The Superconducting Magnet Instrument for Light Isotopes (SMILI) flew
for 19 hours on 1989 September 1 with a residual overburden of 5 g cm-
2. It measured the charge, rigidity, and velocity of 30,000 cosmic-ray
helium nuclei, with velocity determined by time-of-flight and Cerenko
v techniques. Using these data, the flux and isotopic composition of h
elium as a function of energy were determined. The observed isotopic c
omposition is consistent with that expected from interstellar propagat
ion models inferred from the secondaries of CNO, in contrast to earlie
r observations which indicated an overabundance of He-3. We discuss co
nstraints that this result places on cosmic-ray transport and solar mo
dulation models.