Challenger Explosion (Jan. 28): It is bitterly cold by Florida standards. Icicles hang from the gantry adjoining the shuttle Challenger on its launch pad at the Kennedy Space Center. The crew includes a high school teacher, Christa McAuliffe, who is to become the first typical citizen to travel in space. After a two-hour delay to allow the ice to melt, the launch proceeds. For the first minute, it looks like every other launch. Then, after 73 seconds, disaster.

Explosion of the Challenger

"Uh-oh," says co-pilot Michael J. Smith, the last word from the spacecraft. Giant plumes of smoke shoot out as the explosion paints history against a bright blue sky. A presidential commission concludes the explosion was caused by faulty O-ring seals on the solid fuel rocket booster. Tests had shown the seals to be unreliable in cold weather. But NASA failed to heed warnings because of pressure to meet an "overambitious" launch schedule, the commission reports.

Abdication to Aquino (Feb. 27): After defeat at the polls, Ferdinand Marcos agrees to hand over the presidency of the Philippines to Corazon Aquino and flies to Honolulu.

Making His Day (April 8): Movie star Clint Eastwood is elected mayor of Carmel, Calif.

	Chernobyl

Chernobyl Catastrophe (April 28): Scientists in Sweden detect high radiation levels over Stockholm. Finland, Norway and Denmark report similar findings. After first denying an accident, Soviet officials confirm a "mishap" at the Chernobyl nuclear power station in Ukraine. Soviet bureaucrats downplay the damage, but it soon becomes clear that the reactor is not only damaged but on fire. It releases a radiation cloud 10 times more potent than that unleashed by the atomic bomb used on Hiroshima in 1945. More than 200,000 people are evacuated, but only after a 36-hour delay.

American in Paris (July 27): Greg LeMond becomes the first American bicyclist to win the Tour de France race.

Sticking to His Guns (Oct. 13): A summit in Iceland ends in disappointment when President Reagan refuses to accept a Soviet demand that he scrap development of the so-called Star Wars missile defense system.

	Reagan

Operation Uncovered (Nov. 13): President Reagan acknowledges that arms were sold to Iran but denies any exchange for American hostages in Lebanon. On Nov. 25, the White House discloses that some profits from the arms sales were diverted to support the Contras in Nicaragua, sidestepping congressional action to cut aid to the rebels. Reagan says he "was not fully informed," and Marine Lt. Col. Oliver North is fired. Attorney General Ed Meese asks for an investigation by an independent counsel, and Iran-Contra, as it becomes known, is born.