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This Month in Physics History

January 28, 1986: The Challenger Explosion and its aftermath.

On January 28, 1986, the nation eagerly awaited the launch of Space Shuttle Challenger, NASA's pride and joy, in an historic flight: its crew included high school teacher Christa McAuliffe, the first non-astronaut citizen to be launched into space. But a mere 73 seconds into the flight, Challenger made a different kind of history. As millions of Americans watched, it burst into flames, killing all seven crew members, making it the worst space disaster ever.

Investigators viewing slow-motion replays of the shuttle just before the explosion witnessed a jet of flame shooting out of the side of one of the solid rocket boosters, burning straight into the side of the main fuel tank, causing it to explode. During Congressional hearings before the Rogers Commission assigned to investigate the tragedy, it was revealed that the technical problem lay with the rubber O-ring seals between the rear-most segment of the shuttle's right-hand solid rocket booster. The seals were intended to prevent hot exhaust gases from escaping, but because of a design flaw, they were dangerously sensitive to low temperatures. The night before the launch had been a cold one, and frost had formed on the O-ring in question, freezing it and making it brittle. A jet of hot gas escaped through a crack in the O-ring, piercing the main fuel tank in a fraction of a second. The liquid hydrogen and oxygen mixed and exploded, destroying the shuttle instantly.

The highlight of the Rogers Commission hearings was the testimony of Nobel-Prize-winning physicist Richard Feynman, who, frustrated by witnesses' vague answers and slow bureaucratic procedures, conducted an impromptu experiment that proved key to the investigation. He dunked a piece of the rocket booster's O-ring material into a cup of ice water, memorably demonstrating how it lost all resiliency at low temperatures and removing all doubt as to the technical cause of the explosion. In the commission's final report, Feynman accused NASA of "playing Russian roulette" with astronauts' lives.

That caustic observation was sparked by a more alarming finding of the commission: namely, that the safety reporting system at NASA was so weak that the commission termed it "silent", and that the agency's management structure suppressed pre-launch warnings that could have prevented the tragedy. Thiokol Corporation, the company that designed the O-ring, first discovered the flaw in 1977 and reported it to NASA, but the commission in charge of the shuttle project ignored the report, even after significant erosion to the O-rings was discovered during shuttle flights in 1981.

In the wake of the Challenger disaster and subsequent commission report, NASA invested $2 billion in nearly 400 improvements before the first post-Challenger shuttle flight on September 29, 1988, seeking to upgrade equipment, enlarge its safety corps, and inject new accountability into shuttle management. Of these, the most significant change was made to the solid rocket boosters: an internal metal latch was added, along with a third rubber O-ring and a reconfiguration of the insulation, each intended to prevent the escape of combustive exhaust gases from the side of the motor. And the seals were equipped with electric heaters to keep the O-rings from becoming brittle in cold temperatures and losing their sealing capability.

The shuttles were also equipped with a rudimentary escape system permitting a crew to bail out if faced with the prospect of ditching in the ocean. New latches to prevent a premature interruption in fuel flow to the main engines were installed in the fuel lines, the brakes and steering controls were improved, and a drag chute was fitted to the tail to increase control during high-speed landings. Many of the tiles protecting the shuttle from the heat of re-entry were replaced with larger, more durable insulation blankets. Finally, the mission management team responsible for overseeing the countdown, launching and flight operations now included NASA safety personnel and representatives from the major contractors involved in each flight.

NASA had the chance to prove it had learned its lesson in the summer of 1995, just prior to the scheduled August 5 mission of shuttle Endeavor. Thiokol alerted NASA that its inspectors had discovered pencil-point-sized scorch marks on nozzle O-rings recovered from two consecutive shuttle launches - evidence that hot exhaust gases had strayed dangerously within the booster nozzle. This time, NASA responded by postponing the mission until Thiokol scientists could correct the problem. Endeavor launched safely on September 7.

President Reagan, in his address to the nation the night following the Challenger tragedy, called the lost crew members "pioneers" in our continued efforts to explore and master space, and specifically addressed the nation's schoolchildren. "I know it's hard to understand that sometimes painful things like this happen. It's all a part of the process of exploration and discovery, it's all a part of taking a chance and expanding man's horizons," he said. "The future does not belong to the fainthearted. It belongs to the brave."

Birthdays for January:

1: Satyendranath Bose (1894)
8: Stephen Hawking (1942)
22: André Marie Ampère (1775); Lev D. Landau (1908)
23: David Hilbert (1862); Hideki Yukawa (1907)
25: J. L. Lagrange (1736)

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette