Explorer 50, also known as IMP-J or IMP-8, 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, succeedi ...

satellite launched to study the

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo ...

. It was the eighth and last in a series of the Interplanetary Monitoring Platform.

Spacecraft

Explorer 50 was a drum-shaped spacecraft, across and

height Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is). For example, "The height of that building is 50 m" or "The height of an airplane in-flight is ab ...

, with

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

Star-17A, instrumented for

interplanetary medium The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM sto ...

and magnetotail studies of

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 ow ...

s, energetic solar particles, plasma, and

electric Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described b ...

and

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and t ...

s. Its initial orbit was more elliptical than intended, with apogee and perigee distances of about 45.26

Earth radii Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly (equatorial radius, den ...

and 22.11 Earth radii. Its

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values b ...

decreased after launch. Its orbital inclination varied between 0 deg and about 55° with a periodicity of several years. The spacecraft spin axis was normal to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...

plane, and the spin rate was 23

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 dimension ...

. The data

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", and ' ...

rate was 1600 bps. The spacecraft was in the

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the ...

for 7 to 8 days of every 11.99 days orbit. Telemetry coverage was 90% in the early years, but only 60-70% through most of the 1980s and early 1990s. Coverage returned to the 90% range in the mid to late 1990s.

Launch

Explorer 50 was launched on 23 October 1973 at 02:26:03 UTC, by a Thor-Delta 1604

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 pads, supported by a launch control center and syste ...

from Cape Canaveral (

CCAFS 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 ...

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, a ...

. The spacecraft functioned nominally until 7 October 2006. The satellite orbited 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 surf ...

once every 12 days, at 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 28.67°. Its

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 ...

was 25

and

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 el ...

was 45 Earth radii.

Experiments

Charged Particle Measurements Experiment (CPME)

Three solid-state detectors in an anticoincidence plastic scintillator observed

electron The electron (, or in nuclear reactions) 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 partic ...

s between 0.2 and 2.5 MeV; protons between 0.3 and 500 MeV;

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pro ...

s between 2.0 and 200 MeV; heavy particles with Z values ranging from 2 to 5 with energies greater than 8 MeV; heavy particles with Z values ranging between 6 and 8 with energies greater than 32 MeV; and integral protons and alphas of energies greater than 50 MeV/

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons w ...

, all with dynamic ranges of 1 to 1E+6 particles per (cm²-second-sr). Five thin-window

Geiger–Müller tube The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated ...

s observed electrons of energy greater than 15 keV, protons of energy greater than 250 keV, and

X-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

s with wavelengths between 2 and 10 A, all with a dynamic range of 10 to 1E+8 per (cm²-second-sr). Particles and X-rays, primarily of solar origin, were studied, but the dynamic range and resolution of the instrument also permitted observation of

s and magnetotail particles.

Cosmic Ray Nuclear Composition

This experiment used two telescopes to measure the composition and energy spectra of solar (and galactic) particles above about 0.5 MeV/nucleon. The main telescope consisted of five collinear elements (three solid state, one

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 highl ...

(CsI), and one

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sap ...

Cherenkov) surrounded by a plastic anticoincidence shield. The telescope had a 60°, full-angle acceptance cone with its axis approximately normal to the spacecraft spin axis, permitting eight-sectored information on particle arrival direction. Four elements of the main telescope were pulse-height analyzed, and low- and high-gain modes could be selected by command to permit resolution of the elements

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

(H) through

Nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

(Ni) or of electrons and the

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass number ...

s of

(H) and

Helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

(He) and light nuclei. A selection-priority scheme was included to permit sampling of less abundant particle species under normal and solar-flare conditions. The low-energy telescope was essentially a two-element shielded solid-state detector with a 70° full-angle acceptance cone. The first element was pulse-height analyzed, and data were recorded by sectors.

Electrons, Hydrogen and Helium Isotopes

This experiment was designed to measure the differential energy spectra of the isotopes of hydrogen through

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

from 2 to 40-MeV/nucleon, and of electrons from 0.2 to 5-MeV. The instrument consisted of a stack of 11 fully depleted silicon solid-state detectors surrounded by a plastic scintillator anticoincidence cup. The outer two solid-state detectors were annular, permitting measurements in both narrow-geometry (typical geometrical factor was 0.2 cm²-sr) and wide-geometry (typical geometric factor was 1.5 cm²-sr) coincidence modes. Anisotropy data (45° angular and 20 seconds temporal resolution) were obtained.

Electrostatic Fields

The instrument was designed to measure ambient electric fields in the solar wind and the Earth's magnetosheath up to 1 kHz in frequency. The sensor consisted of a pair of wire antennas (, tip-to-tip), which were held rigid by

centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is parallel ...

due to satellite spin (about 24

). The wires were insulated from the plasma, except for their short outer sections, to remove the active probe area from the spacecraft sheath. The antenna served as a double floating probe, and measurements were obtained every 1/4 spacecraft revolution (about 0.75 second).

Ultra low frequency Ultra low frequency (ULF) is the ITU designation for the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz, corresponding to wavelengths between 1,000 to 100 km. In magnetosphere science and seismology, alternative defin ...

(ULF) and

Very low frequency Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave ...

(VLF) measurements were obtained using seven 60% bandwidth filters with center frequencies logarithmically spaced from 1-Hz to 1000-Hz. These frequency channels had an intrinsic sensitivity of 1.0E-5 V/m, and a peak range of 1.0E-2 V/m. However, the effective low-frequency filter threshold was determined by interference due to harmonics of the spacecraft spinning within an asymmetric sheath. The other major limitation was also due to sheath effect. Whenever the electron plasma density was less than about 10 particles/cc, the sheath overlapped the active antenna portions and precluded meaningful measurements of ambient conditions.

Electrostatic Waves and Radio Noise

A wide-band receiver was used to observe high-resolution frequency-time spectra, and a six-channel narrow-band receiver with a variable center frequency was used to observe wave characteristics. The receivers operated from three antenna systems. The first system contained a pair of long dipole antennas (one, extendable to about , normal to the spacecraft spin axis and the other antenna, extendable to about , along the spin axis). The second system contained a boom-mounted triad of orthogonal loop antennas. The third system consisted of a boom-mounted spin-axis dipole. The magnetic and electric field intensities and frequency spectra,

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

, and direction of arrival of naturally occurring

radio noise In radio reception, radio noise is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise near in frequency to the radio signal being receive ...

in the magnetosphere were observed. Phenomena studied were the time-space distribution, origin, propagation, dispersion, and other characteristics of radio noise occurring across and on either side of the magnetospheric boundary region. The frequency range for electric fields was 0.3 Hz to 200 kHz, and for magnetic fields it was 20 Hz to 200 kHz.

Energetic Electrons and Protons

The purposes of this investigation were: (1) to study the propagation characteristics of solar cosmic rays through the interplanetary medium over the energy ranges indicated below, (2) to study electron and proton fluxes throughout the geomagnetic tail and near the flanks of the magnetosphere and (3) to study the entry of solar cosmic rays into the magnetosphere. The instrumentation consisted of a three-element telescope employing fully depleted surface-barrier solid-state detectors and a magnet to deflect electrons. Two side-mounted detectors were used to measure the deflected electrons. Two additional detectors in separate mounts were used to measure charged particles above 15 keV (F), Z greater than or equal to 2 above 0.6 MeV (G1) and above 1.0 MeV (G2), and Z greater than or equal to 3 above 2.0 MeV (G3). The telescope measured protons in three ranges between 2.1 and 25 MeV (14, 15, 16 channels); Z greater than or equal to 1 in three ranges between 0.05 and 2.1 MeV (11, 12, 13 channels); alpha particles between 8.4 and 35.0 MeV in two ranges (111, 112 channels); Z greater than or equal to 2 between 2.2 and 8.4 MeV (110 channel); and a background channel (19 channel). Deflected electrons were measured in two ranges between 30 and 200 keV (17, 18 channels).

Magnetic Field Experiment

This experiment consisted of a boom-mounted triaxial fluxgate magnetometer designed to study the interplanetary and geomagnetic tail magnetic fields. Each sensor had three dynamic ranges of ± 12, ± 36, and ± 108 nT. With the aid of a bit compaction scheme (delta modulation), 25 vector measurements were made and telemetered per second. The experiment operated normally from launch until mid-1975. On 11 July 1975, because of a range indicator problem, the experiment operation was frozen into the 36 nT range. The digitization accuracy in this range is about ± 0.3 nT. On 23 March 1978, the sensor flipper failed. After that time, alternative methods of Z-axis sensor zero-level determination were required. The magnetometer failed 10 June 2000.

Measurement of Low-Energy Protons and Electrons

This experiment was designed to measure the energy spectra of low-energy electrons and protons in the geocentric range of 30 to 40 Earth radii to give further data on

geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...

aurora An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...

, tail and neutral sheet, and other magnetospheric phenomena. The detector was a dual-channel, curved-plate electrostatic analyzer (LEPEDEA - low energy proton and electron differential energy analyzer) with 16 energy intervals between 5 eV and 50 keV. It had an angular field of view of 9° by 25°. The detector could be operated in one of two modes: (1) one providing good angular resolution (16 directions for each particle energy band) once each 272 seconds, and (2) the other providing good temporal resolution in which the entire energy range in four directions was measured every 68 seconds.

Solar and Cosmic-Ray Particles

The

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 emp ...

cosmic-ray experiment was designed to measure energy spectra, composition, and angular distributions of solar and galactic electrons, protons, and heavier nuclei up to Z=30. Three distinct detector systems were used. The first system consisted of a pair of solid-state telescopes that measured integral fluxes of electrons above 150, 350 and 700 keV and of protons above 0.05, 0.15, 0.50, 0.70, 1.0, 1.2, 2.0, 2.5, 5.0, 15, and 25 MeV. Except for the 0.05 MeV proton mode, all counting modes had unique species identification. The second detector system was a solid-state dE/dx versus E telescope that looked perpendicular to the spin axis. This telescope measured Z=1 to 16 nuclei with energies between 4 and 20 MeV/nucleon. Counts of particles in the 0.5 to 4 MeV/nucleon range, with no charge resolution, were obtained as counts in the dE/dx sensor but not in the E sensor. The third detector system was a three-element telescope whose axis made an angle of 39° with respect to the spin axis. The middle element was a CsI scintillator, while the other two elements were solid-state sensors. The instrument responded to electrons between 2 and 12 MeV and to Z=1 to 30 nuclei in the energy range 20 to 500 MeV/nucleon. For particles below 80 MeV, this instrument acted as a dE/dx versus E detector. Above 80 MeV, it acted as a bidirectional triple dE/dx versus E detector. Flux directionality information was obtained by dividing certain portions of the data from each detector into eight angular sectors.

Solar Plasma Electrostatic Analyzer

A hemispherical electrostatic analyzer measured the directional intensity of positive ions and electrons in the solar wind, magnetosheath, and magnetotail. Ions as heavy as oxygen were resolved when the solar wind temperature was low. Energy analysis was accomplished by charging the plates to known voltage levels and allowing them to discharge with known RC time constants. In the solar wind, positive ions from 200-eV to 5 keV (15% spacing, 3% resolution) and electrons from 5 eV to 1 keV (30% spacing, 15% resolution) were studied. In the magnetosheath, positive ions from 200 eV to 5 keV (15% spacing, 3% resolution) and from 200 eV to 20 keV (30% spacing, 15% resolution) and electrons from 5 eV to 1 keV (30% spacing, 15% resolution) were studied. In the magnetotail, positive ions from 200 eV to 20 keV (30% spacing, 15% resolution) and electrons from 5 eV to 1 keV (30% spacing, 15% resolution) and from 100 eV to 20 keV (15% resolution) were studied. No data were obtained from this experiment past October 2001.

Solar Plasma Faraday Cup

A modulated split-collector

Faraday cup A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup was named after Michael Faraday ...

, perpendicular to the spacecraft spin axis, was used to study the directional intensity of positive ions and electrons in the solar wind, transition region, and magnetotail. Electrons were studied in eight logarithmically equispaced energy channels between 17 eV and 7 keV. Positive ions were studied in eight channels between 50 eV and 7 keV. A spectrum was obtained every eight spacecraft revolutions. Angular information was obtained in either 15 equally spaced intervals during a 360° revolution of the satellite or in 15 angular segments centered more closely about the spacecraft-sun line.

Solid-State Detectors

This experiment was designed to determine the composition and energy spectra of low-energy particles observed during solar flares and 27-d recurrent events. The detectors used included: (1) an electrostatic analyzer (to select particles of the desired energy per charge) combined with an array of windowless solid-state detectors (to measure the energy loss) and surrounded by an anticoincidence shield and (2) a thin-window proportional counter, solid-state particle telescope. The experiment measured particle energies from 0.1 to 10 MeV per charge in 12 bands and uniquely identified positrons and electrons as well as nuclei with charges of Z from 1 to 8 (no charge resolution for Z greater than 8). Two 1000-channel pulse-height analyzers, one for each detector, were included in the experiment payload.

Extended mission

The objectives of the extended Explorer 50 (IMP-8) operations were to provide solar wind parameters as input for magnetospheric studies and as a 1-AU baseline for deep space studies, and to continue solar cycle variation studies with a single set of well-calibrated and understood instruments.

End of mission

In October 2001, Expkorer 50 (IMP-8) was terminated as an independent mission. Telemetry acquisition resumed after about three months at Canberra, Australia, only (30-50% coverage), as an adjunct to the Voyager and Ulysses missions. The last useful science data from Explorer 50 (IMP-8) was acquired on 7 October 2006.

References

External links

IMP-8 Project Information

{{Orbital launches in 1973 1973 in spaceflight Explorers Program Spacecraft launched in 1973 Satellites orbiting Earth Spacecraft launched by Delta rockets