Being second is its own kind of test. Alan Shepard's Freedom 7 flight eleven weeks earlier had absorbed the nation's anxiety and adulation; by the time Grissom launched, the novelty had diminished just enough to shift the emphasis from spectacle to performance. Mercury-Redstone 4 was planned as a near-repeat of Shepard's suborbital arc, but with meaningful upgrades. The capsule carried a new centerline trapezoidal window replacing the two small portholes of Freedom 7, giving Grissom a wider view of the Earth below. A new rate stabilization control system provided finer attitude control. And the hatch - the hatch was new too.

McDonnell Aircraft had designed an explosive release mechanism to replace the original bolted side hatch, which required removing 70 bolts and several heat shingles to open. The new system used those same 70 titanium bolts, but each had a tiny hole bored through it to create a weak point. A mild detonating fuse could shear all 70 in an instant, blowing the hatch 25 feet clear in one second. It was an elegant solution to a real problem. It would also become the mission's defining controversy.

Liberty Bell 7 launched at 12:20 UTC on July 21, 1961, after three weather delays that had tested Grissom's patience. On the morning of the fourth attempt, a pad technician discovered a misaligned hatch bolt. Engineers decided that 69 of 70 bolts would suffice. The bolt was left as it was.

Grissom later admitted he was "a bit scared" at liftoff, but confidence grew with the acceleration. At 27,000 feet, cabin pressure sealed. During the climb, the horizon shifted from pale blue to absolute black - a transition that startled him despite his training. The Redstone's engine cut off two minutes and twenty-two seconds after launch. In the sudden silence of weightlessness, Grissom felt a disorienting tumble before steadying himself.

He took manual control and found the hand controller sluggish compared to the simulator, overshooting his marks in both pitch and yaw. Switching to the new rate command system, he got perfect response. Looking down through his trapezoidal window, he spotted land through the clouds - later identified as western Florida near Apalachicola. Then Cape Canaveral itself appeared, startlingly clear from a slant range of over 150 miles. The flight reached an altitude of 190 kilometers and covered 486 kilometers downrange. Everything worked as planned.

Everything worked until it didn't. After splashdown, Grissom lay in the bobbing capsule, running through his post-landing checklist while waiting for helicopter recovery. He armed the rescue aids, removed his helmet neck dam, disconnected his oxygen hose. He had not yet removed the safety pin from the hatch detonator's plunger.

Then the hatch blew. Grissom heard a dull thud, felt the blast, and saw blue sky where a sealed hatch had been. Seawater rushed in. He scrambled through the opening and into the Atlantic, immediately realizing that his spacesuit was taking on water through the open oxygen inlet. The suit grew heavier by the second. The recovery helicopter, already straining to lift the waterlogged capsule, had to cut the spacecraft loose to avoid being dragged down itself. Liberty Bell 7 sank.

Grissom treaded water for several minutes, his suit dragging him under, before a second helicopter hauled him to safety. He insisted he had not touched the plunger. NASA's investigation was inconclusive. In a 1965 interview, Grissom suggested the external release lanyard - held in place by a single screw - may have come loose. In 2021, analysis of recovery video proposed a different theory: static electricity from the helicopter's rotors may have detonated the fuse. The truth rests somewhere on the Atlantic floor.

The floor held it for 38 years. After failed attempts in 1992 and 1993, a team led by salvage expert Curt Newport and financed by the Discovery Channel raised Liberty Bell 7 on July 20, 1999 - the 30th anniversary of the Apollo 11 moon landing, and one day short of the 38th anniversary of the capsule's loss. They found it nearly 16,000 feet down, 300 nautical miles southeast of Cape Canaveral.

The titanium hull had survived the decades intact. Plastic components were fine. But aluminum and steel had corroded catastrophically - the instrument panels had disintegrated entirely, their gauges and controls lying in the astronaut's couch, still connected by wiring harnesses to panels that no longer existed. Among the artifacts recovered were Grissom's logbook, his survival knife, his flashlight, and 35 Mercury dimes he had carried as souvenirs. His camera was found, but the film yielded no usable images.

The Cosmosphere in Hutchinson, Kansas, restored the capsule, which toured nationally before going on permanent display. Grissom himself never saw his spacecraft again. Six years after Liberty Bell 7 sank, he died alongside Ed White and Roger Chaffee in the Apollo 1 fire - a disaster caused in part by a hatch that, this time, would not open fast enough.