the night side of Mercury by the light of Venus" [I just ordered a copy from]) *and* for including the link to Dr. Basil Singer's "Space Pioneer" episode about colonizing Mercury, which I just watched (I'd never heard of him or that series before, and I'm glad that I now have!). Also:

      I'm glad to have learned that I'm not the only one who, today, thinks that exploring and colonizing Mercury (starting first with robotic orbiters, landers, and rovers, of course) is a worthwhile goal. The planet is often forgotten when future missions and colonization are discussed, but while it is a challenging place to reach and live on, it also offers advantages (and lacks some disadvantages). It has water, chemically-bound oxygen (and other minerals, as well as industrial and specialty metals, in abundance), plenty of solar energy, and Mars-like gravity (but no toxic perchlorates or sensible atmosphere as Mars does, which would facilitate "lunatron"-type [hermitron?] electromagnetic launchers for shooting resources and spacecraft elsewhere in the solar system). It is also thought to be seismically stable, which would facilitate the eventual construction of sub-surface colonies where--like on the Moon--the temperatures would be moderate and unvarying between day and night. Here (see: ) are other studies of Mercury colonization.

      -- James *Jason* Wentworth

  2. Mr. Park:

    I'm always a little leery about calling this kind of study a "plan" in the sense that it had any kind of NASA endorsement. That being said, it was from little studies like this one that real plans could grow. This one came along at a bad time, however. Not long after it was published, it became very clear that neither Venus nor Mars - nor the moon, for that matter - was on the agenda for NASA astronauts in the 1980s.

    I wrote this post because I'm supposed to do a brief talk on these kinds of studies for NASA JSC soon. My goal is to get some of the plans I haven't written about written up as posts. The talk will be too short for me to go into much detail, so I want to be able to refer people to more detail in the posts.

    Thanks for the links - I'll look at those shortly - I have to respond to your other comment first!

    I have in my files a study of piloted Jupiter and Mercury flybys. I haven't read it yet - thanks for the reminder. :-)


    1. Phillip and Jason:

      More intriguing links I need to follow!


  3. "he NASA LeRC mathematicians explained that they had discovered a repeating 6.4-year cycle of seven potentially useful dual-stopover mission opportunities. The seven opportunities varied only slightly from one 6.4-year cycle to the next."

    I wonder, if these calculations were re-done using modern NASA computers that solve N-body problems, that the solutions would still be valid?

    Also, the difference between a stop-over and a flyby can be thousands meters per second of deltaV to get into and out of each end. How would it be provided?

    1. It's probable that modern computing methods combined with more precise data would refine the results Willis and Padrutt published in 1969. They are quite vague about the dates of their mission opportunities, presenting them only in a small hand-drawn chart with Julian dates. I could have taken a straight-edge to the chart and converted the Julian dates, but there wouldn't have been much point; the width of my pencil line would span at least a couple of weeks. I suspect that they must have felt the same way, since they leave many questions unanswered.

      That being said, I suspect that their work isn't so far removed from reality as to be meaningless. The analog tools of the period could achieve impressive results, and Willis (and probably Padrutt - though I've not seen his work elsewhere) was no slouch. I guess one could say that their results are good enough to suggest that electronic computers be brought to bear on the problem.

      I might not have been sufficiently explicit - at both Venus and Mars the spacecraft would enter a loosely bound elliptical orbit. That would greatly reduce the capture and escape delta-Vs. This was a common tactic in Mars mission planning in the late 1960s. The crew would also enter Earth's atmosphere directly in a small capsule, with no propulsive braking.

      Use of elliptical orbits was revived in the 1990s and applied to Earth escape. An electric-propulsion stage would slowly boost the piloted Mars spacecraft and a small chemical escape stage into a highly elliptical Earth orbit. A crew would board at apogee (thus saving them from riding out many Van Allen Belt passes) and fire the chemical stage at perigee, raising the apogee (in theory, anyway) to infinity and enabling escape on a minimum-energy path to Mars. This technique was expected to save several heavy-lift launches - three or four comes to mind, though don't quote me on that.

      The biggest savings of the dual-stopover mission would, of course, accrue from launching components for only one spacecraft into Earth orbit and departing circular low-Earth orbit only once.


    2. Thanks for the explanation.

    3. MB:

      I modified the post to make the terminology more consistent and clarify the mission designs.



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