Most research stopped during World War II. After World War II, U.S. interest in high altitude research experiments resumed. The Office of Naval Research (ONR) made plans for a manned balloon flight into the upper atmosphere. Project Helios called for the construction of plastic balloons with a gondola equipped with scientific observation instruments. When the war ended, Jean Piccard returned to his work on plastic balloons. In 1947, he received funding from the Office of Naval Research (ONR) for the Helios project. Helios would consist of 80 to 100 plastic balloons that carried a sealed gondola as high as 100,000 feet (30,480 meters). Working in a wartime-created bombsite laboratory at General Mills in Minneapolis (the cereal company), Piccard worked with Otto Winzen, a young man he had met in 1946 while visiting the University of Minnesota's aeronautical laboratory, to find a suitable plastic for their balloons. They finally decided on polyethylene and then worked on how to manufacture balloons from sheets of this plastic that were only 1/1000 of an inch (0.0254 millimeters) thick.
On 25 September 1947, they launched the first large balloon since the end of World War II. The first in a series of four launches, the polyethylene balloon had a capacity of 100,000 cubic feet (2,832 cubic meters), but carried only 70 pounds (32 kilograms) of equipment. The next two test launches failed. On the fourth launch, the balloon refused to descend for three days, and the high-altitude controls, radio equipment, and insulated containers malfunctioned. The delay was a goldmine for cosmic ray researchers. Two Brookhaven National Laboratory physicists, J. Hornbostel and E.O. Salant, had flown a pair of cosmic ray plates on the mission, and they were delighted with the results that the three-day delay brought. The success of their experiment led the ONR to abandon the idea of human balloon flights and focus on unmanned research.
From 1947 on, polyethylene plastic balloons demonstrated their superiority over natural or synthetic rubber balloons for high-altitude flights. The lightweight and reasonably low-cost means of lofting instrument payloads to altitudes of more than 100,000 feet (30,480 meters) made it easier for researchers to conduct scientific experiments above 99 percent of the Earth's atmospheric mass that could measure atmospheric and cosmic effects without interference. Cosmic ray physicists were the first to use these new plastic balloons. From 1947 to 1957, literally hundreds of cosmic ray instruments and photographic plates were carried aloft under polyethylene balloons.
The ambitious Helios plan was replaced in 1947 by Project Skyhook, which used polyethylene balloons to carry instrument packages to extreme altitudes. Thousands of these balloons were sent into the stratosphere for basic research.
On 28 July 1952, Contract Nonr 675(00) between General Mills, Inc. and the Office of Naval Research was amended to provide for the execution of an experiment designed to carry scientific equipment to high altitudes. The scientific payload was supplied by the Naval Research Laboratory. General Mills supplied "Skyhook" balloons to carry aloft the scientific payload, the balloon controls, and recording and safety equipment. In addition, the flights were launched, tracked, and recovered by General Mills technical personnel.
NRL began studies of cosmic rays in 1949. For the next ten years NRL physicists developed detectors for identifying relativistic particles in cosmic rays. The detectors consisted of stacks of sensitive emulsions, and after a particle passed through, the path could be traced and analyzed. In the 1960s and 1970s, the NRL cosmic-ray-physics program shifted from particle studies to the stars, the origin of these cosmic rays. During the Gemini XI mission, NRL had several detector trays on the spacecraft that yielded important information on the origin and history of cosmic rays. NRL's principal vehicle for studying cosmic rays, however, was the Skyhook balloon, a 10-million-cubic-foot plastic balloon that carried detectors to an altitude of 140,000 feet.
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