I'd prevent the Challenger launch. Manned spaceflight doesn't get shelved for an entire generation, and a young me doesn't lose hope for the future at such an early age.

Through a bizarre series of butterfly effects, the successful launch and its international attention gives bureaucrats in Pripyat an extra nudge to encourage cooperation amongst their engineers and nuclear scientists, and a critical flaw in the operation of the plant at Chernobyl is caught before it causes a catastrophic meltdown.

The cumulative effect is a continued culture of progressive technological expansion into the 90s, and the fading of the anti-intellectualism that threatened to overtake the world during the Reagan and Thatcher administrations. Hand in hand with this is a decreased militarism, as technology is increasingly seen as a tool for the betterment of humanity, and less as a means of building better weapons.

One other immediate result is in the US presidential election of 1988. A lack of meaningful engagement with the public (no "skipped the surly bonds of earth" speech) led to increasing apathy toward the outgoing Reagan administration, giving G.H.W. Bush a tougher hill to climb, and less solid footing on the issue of defense. Dukakis doesn't feel the need to do a silly photo op in a tank, but instead campaigns partly on an expansion of the space program and educational outreach programs similar to the one that brought in Christa McAuliffe.

Neoconservatism and neoliberalism wither together on the vine. Permanent human presence in space continues uninterrupted for the next two decades, with a base on the moon by the end of the century and a manned mission to Mars planned for a decade after that.

No Bushes, no rise of Al-Qaeda in 1988, no Gulf War, no Rush Limbaugh, no Clinton's, and no 9/11.