12 August 2015

Solar Flares and Moondust: The 1962 Proposal for an Interdisciplinary Science Satellite at Earth-Moon L4

Mariner 1 launches atop an Atlas-Agena B rocket, a missile-based workhorse of the early Space Age. Image credit: NASA
James Van Allen discovered the two Earth-circling radiation belts that bear his name in 1958. The discovery, based on data from Explorer 1 (the first U.S. satellite), Explorer 3, and Pioneer 3, was the first fundamentally new finding of the Space Age. In addition to its scientific and practical importance, it constituted a prestige victory in the Cold War space race with the Soviet Union. Time magazine put Van Allen on the cover of its 4 May 1959 issue.

The Van Allen Belts are a feature of Earth's magnetosphere. Though fascinating in its own right, the magnetosphere became a source of frustration for scientists eager to study the Sun. This is because Earth's magnetic envelope blocks solar particles, preventing detailed study of flares and other solar phenomena.

Physicist James Van Allen (center) holds aloft the backup Explorer 1 satellite and upper stage with Jet Propulsion Laboratory director William Pickering (left) and Wernher von Braun. Image credit: NASA
Van Allen chaired the National Academy of Sciences Space Science Board Summer Study in Iowa City, Iowa, between 17 June and 10 August 1962. Iowa City is home to the University of Iowa, where Van Allen was a professor. His role as chair of the two-month study pointed up its intended significance. The Summer Study was meant to chart a course for U.S. space science and to bring together under NASA sponsorship the disparate elements of the nascent space science community. It involved more than 100 scientists from many disciplines.

Among them were Leo Steg, a General Electric scientist whose specialties were missile reentry vehicles and orbital mechanics, and Eugene Shoemaker, a U.S. Geological Survey geologist noted for his study of asteroid impacts, impact and explosion craters, and the cratered lunar surface. Their collaboration on a brief report on the uses of a libration (L) point satellite illustrates the interdisciplinary intent of the Iowa City study. It was also among the earliest proposals to treat the L points as destinations that could be explored and put to good use.

The Earth-moon system contains five L points. They are features as real as the moon and the Earth. In theory, an object parked at one of these "equilibrium" points will remain there indefinitely. In practice, the Sun's gravity perturbs objects parked at the Earth-moon L points, making station-keeping necessary. The propulsive energy (and thus propellants) needed to keep station is, however, quite modest.

Steg and Shoemaker examined the possibility of placing a satellite in orbit about the L4 or L5 point of the Earth-moon system. L4 is located 60° ahead of the moon along its orbit about the Earth; L5 is 60° behind the moon along its Earth-centered orbit.

Beyond Earth's magnetosphere and nearly always in view of the Sun, either L4 or L5 would, they wrote, "be an excellent location for a satellite whose objective is to perform solar-flare observations." Even if the magnetosphere did not interfere with its observations, a solar-observation satellite in low-Earth orbit would spend up to half its time in Earth's shadow, in the night portion of its orbit, so could not monitor the Sun continuously. The L4 or L5 satellite would be eclipsed by the Earth about as often as the moon is - that is, for a few hours each year.

Schematic illustration of Earth-moon system with libration points indicated. Image credit: NASA
In keeping with his scientific discipline, Shoemaker had a geologic interest in the Earth-moon L4 and L5 points. It stemmed from a possible breakthrough made behind the Iron Curtain 14 months before the Iowa City meeting. In March-April 1961, Polish astronomer Kazimierz Kordylewski had succeeded in photographing very faint dust clouds at the Earth-moon L4 and L5 points. He had first observed them in 1956 while peering through a telescope, but had at first been unable to capture them on film. The clouds were thought to be made up of dust knocked off the moon by large asteroid impacts and captured temporarily at the L4 and L5 points.

Had it flown, Steg and Shoemaker's L point mission would have begun with an Atlas-Agena B rocket launch (image at top of post) from Cape Canaveral on 24 October 1963. After arrival in low-Earth parking orbit, the rocket's Agena upper stage would have restarted to boost a nearly 900-pound satellite toward the Earth-moon L4 point. The satellite would travel the 246,781-mile path to L4 in about 78 hours.

Steg and Shoemaker envisioned that their satellite would include a rocket engine and propellants with a total mass of 360 pounds for course corrections, injection into an elliptical orbit around the L4 point, and station-keeping. The satellite's 70-pound science payload would include a 30-pound micrometeorite collector/analyzer for study of Kordylewski cloud dust grains, thus permitting examination of possible lunar surface material without a moon landing. The remaining 40 pounds of instrumentation would be dedicated to solar-flare observations.

Fifty pounds of radio equipment would transmit the L4 satellite's findings to Earth. Steg and Shoemaker noted that their proposed satellite's unique position might enable it to serve as a useful "communication base" for future lunar missions. It might, for example, relay radio signals between Earth and part of the Farside, the lunar hemisphere that is turned always away from the Earth.


A Review of Space Research: The Report of the Summer Study conducted under the auspices of the Space Science Board of the National Academy of Sciences at the State University of Iowa, Iowa City, Iowa, 17 June-10 August 1962, Publication 1079, National Academy of Sciences – National Research Council, Washington, DC, 1962

"Dust-Cloud Moons of the Earth," J. Wesley Simpson, Physics Today, February 1967, p. 39

More Information

Earth-Approaching Asteroids as Targets for Exploration (1978)


  1. Very interesting post as usual but a clarification may be useful.
    Libration points are more commonly nammed Lagrangian points, and I think that the sentence
    "either L point would be an excellent location for a satellite whose objective is to perform solar-flare observations"
    did not refer (or should not have referred) to the Earth-Moon L4 nd L5 points. Indeed these points are on the same orbit as the Moon, so they are in the Earth shadow as often as the Moon itself. The Sun-Earth L1 point, on the other hand, is never in the shadow of the Earth nor of the Moon and is indeed used to observe solar activity , first with mission ISEE-3 in 1978. The Earth-Moon L4 and L5 points could indeed host the (still hypothetical) Kordylewski clouds.

    1. Simon:

      Thanks for pointing that out. I will correct the text. The original paper states that L4 and L5 would be good for solar observations, and compares it favorably with LEO as far as eclipses are concerned, but does not say that there would be no eclipses. Misinterpretation on my part! My apologies, and thanks again for pointing out my error.


  2. I am so glad that I could finally leave a comment on your blog! This required me to allow third-party cookies from all sites. Maybe other readers are encountering the same difficulty ?

    1. Not sure what this is about. Let me see if I have trouble at a different computer.



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