Among the greatest scientific achievements of the opening years of the Space Age was the characterization of Earth’s magnetic field and the discovery of what became known as the Van Allen radiation belts (see “Explorer 1: America’s First Satellite”). When the first probes left the Earth’s magnetosphere and penetrated the Sun-dominated interplanetary environment, scientist discovered the flow of low energy plasma emanating from the Sun, called the solar wind, and its complex interactions with the interplanetary magnetic field (see “Vintage Micro: The First Interplanetary Probe”).
NASA’s first S-3 satellite, Explorer 12, which provided the first data on the interaction between Earth’s magnetosphere and the solar wind. (NASA/GSFC)
Early on scientists believed that the Earth’s magnetosphere would be symmetrical around our home world with no difference between the day and night sides. But this simplified view was quickly shown to be incorrect. NASA launched the 35-kilogram Explorer 10 (designated P-14 in NASA planning documents) on March 25, 1961 into an elongated geocentric orbit with an apogee of 180,100 kilometers which gathered data for about 52 hours before falling silent. This was followed by the launch of the first S-3 satellite series, the 38-kilogram Explorer 12, on August 16 into an elongated orbit with an apogee of 77,620 kilometers. The single penetration of Explorer 10 into interplanetary space and the four months of observations from Explorer 12, with its repeated probing of the Sun-ward interface between the Earth’s magnetosphere and the interplanetary environment, revealed a much more complex picture. The interaction with the flow of the solar wind was found to compress the magnetosphere on the Sun facing portion and draw it out into a long tail stretching back hundreds of thousands of kilometers downwind. With this revelation, it was realized that more data were needed to help fill in the details of this emerging view of the magnetosphere.
Schematic diagram of Earth’s magnetosphere based on data returned by NASA’s Explorer 10 and 12 satellites. Click on image to enlarge. (NASA)
The Interplanetary Monitoring Platform
The Interplanetary Monitoring Platform (IMP) program was started in the fall of 1961 based on a recommendation from NASA’s Goddard Space Flight Center (GSFC) earlier that spring to launch a series of probes to monitor the space environment around the Earth and nearby interplanetary space. Such knowledge was vital for defining the environment astronauts would encounter during flights to the Moon and beyond as well as helping scientists understand the space environment and the forces which shape it. In order to provide these data, the typical IMP satellite was placed into a highly eccentric Earth orbit which would reach out hundreds of thousands of kilometers to probe the magnetosphere, interplanetary space, and the interactions between the two realms. Initially, a seven-satellite series was planned to monitor this environment throughout the 11-year solar cycle and eventually directly support the Apollo lunar missions planned for later in the decade.
The IMP A satellite (NASA/GSFC)
The first three of the initially planned seven satellite series were all designed and built in-house at GSFC with contract support for mechanical assembly, electronic integration, as well as testing of the various systems and instruments provided by EMR Aerospace Sciences, Inc. in College Park, Maryland. IMP A, B and C, as the first three satellites were known before launch, were built to a common design based on GSFC’s experience with earlier S-3 series satellites like Explorer 12. Each satellite consisted of an octagonal shaped main structure 71 centimeters across and 20 centimeters tall which housed all of the spacecraft’s systems.
Diagram of the IMP A spacecraft. Click on image to enlarge. (NASA/GSFC)
Spin-stabilized by a rotation rate of 22 RPM, the spacecraft sported a number of appendages. First were a quartet of 66 by 46-centimeter panels holding a total of 11,520 silicon P-N solar cells which would be deployed after launch. The solar cells were covered by a 0.3-millimeter-thick cover glass to provide protection from energetic protons. With a total area of 2.3 square meters, these panels provided an average of 37 watts to power the spacecraft’s systems. Also included, to provide power during the time the satellite would spend in the Earth’s shadow during each orbit, were 13 sealed silver-cadmium batteries connected in series to provide a total of 5 amp-hours of electrical power. This was sufficient to maintain about 30% of the normal power levels while in shadow and were “magnetically clean” so that readings from the satellite’s instrument suite would not be affected. An automatic undervoltage protection circuit was included which would turn off the batteries before becoming totally depleted allowing an 8-hour recharge cycle before spacecraft systems would come back on line.
Here we see a technician working on IMP A. (NASA/GSFC)
Data from IMP were returned to Earth using a four-watt transmitter operating at 136 MHz in the VHF band using pulsed frequency modulation (PFM) for digital data via tracking stations located at Woomera, Australia; Johannesburg, South Africa; and Santiago, Chile. The normal telemetry sequence consisted of 81.9-second intervals where 795 bits of digital data were generated. IMP had an on-board data recorder which could hold up to a month of continuous instrument measurements or three months of data at a lower sampling rate. In addition to the digital data sent in PFM format, 9 out of the 16 81.9-second telemetry sequences in a standard cycle were used for sending analog signals from selected instruments.
Other appendages on the IMP included antennas and booms which supported sensors for the 15-kilogram suite of seven instruments designed to study the magnetic field, radiation, cosmic rays, and plasma environment – a much more capable instrument suite than was carried by the smaller S-3 series Explorer satellites. The most noticeable of these was a 1.8-meter boom on top of the spacecraft capped with a 33-centimeter in diameter sphere for the IMP’s highly sensitive rubidium vapor magnetometer. Every fourth 81.9-second telemetry sequence would be dedicated to returning data from this sensitive instrument.
The intended orbit for the IMP A (also known as S-72 in NASA planning documents) was an extended 190 by 277,000-kilometer orbit with a period of six days, 8.7 hours. With an intended inclination of 33.0°, the initial apogee would be targeted to occur at about 10:20 local time. Because the orientation of the satellite’s elongated orbit would remain basically fixed in inertial space, IMP would sample different parts of Earth’s magnetosphere as our planet revolved around the Sun during the satellite’s intended 6 to 12-month lifetime. The spin axis of the satellite, which would remain essentially fixed once released by the launch vehicle, would be aimed at right ascension 7h 40m, declination -25°in the southern constellation of Puppis, the Ship Stern. IMP included an aspect sensor system to determine the orientation of the various sensors as the satellite rotated.
IMP A shown prior to the last solar panel being folded down in preparation for launch. (NASA)
Because of the extended geocentric orbit and the 62-kilogram mass of IMP A, a more capable launch vehicle than the Delta A used to launch Explorer 10 and 12 was required. The launch vehicle for the first group of IMP satellites was NASA’s Delta C. The three-stage Delta C, or DSV-3C, launch vehicle was the latest evolutionary upgrade based on the USAF Thor-Able rocket which launched many of NASA’s earliest spacecraft (see “Pioneer 1 – NASA First Space Mission”). The first stage of the Delta C consisted of an updated Thor rocket with a base diameter of 2.4 meters and a length of 17 meters. Originally developed by the Douglas Aircraft Company (which became McDonnell-Douglas in 1967 and three decades later merged with Boeing) for use as an IRBM by the USAF, the Thor used an improved version of Rocketdyne’s MB-3 power plant burning kerosene and liquid oxygen (LOX) to produce 755 kilonewtons of thrust at liftoff. The second stage was lengthened by 0.9 meters compared to the original design and now used an improved Aerojet AJ10-118 engine burning unsymmetrical dimethyl hydrazine (UDMH) and inhibited red fuming nitric acid (IRFNA) to produce 35 kilonewtons of thrust.
By far the most significant upgrade of the Delta C was the substitution of the original X-248 solid rocket motor for the third stage with a Hercules X-258-A5DM motor with a higher thrust of 12 kilonewtons. Based on Hercules’ work on the second stage of the Polaris A2 SLBM, the spin-stabilized X-258 motor (also known by the name Altair 2) had already been used as the final stage of NASA’s all-solid Scout launch vehicle. A despin mechanism mounted on the IMP, consisting of weights deployed on long cables, would be used to decrease the satellite spin rate to 22 RPM following separation from the third stage. The Delta C had a launch mass of about 52 metric tons and a total height of 27.4 meters including its streamline nose fairing. IMP A a would be the first launch of this improved Delta variant.
IMP A shown being enclosed in its nose fairing prior to launch. (NASA/GSFC)
The Explorer 18 Mission & Results
After a couple of day delay to attend to issue with Delta’s new third stage, the inaugural flight of the Delta C, Delta number 21, lifted off at 9:30 PM EST on November 26, 1963 carrying IMP A from what would be renamed as “Cape Kennedy” just two days later in commemoration to the recently slain president who had committed the nation to reaching the Moon. Also known as Explorer 18 after its launch, the 62.4-kilogram IMP 1 was initially placed into a 192 by 197,616-kilometer orbit with a period of 94 hours, 26 minutes, and an inclination of 33.3°. Although the apogee was well short of the planned 277,000 kilometers due to an underperformance of the X-258 third stage, it was still sufficient to meet mission requirements.
The launch of Delta 21 on November 26, 1963 from LC-17 carrying Explorer 18 (IMP A). (NASA)
After separation from the launch vehicles third stage, the despin mechanism went into action and the satellites appendages were deployed leading to an initial spin rate of 22.27 RPM. While the spin rate decreased slightly during IMP’s first orbit (presumably as its appendage settle into their final positions), it was observed that the spin rate increased to 24.19 RPM after 68 days in orbit because of the windmill effect of sunlight reflecting off the canted solar panels. This spin up continued until a maximum of 27.5 RPM was measured 151 days after launch. In late April 1964, the spin rate was observed to decrease as a result of the changing aspect of the Sun with respect to the satellite.
As Explorer 18 continued in its geocentric orbit, it probed different parts of the magnetosphere with respect to the Earth-Sun line as our planet revolved around the Sun over the course of a year. From launch through May 6, 1964, Explorer 18 returned excellent data from all but one of is experiments. The Thermal-Ion Electron Experiment suffered a malfunction in its mechanical programmer about 20 hours after launch. Eventually it proved possible to recover about 10% of the data it recorded. A gradual degradation in one of the two redundant programmer cards in the satellite starting on February 3, 1964 lead to the loss of about half of the data from the rubidium vapor magnetometer, which did not compromise the experiment results. In April an intermittent failure of the proton analyzer resulted in occasional data outages last several hours to several days.
This diagram shows how the orbit of IMP A sweeps around the Earth over the course of a year as our planet orbits the Sun. Click on image to enlarge. (NASA/GSFC)
As predicted, on May 6, 1964 Explorer 18 entered the Earth’s shadow for an extended period of 8 hours with spacecraft temperature dropping to as low as -60° C. After automatically shutting itself down, the satellite was reacquired by the NASA tracking station in Santiago, Chile the following day. While IMP 1 had survived, it had lost the use of the Cosmic Ray Experiment and began to experience unexplained episodes where the satellite would shut itself down unexpectedly. After May 30, 1964, Explorer 18 only maintained intermittent contact with NASA ground stations. As the angle of the Sun changed with respect to the satellite, another 600 hours of data were received between November 12 and December 15. The final period of operation running from February 21 to March 25, 1965 only returned a small amount of data. With no further contact afterwards, the orbit of Explorer 18 finally decayed on December 30, 1965.
Diagram showing the structure of the Earth’s magnetosphere as observed by Explorer 18 (IMP A). Click on image to enlarge. (NASA)
While it had encountered some operational issues, Explorer 18 managed to return almost 6,000 hours of data up until its last operation on March 25, 1965. With its most useful data returned during the first six month of operation, IMP 1 proved to be a highly successful mission which, when combined with data from other satellites, allowed scientists to map some of the details of Earth’s complex magnetosphere for the first time. Subsequent IMP missions, starting with the launch of IMP 2 on October 3, 1964 to become Explorer 21, would continue the much-needed exploration of this new realm.
General References
“Results of Space Research: The Solar Wind”, STL Space Log, Vol. 3, No. 1, pp. 32-38, March 1963
“Explorer 18”, Space Log, Vol. 4, No. 2, pp. 9-10, Summer 1964
Interplanetary Monitoring Platform: Engineering History and Achievements, NASA TM-80758, NASA/GSFC, May 1980
Roderick D. Hibben, “IMP Will Warn of Dangerous Solar Flares”, Aviation Week & Space Technology, Vol. 79, No. 19, pp. 28-29, November 4, 1963
Related Reading
“AIMP: The Forgotten Lunar Orbiters”, Drew Ex Machina, July 28, 2017 [Post]
“Vintage Micro: The First Interplanetary Probe”, Drew Ex Machina, April 15, 2015 [Post]