ENHANCED RADAR BACKSCATTER FROM SPACE-SHUTTLE EXHAUST IN THE IONOSPHERE

Enhancements in the backscatter from the 430-MHz radar at Arecibo were recorded during the Spacelab 2 mission when the space shuttle orbital maneuver system (OMS) engines were fired in the ionosphere. The modifications in the backscatter could have been the result of (1) compression of the electrons to produce higher densities, (2) generation of ion acoustic waves, (3) variations in the electron to ion temperature ratio, (4) enhanced scatter cross section by charging of ice particles in the exhaust, or (5) excitation of dust acoustic waves. Rapid cooling and condensation of the exhaust are important in determining the scattering properties of the modified ionosphere. A dusty plasma is formed when electrons are attached to ice particles in the exhaust plume. The calculated neutral temperature inside the exhaust plume is 120 K. Charge exchange between ambient O+ and the cold exhaust molecules yields low-temperature ion beams that excite weakly damped, ion acoustic waves. The enhanced radar echoes are probably the result of scatter from these waves, but the effects of the dusty plasma may be important. During future experiments, the space shuttle will fire the OMS engines over radars located at Arecibo, Puerto Rico; Jicarmarca, Peru; or Kwajalein, Marshall Islands. Measurements of the spectra from these radars will provide the means to distinguish between the various backscatter processes.