to the invention of hydrogen balloons. Various theories in mechanics by physicists during the same period of time—notably fluid dynamics and Newton's laws of motion—led to the foundation of modern aerodynamics. Tethered balloons filled with hot air were used in the first half of the 19th century and saw considerable action in several mid-century wars, most notably the American Civil War, where balloons provided observation during the Siege of Petersburg. Apart from some scattered reference in ancient and medieval records, resting on slender evidence and in need of interpretation, the earliest clearly verifiable human flight took place in Paris in 1783, when Jean-François Pilâtre de Rozier and François Laurent d'Arlandes went in a hot air balloon invented by the Montgolfier brothers. The Wright brothers made the first sustained, controlled and powered heavier-than-air flight on 17 December 1903, in their revolutionary aircraft, the Wright Flyer. World War II saw a drastic increase in the pace of aircraft development and production. All countries involved in the war stepped up development and production of aircraft and flight-based weapon delivery systems, such as the first long-range bomber. After the war ended, commercial aviation grew rapidly, using mostly ex-military aircraft to transport people and cargo. This growth was accelerated by the glut of heavy and super-heavy bomber airframes like the Lancaster that could be converted into commercial aircraft. The first commercial jet airliner to fly was the British De Havilland Comet. This marked the beginning of the Jet Age, a period of relatively cheap and fast international travel. In the beginning of the 21st century, subsonic military aviation focused on eliminating the pilot in favor of remotely operated or completely autonomous vehicles. Several unmanned aerial vehicles or UAVs have been developed. In April 2001, the unmanned aircraft Global Hawk flew from Edwards AFB in the US to Australia non-stop and unrefueled. This is the longest point-to-point flight ever undertaken by an unmanned aircraft, and took 23 hours and 23 minutes. In October 2003, the first totally autonomous flight across the Atlantic by a computer-controlled model aircraft occurred. Major disruptions to air travel in the 21st century included the closing of U.S. airspace following the September 11 attacks, the closing of northern European airspace after the 2010 eruptions of Eyjafjallajökull, and the COVID-19 pandemic. Space The realistic dream of spaceflight dated back to Konstantin Tsiolkovsky, however Tsiolkovsky wrote in Russian, and this was not widely influential