Today marks the 28th commemoration of the space shuttle Challenger disaster, which occurred on January 28, 1986, some 73 seconds after the orbiter blasted off from the Kennedy Space Center, in Florida. A total of seven astronauts lost their lives when the spacecraft disintegrated due to aerodynamic forces. The STS-51-L mission Challenger was running was supposed to deliver a series of NASA Tracking and Data Relay Satellites to low-Earth orbit, and to delivery the Shuttle-Pointed Tool for Astronomy / Halley's Comet Experiment Deployable into space for the first time. A failure that occurred in an O-ring seal on the starboard Solid Rocket Booster the white rockets attached to either side of the orange External Fuel Tank allowed the tip of the Solid Rocket Booster to lean to the side, while hot gases burnt away at the External Fuel Tank. The massive tank then catastrophically exploded. The massive blast occurred off the coasts of central Florida, about 3 nautical miles over the Atlantic Ocean, at 16:38 UTC. Both Solid Rocket Booster and Challenger flew away from the blast location, and overwhelming aerodynamic forces broke the shuttle apart soon thereafter. On March 7, divers deployed from aboard the USS Preserver were able to located the crew cabin on the bottom of the Atlantic Ocean. Subsequent dives determined that not all of the seven astronauts had lost their lives in the blast. Three of the four Personal Egress Air Packs had been manually activated. The crew spent a total of 2 minutes and 45 seconds in free-fall before hitting the ocean surface. It is still unclear how many of the astronauts survived up to this point. The loss in cabin pressure that prompted them to use the Personal Egress Air Packs would have rendered them unconscious long before that. The Challenger disaster represented the first time American astronauts died during a space mission. All other fatalities, such as the ones registered in the early stages of the Apollo program, were on the ground or in the atmosphere. All shuttle flights were subsequently grounded for almost three years. As a direct result of the explosion, the US Air Force decided to scrap plans it had of launching classified military satellites aboard space shuttles, from the Vandenberg Air Force Base, in California, opting instead to use the Titan IV delivery system.