The Relikt missions : results and prospects for detection of the microwave background anisotropy.

We review the Soviet space programme Relikt to measure anisotropies in the cosmic microwave background (CMB) at millimetre wavelengths. The first experiment - Relikt-1 - was successfully carried out in 1983-84. It produced a radio brightness map of the sky at 8 mm with an angular resolution of 5.8^deg^, which was used to find the dipole component of the CMB, to measure the flux from the galactic plane, and to set constraints on large-scale angular fluctuations. For the Harrison- Zeldovich spectrum of primordial fluctuations, the upper limit on the quadrupole component of the spectrum of angular fluctuations was found to be ({DELTA}T/T)_quad_ < 1.6 X 10^-5^ at a 95 percent confidence level. The model-independent upper limit on the quadrupole was ({DELTA}T/T)_quad_ < 3.0 X 10^-5^. The angular correlation function of fluctuations was constrained at the level <{DELTA}T{DELTA}T(θ)> <= 5x10^-9^ for angles θ from 20^deg^ to 160^deg^. Results of the Relikt-1 experiment were close to those of the COBE differential microwave radiometer (DMR). In mid-1993 another satellite carrying the Relikt-2 experiment will be launched. Relikt-2 will have much better sensitivity and more channels than the COBE DMR. Galactic emission (synchrotron, thermal and dust radiation) will probably be the main obstacle to detection of fluctuations of the CMB. We discuss the expected level of this emission and methods for correcting for it.