Pioneer was a diverse series of NASA spacecraft designed for lunar and interplanetary exploration. The first few to be launched were either total or partial failures, although Pioneer 3 did send back data leading to the discovery of the outer of the Van Allen Belts. Pioneer 5 returned important data on solar flares, the solar wind, and galactic cosmic rays, and also established a record at the time of 36.2 million kilometers for radio communication in space. Pioneers 6 through 9 went into elliptical orbit around the Sun and carried out further observations of the solar wind and interplanetary magnetic field. Pioneer 10 and Pioneer 11 were the first probes to explore the outer solar system, while the Pioneer Venus probes sent back valuable data from Earth's inner neighbor.
Early Pioneers: a failed lunar programThe Pioneer program began in 1958 when the Advanced Research Projects Agency of the United States Department of Defense authorized the launch of five spacecraft toward the Moon. The first was lost in a launch failure just 77 sec after liftoff on Aug. 17, 1958, and is referred to as Pioneer 0 or Able 1. The second, designated Pioneer 1, was the newly-formed NASA's inaugural mission but failed to reach escape velocity and crashed into the South Pacific. Likewise, Pioneer 2 and 3 fell back to Earth. Pioneer 4 did at least manage to fly past the Moon, but its closest approach of 60,000 km was too remote for any lunar data to be returned. Four more probes followed, all of which failed, before the Pioneer lunar program was abandoned.
Pioneer 5–9: success further afieldBeginning with Pioneer 5, the program shifted focus to interplanetary space – and immediately became more successful. Pioneer 5 was placed in a solar orbit which ranged between Earth and Venus and provided the first experience of communicating with a spacecraft at distances of tens of millions of km. It was followed by a series of identical, spin-stabilized probes equipped with sensors to monitor the solar wind, magnetic fields, and cosmic rays. All vastly exceeded their design lifetimes of six months. Pioneer 9 finally failed in 1983 after 15 years service, contact with Pioneer 6 was reestablished on Dec. 8, 2000 – 35 years after launch, and data from one science instrument on each of Pioneer 7 and 8 are still being received. For more details, see Pioneer 6, Pioneer 7, Pioneer 8, and Pioneer 9.
Pioneer 10 and 11: Jupiter and beyondTwin probes that became the first to cross the asteroid belt and fly past Jupiter. Pioneer 11 used a Jupiter gravity-assist to redirect it to an encounter with Saturn. Both spacecraft, along with Voyager 1 and Voyager 2, are now leaving the solar system. Of this interstellar quartet, only Pioneer 10 is heading in the opposite direction to the Sun's motion through the Galaxy. It continues to be tracked in an effort to learn more about the interaction between the heliosphere and the local interstellar medium. Pioneer 10's course is taking it generally toward Aldebaran (65 light-years away) in the constellation of Taurus, and a remote encounter about two million years from now. It was superseded as the most distant human-made object by Voyager 1 in mid-1998. The last communication from Pioneer 11 was received on Nov. 30, 1995. With its power source exhausted, it can no longer operate any of its experiments or point its antenna toward Earth but continues its trek in the direction of the constellation of Aquila.
Pioneer VenusAfter a six-month journey, Pioneer Venus 1 entered an elliptical orbit around Venus in December 1978 and began a lengthy reconnaissance of the planet. The spacecraft returned global maps of the Venusian clouds, atmosphere, and ionosphere, measurements of the interaction between the atmosphere and the solar wind, and radar maps of 93% of the planet's surface. In 1991 the Radar Mapper was reactivated to investigate previously inaccessible southern portions of the planet. In May 1992 Pioneer Venus began the final phase of its mission, in which the periapsis (orbital low point) was held at 150–250 km until the fuel ran out and atmospheric entry destroyed the spacecraft. Despite a planned primary mission duration of only eight months, the probe remained in operation until Oct. 8, 1992.
Pioneer Venus 2 consisted of a bus which carried one large and three small atmospheric probes. The large probe was released on Nov. 16, 1978, and the three small probes on Nov. 20. All four entered the Venusian atmosphere on Dec. 9, followed by the bus. The small probes were each targeted at different parts of the planet and were named accordingly. The North probe entered the atmosphere at about 60° latitude on the day side. The Night probe entered on the night side. The Day probe entered well into the day side, and was the only one of the four probes that continued to send radio signals back after impact, for over an hour. With no heat-shield or parachute, the bus survived and made measurements only to about 110 km altitude before burning up. It afforded the only direct view of the upper atmosphere of Venus, as the probes did not begin making direct measurements until they had decelerated lower in the atmosphere.
Related category SATELLITES AND SPACE PROBES
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