Approaching the knee -- balloon-borne observations of cosmic ray   composition
  Below the knee in the cosmic ray spectrum, balloon and spacecraft experiments offer the capability of direct composition and energy measurements on the primary particles. A major difficulty is obtaining enough exposure to extend the range of direct measurements sufficiently high in energy to permit overlap with ground-based observations. Presently, balloon and space measurements extend only up to ~100 TeV, well below the range of ground-based experiments. The prospect of Ultra-Long Duration Balloon missions offers the promise of multiple long flights that can build up exposure. The status of balloon measurements to measure the high energy proton and nuclear composition and spectrum is reviewed, and the statistical considerations involved in searching for a steepening in the spectrum are discussed. Given the very steeply falling spectrum, it appears unlikely that balloon experiments will be able to extend the range of direct measurements beyond 1000 TeV any time in the near future. Especially given the recent suggestions from KASCADE that the proton spectrum steepens only at 4000-5000 TeV, the chance of detecting the knee with direct measurements of protons to iron on balloons is not likely to occur without significant increases in the payload and flight duration capabilities of high altitude balloons. 
Michael L. Cherry