The first medium-class mission was selected from proposals formulated by ESA in 1982, prior to Horizon 2000. A Titan Probe that would piggyback on the American Cassini spacecraft was proposed by a group of American and European scientists, and was selected alongside the American-European LYMAN ultraviolet and QUASAT very-long-baseline interferometry observatories as finalists. The European-Soviet Vesta multiple-flyby minor planet mission and GRASP gamma-ray observatory competed, but were rejected by the AWG and SSWG. After budget cuts resulting from the Challenger disaster forced NASA to retract its support for LYMAN and QUASAT, the Titan Probe was selected by the SPC in November 1988, and renamed Huygens in honor of Christiaan Huygens, who discovered Titan in 1655, per the suggestion of Swiss astronomers at the meeting. In the competition for the second medium-class mission in June 1989, a consortium of American and European institutions proposed INTEGRAL, a gamma-ray observatory that combined GRASP with the American Nuclear Astrophysics Explorer (NAE), which had lost selection for NASA's Explorers Programme that year. NASA supported the proposal, and the Russian Academy of Sciences later offered launch aboard a Proton launch vehicle in exchange for observation time on the mission. Despite concerns about NASA's commitment to the mission and their funding sources, INTEGRAL was selected by the SPC in June 1993, with NASA contributing Deep Space Network services and a spectrometer. In response, INTEGRAL was selected by NASA as an Explorers mission without competition. This, along with concerns about the sensitivity of the spectrometer designed for the mission, proved controversial among NASA's advisory bodies. In September 1994, ESA and NASA resolved to end NASA's involvement with the spectrometer, citing a lack of financial support. CNES promptly assumed the financial burden, and led the design and manufacturing of a new spectrometer.
