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 federal government of the United States, US federal government responsible for the United States ...

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

low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...

with a

periapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

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. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

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 that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

designed to extend the astronomical studies in the

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

gamma-ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists ...

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

, 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 through an electric potential difference of one volt in vacuum. When us ...

. 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 corpuscle 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 s ...

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. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

, of

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

, 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 Malindi Space Center (LBMSC) located near Malindi, Kenya, is an Italian Space Agency (ASI) Spaceport. It was named after its founder and Italian space pioneer Luigi Broglio. Developed in the 1960s through a partnership betwe ...

off the coast of

Kenya Kenya, officially the Republic of Kenya, is a country located in East Africa. With an estimated population of more than 52.4 million as of mid-2024, Kenya is the 27th-most-populous country in the world and the 7th most populous in Africa. ...

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, 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) an ...

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 r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...

. 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 bot ...

, a

pulsar A pulsar (''pulsating star, on the model of quasar'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its Poles of astronomical bodies#Magnetic poles, magnetic poles. This radiation can be obse ...

believed to be the remnant of a

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

that exploded 300,000 years ago.Geminga
The

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

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 in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...

pair and provided a means of determining the energy of these particles by measuring their

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

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