Explorer 27 (or BE-C or Beacon Explorer-C, Beacon-C or S-66C) was a small
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, launched in 1965, designed to conduct scientific research in the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
.
[ ] It was powered by 4
solar panel
A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...
s. One goal of the mission was to study in detail the shape of 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 ...
by way of investigating variations in its
gravitational field
In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
.
It was the third and last of the Beacons in the
Explorers 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 ...
. The satellite was shut off in July 1973 so that its transmission band could be used by higher-priority spacecraft.
Spacecraft
![S66SN35_Mounted_on_Vibration_Table](https://upload.wikimedia.org/wikipedia/commons/d/d6/S66SN35_Mounted_on_Vibration_Table.jpg)
Built at the
Applied Physics Laboratory
The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit University Affiliated Research Center, university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated w ...
(APL),
under the direction of
Goddard Space Flight Center
The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C. in Greenbelt, Maryland, United States. Established on May 1, 1959 as NASA's first space flight center, GSFC empl ...
,
[ Explorer 27 began as S-66C, the last of the five satellites in NASA's first stage of ionospheric exploration, and the first of five NASA geodetic satellites.][ ] Its primary mission was "to conduct ionospheric measurements on a worldwide basis. The program will determine the total electron content
Total electron content (TEC) is an important descriptive quantity for the ionosphere of the Earth. TEC is the total number of electrons integrated between two points, along a tube of one meter squared cross section, i.e., the electron columnar nu ...
of a vertical cross-section of the ionosphere located between the satellite and the Earth. Accomplishing this objective will aid in establishing the behavior pattern of the ionosphere as a function of latitude, time of day, season, and solar cycle".[
Weighing , the satellite was an octagonal spacecraft with a honeycomb ]nylon
Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic.
Nylon is a silk-like thermoplastic, generally made from petro ...
and 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 ...
hull, in diameter, high, with four solar panels wide and long.
A three-axis magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
and Sun sensors provided information on the satellite attitude and spin rate.[Ludwig Combrinck, 2010. Sciences of Geodesy (Cap. 9) ''Springer-Verlag'' Retrieved June 9, 2018] There was no 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- ...
aboard so that satellite could be received only when the satellite was within range of a ground telemetry
Telemetry is the in situ data collection, collection of measurements or other data at remote points and their automatic data transmission, transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Gr ...
station. Continuous Doppler transmitters operated at 162 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 sā1, meaning that one he ...
and 324-MHz to permit precise tracking by Transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
tracking stations for navigation and geodetic studies. Four other transmitters operated on 20, 40, 41, and 360-MHz to measure ionospheric density. Explorer 27's last experiment was an Electron Density Experiment designed for measuring charged particles in the immediate vicinity of the satellite.[
S-66 mounted 360 "cube corner" ]retroreflectors
A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflection (physics), reflects radiation (usually light) back to its source with minimum scattering. This works at a wide range of angle of incidence (opt ...
made of fused silica
Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...
,[ so that the satellite could be tracked via lasers beamed from mobile stations at ]Wallops Flight Facility
Wallops Flight Facility (WFF) is a rocket launch site on Wallops Island on the Eastern Shore of Virginia, United States, just east of the Delmarva Peninsula and approximately north-northeast of Norfolk. The facility is operated by the Goddard ...
(WFF).
History
The first S-66 was scheduled for launch in late 1963. However, due to problems with the Scout X-4
Scout may refer to:
Youth movement
*Scout (Scouting), a child, usually 10ā18 years of age, participating in the worldwide Scouting movement
**Scouts (The Scout Association), section for 10-14 year olds in the United Kingdom
**Scouts BSA, sectio ...
,[ the flight was rescheduled for the following year on a ]Delta B
The Delta B, or Thor-Delta B was an American expendable launch system used for nine orbital launches between 1962 and 1964. A derivative of the Thor-Delta, it was a member of the Delta family of rockets.
The first stage was a Thor missile in the ...
, from Cape Canaveral
, image = cape canaveral.jpg
, image_size = 300
, caption = View of Cape Canaveral from space in 1991
, map = Florida#USA
, map_width = 300
, type =Cape
, map_caption = Location in Florida
, location ...
LC-17A. On 19 March 1964, the first attempt to this S-66 ended in failure when the third stage of its Delta launch vehicle
A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pad, launch pads, supported by a missile launch contro ...
burned just 22 seconds instead of the programmed 40 seconds. This was only the second time the Delta launch vehicle had failed, and the incident followed 22 prior successes.[
A second S-66 was launched, this time successfully, via a Scout X-4 launch vehicle at 03:01 ]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 9 October 1964, from the PALC-D launch facility at Vandenberg Air Force Base Vandenberg may refer to:
* Vandenberg (surname), including a list of people with the name
* USS General Harry Taylor (AP-145), USNS ''General Hoyt S. Vandenberg'' (T-AGM-10), transport ship in the United States Navy, sank as an artificial reef in K ...
. Once in space, it became known as Explorer 22. Explorer 22's as-yet unnamed backup was scheduled for launch in March/April 1965 to extend the geodetic experiments of its predecessor. Upon the backup's launch on 29 April 1965 at 14:17:00 GMT via Scout X-4
Scout may refer to:
Youth movement
*Scout (Scouting), a child, usually 10ā18 years of age, participating in the worldwide Scouting movement
**Scouts (The Scout Association), section for 10-14 year olds in the United Kingdom
**Scouts BSA, sectio ...
from Wallops Island
Wallops Island is a island in Accomack County, Virginia, part of the Virginia Barrier Islands that stretch along the eastern seaboard of the United States of America. It is just south of Chincoteague Island, a popular tourist destination.
W ...
, this third S-66 satellite was designated Explorer 27.
Experiments
Langmuir Probes
Two cylindrical electrostatic probes of the Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
type were used. They consisted of a collector electrode extending from the central axis of a cylindrical guard ring. The guard ring extended from the spacecraft and the probe extended . A 2- Hz sawtooth voltage of -3 to +5 volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745ā1827).
Defi ...
s was swept to either of the probes, and the resulting current profile to the probe was telemetered. From this profile, the electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
, electron temperature
Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or char ...
, and mean ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
mass were determined. This experiment performed nominally from launch until 13 August 1968, when solar cell degradation resulting from radiation prevented operation of all systems on the satellite. The probe was not operated after that time. No archival data were produced since the experiment was a back-up for the Explorer 22 (BE-B) mission, which had been flown successfully.[ ]
Laser Tracking Reflectors
The passive optical laser experiment, which consisted of nine panels on the spacecraft, was used to determine the spacecraft range and angle. Each panel was covered with 40 quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
cube-corner prisms that provided laser tracking capabilities for optical tracking studies. The ground-based optical transmitter was a pulsed 1-ms ruby laser
A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960.
Ruby lasers produce pulses of c ...
. A photodetector
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...
determined whether the laser beam interrupted the spacecraft.[ ]
Radio Beacon
A radio beacon radiated a plane-polarized signal at 20.005 MHz, 40.010 MHz, 41.010 MHz, and 360.090 MHz, all harmonics of 1.00025 MHz. The plane of polarization of the three lower frequencies underwent an appreciable number of rotations due to electron concentration. The polarization plane of highest frequency did not rotate appreciably. Several methods were used to analyze these rotations and determine the total electron content between the satellite and a ground receiver. The beacons were on until the satellite operation terminated on 6 May 1968. On 13 February 1970, the beacons were again turned on to replace the Explorer 22 (1964-064A - BE-B) beacons which had completely failed by the end of January 1970.[ ]
Radio Doppler System
Two coherent, unmodulated CW transmitters, operating on frequencies of 162 and 324 MHz, allowed the Tranet Doppler Network to obtain data for studies of dynamic geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
. The frequencies were generated from redundant, dual, ultra-stable crystal oscillators operating on a frequency of 5 MHz minus 80 ppm. The system operated as planned.[ ]
Results
Explorer 27 was turned off on 20 July 1973 because it was interfering with other, more important satellites. Tracking of the satellite via its passive laser reflectors continued at least into the 21st century.[ ]
See also
* Explorer 22
* 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 1965
Explorers Program
Spacecraft launched in 1965
Laser ranging satellites
Geospace monitoring satellites