To Mars by Way of Eros (1966)

True-color image of Eros, the second-largest Near-Earth Asteroid, from the NEAR Shoemaker spacecraft. Image credit: NASA.
German astronomer Gustav Witt discovered the asteroid Eros on 13 August 1898. Eros was both the first asteroid found to orbit entirely outside of the Main Belt of asteroids between Mars and Jupiter and the first known planet-crosser; it crosses the orbit of Mars. Eros orbits the Sun in a little more than 643 days. Eros and Earth pass nearest each other — at a distance of about 14 million miles — every 81 years.

In March 1966, Eugene Smith, an engineer with Northrop Space Laboratories in Hawthorne, California, described a piloted Eros flyby mission. The mission would, he explained, help to prepare NASA for piloted missions to Mars. He wrote that "the value of the Eros mission to subsequent manned planetary flights having a higher level of difficulty and complexity is of no small consequence."

Eros exploration might also help scientists to understand Main Belt asteroids and small planetary moons (for example, the martian satellites Deimos and Phobos). Smith noted that Eros — which he described as "brick-shaped" — would pass within 14 million miles of Earth on 23 January 1975, its closest approach of the 20th century.

This proposal might seem prescient to readers familiar with current NASA Mars plans, which include a peculiar scheme to capture a boulder from the surface of a Near-Earth Asteroid using a robotic spacecraft and then send a crew to rendezvous with it in lunar orbit. The astronauts would perform spacewalks to sample the boulder. The mission, it is argued, would test a variety of technologies with potential piloted Mars mission and asteroid deflection applications.

Smith's mission was, however, part of a distinctly different piloted Mars program evolutionary strategy. At the time Smith presented his paper, NASA and its contractors devoted considerable effort to studies of piloted free-return Mars/Venus flyby missions based on Apollo technology. The first of these was expected to depart Earth for Mars in late 1975. Among other expected benefits, a piloted Mars flyby would provide interplanetary flight experience ahead of 1980s piloted Mars landings.

The Northrop engineer expected that a Mars flyby spacecraft would likely be so heavy that placing all of its components and propellants into space would need either a Saturn V rocket with a nuclear-thermal upper stage or multiple all-chemical Saturn V launches followed by assembly through multiple dockings in Earth orbit. He called instead for a 1975 piloted Eros flyby that would provide experience applicable to Mars landings, yet could depart Earth on a single uprated Saturn V rocket.

The 527-day Eros flyby mission would begin with launch from Cape Canaveral on 3 May 1974, at the opening of a 30-day launch window. The Eros Saturn V and its payload, the Eros Flyby Spacecraft Vehicle (EFSV), would stand 21 feet taller than the 363-foot-tall Apollo Saturn V.

Apollo (left) and Eros spacecraft configurations compared. With one exception (D is the Eros Mission Module while d is the Spacecraft Launch Adapter housing the Apollo Lunar Module), the lower-case and upper-case letters identify equivalent systems. a/A = Launch Escape System; b/B = Command Module; c/C = Service Module; e/E = Saturn V rocket Instrument Unit; f/F  = Saturn V S-IVB third stage; g/G = J-2 rocket motor. The Apollo configuration would measure 143 feet long; its Eros counterpart, 164 feet. Image credit: David S. F. Portree/Northrop Space Laboratories.
Eros Command Module/Eros Service Module. Image credit: Northrop Space Laboratories.
Smith's EFSV would comprise the conical Eros Command Module (ECM), outwardly a twin of the Apollo Command Module, but bearing a six-man crew; the Eros Service Module (ESM), a 21.5-foot-diameter, 34.3-foot-long substitute for the 12.8-foot-diameter, 25.7-foot-long Apollo Service Module; and the cylindrical, 21.5-foot-diameter, 30-foot-long Eros Mission Module (EMM). An S-IVB stage and Instrument Unit — respectively the third stage and the "electronic brain" of the Saturn V rocket — would inject the EFSV into 100-nautical-mile (n-mi) Earth orbit. Mass injected into orbit including the S-IVB and IU would total about 165 tons.

When Smith presented his paper, the Apollo Saturn V was still more than a year away from its maiden flight. NASA expected that it would be able to launch about 130 tons into 100-n-mi Earth orbit.

Smith suggested that NASA boost Saturn V capacity to 165 tons by uprating the five J-2 engines in its S-II second stage. Alternately, the rocket's S-IC first stage might be fitted with twin 260-inch-diameter solid-propellant strap-on rocket motors, increasing its capacity to a whopping 215 tons. The latter alternative, Smith wrote, would provide ample margin for EFSV weight growth during development. It would, of course, also constitute a more radical (and thus more costly) change in the basic Saturn V design than would S-II engine uprating.

During Apollo Moon missions, an S-IVB would ignite following S-II stage separation and burn for 2.5 minutes to place itself, the IU, a shroud housing the Apollo Lunar Module (LM), and the Apollo Command and Service Module (CSM) into 115-n-mi parking orbit about the Earth. About two hours and 44 minutes after launch, the S-IVB would ignite a second time and burn for six minutes to put the CSM and LM on course for the Moon. The stage and the IU would then separate.

When used as part of Smith's EFSV, the S-IVB would carry out its first burn much as in the Apollo Moon missions, but its second burn would be different. Upon arrival in parking orbit, the crew in the ECM would check out the EFSV's systems. Assuming that all appeared normal, they would then ignite the S-IVB engine at perigee (the low point in its Earth-centered orbit) to raise the ESFV's apogee (the high point in its Earth-centered orbit) and gain over 90% of the velocity needed to depart Earth orbit for Eros. At S-IVB burnout they would still orbit Earth, but in an elliptical "Intermediate Departure Orbit" with an orbital period of two days.

Eros flyby mission plan. Please click on image to enlarge. Image credit: Northrop Space Laboratories.
The astronauts would next separate the ECM/ESM from the EMM/spent S-IVB and turn it end for end so that it could link its nose-mounted probe docking unit with a drogue unit at the bottom of a conical recess in the top of the EMM. After casting off the spent S-IVB stage, the crew would transfer to the EMM, their main living and working space during the Eros flyby mission.

There they would deploy the EMM's eight solar panels, a steerable "sensor turret," a large dish antenna, and a "support structure" which, along with the conical recess in the top of the EMM, would shield the ECM from harsh sunlight and micrometeoroids. The disk-shaped solar panels would ride to Earth orbit folded and stacked under the aft end of the EMM. After linking the EMM and ECM/ESM electrical and control systems, they would check out all EFSV systems a second time.

The ESM would include two RL-10A-3 main engines that would burn high-performance cryogenic liquid hydrogen/liquid oxygen propellants. Smith calculated that the ESM would need only one engine to perform most Eros flyby mission maneuvers, but he included separate twin engines in his design for redundancy.

If the EFSV failed its second checkout, the astronauts could abort their mission by separating from the EMM in the ECM/ESM, pointing the ESM engines forward, and firing them at next perigee on 5 May 1974 to reduce speed. They would then separate from the ESM in the ECM and reenter Earth's atmosphere. If EFSV systems continued to function normally, however, the astronauts would fire the ESM engines at perigee to add enough velocity to place their spacecraft on course for Eros.

Eros Flyby Spacecraft Vehicle configured for Earth-orbit departure and interplanetary flight. A = twin RL-10A-3 engines; B = Eros Service Module; C = Eros Command Module; D = Eros Mission Module; E = high-gain antenna; F = sensor turret; G = four-panel solar array. The spacecraft would orbit the Sun with its twin solar arrays pointed Sunward and its high-gain antenna pointed toward Earth. Image credit: David S. F. Portree/Northrop Space Laboratories.
The EMM would contain near its center a spherical pressurized habitat module similar to the one in NASA Marshall Space Flight Center's February 1965 Mars/Venus piloted flyby study (see "More Information" below). In the event of a solar flare, the crew would retreat to a "storm cellar" with hatches at both ends. The forward hatch would lead to the ECM and the aft hatch to the habitat sphere.

A centrifuge would divide the sphere into forward (crew quarters) and aft ("mission task area") halves. Smith hoped that periodic "centrifugation" in the small centrifuge would be sufficient to maintain crew health during the 17.5-month Eros voyage, since spinning the entire EFSV to create acceleration which the crew would feel as gravity would create engineering challenges — for example, designing solar arrays that would track on the Sun as the spacecraft rotated. Smith wrote that meeting these challenges would increase the EFSV's mass so that it no longer could depart Earth on a single uprated Saturn V.

A hatch in the aft end of the habitat sphere would lead to a pressurized equipment room, which would in turn lead to a round "probe hatch" in the aft end of the EFSV. The probe hatch would open into space.

The solar arrays and aft end of the EFSV would point toward toward the Sun during most of the mission. This would place the ECM/ESM in shadow, which, along with heavy insulation, would prevent the cryogenic propellants in the ESM from boiling away during the long voyage.

On 18 January 1975, the astronauts would begin tracking Eros using radar, a reflecting telescope with a 30-inch primary mirror, and other instruments mounted in the sensor turret. On 23 January 1975, they would adjust their course using the ESM engines to ensure an Eros close-approach distance of about 50 miles and would begin gathering Eros science data.

About eight hours before closest approach, the astronauts would "catapult" a 200-pound automated probe out of the probe hatch toward the asteroid. The probe would function much as the Block III Ranger lunar probes had been meant to do; that is, it would image Eros until it smashed into its surface and was destroyed, yielding detailed close-up images in its final seconds. The EMM's dish antenna would relay to Earth data from the probe's TV camera and other instruments.

Closest approach to Eros would occur about 14 million miles from Earth on 28 January, just five days after the Earth-Eros close approach. Close proximity would permit a higher rate of data transmission from the EFSV to Earth during the flyby than would otherwise be possible.

The piloted Eros flyby spacecraft would spend about 90 seconds within 200 miles of the asteroid's sunlit side and about 30 seconds within 100 miles. On 30 January 1975, the crew would end Eros tracking and fire the ESM engines to correct course deviations imparted by the 23 January maneuver, the automated probe launch, and the weak tug of the asteroid's gravity.

The astronauts would load the ECM with scientific data — mainly film — and check out its systems beginning on 10 October 1975. On 12 October, they would abandon the EMM and use the ESM engines to place the ECM on course for Earth atmosphere reentry. They would then jettison the ESM, reenter the atmosphere at about 40,000 feet per second — about 3500 feet per second faster than Apollo lunar-return speed — and descend to a landing on parachutes.

Image credit: NASA.
Congress killed NASA's plans for piloted Mars and Venus flyby missions in August 1967, in the aftermath of the January 1967 Apollo 1 fire. Smith's piloted Eros flyby proposal received little attention. The only U.S. piloted mission of 1975 was the Apollo-Soyuz Test Project, which saw the final Apollo CSM dock with the Soviet Soyuz 19 spacecraft in low-Earth orbit.

When NASA at last explored a near-Earth asteroid, it explored Eros. The $112-million Near-Earth Asteroid Rendezvous (NEAR) robotic mission — the first mission in NASA's low-cost Discovery Program — left Earth on 17 February 1996, 22 years after the planned launch date of Smith's piloted Eros flyby.

On 20 December 1998, NEAR failed to enter Eros orbit because its computer aborted a crucial engine burn. Three days later, after some quick reprogramming, NEAR flew past the 22-mile-long, 13-mile-wide asteroid at a distance of 2375 miles. It returned 222 images. They revealed that Eros is shaped like a ballet slipper or, as some would have it, a banana.

On 14 February 2000, after another revolution around the Sun, NEAR at last orbited its target. NASA renamed the spacecraft NEAR Shoemaker in March 2000 to commemorate renowned planetary geologist and asteroid and comet discoverer Eugene Shoemaker, who had died in a car crash in Western Australia in July 1997 while looking for ancient asteroid impact craters. During the year that followed, the spacecraft radioed to Earth more than 160,000 close-up images of Eros. New images revealed many odd smooth "ponds" made of dust.

Though designed as an orbiter, NEAR Shoemaker succeeded in landing on Eros on 12 February 2001. It may have landed in a dust pond, cushioning its impact. It returned gamma-ray spectrometer data from the asteroid's surface until 28 February 2001.

Eros flew past Earth at a distance of 16.6 million miles on 31 January 2012, its closest approach since 1975. The asteroid will pass slightly closer to Earth than it did in 1975 on 24 January 2056.


"A Manned Flyby Mission to Eros," Eugene A. Smith, Proceedings of the Third Space Congress, "The Challenge of Space,” pp. 137-155; paper presented at the Third Space Congress in Cocoa Beach, Florida, 7-10 March 1966.

The Near Earth Asteroid Rendezvous Mission: A Guide to the Mission, the Spacecraft, and the People, Johns Hopkins University Applied Physics Laboratory, December 1999.

NASA Press Release 01-29, "Asteroid Mission Not Yet 'NEAR' An End," D. Savage, NASA Headquarters, 14 February 2001.

"NEAR Shoemaker Weekly Report," Michelle Stevens (for Debra Fletcher), 2 March 2001.

More Information

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

Missions to Comet d'Arrest & Asteroid Eros in the 1970s (1966)

Earth-Approaching Asteroids as Targets for Exploration (1978)


  1. Thanks for reporting this other unknown and fascinating study, David. But as often, I am curious to know your personal opinion on such plans.
    To me, this mission concept illustrates well of the largest missions achievable with mainstream Appolo technologies. It is very interesting from an engineering point of view, but the scientific usefulness seems quite limited: to send 6 astronauts in deep space for 1.5 years, and gain only a quick flyby of a large piece of rock... the scientific return on investment would have been as weak as for the current crazy plan "get boulder around Moon and catch it".

  2. I think there are a couple of ways to look at this. One can think of this as a science mission, in which case, as you say, the science return per dollar is lousy. Or, one can think of it as a preparatory mission for testing technologies and techniques of interplanetary spaceflight. I think that if one compares this to Apollo missions, it would be equivalent to one of the early Apollos. I assert that Apollo missions 7-12 were preparatory missions; 13/14 were science missions using the basic Apollo system, and the J-class missions were where all that preparation was meant to really pay off.

    That said, this particular mission is peculiar in many respects, which is why I mentioned the (equally peculiar) Orion lunar-orbital asteroid boulder mission in my third paragraph. It is actually of greater duration than many of the Mars/Venus flybys and assumes more modifications to the basic Saturn V than most. The one plus - according to Smith - is that it avoids Earth-orbital rendezvous, but one has to ask whether one would want to avoid EOR if EOR would be a part of all subsequent missions.

    Smith fails to mention in-flight science, which might dramatically improve the science payback of his long Eros mission. The crew would study themselves, of course, and could use their telescope to perform astronomical observations. They could also observe the Sun, capture interplanetary dust and meteoroid samples, observe the Earth from a distance, etc. Many other in-flight experiments were proposed in the mid-1960s. This is a curious and unfortunate omission.

    So, in a nutshell, I think this is a not a desirable mission as described. If it could be made shorter - say, a year instead of nearly two years - even at the cost of losing the asteroid flyby, it might be a good test of technology and techniques. If it could be made lighter, or if it used EOR - say, split the EMM and ECM/ESM between two Saturn Vs - then it would fit better with the probable evolution of the piloted Mars program.

  3. I've already red this plan when your blog was at wired. Every time I read I think is is a nice additionto the Venus and Mars flybys planned at this time. It is actually clever - flyby of an asteroid slighty more make sense than flybying a planet.

    What I wonder if whether they could actually extend the plan and add a Lunar Module to land on Eros. The LM had an enormous delta, 2.5 km/s for each stage.
    The LM with two astronauts would separated from the flyby craft some day ahead of the Eros encounter, and then land. After some hours, if days of exploration the LM upper stage would blast off in pursuit of the flyby ship.
    I suppose that the flyby craft would be in orbit around the sun, on an hyperblic trajectory. There was a similar plan for Mars called FLEM - Flyby Landing and Excursion Mode. You already mentionned it in your blogs.

    Wouldn't it be cool to have a LM standing on Eros ? even better the weaker gravity and lack of atmosphere would entail very little modification to the LM. IT could be unmodified.

    1. I don't know - I'm not enough of a spacefarer to compute whether your FLEM-Eros scheme would work. Perhaps someone else could step in at this point.

      I can say that adding ~25 tons to the mission - the weight of the LM - would affect the mission design. The beefier version of the Saturn V would be necessary, plus perhaps a nuclear stage in place of the S-IVB. I'm not sure how early the LM would need to separate from the EFSV - going by the FLEM plans, it might have to be weeks before the flyby. If the total time from LM sep to LM ascent stage docking were longer than 3 days, the LM as flown for J missions would run short of consumables - life support, cooling water, etc.

      I think what might work, however, would be a robotic sample collector.

      This is actually the latest version of one of my oldest space history posts. It has been rewritten several times. When it moved from Romance to Reality (my first space webpage) it was a straightforward tech description with no context. When it moved to Beyond Apollo, I added the NEAR Shoemaker stuff, since that had happened in the meantime. This adds some stuff linking Smith's proposed mission to current NASA asteroid/Mars planning. I hadn't planned on reworking this one, but the asteroid component of current NASA Mars planning made it seem relevant. So, I polished it up and added some new text.

  4. The LM had probably much more delta-V than needed for Eros. So it could fire it descent engine, not for landing, but to shorten the transit time to Eros. You'll need very little fuel to land on Eros, Same goes for the ascent stage: escaping Eros gravity field isn't comparable to a trasn-Earth injection.

    1. The LM ascent stage needed only enough delta-V to reach lunar orbit - I suspect you are aware of that and your mention of TEI is an oversight. I'm intrigued by your suggestion that the descent engine could accelerate the LM to Eros. What if it were expended and only the LM ascent stage maneuvered near the asteroid? Then it could chase after the EFSV and perhaps rendezvous in a short time with it.

      I'm not sure the piloted LM landing on Eros would be worth the risk. I think the LM would need a lot of novel mods, which would add to cost and reduce reliability. I suppose, however, that one might park a less modified LM inside a hangar attached to the EFSV. The hangar would protect it against long exposure to interplanetary space conditions.

      How would the LM actually land? Small-body landings seem to be kind of tricky. Things tend to bounce off. NEAR Shoemaker is probably the only exception - hey, maybe that's the key to landing on an asteroid - don't design your spacecraft to land on an asteroid! :-)

    2. I don't know for other missions but Philae was to fire harpoons at the comet. Kind of rocket harpoons getting fixed within the crust. I think other probes used similar systems, but I can't remember wich.

    3. My bad for TEI - it was the CSM SPS that did the job, not the LM. Still it takes 2.5 km/s of delta-v to land (and to return to orbit) on the Moon.

    4. You're right - I've forgotten that the LM might have to separate days or weeks before the Eros encounter, thus it couldn't do the job - not enough endurance.
      I'm rather lousy at maths, so don't know much about possible trajectories.

    5. So that it's on the record (7 years later) but the delta-vee of the LEM was 4 km/s total. About 1.9 km/s ascent and 2.1 km/s descent. The excursion to Eros was probably doable. Might need to change the total crew to 4 persons, to reduce the other system masses. Adapting the LEM for a milli-gee landing would mean claws, not legs, and reducing the various engine masses. No need for 1 gee thrust.

  5. Does anyone have any book recommendations about the Apollo-Soyuz Test Project? I know the basics, but I've not read any good in-depth pieces about it.

    1. What about the NASA history, THE PARTNERSHIP? I think it's a good place to start. ASTP hasn't received a lot of attention over the years compared with even Skylab. For one thing, it was just one mission, which might make for a skinny book. Of course, if one likes all the fiddly background details, there's a lot more to the story.


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