Can intensity interferometry play a useful role in a space environment?

The intensity interferometer was developed by Hanbury Brown and Twiss to use correlations between intensity fluctuations in order to measure the coherence between light waves. Correlating the intensities of the waves and not their amplitudes can overcome the main disadvantage of the Michelson Stellar Interferometer in a space environment, which is the present inability to maintain sufficient stability in the optical path difference through apertures which are located on different spacecrafts. Using the Michelson Stellar Interferometer, path length equalization required between the interfering waves is of the order of the wavelength of interest. Since this is in the visible or infrared, (10 - 0.5μm) the required control of the distance between the spacecraft is yet to be demonstrated. However, since an intensity interferometry could today measure correlation between fluctuations on a time scale of 5.10^-10 sec, stability of the order of cm is required, and this is achievable at the present time. The disadvantage of intensity interferometry, its lack of photon sensitivity, can be compensated in space by long integration times. We show that intensity interferometry should be applicable to stellar sources as weak as 8^th magnitude, using techniques which are theoretically feasible today. Furthermore, the use of three or more satellites for intensity interferometry was considered. With this system a triple correlation can be constructed. From the triple correlation, information about the phase can be found and together with the amplitude information as obtained from the second order correlation, imaging is in principle made possible.