Vintage Micro: The Apollo Particles and Fields Subsatellites

There has been increasing interest in recent years in using the technology employed in miniaturized Earth-orbiting satellites for lunar and planetary exploration. NASA’s recent call for studies in using nanosatellites to explore Europa and the launch of the 14-kilogram German-built 4M (Manfred Memorial Moon Mission) payload that flew piggyback on the final stage of the Chinese launch vehicle that sent the recent Chang’e 5 T1 mission to the Moon are just two recent examples of this trend.

As I have pointed out before, the use of miniaturized payloads to perform lunar exploration is hardly new. The first successful American lunar probe, Pioneer 4 launched in March 1959 with a mass of just six kilograms, is a nanosatellite by today’s definition yet performed important scientific and technological investigations on its way past the Moon and into solar orbit (see “Vintage Micro: The Pioneer 4 Lunar Probe”). There are many lessons today’s mission planners can learn from these early missions that were flown with technology that was much more primitive than what is available today. One of those little-known lunar missions that involved miniaturized satellites was the Apollo program’s Particles and Fields Subsatellite (PFS) flown in the early 1970s.

The PFS & Its Experiments

The PFS was one small part of the expanded suite of science instruments carried by the J-series Apollo missions starting with Apollo 15. The purpose of the PFS was to characterize the magnetic field as well as plasma and energetic particle intensities from a moderate altitude lunar orbit with a nominal period of about two hours. These measurements were to complement those being made from the lunar surface using the ALSEP (Apollo Lunar Surface Experiment Packages) instruments left on the Moon by the Apollo astronauts and from Explorer 35 (also known as the Anchored Interplanetary Monitoring Platform 2) in its extended lunar orbit (see “AIMP: The Forgotten Lunar Orbiters“). To meet these objectives, each PFS carried out three experiments.

Diagram showing the layout of experiments in the Apollo 15 and 16 J-series science instrument module (SIM) on Service Module (SM). The PFS was stowed in a compartment in the lower left. Click on image to enlarge. (NASA)

The first experiment was officially designated the “Particles and Fields Subsatellite Particles Experiment (S-173)” with Kinsey A. Anderson (University of California – Berkeley) as the Principal Investigator (PI). The instruments for this experiment were a pair of solid state telescopes designed to measure electrons and protons and a set of five electrostatic analyzers to measure lower energy electrons. This suite of instruments could measure the kinetic energy of electrons in nine intervals over the 0.53 to 300 keV range and protons in six intervals in the 40 keV to 6 MeV range. The second experiment, with Paul J. Coleman (UCLA) as PI, was the “Particles and Fields Subsatellite Fields Experiment (S-174)”. Its sensor was a two-axis fluxgate magnetometer capable of measuring magnetic fields with a flux densities in the ±200 nanoteslas (nT) with a resolution as good as 0.4 nT.

The final experiment was the “S-Band Transponder (S-164)” whose PI was William Sjogren (NASA-JPL). This experiment would retransmit a precise signal sent to it at a frequency 2.115 GHz generated from a cesium reference on the ground. The round trip timing allowed the position of the satellite to be determined while Doppler measurements allowed its line-of-sight velocity to be measured to an accuracy of 6.5×10^-4 meters per second. With this information, the orbit of the satellite could be determined allowing the lunar gravitational field to be characterized as well as provide information on the position of the satellite to place its particles and fields measurements into context.

Diagram showing the Apollo Particles and Fields Subsatellite in its stowed (lower) and deployed (upper) configurations. Click on image to enlarge. (NASA)

The PFS, which was designed and built by TRW, was a hexagonal prism with a length of 78 cm about 36 cm across its cross section with a mass of just 36 kilograms (a microsatellite by today’s definition). Each of its six long faces were covered with solar cells that produced a peak of 25 watts of electricity in sunlight to recharge 11 silver-cadmium batteries that powered the satellite’s systems. At one end of the satellite was a trio of 1.83 meter long, deployable booms one of which carried the magnetometer sensor. Since the satellite had no active attitude control system, it was spin-stabilized requiring the other two booms to carry balancing masses. The satellite was ejected at a relative velocity of 1.2 meters per second from the scientific instrument module (SIM) of the J-series Apollo Service Module upon command of the astronauts by means of a compression spring which also imparted an initial spin of 140 RPM. After the satellite deployed its booms, the spin rate decreased to just 12 RPM. The satellite was precisely deployed so that its spin axis would be aligned perpendicular to the ecliptic plane to an accuracy of about 1°. A wobble damper inside the satellite removed an precessional and nutational motions of the satellite resulting from its ejection from the SM or the deployment of its booms.

PFS 1 with its booms stowed being prepared to be inserted into the Apollo 15 SIM. (NASA)

The satellite was capable of transmitting data at a rate of 128 bits per second via an antenna on the end of the PFS opposite of its three booms. Since the objectives of the PFS experiments required data to be gathered from anywhere around the Moon, the satellite included a (primitive by today’s standards) magnetic core memory with a storage capacity of 49,152 bits which could be recorded in real time at speeds of 8 or 16 bits per second allowing data to be recorded for a full or half of an orbit, respectively. The contents of the memory could be downloaded upon ground command in 512 seconds. The expected lifetime of the PFS was one year.

The PFS Missions

The first subsatellite, PFS 1, was carried by Apollo 15 launched on July 26, 1971. PFS 1 was deployed at 21:00:31 GMT on August 4 after Apollo 15 had completed its surface operations and was preparing for return to Earth. Its initial orbit around the moon was 102 by 139 kilometers inclined 28.5° to the equator (actually, since the satellite was travelling clockwise as viewed from the Moon’s north pole, it was technically in a retrograde orbit with an inclination of 151.5°).

A portrait of the crew of Apollo 15 – (l to r) David Scott, Alfred Worden and James Irwin – shown with a mockup of the PFS. (NASA)

The day after deployment, the process of activating the experiments on PFS 1 commenced and it began to take data. Because of perturbations from the Moon’s lumpy gravitational field (and to a lesser extent, the influence of the Sun and Earth), the orbit of PFS 1 slowly evolved over time but it remained safely above the lunar surface. Failures of two telemetry channels on February 4 and 26, 1972 curtailed the satellite’s ability to return data and disabled the magnetometer experiment. The surviving data channels were monitored intermittently until June 1972 and ground support was finally terminated on January 22, 1973 – 537 days after PFS 1 was deployed from Apollo 15 and the month after the last Apollo lunar mission was completed. PFS 1 is believed to have eventually impacted the lunar surface at some later date in an unknown location.

Image of PFS 1 after it was deployed from Apollo 15 on August 4, 1971. (NASA)

The second subsatellite, PFS 2, was carried by Apollo 16 launched on April 16, 1972. Because of an issue with the backup thrust vector control of the Apollo 16 SM’s main engine, the final maneuver to shape the orbit of the spacecraft before the deployment of PFS 2 could not be performed. As a result, it was deployed into a less-than-ideal orbit of 90 by 130 kilometers inclined 10° to the lunar equator (actually, a retrograde orbit with an inclination of 170°) at 21:56:09 GMT on April 24 with its spin axis misaligned by 5.5°.

Unlike its sister, the orbit of PFS 2 very quickly changed in a matter of days and after just two and a half weeks in orbit, it was swooping to within 10 kilometers of the lunar surface. Over the following days its closest approach backed off to almost 20 kilometers but its fate was already sealed. Tracking indicated that on its 416^th revolution, PFS 2 had come to within 5 kilometers of the surface on the lunar farside. PFS 2 failed to reappear for revolution 426 on May 29, 1972 and is believed to have crashed into the Moon around 10.2° N, 112° E at about 21:00 GMT. PFS 2 had lasted only 34 days in lunar orbit instead of the planned year.

A Mercator projection map of the lumpy gravitational field of the Moon as measured by NASA’s GRAIL mission launched in September 2011. The units are in milliGalileos (mGal) which correspond to 10 microns per second squared. Click on image to enlarge. (NASA/JPL-Caltech/GSFC/MIT)

Subsequent study of the unusually lumpy lunar gravitational field, which was ironically aided by the unintended low altitude passes of PFS 2, showed that perturbations from mass concentrations (called “mascons”) had caused the orbit of PFS 2 to change more quickly than anticipated. While the mascons had first been detected in 1966 by the Soviet Luna 10 (see “Luna 10: The First Lunar Satellite“) and roughly mapped out by tracking the orbits of NASA’s Lunar Orbiter satellites over the next couple of years, the full effects of their influence on the evolution of the eccentricity of low orbiting lunar satellites was not fully appreciated until later. Eventually it was found that there were four special orbit inclinations for so called “frozen orbits” where the orbit eccentricity would not change under the influence of the mascons: 27°, 50°, 76° and 86° as well as their corresponding retrograde orbits. By chance, PFS 1 was deployed into an orbit with an inclination near one of these special values so it remained in orbit much longer than PFS 2.

In addition to mapping the lunar gravitational filed, the pair of PFS missions also gathered much useful data about the particles and fields environment around the Moon. For example, unlike the Earth with its strong magnetic field which diverts the stream of charged particle coming from the Sun called the solar wind, the Moon essentially punches a hole in the flow by physically blocking it. Remnants of the Moon’s ancient magnetic field were also detected during the parts of the lunar orbit when the Moon was outside the relatively noisy magnetosphere of the Earth. The presence of one especially strong magnetic anomaly was noted on the farside of the Moon near the crater named Van der Graff. With the increasing interest in exploring space beyond Earth orbit with miniaturized spacecraft, hopefully the lessons learned from the PFS missions will be of value to today’s mission planners.

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General References

Robert Goodwin (editor), Apollo 15 – The NASA Mission Reports Volume 1, Apogee Books, 2001

Robert Goodwin (editor), Apollo 16 – The NASA Mission Reports Volume 1, Apogee Books, 2002

Andrew Wilson, Solar System Log, Jane’s, 1987

Apollo 16 Science Handbook – Revision, MSC-0894, NASA – Manned Spacecraft Center, April 7, 1972

Apollo Scientific Experiments Data Handbook, JSC-09166, NASA Technical Memorandum TM X-58131, August 1974