Explorer 7 was a NASA satellite launched on 13 October 1959, at 15:30:04
GMT
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
,
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 ...
launch vehicle from
Cape Canaveral Air Force Station
Cape Canaveral Space Force Station (CCSFS) is an installation of the United States Space Force's Space Launch Delta 45, located on Cape Canaveral in Brevard County, Florida.
Headquartered at the nearby Patrick Space Force Base, the statio ...
(CCAFS) to an orbit of and
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 50.27°.
[ ] It was designed to measure
solar X-ray and
Lyman-alpha flux, trapped
energetic particles, and heavy primary
cosmic ray
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 ...
s. Secondary objectives included collecting data on
micrometeoroid penetration, molecular sputtering and studying the Earth-atmosphere heat balance.
[ ]
Satellite description
The spin-stabilized satellite's external structure consisted of two truncated conical
fiberglass
Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...
shells joined by a cylindrical
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
center section. The spacecraft was wide at its equator and about high with a payload mass of about . The spacecraft was powered by approximately 3000
solar cell
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. s mounted on both the upper and lower shells. Additional power was provided by 15 rechargeable
nickel-cadmium batteries that were positioned on its equator near the outer skin as an aid in maintaining a proper spin rate. Two crossed dipole (1 W, 20-MHz) telemetry antennas projected outward from the center section, and a 108-MHz antenna used for tracking was mounted on the bottom of the lower shell.
Located around the periphery of the center section were five
bolometer
A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.
Principle of operation
A bolometer ...
s for thermal radiation measurements and three
cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid.Egon Wiberg, Arnold Frederick Holleman (2001''Inorganic Chemistry'' Elsevier It occurs in nature with two different crystal structures as the rare m ...
micrometeoroid detector cells. A cylindrical ion chamber (lithium fluoride window) and a beryllium window X-ray chamber were located on opposite sides of the upper cone, and a cosmic-ray
Geiger counter
A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental ph ...
was located on the very top. A primary cosmic-ray ionization chamber was located within the center portion of the upper cone. Communications and tracking were provided by a 108-MHz transmitter at 15-mW designed to operate for 2 months and a 20-MHz transmitter at 600-mW powered by the solar cells designed for cut-off after about one year.
Experiments
Ground Based Ionospheric
An all-transistor beacon telemetry transmitter was operated at a fundamental frequency of 19.9915
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
. This transmitter, which was powered by solar cells and rechargeable nickel-cadmium batteries, provided 660-mW of power at the fundamental frequency. It also radiated the second and third harmonics (15-mW each) at 39.9830 and 59.9745-MHz. Beacon receivers were known to have been located in
Washington, D.C.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
, and
Chicago
(''City in a Garden''); I Will
, image_map =
, map_caption = Interactive Map of Chicago
, coordinates =
, coordinates_footnotes =
, subdivision_type = Country
, subdivision_name ...
,
Illinois
Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...
. Interpretation of the changes of beacon signal characteristics between the satellite and the ground station showed irregularities to be present at all times of the day with horizontal dimensions of to . Data were received over a period of 16 months.
[ ]
Heavy Primary Cosmic Rays
This experiment was designed to measure the omnidirectional flux of heavy primary cosmic rays in the rigidity range 1 to 15.5
GV. Particles with atomic numbers Z >5, Z >8, and Z >15 were counted separately by an ionization chamber in which each incident particle yielded a pulse. Pulse amplitude was substantially independent of the energy of the incident particle but was proportional to the square of its Z value. Each of the three counting rates was determined every 15 seconds. The experiment performed as planned from launch until 25 October 1959. About 80% of the data acquired for the 25 October 1959, to 31 May 1960, period are useful, with most problems occurring in the lowest Z mode. Very little useful data were acquired after 31 May 1960.
[ ]
Micrometeorite
Three photoconducting
cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid.Egon Wiberg, Arnold Frederick Holleman (2001''Inorganic Chemistry'' Elsevier It occurs in nature with two different crystal structures as the rare m ...
cells were used to measure
micrometeorite
A micrometeorite is a micrometeoroid that has survived entry through the Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more abundant, and different in composition. T ...
penetration and molecular sputtering. The three cells were identical in design and effective area (18 sq mm) and were mounted on a
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
plate on the satellite's equator facing outward perpendicular to the satellite's spin axis. The experiment was exposed to the space environment of micrometeorites, trapped radiation, and sputtering for 38 days of active life. The three cells and a thermistor that was included as part of the experiment performed normally. One cell was penetrated on the 16th day by a particle approximately 10 microns in diameter.
[ ]
Solar X-Ray and Lyman-Alpha Radiation
The solar X-ray and Lyman-alpha radiation were measured by means of gas ionization chambers mounted on opposite sides of the upper portion of the double cone configuration of the Explorer 7 satellite. Intensities were monitored in order to obtain a long-term history of solar X-ray and
Lyman-alpha
The Lyman-alpha line, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. It is emitted when the atomic electron transitions from an ''n'' = 2 orbital to the gr ...
fluxes and to correlate these with terrestrial atmospheric responses. The two X-ray detectors ( deep) were filled with argon gas and had beryllium windows (.021 g/sq cm) resulting in a sensitivity to X-rays in the 2 to 8
A range. The Lyman-alpha detectors (on the opposite side), which were circular ionization chambers ( in diameter) filled with
nitric oxide gas, had
lithium fluoride
Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to ...
windows. Their sensitivity was in the 1050 to 1350 A interval. The data, however, were impossible to interpret in terms of incident solar radiation due to both the saturation of detector circuits by Van Allen radiation (150 keV
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) and electronic difficulties in the feedback amplifier.
[ ]
Thermal Radiation
The Explorer 7 thermal radiation experiment was designed to measure incident and reflected solar
UV radiation
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 i ...
and terrestrial
IR radiation in order to obtain a better understanding of the driving forces of the Earth-atmosphere system. The primary instrumentation consisted of five bolometers in the form of hollow
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
hemispheres that were thermally insulated from, but in close proximity to specially
aluminized mirrors. The hemispheres thereby behaved very much like isolated spheres in space. Two of the hemispheres had black coatings and responded about equally to solar and terrestrial radiation. A third hemisphere, coated white, was more sensitive to terrestrial radiation than to solar radiation. A fourth, which had a
gold metal surface, was more sensitive to solar radiation than to terrestrial radiation. The fifth hemisphere, protected from direct sunlight, was used to measure the reflected sunlight. A glass-coated bead thermistor was mounted on the top of each hemisphere to measure the temperature. A complete set of four temperature observations and one reference sample required 30 seconds. Thus, in each orbit, about 180 temperature measurements could be obtained. The experiment was a success, and usable data were obtained from launch until 28 February 1961.
[ ]
Significantly, it also carried
Dr. Verner E. Suomi's flat-plate
radiometer
A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the microwave w ...
, improved with the help of
Robert Parent, that took the first
Earth radiation budget
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...
measurements from space and initiated the era of
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
studies of the
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
. It made the
first coarse maps of "the solar radiation reflected by the Earth and the infrared radiation emitted by the Earth".
Using both satellite observations of the Earth's heat balance and
atmospheric
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
cooling rates measured by net flux radiosondes, Suomi established the important role played by
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may co ...
s in absorbing radiated
solar energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
. These observations established that Earth's energy budget varies markedly due to the effect of clouds, the surface albedo, and other absorbing constituents. Using these instruments, Suomi and his team discovered that the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
absorbed more of 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 ...
's energy than originally thought and demonstrated that it was possible to measure and quantify seasonal changes in the global heat budget. Explorer 7 was unable to detect solar X-rays due to its sensors being saturated by background radiation in the
Van Allen radiation belt.
[ ]
Trapped Radiation and Solar Protons
Two omnidirectional
Geiger counter
A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental ph ...
s (Anton 302 and 112) were used to conduct a comprehensive spatial and temporal monitoring of total cosmic-ray intensity, geomagnetically trapped corpuscular radiation, and
solar protons. The detector was sensitive to protons (E >20 MeV) and electrons (E >30 keV). The experiment operated satisfactorily from launch until 28 February 1961, except for a brief period in September and October 1960.
[ ]
Launch
Launch was originally scheduled for late September 1959, but the mission was delayed for a week after a
Jupiter IRBM
The PGM-19 Jupiter was the first nuclear armed, medium-range ballistic missile (MRBM) of the United States Air Force (USAF). It was a liquid-propellant rocket using RP-1 fuel and LOX oxidizer, with a single Rocketdyne LR79-NA (model S-3D) roc ...
test on an adjacent pad failed shortly after liftoff, causing flying debris to hit Explorer 7's launch vehicle. However, the damage to the booster was minor and could be easily repaired.
Explorer 7 was launched on 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 ...
at 15:31
GMT
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
on 13 October 1959 from the
Atlantic Missile Range
The Eastern Range (ER) is an American rocket range (Spaceport) that supports missile and rocket launches from the two major launch heads located at Cape Canaveral Space Force Station and the Kennedy Space Center (KSC), Florida. The range ha ...
into a orbit. On 16 June 1960, NASA announced one of the four frequency modulated subcarriers on the second transmitter had become erratic and the information it was transmitting on 3 of the 7 experiments was no longer intelligible. The tracking beacon ceased transmitting on 5 December 1959. As of September 1960 the orbit was with 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 50.3° and 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 101.2 minutes. Useful real-time data were transmitted from launch through February 1961 and intermittently until 24 August 1961. The spacecraft provided significant geophysical information on radiation and magnetic storms, demonstrated methods of controlling internal temperatures, recorded the first micrometeorite penetration of a sensor in flight, and detected large scale weather patterns.
Mission results
It transmitted data continuously through to February 1961 and went dead on 24 August 1961. It is still in orbit.
[ ]
See also
*
S-1 (satellite)
*
Explorer program
The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...
References
{{Orbital launches in 1959
Spacecraft launched in 1959
Explorers Program
Satellites orbiting Earth
1959 in the United States
Earth observation satellites