The Small Astronomy Satellite 2, also known also as SAS-2, SAS B or Explorer 48, was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

gamma ray telescope. It was launched on 15 November 1972 into the

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...

with a

periapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

of 443 km and an

apoapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

of 632 km. It completed its observations on 8 June 1973.

Mission

SAS 2 was the second in the series of small

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

designed to extend the astronomical studies in the

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

, visible, and infrared regions. The primary objective of the SAS-B was to measure the spatial and energy distribution of primary galactic and extragalactic gamma radiation with energies between 20 and 300

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...

. The instrumentation consisted principally of a guard scintillation detector, an upper and a lower spark chamber, and a charged

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

telescope.

Launch

The spacecraft was launched on 15 November 1972 into an initial orbit of about of

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

, of

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

, 1.90° of

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

, with an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of 95.40 minutes. from the

San Marco platform The Luigi Broglio Space Center (BSC) is an Italian-owned spaceport near Malindi, Kenya, named after its founder and Italian space pioneer Luigi Broglio. Developed in the 1960s through a partnership between the Sapienza University of Rome's Aero ...

off the coast of

Kenya ) , national_anthem = " Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi ...

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, into a nearly equatorial orbit. The orbiting spacecraft was in the shape of a cylinder approximately in diameter and in length. Four solar paddles were used to recharge a 6 amp-h, eight-cell,

nickel–cadmium battery The nickel–cadmium battery (Ni–Cd battery or NiCad battery) is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. The abbreviation ''Ni-Cd'' is derived from the chemical symbols of nickel (Ni) and ...

and provide power to the spacecraft and telescope experiment. The spacecraft was spin 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 within approximately 1°. The experiment axis lay along this axis allowing the telescope to look at any selected region of the sky with its ± 30° acceptance aperture. The nominal spin rate was 1/12

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

. Data were taken at 1000 bps and could be recorded on an onboard

tape recorder An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...

and simultaneously transmitted in real time. The recorded data were transmitted once per orbit. This required approximately 5 minutes.

Experiment

The telescope experiment was initially turned on 20 November 1972 and by 27 November 1972, the spacecraft became fully operational. The low-voltage power supply for the experiment failed on 8 June 1973. No useful scientific data were obtained after that date. With the exception of a slightly degraded star sensor, the spacecraft control section performed in an excellent manner. SAS-2 first detected

Geminga Geminga is a gamma ray and x-ray pulsar source thought to be a neutron star approximately 250 parsecs (around 800 light-years) from the Sun in the constellation Gemini. Its name, attributed by its discoverer Giovanni Bignami, is both ...

, a pulsar believed to be the remnant of a supernova that exploded 300,000 years ago.Geminga
The

Internet Encyclopedia of Science David Darling (born 29 July 1953 in Glossop, Derbyshire) is an English astronomer, freelance science writer, and musician. Darling has published numerous popular science works, including ''Life Everywhere: The Maverick Science of Astrobiolog ...

Gamma-Ray Telescope

The instrument consisted of two spark-chamber assemblies, four plastic scintillation counters, four Cherenkov counters, and an anticoincidence scintillation counter dome assembled to form a telescope. The spark chamber assembly consisted of 16-wire spark-chamber modules with a magnetic core readout system. Sandwiched between these two assemblies was a plane of plastic scintillator formed by the four scintillation counters. Thin tungsten plates, averaging thick, were interleaved between the spark chamber modules, which had an active area of 640-cm². These plates provided the material for the gamma ray to convert into an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

- positron pair and provided a means of determining the energy of these particles by measuring their coulomb scattering. The spark chamber modules revealed the position and direction of the particles; from this information, the energy and direction of the gamma ray was determined. The scintillation counters and the four directional Cerenkov counters that were placed below the second spark chamber assembly constituted four independent counter coincidence systems. The single-piece plastic scintillator dome surrounded the whole assembly except at the bottom to discriminate against charged particles. The threshold of the instrument was about 30-MeV, and energies up to about 200-MeV could be measured along with the integral flux above 200 MeV. The angular resolution of the telescope varied as a function of energy and arrival direction from 1.5° to 5°. During the lifetime of the experiment from 15 November 1972 to 8 June 1973, approximately 55% of the celestial sphere, including most of the galactic plane, was surveyed.

Notes

References