Back during the days of the Apollo lunar missions, young budding space enthusiasts like myself were all aware of the trio of unmanned lunar programs which had paved the way to the Moon during the 1960s: the Ranger impact missions, the Surveyor missions to soft land on the Moon and the Lunar Orbiter missions to map possible landing sites. It was not until the early 1980s when I was a budding space historian interested in the technical details of spaceflight that I became aware of another, lesser known program to orbit the Moon: NASA’s Anchored Interplanetary Monitoring Platforms (AIMP) launched as part of the Explorer program in 1966 and 1967. Although not as well known as NASA’s “big three” lunar programs, the data returned by these spacecraft as well as the other spacecraft of the Interplanetary Monitoring Platform (IMP) program were nonetheless vital to the success of Apollo.

Origins of the IMP Program

The 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.

The first three of the initially planned seven satellite series were all built in-house at GSFC to a common design based on their experience with earlier satellites of the Explorer series. IMP A, B and C, as the first three satellites were known before launch, consisted of an octagonal shaped main structure 71 centimeters across and 20 centimeters tall which housed all of the spacecraft’s systems. Spin-stabilized by a rotation rate of 20 RPM, the spacecraft sported a number of appendages. First were a quartet of 66 by 46 centimeter panels holding 11,500 solar cells which would be deployed after launch. With a total area of about three square meters, these panels provided 38 watts to power the spacecraft’s systems and recharge its 5 amp-hour silver-cadmium battery. Other appendages included antennas and booms which supported sensors for the suite of seven instruments designed to study the magnetic field, radiation and plasma environment. 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. With a typical mass of about 60 kilograms (a microsatellite by today’s standards), the first IMP satellites carried about 15 kilograms of scientific instrumentation.

The launch vehicle for this 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 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. The Delta C had a launch mass of about 52 metric tons and a total height of 27.4 meters including its new streamline nose fairing.

The inaugural flight of the Delta C, Delta number 21, lifted off 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 and 26 minutes. Although this was shorter than the intended 102 hour, 44 minute orbit with a higher apogee of 277,184 kilometers due to an underperformance of the X-258 third stage, it was still sufficient to meet mission requirements. While it had encountered some operational issues after six months in orbit, Explorer 18 managed to return almost 6,000 hours of data up until its last operation on March 25, 1965.

The 61.4-kilogram IMP B was launched from Cape Kennedy on October 3, 1964 to become Explorer 21. A malfunction in the third stage of Delta 31 left IMP 2 in a 193 by 95,400 kilometer orbit with a period just shy of 35 hours – much lower than the planned orbit with a 203,540 kilometer apogee. The lower orbit prevented Explorer 21 from penetrating Earth’s bow shock to monitor conditions in interplanetary space. A battery failure two months into the mission meant that IMP would shut down for 8 hours to recycle its power systems every time it entered Earth’s shadow. The low orbit and less than ideal orientation with respect to the Sun severely affected the spacecraft’s systems but it still managed to return data for a total of five months and was last heard from on October 13, 1965.

Based on experience gained during the first two flights, a series of improvements were made to the last spacecraft of the initial batch of IMP satellites. Modifications to various components of its power system brought the mass of IMP C down to 58.4 kilograms at the same time that the mass of its instrument suite had grown to 20 kilograms. IMP C was launched by Delta 31 from Cape Kennedy on May 29, 1965 to become Explorer 28 after entering a higher than planned 205 by 260,799 kilometer orbit with a period just 84 seconds shy of 140 hours. Although some issues were encountered, IMP 3 managed to return large quantities of scientific data until it was last heard from on May 12, 1967.

The Anchored IMPs

While the first three IMPs were able to return much useful data from their elongated geocentric orbits, they did have limitations. The comparatively slow motion of the spacecraft near apogee meant that they would return much of their data far from the Earth, however, the IMPs were unable to provide continuous data from these great distances because their orbits would periodically swing in close to the Earth. In addition, the orientation of these elongated orbits tended to stay fixed in inertial space. As a result, the spacecraft would take a year as the Earth moved in its orbit to sample the environment at all angles with respect to the Sun from the magnetotail around to the interplanetary environment ahead of Earth’s bow shock. What was needed was a spacecraft in a much more distant orbit which could take continuous measurements far from the Earth yet sample all parts of its magnetosphere and surrounding interplanetary space in a reasonable time.

A January 1964 feasibility study suggested a solution to IMP’s limitations: place an IMP in a distant orbit around the Moon. With the Moon serving as an anchor, this Anchored IMP or AIMP would be able to monitor the environment continuously at lunar distances with the orbital motion of the Moon allowing everything from interplanetary space to Earth’s long magnetotail to be observed during the course of every lunar month. In addition, the AIMP would provide data on the lunar environment itself more directly supporting NASA’s Apollo program.

For the second batch of spacecraft in the IMP program, a pair of AIMP missions designated AIMP D and E before launch were approved for development. Like the earlier IMP spacecraft, the AIMP was built around an octagonal bus 71 centimeters across to house all of the spacecraft’s systems which now was 31 centimeters tall and reinforced to handle greater loads. Based on experience with the earlier spacecraft of the series, the heat-producing 7-watt VHF transmitter and the power converter subsystem were placed near the outer edge of the spacecraft bus on opposite sides in order to spread out the heat load and aid in thermal control. As with the IMPs, the AIMP spacecraft included four solar panels which were now enlarged to 70 by 64 centimeters with a total of 7,680 improved solar cells. Initially these panels provided an average of 66 watts of electrical power and at least 49 watts at the end of the spacecraft’s six-month design life. The power system now included an 11 amp-hour rechargeable silver-cadmium battery pack which could provide 2½ to 3 hours of power when the spacecraft was in shadow. A total of six instruments with a combined mass of nine kilograms were included to monitor the magnetic field, energetic particles and plasma in the surrounding environment. The spacecraft’s transmitter would also provide data for passive experiments to characterize the lunar ionosphere and map the Moon’s gravitational field by tracking the spacecraft’s orbit around the Moon.

By far the biggest difference between the original IMP and the AIMP spacecraft was the inclusion of a Thiokol TE-M-458 solid rocket motor on the top of the spacecraft in place of the early IMPs’ iconic boom with its ball-shaped magnetometer. This motor would act as a retrorocket to slow AIMP enough to enter lunar orbit about 72 hours after launch. With a mass of 36 kilograms, the TE-M-458 would produce 4.1 kilonewtons of thrust for 20 to 22 seconds when commanded to ignite as AIMP passed about 4,800 kilometers ahead of the Moon. About two hours after entering a nominal 1,300 by 6,400 kilometer orbit inclined 175° to the lunar equator, the TE-M-458 motor would be jettisoned. The AIMP would then be able to make its measurements as it circled the Moon once every ten hours. In order to simplify the design of the AIMP, no additional propulsion system was carried to perform a midcourse maneuver. Instead, AIMP would rely on the accuracy of its launch vehicle to set it on the proper course to enter lunar orbit. The total height of the AIMP from its interface with the launch vehicle to the top of its retrorocket was about 88 centimeters and it had a total launch mass of 94 kilograms – still a microsat by today’s standards.

Because of its increased launch mass and the greater distances the AIMP would need to travel to reach the Moon, a much more capable launch vehicle was required than the Delta C employed by the first IMP missions. Thankfully, NASA had already been working on a series of upgrades to their Delta launch vehicle resulting in the new Delta E which was also known by the designation DSV-3E. The most significant difference between the Delta E and the earlier C-model used by the original IMPs was the addition of three Castor solid rocket motors strapped onto the base of the Thor first stage. Built by Thiokol, these motors were 79 centimeters in diameter, six meters long with a mass of 4,160 kilograms each. Based on Thiokol’s earlier work on the Sergeant missile and also employed as the second stage of NASA’s all-solid Scout launch vehicle, these three motors were ignited on the pad along with the Thor’s improved Block III MB-3 engine to increase the liftoff thrust from 763 kilonewtons to 1,481 kilonewtons. These motors, which were jettisoned after they burned out during ascent, were first employed in 1964 on the Thor-Agena D rocket used by the USAF to create the Thrust Augmented Thor or TAT. Likewise, these early versions of the imporved Delta which used the Castor strap on boosters were also known at the time as the Thrust Augmented Improved Delta or TAID.

In addition to the strap on rocket motors, the Delta E sported some less obvious but still significant improvements. The diameter of the second stage was increased from 0.8 to 1.4 meters resulting in a mass growth from 2,900 to about 5,900 kilograms fully fueled. The huge increase in propellant load now meant that the burn time of the pressure-fed AJ10-118 engine had gone from 178 to 398 seconds. While the baseline Delta E used an X-258 Altair 2 motor as a third stage, the Delta E1 variant to be used to launch the AIMPs instead sported a 25-kilonewton FW-4D solid rocket motor built by United Technology Corporation (UTC) which had also developed the three-meter solid rocket motors of the USAF Titan IIIC launch vehicle (see “The First Missions of the Titan IIIC”). As with the X-258, the FW-4S model of this motor was used as the fourth stage of some advanced versions of NASA’s Scout launch vehicle. With its nose fairing in place, the Delta E stood 28 meters tall and had a launch mass of about 68 metric tons. The substantial improvements in the Delta E had increased its payload capacity for geosynchronous transfer orbits (GTO) to 150 kilograms from 82 kilograms for the earlier Delta C – more than enough to handle the increased payload demands of the new AIMP satellites.

The First Attempt – Explorer 33

Because of the requirements of the AIMP mission, groups of daily launch windows just a few minutes long only existed around the time of the solstices. With this in mind, the launch of AIMP D was slated for the early summer of 1966 just as NASA was also pushing ahead to launch the first of its much more ambitious Lunar Orbiter spacecraft to map the surface of the Moon (see “Lunar Orbiter 1: America’s First Lunar Satellite”). Three-minute launch windows (the most restrictive for any lunar mission attempted by NASA) were available daily from June 30 to July 3 and again for a three day period late in July. Although studies suggested that there was about a 70% chance that AIMP would be launched accurately enough to attain a suitable lunar orbit, a contingency plan was available to fire the retrorocket about six hours after launch to place AIMP D into an extended 39,000 by 434,000 kilometer geocentric orbit if this proved not be possible.

The launch vehicle for the AIMP D mission was designated Delta 39. Launched from Pad A of Launch Complex 17 on Cape Kennedy, Delta 39 would be the fourth launch of the TAID and the first of the Delta E1 variant using the FW-4D upper stage (although the FW-4D had been successfully flown once before on May 25, 1966 for the launch of Explorer 32 using a Delta C1). The AIMP D spacecraft was prepared for launch and fitted with one last experiment. Four panels each consisting of 16 one by two centimeter solar cells were attached to the body of the spacecraft. With one panel’s solar cells unshielded and the other three incorporating various types of transparent protective covers, the objective of this engineering experiment was to monitor how radiation affected the performance of n-on-p silicon solar cells. The AIMP D spacecraft was mounted onto Delta 39 on June 22 and final preparations were made for launch.

The countdown for the first launch attempt started on June 28, 1966. The countdown for a launch at 10:08:13 AM EST on June 30 proceeded well until T-3 minutes when a hold was called for weather. While a heavy Florida rain shower had cleared from LC-17A and the countdown proceeded to a hold at T-0 for an instant launch upon command, heavy rain over the nearby tracking site responsible for radio guidance during the early phases of flight for Delta 39 as well as a problem with an air conditioning duct on the nose fairing forced a last second scrub. The countdown was recycled for a launch the next day. The second time proved to be the charm as the countdown proceeded as planned with Delta 39 lifting off from LC-17A at 11:02:25.5 EST (16:02:25.5 GMT) at the beginning of that day’s launch window.

All three stages of Delta 39 operated well within specifications with AIMP D, now called Explorer 33, separating from the final stage of its launch vehicle 19 minutes after launch. While Explorer 33 seemed healthy, early tracking indicated that the probe was in 6,671 by 867,119 kilometer orbit – much higher than the planned 650,000 kilometer apogee orbit required to guarantee lunar orbit could be achieved. A detailed postflight inspection of the Delta’s ascent telemetry showed that a minor guidance tracking error during the second stage burn combined with a 0.3% above-nominal performance of the third stage had left Explorer 33 with an excess velocity of 21.3 meters per second. Although all of Delta’s systems had worked well within their specifications, the sum of the minor deviations had conspired to leave AIMP unable to enter lunar orbit. With the original goal of entering lunar orbit not possible, Explorer 33 ignited its TE-M-458 retrorocket upon ground command at 22:32:57 GMT to enter a 50,000 by 450,000 kilometer orbit with a period of 15.5 days. At 00:33:47 GMT on July 2, the AIMP jettisoned its now spent retrorocket to begin its scientific mission 8½ hours after launch. GSFC had just missed out beating Lunar Orbiter 1 developed by NASA’s Langley Research Center to the Moon by six weeks although the Soviet Luna 10 mission had already beaten everyone to lunar orbit three months earlier (see “Luna 10: The First Lunar Satellite”).

Explorer 33 commenced its alternate mission in high orbit returning valuable data about Earth’s magnetosphere and nearby interplanetary environment. While it failed to enter lunar orbit, Explorer 33 did make distant passes of the Moon starting with a 35,000 kilometer encounter on July 9, 1966. This and subsequent distant lunar encounters strongly perturbed the orbit of Explorer 33 causing the perigee altitude to vary from 32,200 to 262,000 kilometers and the apogee to range from 432,000 to as much as 591,00 kilometers during the first two years of its mission as the Moon added and removed energy from the high flying satellite. Although issues with spacecraft systems and instruments were encountered and dealt with, Explorer 33 sent back data almost continuously until January 14, 1970 and then intermittently until it was last heard from on September 21, 1971. This five-plus year lifetime far surpassed the six month design life of the AIMP.

Explorer 35 Finds Its Anchor

While Explorer 33 failed to attain its intended goal, postflight analysis indicated that if the AIMP had changed its attitude by just 10° after separation from the last stage of its launch vehicle, the delta-v from its retrorocket would have been in the right direction to allow the spacecraft to enter lunar orbit. Unfortunately, AIMP (like the earlier IMPs) did not include an active attitude control system in order to simplify the spacecraft’s design. Instead, the AIMP relied on its 28 RPM spin to keep it stably pointing in the same direction after its release from the Delta third stage. Only the tiny torques from sunlight reflecting off of the solar panels and other parts of the spacecraft would slowly alter the spin rate and slightly change the direction of the spin axis over time. The availability of an active attitude control system would greatly improve the chances that AIMP E could enter lunar orbit as well as add flexibility in its mission.

During the course of engineering studies for more advanced IMP spacecraft to follow the originally planned seven missions, engineers at GSFC had considered the inclusion of an active attitude control system in the proposed IMP H and J spacecraft starting in mid-1965. Such a system would eliminate the need for paddle-mounted solar cells freeing up more mass and volume for other equipment and simplifying the thermal design. Following the launch of Explorer 33, it was decided to modify the AIMP E design to include a simple attitude control system (ACS) based on these studies. Not only would this be helpful for the AIMP E mission but it would allow GSFC engineers a chance to flight test components that would be incorporated in future IMP designs.

The ACS on AIMP E consisted of a pair of 71-millinewton cold-gas thrusters mounted on the tips of solar panels #2 and #4 some 2.5 meters apart. Using Freon-14 as a propellant, the thrusters would be fired upon ground command using information provided by a solar aspect sensor already included in the AIMP design. The ACS along with modifications to the ascent trajectory of the Delta E1 greatly improved the chances that AIMP E could attain lunar orbit. Other modifications to the last AIMP included changes in the thermal control scheme for the TE-M-458 retrorocket and the inclusion of two more instruments. One was a contamination monitor on the top deck of the spacecraft body to determine the source of contamination which degraded the thermal coating on Explorer 33. In addition to the suite of scientific instruments previously carried on Explorer 33, AIMP E also included a micrometeorite flux experiment to complement measurements being made closer to the Moon by the Lunar Orbiter missions. With all the changes, the design life of AIMP E was now extended to one year and its launch mass had grown to 104 kilograms with about 13 kilograms being instruments.

The target for the AIMP E mission was a lunar orbit with a perilune no lower than about 480 kilometers and an apolune of less than 45,900 kilometers. With a planned inclination in the 140° to 180° range, the orbital period could be anywhere from ten to as much as 70 hours depending on the actual approach trajectory of AIMP E. If during the first six hours of tracking after launch it was found that the AIMP E could not achieve lunar orbit, the TE-M-458 retrorocket could be fired as it was on Explorer 33 to place the spacecraft into an extended 30,000 by 450,000 kilometer geocentric orbit with a two week period.

The launch vehicle for the AIMP E mission was Delta 50 to be launch from LC-17B at Cape Kennedy. This would be the seventh flight of the Delta E1 with six previous launches being successful including the last one which placed the 74-kilogram IMP F or Explorer 34 into an extended 242 by 214,382 kilogram Earth orbit on May 24, 1967. In order to reach lunar orbit, AIMP E had daily three-minute launch windows available from July 19 to 22.

Delta 50 successfully lifted off at 10:19:02 AM EDT (14:19:02 GMT) during the first available launch window on July 19, 1967. Unlike its predecessor, AIMP E had been placed into a trajectory which would allow what was now designated Explorer 35 to enter lunar orbit despite a slightly faster than desired approach. Beginning about 38 hours after launch, the ACS was commanded to make a series of carefully timed bursts over the course of three hours to change the spacecraft’s attitude by 5.55° in order to optimize the alignment of the retrorocket to enter the desired lunar orbit. On July 22 at 9:19 GMT, the TE-M-458 retrorocket was fired allowing Explorer 35 to enter a 2,538 by 9,429 kilometer orbit with an inclination of 169° and a period of 11.5 hours. The team at GSFC had finally succeeded in placing an AIMP in lunar orbit. After the spent retrorocket was jettisoned two hours following orbit insertion, the ACS was employed again over the following days to align the spin axis of Explorer 35 to within 4° of the south ecliptic pole so that it could begin its science mission.

Measurements made by Explorer 35 clearly showed that the Moon lacked a magnetic dipole of its own and possessed no radiation belts confirming measurements made by earlier lunar probes. No bow shock was present but instead, the Moon simply punched a hole in the flow of the solar wind with a tail estimated to exceed 160,000 kilometers in length. Calculations indicated that the Moon had a low bulk conductivity and suggested an internal temperature less than 1,000 K. Explorer 35 would continue returning data from its perch in high lunar orbit into the manned Apollo missions of 1969 to 1972. Coordinated measurements with the ALSEP (Apollo Lunar Surface Experiments Package) instruments left on the lunar surface by the Apollo landing missions as well as the Particles and Fields Subsatellites left in moderate-altitude lunar orbits during the Apollo 15 and 16 missions allowed simultaneous sampling of the lunar environment at multiple levels giving scientists additional insights into the processes at work on the Moon (see “Vintage Micro: The Apollo Particles and Fields Subsatellite”). Explorer 35 was finally shutdown on June 24, 1973 after operating for almost six years in lunar orbit.

Afterwards

The IMP program would continue after the launch of Explorer 35 and the originally approved seven satellites. Because of the value of the data returned, a total of ten spacecraft were eventually flown culminating with the launch of IMP J on October 25, 1973. Explorer 50, as the last IMP was designated after launch, would continue to return science data until October 7, 2006. The experience gained from the IMP program was eventually applied to the new ISEE (International Sun Earth Explorer) program whose first pair of satellites – one built by NASA and the other by ESA – were launched together on October 22, 1977. The American ISEE 3 was launched on August 12, 1978 to monitor the interplanetary environment from a halo orbit around the L1 Sun-Earth Lagrange point 1.5 million kilometers from the Earth towards the Sun. After ISEE 3 completed its mission, it was redirected by a series of propulsive maneuvers and multiple lunar flybys to make the first ever comet flyby as part of the new ICE (International Cometary Explorer) mission (see “ICE: The First Comet Flyby”). The September 11, 1985 encounter of ICE with the short-period comet 21P/Giacobini-Zinner would continue the tradition of scientific exploration started by the IMP program almost a quarter of a century earlier.

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Related Reading

“Lunar Orbiter 1: America’s First Lunar Satellite”, Drew Ex Machina, August 14, 2016 [Post]

“Vintage Micro: The Apollo Particles and Fields Subsatellite”, Drew Ex Machina, November 23, 2014 [Post]

“ICE: The First Comet Flyby”, Drew Ex Machina, September 11, 2015 [Post]

General References

J. V. Fedor, T.W. Flatley, M.F. Federline and J.R. Metger, Explorer XXXV Attitude Control System, NASA TN D-5187, NASA/GSFC, June 1969

J. Hunley, U.S. Space-Launch Vehicle Technology: Viking to Space Shuttle, University Press of Florida, 2008

J. J. Madden, Interim Flight Report Anchored Interplanetary Monitoring Platform AIMP 1 – Explorer XXXIII, NASA TM X-55663, NASA/GSFC, December 1966

Paul G. Marcotte, IMP D & E Feasibility Study, NASA TM X-55166, NASA/GSFC, January 1964

Norman F. Ness, “Lunar Explorer 35”, XIth COSPAR Meeting (Tokyo, Japan), May 16, 1968

Moon Orbiting Explorer Scheduled for June 30 Launch, NASA Press Release 66-162, June 27, 1966

AIMP (IMP-D) Technical Summary Description, NASA TM X-55770, NASA/GSFC, March 1967

IMP-E Launch Set July 19 at Cape Kennedy, NASA Press Release 67-178, July 13, 1967

“Explorer 35 (IMP E)”, TRW Spacelog, Vol 7, No. 4, pp. 21-23, Winter 1967-68

Interplanetary Monitoring Platform: Engineering History and Achievements, NASA TM-80758, NASA/GSFC, May 1980