View source for Explorer 33

{{Short description|NASA satellite of the Explorer program (1966–)}}
{{Use American English|date=January 2014}}
{{Use dmy dates|date=November 2021}}
{{Infobox spaceflight
| name              = Explorer 33
| names_list        = IMP-D<br/>AIMP-1<br/>Anchored Interplanetary Monitoring Platform-1
| image             = IMP-D.jpg
| image_caption     = Explorer 33 satellite
| image_size        = 300px

| mission_type      = [[Magnetosphere|Magnetospheric research]]
| operator          = [[NASA]]
| COSPAR_ID         = 1966-058A 
| SATCAT            = 02258
| mission_duration  = {{time interval|July 1, 1966, 16:02:25|September 21, 1971}} (achieved) <br/>{{time interval|July 1, 1966, 16:02:25}} (in orbit)

| spacecraft        = Explorer XXXIII
| spacecraft_type   = Anchored Interplanetary Monitoring Platform
| spacecraft_bus    = AIMP
| manufacturer      = [[Goddard Space Flight Center]]
| launch_mass       = {{cvt|93.4|kg}}
| dimensions        = {{cvt|71|xx|20.3|cm}}
| power             = 43 [[watt]]s

| launch_date       = 1 July 1966, 16:02:25 [[Greenwich Mean Time|GMT]]
| launch_rocket     = [[Delta (rocket family)|Delta E1]] (Thor 467 / Delta 039)
| launch_site       = [[Cape Canaveral Space Force Station|Cape Canaveral]], [[Cape Canaveral Launch Complex 17|LC-17A]]
| launch_contractor = [[Douglas Aircraft Company]]

| entered_service   = 1 July 1966
| last_contact      = 21 September 1971
| destroyed         = 
| decay_date        =

| orbit_reference   = [[Geocentric orbit]]<ref name="Trajectory">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1966-058A|title=Trajectory: Explorer 33 (AIMP-1) 1966-058A)|publisher=NASA|date=28 October 2021|access-date=10 November 2021}} {{PD-notice}}</ref>
| orbit_regime      = [[High Earth orbit]]
| orbit_periapsis   = {{cvt|265680|km}}
| orbit_apoapsis    = {{cvt|480763|km}}
| orbit_inclination = 24.40°
| orbit_period      = 26d 22hr 32min
| apsis             = gee

| instruments       = Ames Magnetic Fields<br/>Electron and Proton Detectors<br/>GSFC Magnetometer<br/>Ion Chamber and Geiger–Müller Counters<br/>Low-Energy Integral Spectrum Measurement Experiment<br/>Plasma Probe<br/>Solar Cell Damage

| programme         = '''[[Explorers program]]'''
| previous_mission  = [[Explorer 32]]
| next_mission      = [[Explorer 34]]
| programme2= [[Interplanetary Monitoring Platform]]
| previous_mission2= [[Explorer 28|IMP-C]]
| next_mission2= [[Explorer 34|IMP-F]]
}}

'''Explorer 33''', also known as '''IMP-D''' and '''AIMP-1''', is a [[spacecraft]] in the [[Explorer program]] launched by [[NASA]] on 1 July 1966 on a mission of scientific exploration. It was the fourth satellite launched as part of the [[Interplanetary Monitoring Platform]] series, and the first of two "Anchored IMP" spacecraft to study the environment around [[Earth]] at lunar distances, aiding the [[Apollo program]]. It marked a departure in design from its predecessors, IMP-A ([[Explorer 18]]) through IMP-C ([[Explorer 28]]). [[Explorer 35]] (AIMP-E, AIMP 2) was the companion spacecraft to Explorer 33 in the Anchored IMP program, but [[Explorer 34]] (IMP-F) was the next spacecraft to fly, launching about two months before AIMP-E, both in 1967.<ref name="explorer-series-images">{{cite web |url=https://rammb.cira.colostate.edu/dev/hillger/Explorer_reference.htm|title=Explorer-series reference images|access-date=2021-07-04}}</ref>

== Spacecraft ==
Explorer 33 (IMP-D) is a spin-stabilized (spin axis parallel to the [[Ecliptic|ecliptic plane]], spin period varying between 2.2 and 3.6 seconds) spacecraft instrumented for studies of interplanetary plasma, energetic charged particles ([[electron]]s, [[proton]]s, and [[Alpha particle|alpha]]s), [[magnetic field]]s, and [[Solar Ray|solar X ray]]s at lunar distances. The spacecraft failed to achieve [[lunar orbit]] but did achieve mission objectives. Explorer 33 is also known as Interplanetary Monitoring Platform D (IMP-D) or Anchored Interplanetary Monitoring Platform 1 (AIMP-1).<ref name="Display">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-058A|title=Display: Explorer 33 (AIMP-1) 1966-058A|publisher=NASA|date=28 October 2021|access-date=10 November 2021}} {{PD-notice}}</ref>

Explorer 33 is similar in design to [[Explorer 28]]. The spacecraft has a mass of 93.4&nbsp;kg. The main body of the spacecraft is an octagonal prism, {{cvt|71|cm}} across and {{cvt|20.3|cm}} high. Four n/p solar cell arrays that produced an average of 43 [[watt]]s, extend from the main bus, along with two {{cvt|183|cm}} magnetometer booms. Four whip antennas are mounted on top of the spacecraft. A {{cvt|35.8|kgf}} thrust [[retrorocket]] (Thiokol TE-M-458) is mounted on top of the bus. Power was stored in [[Silver–cadmium battery|silver-cadmium batteries]] (Ag-Cd). Communication (PFM-PM telemetry) was via a 7-watts transmitter and a digital data processor.<ref name="Display"/>

== Mission ==
Explorer 33 was intended to be the first U.S. spacecraft to enter lunar orbit. The science objectives were to study the near-lunar magnetic field, ionosphere, solar plasma flux, energetic particle population, [[cosmic dust]], and variations of the [[gravitational field]] from lunar orbit. After failing to achieve the intended lunar orbit, it made measurements from a highly elliptical Earth orbit of the interplanetary magnetic and radiation environment.<ref name="Display"/>

== Instruments ==
The scientific payload comprised seven experiments: two fluxgate magnetometers, an energetic particles experiment, an electron and proton experiment, a thermal ion and electron experiment, a plasma probe, and a solar cell damage experiment.<ref name="Display"/>

== Experiments ==
=== Ames Magnetic Fields ===
The Ames [[magnetometer]] experiment consisted of a boom-mounted triaxial fluxgate magnetometer and an electronics package. The sensors were orthogonally mounted with one sensor oriented along the spin axis of the spacecraft. A motor interchanged a sensor in the spin plane with the sensor along the spin axis every 24 hours, allowing inflight zero-level determination. The instrument package included a circuit for spin-demodulating the outputs from the sensors in the spin plane. The noise threshold was about 0.2 nT. The instrument had three ranges covering ± 20, 60, and 200 nT full scale for each vector component. The digitization accuracy for each range was 1% of the entire range covered. The magnetic field vector was measured instantaneously, and the instrument range was changed after each measurement. A period of 2.05-seconds elapsed between adjacent measurements and 6.14-seconds between measurements using the same range.<ref name="Experiment3">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-03|title=Experiment: Ames Magnetic Fields|publisher=NASA|date=28 October 2021|access-date=10 November 2021}} {{PD-notice}}</ref>

=== Electron and Proton Detectors ===
Three EON type 6213 [[Geiger–Müller tube]]s (GM1, GM2, and GM3) and a [[silicon]] solid-state detector (SSD) provided measurements of solar X rays (Geiger–Müller (GM) tubes only, between 2 and 12 A) and of solar, galactic, and magnetospheric charged particles. The Geiger–Müller tubes measured electrons of energies greater than 45 to 50 [[Electronvolt|keV]] and protons of energies greater than 730 to 830 keV. The SSD output was discriminated at four thresholds: (1) PN1, which detected protons between 0.31 and 10 MeV and alphas between 0.59 and 225 MeV, (2) PN2, which detected protons between 0.50 and 4 MeV and alphas between 0.78 and 98 MeV, (3) PN3, which detected protons between 0.82 and 1.9 MeV and alphas between 1.13 and 46 MeV, and (4) PN4, which detected alphas between 2.1 and 17 MeV. GM1 and the SSD were oriented parallel to the spin axis, and GM3 was oriented antiparallel to the spin axis. Data from GM1 and PN1 were divided into data from quadrants oriented with respect to the [[Sun]] (sectors I, II, III, and IV were centered 180°, 270°, 0° and 90° from the Sun, respectively). Data were read out in either 82-seconds or 164-seconds intervals. High temperatures adversely affected the SSD particle data during the periods from 16 September to 14 January and from 16 March to 14 July of each year following 16 September 1966. However, the alpha particle data are believed to be unaffected. On rare occasions (less than 10), a GM tube would produce a high, spurious count rate for a period of several hours. This effect apparently was produced only during periods of extremely high particle and X-ray fluxes. Accumulator failures occurred on 21 July 1967 and 24 September 1967.<ref name="Experiment5">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-05|title=Experiment: Electron and Proton Detectors|publisher=NASA |date=28 October 2021|access-date=11 November 2021}} {{PD-notice}}</ref>

=== GSFC Magnetometer ===
The instrumentation for this experiment consisted of a boom-mounted triaxial fluxgate magnetometer. Each of the three sensors had a range of ± 64 nT and a digitization resolution of ± 0.25 nT. Zero-level drift was checked by periodic reorientation of the sensors. Spacecraft fields at the sensors were not greater than the digitization uncertainty. One vector measurement was obtained each 5.12-seconds. The bandpass of the magnetometer was 0 to 5&nbsp;Hz, with a 20-[[Decibel|dB]] per decade decrease for higher frequencies. The detector functioned well between launch and 10 October 1968, when the [[Direct current|DC]] power converter failed. No useful data were obtained after that date.<ref name="Experiment1">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-01|title=Experiment: GSFC Magnetometer|publisher=NASA|date=28 October 2021|access-date=11 November 2021}} {{PD-notice}}</ref>

=== Ion Chamber and Geiger–Müller Counters ===
This experiment consisted of a {{cvt|10.2|cm}}, Neher-type [[ionization chamber]] and two Lionel type 205 HT [[Geiger–Müller tube]]s (GM). The ion chamber responded omnidirectionally to electrons above 0.7 MeV and protons above 12 MeV. Both GM tubes were mounted perpendicular to the spacecraft spin axis. GM tube A detected electrons above 45 keV which were scattered off a [[Gold leaf|gold foil]]. The acceptance cone for these electrons had a full-angle of 61° and axis of symmetry which was perpendicular to the spacecraft spin axis. GM tube B responded to electrons and protons above 22 and 300 keV, respectively, in an acceptance cone of 45° full-angle with axis of symmetry perpendicular to the spacecraft spin axis. Both GM tubes responded omnidirectionally to electrons and protons of energies above 2.5 and 35 MeV, respectively. Pulses from the ion chamber and counts from each GM tube were accumulated for 39.72-seconds and read out every 40.96-seconds. The time between the first two ion-chamber pulses in an accumulation period was also telemetered. The ion chamber operated normally from launch through 2 September 1966. From 2 September 1966, the ion chamber operated at a lower threshold voltage.<ref name="Experiment4">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-04|title=Experiment: Ion Chamber and Geiger–Müller Counters|publisher=NASA|date=28 October 2021|access-date=11 November 2021}} {{PD-notice}}</ref>

=== Low-Energy Integral Spectrum Measurement Experiment ===
A wide-aperture, multi-grid potential analyzer was used to observe the intensity of the electron and ion components of the low-energy plasma in interplanetary space and near [[Earth]]. Integral spectra were obtained for both ions and electrons in the energy ranges from 0 to 45 eV (15 steps) and 0 to 15 eV (15 steps). Complete spectra for protons and electrons were obtained every 80-seconds. The experiment operated until 29 June 1967.<ref name="Experiment2">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-02|title=Experiment: Low-Energy Integral Spectrum Measurement Experiment |publisher=NASA|date=28 October 2021|access-date=11 November 2021}} {{PD-notice}}</ref>

=== Plasma Probe ===
A split-collector [[Faraday cup]] mounted on the spacecraft equator was used to study the directional intensity of solar wind ions and electrons. The following 25-seconds sequence was executed three times for ions and once for electrons each 328-seconds. Twenty-seven directional current samples from the two collectors were taken in the energy per charge (E/Q) window from 80 to 2850 eV. The currents in the two collectors were then sampled in eight E/Q windows between 50 and 5400 eV at the azimuth at which peak current appeared in the previous 27 measurements. Due to telemetry limitations, only the following data were returned to Earth every 328-seconds: for ions, the sums of currents measured on the two collector plates twice and the difference once, and for electrons, the sums once. The experiment worked well from launch until the final spacecraft data transmission (21 September 1971).<ref name="Experiment6">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-06|title=Experiment: Plasma Probe|publisher=NASA |date=28 October 2021|access-date=11 November 2021}} {{PD-notice}}</ref>

== Launch ==
Explorer 33 was launched on 1 July 1966 from [[Cape Canaveral|Cape Kennedy]], [[Florida]]. The [[Delta (rocket family)|Thor-Delta E1]] second and third stages both delivered too much thrust, resulting in an excess velocity of about {{cvt|21.3|m/s}} towards the [[Moon]]. This was too much for the retrorocket to overcome to put the spacecraft into the intended lunar orbit ({{cvt|1300|xx|6440|km}} with 175°. [[Orbital inclination|inclination]]). Instead, the retrorocket was used to put Explorer 33 into a [[Highly elliptical orbit|highly elliptical initial Earth orbit]] of {{cvt|449174|xx|30550|km}} with an inclination of 28.9° and an apogee beyond lunar orbit. It came within {{cvt|35000|km}} of the Moon on its first orbit, and came within {{cvt|40000|xx|60000|km}} on subsequent approaches in September, November and December 1966. All experiments operated successfully until September 1971.<ref name="Display"/>

When it was launched, AIMP-1 achieved the highest orbit of any satellite up to that time, with an [[Apsis|apogee]] of {{cvt|480763|km}} and a [[Apsis|perigee]] of {{cvt|265680|km}}.<ref name="imp-history">{{cite book|url=https://ntrs.nasa.gov/citations/19800012928|title=Interplanetary Monitoring Platform Engineering History and Achievements|publisher=NASA|pages=11, 63, 138|date=29 August 1989|access-date=2021-07-05|last1=Butler |first1=P. M. }} {{PD-notice}}</ref>

== Orbit ==
Originally intended for a lunar orbit, mission controllers worried that the spacecraft's velocity was too fast to guarantee lunar capture.<ref>{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670009370_1967009370.pdf|title=Interim Flight Report, Anchored Interplanetary Monitoring Platform, AIMP-1 - Explorer XXXIII|author=J. J. Madden
|publisher=NASA|date=December 1966}} {{PD-notice}}</ref> Consequently, mission managers opted for a backup plan of placing the craft into an eccentric Earth orbit with a perigee of {{cvt|265680|km}} and an apogee of {{cvt|480763|km}}—still reaching beyond the Moon's orbit.<ref>{{cite web|url=http://www.astronautix.com/craft/imp.htm|title=IMP Chronology|publisher=Encyclopedia Astronautica|url-status=dead|archive-url=https://web.archive.org/web/20100116222655/http://astronautix.com/craft/imp.htm|archive-date=January 16, 2010}}</ref>

Despite not attaining the intended lunar orbit, the mission met many of its original goals in exploring [[solar wind]], interplanetary plasma, and solar X-rays.<ref>{{cite web
|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-058A|title=Display: Explorer 33 1966-058A|publisher=NASA|date=April 2, 2008|access-date=July 4, 2008}} {{PD-notice}}</ref> [[Principal investigator]] [[James Van Allen]] used [[electron]] and [[proton]] detectors aboard the spacecraft to investigate charged particle and X-ray activity.<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1966-058A-05|title=Explorer 33 -- Electron and Proton Detectors|publisher=NASA|date=April 2, 2008|access-date=July 4, 2008}} {{PD-notice}}</ref> Astrophysicists N. U. Crooker, [[Joan Feynman]], and J. T. Gosling used data from ''Explorer 33'' to establish relationships between the [[Earth's magnetic field]] and the [[solar wind]] speed near Earth.<ref>{{cite web|url=https://ntrs.nasa.gov/search.jsp?R=238288&id=9&qs=Ne%3D20%26N%3D4294924156%26Ns%3DPublicationYear%257C0|title=On the high correlation between long-term averages of solar wind speed and eomagnetic activity|author1=Crooker, N. U.|author2=Feynman, J.|author3=Gosling, J. T.|date=May 1, 1977|access-date=July 4, 2008|publisher=NASA}} {{PD-notice}}</ref>

== MOSFET-based telemetry system ==
The first of Explorer 33's predecessors in the Interplanetary Monitoring Platform series, Explorer 18 (IMP-A), had been the first spacecraft to fly with [[integrated circuit]]s on board.<ref name="butrica">{{cite book|last1=Butrica|first1=Andrew J.|chapter=Chapter 3: NASA's Role in the Manufacture of Integrated Circuits|editor-last1=Dick|editor-first1=Steven J.|title=Historical Studies in the Societal Impact of Spaceflight|date=2015|publisher=NASA|isbn=978-1-62683-027-1|pages=149-250 (237-242)|chapter-url=https://www.nasa.gov/sites/default/files/atoms/files/historical-studies-societal-impact-spaceflight-ebook_tagged.pdf#page=237}} {{PD-notice}}</ref> AIMP-1 advanced the state of the art again when it was the first spacecraft to use the [[MOSFET]] (metal–oxide–semiconductor field-effect transistor, or MOS transistor), which was adopted by NASA for the IMP program in 1964.<ref>{{cite journal|last1=White|first1=H. D.|last2=Lokerson|first2=D. C.|title=The Evolution of IMP Spacecraft Mosfet Data Systems|journal=IEEE Transactions on Nuclear Science|date=1971|volume=18|issue=1|pages=233–236|doi=10.1109/TNS.1971.4325871|bibcode=1971ITNS...18..233W |issn=0018-9499}}</ref> The use of MOSFETs was a major step forward in spacecraft [[electronics]] design. The MOSFET blocks were manufactured by [[General Microelectronics]], which had NASA as its first MOS contract shortly after it had commercialized MOS technology in 1964.<ref name="butrica"/>

MOSFETs had first been demonstrated in 1960 and publicly revealed in 1963. Metal–oxide–semiconductor technology simplified [[semiconductor device fabrication]] and manufacturing, enabling higher [[transistor count]]s on [[integrated circuit]] chips.<ref name="imp-history"/> This resolved a growing problem facing spacecraft designers at the time, the need for greater on-board electronic capability for [[telecommunications]] and other functions. The [[Goddard Space Flight Center]] used MOSFETs in building block circuits, with MOSFET blocks and [[resistors]] accounting for 93% of the AIMP-D's electronics. MOS technology allowed for a substantial increase in the overall number of [[transistor]]s and [[communication channel]]s, from 1,200 transistors and 175 channels on the first three IMP spacecraft up to 2,000 transistors and 256 channels on the AIMP-D. MOS technology also greatly reduced the number of [[electrical]] parts required on a spaceship, from 3,000 non-resistor parts on IMP-A down to 1,000 non-resistor parts on the AIMP-1, despite the satellite having twice the electrical complexity of IMP-A.<ref name="butrica"/><ref name="white-1966">{{cite report|url=https://ntrs.nasa.gov/citations/19670004452 |title=Evolution of satellite PFM encoding systems from 1960 to 1965|author=Hosea D. White Jr. |publisher=NASA|date=December 1966|access-date=2021-07-04}} {{PD-notice}}</ref> While IMP-A through IMP-C had made some use of integrated circuits, the encoders still primarily used discrete transistors (one per package). AIMP-1's design put 4,200 semiconductors into 700 packages, reducing the number of individual components used and the amount of space they occupied. Components were combined into [[printed circuit board#Cordwood construction|cordwood]] modules.<ref name="imp-history"/>

AIMP-1 (IMP-D) improved upon its predecessors' Digital Data Processors (DDPs) and had an Optical Aspect Computer capable of operating in different power-saving modes to reduce load on the satellite's batteries and solar panels.<ref name="cliff-1966">{{cite report|url=https://ntrs.nasa.gov/citations/19660021074|title=Power Switching in Digital Systems|author=Rodger A. Cliff|publisher=NASA|date=July 1966|access-date=2021-07-04}}</ref> As in previous IMP spacecraft, experiments stored data into [[accumulator (computing)|accumulator]]s which were then read out on a repeating cycle and encoded into [[pulse-frequency modulation]] (PFM) signals to be sent to ground stations. This cycle was also interleaved with analog transmissions for certain experiments.<ref name="feasibility">{{cite report |url=https://ntrs.nasa.gov/citations/19650010243|title=IMP D and IMP E Feasibility Study|author=Paul G. Marcotte|publisher=NASA|date=January 1964|access-date=2021-07-04}} {{PD-notice}}</ref>

== See also ==
{{Portal|Spaceflight}}
* [[Explorer 35]] (AIMP-2)
* [[1966 in spaceflight]]
* [[Explorer 18]]
* [[Explorer 21]]
* [[Explorer 28]]
* [[Explorer program]]

== References ==
{{Reflist|30}}

== External links ==
* [https://ntrs.nasa.gov/citations/19670017263 AIMP-D Technical Summary Description]
* [https://ntrs.nasa.gov/citations/19670022978 Second Interim Flight Report - AIMP-1 - Explorer XXXIII]
* [https://ntrs.nasa.gov/citations/19670009321 Observations of the Earth's magnetic tail and neutral sheet at 510,000 km by Explorer 33 - 1966]
* [https://ntrs.nasa.gov/citations/19670023519 Mapping of the Earth's bow shock and magnetic tail by Explorer 33]
* [https://ntrs.nasa.gov/citations/19680019293 Energetic particles in the outer magnetosphere - Explorer 33]

{{Explorer program}}
{{Orbital launches in 1966}}

[[Category:Explorers Program]]
[[Category:Satellite launch failures]]
[[Category:Spacecraft launched in 1966]]
[[Category:Interplanetary Monitoring Platform]]