Explorer 11 (also known as S-15) was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

satellite that carried the first space-borne

gamma-ray telescope Gamma-ray astronomy is the astronomical observation of gamma rays,Astronomical literature generally hyphenates "gamma-ray" when used as an adjective, but uses "gamma ray" without a hyphen for the noun. the most energetic form of electromagneti ...

. This marked the beginning of space gamma-ray astronomy. Launched on 27 April 1961 by a

Juno II Juno II was an American space launch vehicle used during the late 1950s and early 1960s. It was derived from the Jupiter missile, which was used as the first stage. Development Solid rocket motors derived from the MGM-29 Sergeant were used ...

, the satellite returned data until 17 November 1961, when power supply problems ended the science mission. During the spacecraft's seven-month lifespan it detected twenty-two events from gamma-rays and approximately 22,000 events from

cosmic radiation Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

Launch

Explorer 11 was launched for the purpose of detecting the sources of high-energy gamma rays. The spacecraft achieved an orbit with an

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 ellip ...

of , a

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 ellip ...

of , a

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

of 108.10 minutes, and an

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

of 28.90°. In addition to detecting gamma rays, Explorer 11 was designed to map their direction with emphasis on the plane of the

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

, the

galactic center The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...

, the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, and other known

radio noise source A radio noise source is a device that emits radio waves at a certain frequency, used to calibrate radio telescopes such that received data may be compared to a known value, as well as to find the focal point of a telescope soon after construction, ...

s; to relate the measurements to the cosmic-ray flux density and the density of interstellar matter; and to measure the high-energy gamma-ray albedo of the Earth's atmosphere. The satellite was a spin-stabilized octagonal aluminum box () on a cylinder ( in diameter and long). Explorer 11 was constructed so that its stable motion was an end-over-end tumble about the transverse principal axis that had the largest moment of inertia. The gamma-ray telescope assembly was mounted so that its axis of sensitivity, which was parallel to the long axis of the satellite, would rotate in the plane of tumble. The orientation of this axis in space was determined to approximately 5° by means of optical aspect detectors and the use of the known radiation pattern of the vehicle antenna. Telemetry was provided only in real time by two PM transmitters, since the onboard tape recorder failed at launch.

Instruments

Crystal Sandwich / Cherenkov Counter

The instrumentation for the Explorer 11 crystal sandwich Cherenkov detector experiment was the same as that used for the gamma-ray telescope experiment. The gamma-ray experiment was designed by researchers at the

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...

(MIT) primarily to detect cosmic gamma rays greater than 50

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 ...

. Charged particle data were collected using the same instrument. This telescope was used to determine the intensity and pitch-angle distribution of geomagnetically trapped

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s. The telescope consisted of an anticoincidence plastic shield, layers of

Sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...

(NaI) and

Caesium iodide Caesium iodide or cesium iodide (chemical formula CsI) is the ionic compound of caesium and iodine. It is often used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are hi ...

(CsI) crystals, and a cylindrical Cherenkov detector. When the anticoincidence requirement of the plastic scintillator shield was removed, charged particle information was recorded by all three counters. In addition, charged particle coincidences between the crystal sandwich and Cherenkov detectors were recorded. In this mode, directional information was obtained. The solid-angle-area factor of the telescope was about . The look direction of the telescope was identical to the symmetry axis of the spacecraft. For a beam incident parallel to the look direction of the telescope, the detection efficiency fell to 0 at 15° from this direction. The energy thresholds for each detector were as follows: (1) scintillation plastic (upper portion),

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 kn ...

s -350 keV, protons -3.5 MeV, (2) scintillation plastic (lower portion), electrons -400 keV, protons -35 MeV, (3) crystal sandwich, electrons -400 keV, protons -75 MeV, and (4) Cherenkov, electrons -15 MeV, protons -350 MeV. The accumulation time for the charged particle data was approximately 30 seconds. A single scaling circuit in Explorer 11 permitted one channel at a time to be monitored. During the 7 months in which the instrument was turned on and working in orbit, only 141 hours (3%) were considered useful observing time. During this time, the telescope was monitored for gamma rays and charged particles.

Phoswich-Cherenkov Counter Telescope

This experiment was designed to search for high-energy gamma rays (greater than 50

) from the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

. The basic detector scheme consisted of a sandwich of NaI and CsI scintillating crystals (20 g/cm²), viewed by a single

photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sho ...

, and a Lucite Cherenkov counter, viewed by two photomultipliers. It was completely surrounded by a shield of scintillating plastic viewed by five photomultipliers. The sandwich detector provided high-atomic-number material for the pair production process, i.e., energetic gamma rays were converted into

positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...

pairs. The electrons and positrons then entered the Cherenkov counter, which detected particles traversing its volume in only the downward sense. The simultaneous electric signals from its two PMTs indicated that one or more charged particles had traversed the telescope. The signals or lack of signals from the surrounding plastic shield at the instant the telescope had been triggered indicated whether the triggering was caused by an uncharged or charged particle. Also, the "last" and "total" components of the signals from the photomultiplier that viewed the sandwich detector afforded a method of distinguishing neutron and gamma-ray induced events. The experiment performed normally from launch until 17 November 1961. The Explorer 11 telescope, developed at

(MIT), used a combination of a sandwich scintillator detector along with a Cherenkov counter to measure the arrival directions and energies of high-energy gamma rays. Since the telescope could not be aimed, the spacecraft was set in a slow spin to scan the celestial sphere. Due to a higher than planned orbit that carried the spacecraft into the detector-jamming radiation of the Van Allen radiation belt, and an early failure of the on-board tape recorder, only 141 hours of useful observing time could be culled from about 7 months during which the instrument operated. During this time thirty-one "gamma-ray signature" events were recorded when the telescope was pointing in directions well away from the

atmosphere of Earth The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

, which is a relatively bright source of gamma rays produced in interactions of ordinary cosmic ray

s with air atoms. The celestial distribution of the thirty-one arrival directions showed no statistically significant correlation with the direction of any potential cosmic source. Lacking such a correlation, the identification of the cause of the thirty-one events as gamma rays of cosmic origin could not be established. The results of the experiment were therefore reported as upper limits that were significantly lower than the limits obtained from previous balloon-borne experiments. An improved gamma-ray telescope, also developed at MIT, was flown on the Orbiting Solar Observatory 3 (

OSO 3 OSO 3 (Orbiting Solar Observatory 3), or Third Orbiting Solar Observatory (known as OSO E2 before launch) was launched on March 8, 1967, into a nearly circular orbit of mean altitude 550 km, inclined at 33° to the equatorial plane. Its on- ...

), which was launched in 1967. It achieved the first definitive observation of high-energy cosmic gamma rays from both galactic and extragalactic sources. Later experiments, both in orbit and on the ground, have identified numerous discrete sources of cosmic gamma rays in our

and beyond.

References

External links