Four decades have passed since the first trans-oceanic supersonic passenger flight took off from London Heathrow Airport in 1976. Subsequently, more than 2.5 million passengers flew supersonically until British Airways and Air France took the Concorde out of service in 2003. Although no su personic passenger aircraft have flown since then, aviation enthusiasts, aircraft and parts manufacturers, airlines, and some Members of Congress have expressed interest in restarting supersonic air travel. Several U.S. startup companies are now developing supersonic commercial and business jets. The major issues affecting the introduction of supersonic aircraft appear to remain the same as in the Concorde era—how to translate technological advances into commercial ventures that are economically viable and acceptable to regulators and the public. Gaining international consensus and approvals to fly supersonically over other countries besides the United States may be a critical element in determining the market viability of future civil supersonic aircraft designs. International agreements would also need to address permissible conditions for supersonic flight operations over water and over polar regions that have opened up to civil aircraft operations over the past decade and offer shorter flights between the United States and Asia. Supersonic flight means flight that is faster than the speed of sound. The speed of sound in Earth's atmosphere varies depending on temperature and other atmospheric conditions. Near sea level, it is typically about 760 miles per hour (mph). At the cruising altitude of commercial aircraft, where the air is much colder, it is often less than 700 mph. The ratio of an aircraft's speed divided by the speed of sound is known as its Mach number All current commercial aircraft are subsonic, with Mach number less than 1. For example, the typical cruising speed of a Boeing 777 airliner is Mach 0.84. Flight near Mach 1 is called transonic. Aircraft typically fly at such speeds only briefly while they accelerate from subsonic to supersonic or vice versa. They do not cruise near Mach 1 because they would experience high drag. Supersonic flight is faster than Mach 1. The Concorde cruised at about Mach 2.02 (roughly twice the speed of sound) when not over land. Some military aircraft fly at even higher supersonic speeds. Flight faster than Mach 5 is known as hypersonic. Hypersonic flight is currently limited to experimental aircraft and missiles as well as spacecraft reentering the atmosphere from orbit (the space shuttle during reentry flew at about Mach 25). As an aircraft flies, it disturbs the air through which it moves. The disturbance includes air flow around the aircraft as well as traveling pressure waves that humans perceive as sound. In subsonic flight, sound waves may be emitted in all directions. In supersonic flight, because the aircraft is flying faster than sound travels, all disturbances are behind the aircraft. Instead of sound waves, the pressure waves combine to form a shock wave, which people on the ground perceive as a sudden sonic boom after the aircraft passes (see Figure 1 ). Boom-related environmental impacts and community objections have been major issues for supersonic flight. Companies and government research programs are attempting to address these concerns by designing aircraft so that the shock waves produced by different components (such as the nose, wings, and engine) spread out in space and time, producing a longer but quieter "thump" rather than combining into a single loud boom. Flying faster than the speed of sound is not a novel concept. In 1947, a U.S. Air Force experimental aircraft became the first manned aircraft to exceed Mach 1, breaking the "sound barrier." This represented an important milestone for the burgeoning post-World War II aviation industry and set the stage for fierce international competition for speed and prestige. Notable supersonic developments include the Mach 2 British/Franco Concorde supersonic aircraft and the Mach 3.3 Lockheed SR-71 reconnaissance aircraft. While early research and development focused on military aircraft, by the early 1960s interest in developing supersonic civil aircraft grew worldwide. The Soviet Union became the first country to fly a supersonic passenger plane, the Tupolev TU-144, in 1968. The aircraft, which was designed to fly at Mach 2.2 and carry 140 passengers, went into production in 1972. However, a fatal crash at the 1973 Paris Air Show ended the Soviet Union's supersonic passenger ambition. In the United States, the supersonic technology developed in military aircraft programs led to interest in developing a supersonic transport for civilian applications. In June 1963, the government announced a major program to develop a supersonic passenger aircraft under the direction of the Federal Aviation Administration (FAA). However, several serious problems soon surfaced, including the need for considerable federal funding because of a development cost beyond the capabilities of any aircraft manufacturer, the lack of interest by the airlines due to their heavy investment in subsonic jets and their doubts about the financial viability of supersonic passenger aircraft, and the major challenges of addressing environmental concerns. The FAA program was eventually terminated by Congress in 1971, amid delays in prototype development and opposition on cost and environmental grounds. The Franco-British Concorde was the product of a costly joint project of the British and the French governments. In January 1976, the first flight of the Concorde, also the world's first trans-oceanic supersonic passenger flight, took off from London Heathrow to Bahrain. More than 2.5 million passengers flew supersonically before Concorde was taken out of service in 2003. With a cruising altitude of about 65,000 feet (nearly twice as high as subsonic airliners) and a speed of over twice the speed of sound, a typical journey between London and New York on the Concorde took about three and a half hours, as opposed to about seven hours on a subsonic nonstop flight. Although the Concorde was considered an aeronautical achievement and a symbol of national prestige by many, it did not turn out to be a commercial success for a variety of reasons. As a government endeavor, the Concorde was a very costly project. Although there has not been an accurate accounting of the costs, it was argued in 1976 that the official figure of £1.46 billion had been a drastic underestimate, and that the program cost of Concorde was nearly £4.26 billion. This was approximately £29.15 billion in 2017 pounds, equivalent to about $37.52 billion in U.S. dollars. Concorde aircraft were also expensive to operate, reportedly using almost three times as much fuel per passenger mile as subsonic aircraft. This drove up operating costs considerably, especially during the period of high oil prices in the 1970s and early 1980s. High subsonic noise levels during takeoffs and landings and sonic boom impacts from cruise flight generated considerable concern. Many countries banned Concorde flights from their airspace—it was reported that nearly half the planned routes, especially those over land, were prohibited. U.S. civil aviation regulations did and still do prohibit overland supersonic flights in the continental United States. This contributed to Concorde's low utilization rate and effectively limited its flights to a limited number of oceanic routes between big cities, including scheduled trans-Atlantic flights between London and New York. Providing premium air travel on selected routes, however, failed to make Concorde flights a sustainable business. Even as the development costs of the Concorde were written off by the British and French governments, few airlines were interested in purchasing a Concorde aircraft. Of the 20 Concordes ever manufactured, 14 were sold to the state-owned carriers of the two countries involved in building the planes: seven to British Airways and seven to Air France. The remainder were built as prototypes and flight test aircraft. All other orders for the Concorde were canceled. Filling the seats on Concorde flights with paying customers was not easy. Concorde tickets were generally priced at about twice the regular first-class airfare on a comparable subsonic flight. For example, in 2003, a round trip across the Atlantic on the Concorde cost £8,000, equivalent to about $15,475 in 2017 U.S. dollars, almost twice the first-class ticket price on a Boeing 747. Once the attraction of novelty wore off, the airlines found it difficult to fill the seats, often flying at less than half capacity. The plane was also impractical for carrying cargo or mail, given the limited cargo space on the Concorde. The airlines were therefore unable to generate additional revenue from these sources, which are important supplemental revenue streams for subsonic transoceanic passenger flights. According to figures from the British government, during the first five years of Concorde operations, British Airways recorded a loss of £10.4 million and Air France a loss of £36.7 million. However, the airlines claimed that in some years the SST operations were profitable. This occasional profitability was based on the fact that Concorde's development and capital costs were absorbed by the British and the French governments. In essence, the Concorde was too expensive for the airlines to operate and maintain with consistent profitability, even though they bore none of the cost of designing and building it. On July 25, 2000, Air France New York-bound flight 4590 took off from Charles de Gaulle airport in Paris. During the take-off acceleration, one of the tires ran over a strip of metal on the runway that had fallen from a previous aircraft. The metal strip shredded the tire. Part of the rubber hit a fuel tank, sending shock waves that burst a valve. Fuel started to pour out and was ignited by sparks from the landing gear damaged by the debris.