Project ICEF—the Skyhook 60 flights

Late last January, an ambitious cosmic‐ray experiment conducted by scientists from fourteen nations and sponsored jointly by the National Science Foundation and the Office of Naval Research was set in motion from the flight deck of the “USS Valley Forge”, then cruising just east of the Leeward Islands and the Caribbean. The object was to send two Navy Skyhook balloons of unprecedented size to an altitude of 18–22 miles and to keep them aloft for two full days. Each balloon was to carry in its gondola an 800‐pound stack of photographic emulsion sheets for recording primary cosmic‐ray events involving energies as high as ${10}^{15}$ electron volts—energies thousands of times greater than are available in the world’s largest high‐energy accelerators. The Caribbean area was chosen because of its proximity to the equator, where the earth’s magnetic field serves as a barrier to cosmic rays of energies lower than about 10 BeV, effectively filtering them out.