Complex sentence: Mercury or caesium atoms were used as fuels during tests in the 1960s and 1970s, these fuels would adhere to and eroded the grids; xenon atoms on the other hand were far less corrosive, and became the fuel of choice for virtually all ion thruster types.
Split into simpler sentences: Mercury or caesium atoms were used as fuels during tests in the 1960s and 1970s, but these fuels adhered to and eroded the grids. Xenon atoms, on the other hand, are far less corrosive, and became the fuel of choice for virtually all ion thruster types.