In December 1986, Gorbachev allowed exiled dissident Sakharov to return to Moscow and when he encountered foot-dragging at a January 1987 Central Committee plenum on democratic change, Gorbachev stepped up glasnost to mobilize public opinion to outflank conservative opponents. An explosion of formerly secret information about Soviet history and contemporary reality undermined belief in the system Gorbachev was seeking to rejuvenate. In 1987, the Soviet economy entered a decline from which it never emerged. In October Moscow party boss Boris Yeltsin was fired after he attacked Gorbachev for slowness in reform. In the spring of 1988, Gorbachev overcame a conservative challenge launched through a critical letter by Nina Andreyeva. In May Gorbachev used the nineteenth party conference to launch changes to party procedures intended to make perestroika irreversible.