In November 2001, on the occasion of the Enterprise's 40th birthday and as they returned from a deployment in support of Operation Enduring Freedom, sailors aboard the aircraft carrier spell out Einstein's famous formula for its tie-in to nuclear power. The ship packs eight nuclear reactors, connected in pairs to drive four propeller shafts. "This was a daring undertaking," reads a U.S. Navy blurb on the design and construction of the ship, "for never before had two nuclear reactors ever been harnessed together. As such, when the engineers first started planning the ship's propulsion system, they were uncertain how it would work, or even if it would work according to their theories."