The 1970s would see the end of the Vietnam War and a relaxing of Cold War tensions. Political scandal would bring down a president and change politics for decades to come. After two successful Moon landings, the public's interest and enthusiasm for the Apollo program was beginning to wane, but on 11 April 1970, an incident during the Apollo 13 mission would once again grab the world's attention: an oxygen tank in the command service module ruptured en route to the Moon, causing an explosion. The lunar landing was canceled, and the astronauts struggled to survive as they guided their damaged craft around the Moon in order to gain enough momentum to safely return to Earth. Two weeks later, on 4 May, Ohio National Guardsmen opened fire on a crowd of antiwar protesters at Kent State University, killing four and wounding nine others; a few days after that, Mississippi policemen would kill two and wound eleven at Jackson State. Also in 1970, a government report on the military recommended an all-volunteer force, the Environmental Protection Agency was established, Congress passed the Occupational Safety and Health Act (OSHA), the floppy disk was invented, and, in accord with the changes set forth by the Second Vatican Council, the entire Roman Catholic Mass began to be recited in the common language of each parish. On 31 December, as 1970 drew to a close, Congress repealed the Gulf of Tonkin Resolution, which had given the president broad authority in executing the war in Vietnam. Following a safety review of the Apollo program, on 31 January 1971, Apollo 14 marked humanity's return to manned exploration of the Moon. Several months later, on 6 June 1971, three Soviet cosmonauts became the first crew from Earth to work in a space station. After 23 days on the station, they returned to Earth, but the cramped conditions of the Soyuz capsules prohibited them from wearing their space suits during the flight, and all three died when an air leak developed during re-entry. A month after the tragedy, Apollo 15 marked the fourth successful Moon landing on 26 July. Also in 1971, The Pentagon Papers, which revealed deeper American involvement in the Vietnam War than the executive branch had ever acknowledged, were leaked by former Defense Department aide Daniel Ellsberg and published in The New York Times; the draft was extended for two more years, and student deferments were eliminated; Texas Instruments developed the first pocket calculator; Intel introduced the first microprocessor, and the 26th Amendment lowered the voting age in the United States to eighteen. In an effort to slow down spiraling inflation, President Richard Nixon imposed a 90-day freeze on wages and prices and severed the dollar's tie to gold, which meant that, from that point onward, the dollar would have no fixed value and would float on the world currency market. On 16 April 1972, Apollo 16 landed on the Moon. That year, the Supreme Court declared capital punishment to be unconstitutional, and President Nixon traveled to China in what was widely considered to be the foreign relations coup of his presidency. In November, Nixon would be reelected, but a burglary at the Democratic National Headquarters by representatives of the Republican Party would ultimately mar his victory; the affair and its subsequent cover-up, known as the Watergate scandal, would eventually bring down his presidency and would color American politics for decades to come. As the year drew to a close, the final Moon landing of the Apollo program, Apollo 17, took place on 7 December. American involvement in the Vietnam War ended in 1973, following the signing of peace accords on 27 January. The same day, President Nixon announced an end to the regular draft, though the draft instituted by the Superhuman Induction Act, which was separate from that created by the Selective Service Act, would remain in effect. The War Powers Resolution, passed by Congress over Nixon's veto, limited presidential war-making power by establishing guidelines for military emergencies in the hope of preventing the continuation of another unpopular war like Vietnam had become. On 14 May, Skylab, an American space station, was launched, unmanned, into orbit; it would see three astronaut visits between 1973 and 1974 before falling into disuse due to budget cuts, and would eventually be allowed to burn up on reentry into the atmosphere in July 1979. In the wake of the October 1973 Yom Kippur War, Arab leaders, frustrated by Israeli success and believing that success to be due largely to American aid, declared an embargo on shipments of oil to the United States, other Western nations, and Japan. The embargo would last until March 1974 and would cause soaring fuel prices and gas lines in the United States. Also in 1973, Vice President Spiro Agnew, charged with tax evasion, resigned and was replaced by Gerald Ford, the first person to ever hold the office without being elected; the Supreme Court ruled in Roe v. Wade that women had the legal right to obtain abortions; Intel marketed the 8080 microprocessor, which became the central processing unit of several microcomputers; and, as the year drew to a close, the Watergate hearings began. In 1974, the House Judiciary Committee instituted impeachment proceedings against President Nixon, but he resigned from the presidency—the first person to do so—before hearings could begin.Vice President Ford assumed the presidency, and one of his first acts was to pardon Nixon. Also that year, Congress established guidelines for research conducted on humans with the passage of the National Research Act. President Ford halted compulsory draft registration in 1975 and also ended the superhuman draft under the Superhuman Induction Act, though the most powerful superhumans would continue to be drafted, and superhuman registration was still required. The first prison specifically designed to hold superhuman criminals opened in a remote area of the Arizona desert in early 1975, and North Vietnamese forces captured Saigon, the capital of South Vietnam, in April 1975, bringing an end to the Vietnam War. Also in 1975, in the final Apollo mission, the first U.S.-Soviet space link occurred 140 miles above Earth when an Apollo capsule docked with a Soyuz capsule. President Jimmy Carter, who had been elected in November 1976, issued a blanket pardon to draft evaders shortly after his inauguration in January 1977, allowing them to return to the United States without fear of arrest. The Supreme Court in 1977 reversed itself, now ruling that the death penalty was legal. On 12 August 1977, the National Aeronautics and Space Administration unveiled the prototype Space Shuttle. Though NASA officials wanted to name the prototype Constitution, President Ford had ordered that it be named Enterprise after receiving 100 thousand letters from fans of the television series Star Trek, begging him to name the first reusable spacecraft after the ship featured in the show. The prototype shuttle had no engines and was used for test gliding flights and a publicity tour before being sent first to Kennedy Space Center at Cape Canaveral, Florida, and then to the Smithsonian Institution in Washington, D.C., where it would sit in the open and unmaintained for decades before a planned museum complex is built around it. America and Iran would come to be at odds in 1979, when religious conservatives ousted the shah, a longtime friend of the United States. In the aftermath of the revolution, Iranian islamic militants seized the U.S. embassy in Tehran and would hold 52 embassy hostage for 444 days; the humiliation of the hostage crisis would contribute to Carter's defeat in the 1980 election.