may be that they sincerely believed it to be true, demonstrating an almost incredible lack of communication between themselves and their working engineers." Despite Feynman's warnings, and despite the fact that Vaughan served on safety boards and committees at NASA, the subsequent press coverage has found some evidence that NASA's relative disregard for safety might persist to this day. For example, leading up to the Columbia disaster, NASA discounted the risk from small foam chunk breakage at launch and assumed that the lack of damage from prior foam collisions suggested the future risk was low. Shuttle operations The Shuttle was originally conceived to operate somewhat like an airliner. After landing, the orbiter would be checked out and start being mated to the External Tank and Solid Rocket Boosters, and be ready for launch in as little as two weeks. In practice, before the loss of Challenger, about half of the turnaround time after a mission was unplanned tests and modifications based on unexpected events that occurred during flight. The process usually took months; Atlantis set the pre-Challenger record by launching twice within 54 days, while Columbia set the post-Challenger record of 88 days. The Shuttle program's goal of returning its crew to Earth safely conflicted with the goal of a rapid and inexpensive payload launch. Furthermore, because in many cases there were no survivable abort modes, many pieces of hardware had to function perfectly and so required careful inspection before each flight. The result was high labor cost, with around 25,000 workers in Shuttle operations and labor costs of about $1 billion per year. Some shuttle features initially presented as important to Space Station support have proved superfluous: As the Soviets demonstrated, capsules and uncrewed supply rockets are sufficient to supply a space station. NASA's initial policy of using the Shuttle to launch all crewless payloads declined in practice, and eventually was discontinued. Expendable launch vehicles (ELVs) proved much cheaper and more flexible. Following the Challenger disaster, use of the Shuttle to carry the powerful liquid fueled Centaur upper stages planned for interplanetary probes was ruled out for Shuttle safety reasons. The Shuttle's history of unexpected delays also made it liable to miss narrow launch windows. Advances in technology have made probes smaller and lighter. As a result, many robotic probes and communications satellites can now use expendable launch vehicles, such as the Delta and Atlas V, which are less