Complex sentence: It is not clear whether other cosmonauts were told of his death immediately, or several weeks later to preserve morale; the media didn't learn of Bondarenko's death--or even of his existence--until many years later, in 1986.
Split into simpler sentences: It is not clear whether other cosmonauts were told of his death immediately. \\; the media didn't learn of Bondarenko's death--or even of his existence--until many years later, in 1986.