After the publication of "Ringworld", many fans identified numerous engineering problems in the Ringworld as described in the novel. One major one was that the Ringworld, being a rigid structure, was not actually in orbit around the star it encircled and would eventually drift, ultimately colliding with its sun and disintegrating. This led MIT students attending the 1971 Worldcon to chant, "The Ringworld is unstable!" Niven wrote the 1980 sequel "The Ringworld Engineers" in part to address these engineering issues. In it, the ring is found to have a system of attitude jets atop the rim walls, but the Ringworld has become gravely endangered because most of the jets have been removed by the natives, to power their interstellar ships. (The natives had forgotten the original purpose of the jets.)