Explorer 42

Explorer 42 satellite

The satellite's name, "Uhuru", is the Swahili word for "freedom". It was named in recognition of the hospitality of Kenya from where it was launched, from the Italian San Marco launch platform near Mombasa.

Explorer 42 was the first in the series of small spacecraft whose objectives were to survey the celestial sphere and search for sources radiating in the X-ray, gamma-ray, Ultraviolet, and other spectral regions. The primary mission of Explorer 42 was to develop a catalog of celestial X-ray sources by systematic scanning of the celestial sphere in the energy range from 2-20 keV. The Uhuru Catalog, issued in four successive versions, the last being the 4U catalog, was the first comprehensive X-ray catalog, contains 339 objects and covers the whole sky in the 2–6 keV band. The final version of the source catalog is known as the 4U Catalog;[2] earlier versions were the 2U and 3U catalogs. Sources are referenced as "4U 1700-37".[3]

The spacecraft was launched 12 December 1970 from the San Marco platform off the coast of Kenya, Africa, into a near-circular equatorial orbit. The orbiting spacecraft was in the shape of a cylinder approximately 56 cm (22 in) in diameter and 116 cm (46 in) in length. Four solar paddles were used to recharge a 6 amp-h, eight-cell, nickel–cadmium battery, and to provide power to the spacecraft and experiment. The spacecraft was stabilized by an internal wheel, and a magnetically torqued commandable control system was used to point the spin axis of the spacecraft to any point of the sky. The aspect sensing system consisted of both a star sensor and Sun sensor that shared the same processing electronics. The system was designed with heavy emphasis on redundancy, not only in the more obvious areas such as aspect sensors and high- and low-voltage power supplies, but also in signal switching and high-voltage distribution. The resulting instrument was capable of sustaining several simultaneous major failures without seriously compromising the scientific objectives. Data were stored on an one-orbit storage tape recorder and telemetered during a 3.4 minutes playback cycle. A 1000 bps PCM/PM system was used.[4]

The main objectives of the mission were:[5]

• To survey the sky for cosmic X-ray sources in the 2–20 keV range to a limiting sensitivity of 1.5 × 10⁻¹⁸ joule/(cm²-sec), 5 × 10⁻⁴ the flux from the Crab Nebula
• To determine discrete source locations with a precision of a few square minutes of arc for strong sources and a few tenths of a square degree at the sensitivity limit
• To study the structure of extended sources or complex regions with a resolution of about 30 arc-minutes
• To determine gross spectral features and variability of X-ray sources
• To perform, wherever possible, coordinated and/or simultaneous observations of X-ray objects with other observers.

The payload consisted of two sets of proportional counters, each with ~0.084 m² effective area. The counters were sensitive with more than 10% efficiency to X-ray photons in the ~2–20 keV range. The lower energy limit was determined by the attenuation of the beryllium windows of the counter plus a thin thermal shroud that was needed to maintain temperature stability of the spacecraft. The upper energy limit was determined by the transmission properties of the counter filling gas. Pulse-shape discrimination and anticoincidence techniques were used to filter out emissions of particles and undesirable high-energy photons in the background. Pulse-height analysis in eight energy channels was used to obtain information on the energy spectrum of the incident photons. The two sets of counters were placed back to back and were collimated to 0.52° × 0.52° and 5.2° × 5.2° (full width at half maximum) respectively. While the 0.52° detector gave finer angular resolution, the 5.2° detector had higher sensitivity for isolated sources.[5]

Explorer 42 achieved several outstanding scientific advances, including the discovery and detailed study of the pulsing accretion-powered binary X-Ray sources such as Cen X-3, Vela X-1, and Her X-1, the identification of Cygnus X-1, the first strong candidate for an astrophysical black hole, and many important extragalactic sources.

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• Explorer 53
• Explorer program
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