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Suprathermal Ion Detector Experiment (SIDE)

NSSDCA ID: 1971-063C-05

Mission Name: Apollo 15 Lunar Module /ALSEP
Principal Investigator:Dr. John W. Freeman


The Suprathermal Ion Detector Experiment (SIDE), part of the ALSEP package, measured positive ions reaching the lunar surface, including magnetospheric ions and those generated from ultraviolet ionization of the lunar atmosphere and from the free-streaming solar wind/lunar surface interaction. Flux, number density, velocity, and energy/unit charge were determined for these ions. The scientific objectives of the experiment were: to provide information on the energy and mass spectra of the positive ions close to the lunar surface; measure the flux and energy spectrum of positive ions in the Earth's magnetotail and magnetosheath during those periods when the Moon passes through the magnetic tail of the Earth; provide data on the plasma interaction between the solar wind and the Moon; and determine a preliminary value for the electric potential of the lunar surface. Similar instruments, differing only in look direction and mass range, were also flown on Apollo 12 and 14.

The experiment was housed in a rectangular box which was deployed on the surface of the Moon by the astronauts during their first EVA. A bubble level on top of the box was used to ensure proper leveling. The Apollo 15 SIDE was tilted 26 degrees towards the equator on special tripod legs so that the detectors looked upward, in a plane parallel to the lunar equator, and 15 deg eastward from the local meridian plane. The sensors looked approximately 18.6 deg to the left of Earth, so solar wind ions were not directly observable while the moon was outside the magnetosphere. Streaming ions in the downstream dusk-side magnetosheath were observed, as were ions upstream from the bow shock. The SIDE is connected to the ALSEP central station by a ribbon cable. A wire screen is spread out on the surface under the tripod to compensate for a possibly large (tens of volts) lunar surface electric potential. The screen is connected to one side of a stepped voltage supply, the other side of which is connected to the internal ground of the detector and to a grounded grid mounted immediately above the instrument and in front of the ion entrance apertures. The top of the instrument is roughly 45 cm above the surface.

The SIDE consisted of two positive ion detectors. The first, the Mass Analyzer (MA), consisted of a velocity filter of crossed E and B fields (a Wien filter) in tandem with a curved-plate electrostatic energy-per-unit-charge filter and a channel electron multiplier behind both filters. The multiplier was operated as an ion counter that yielded saturated pulses for each input ion. The Apollo 15 MA determined the ion flux in 20 mass channels from 1 to approximately 90 amu per charge for 6 energies: 0.2, 0.6, 1.8, 5.4, 16.2, and 48.6 eV. Only the two highest of these energy levels were well calibrated in the laboratory, although data from the other levels are still useful. The other analyzer, the Total Ion Detector (TID), did not have a velocity filter and used a channel electron multiplier to detect higher energy ions in 20 steps over the range 10 to 3500 eV. Both multipliers were biased with the input ends at -3.5 kV to boost the positive ion energies to improve detection efficiency. A mass spectrum (from MA) and an energy spectrum (from TID) were obtained each 24 s in normal mode. The potential of the entrance apertures relative to the grid deployed on the lunar surface was normally varied through 24 steps (of the following voltages: 0, 0.6, 1.2, 1.8, 2.4, 3.6, 5.4, 7.8, 10.2, 16.2, 19.8, 27.6, 0, -0.6, -1.2, -1.8, -2.4, -3.6, -5.4, -7.8, -10.2, -16.2, -19.8, and -27.6) at 2.58 min/step, in order to monitor the lunar surface potential.

The Cold Cathode Ion Gauge (CCIG, also called Cold Cathode Gauge Experiment - CCGE) was carried in a compartment of the SIDE instrument and was removed and set up on the lunar surface by the deploying astronaut. The gauge was connected by a cable to the SIDE package and attached to the end of the SIDE ground screen holding tube, which was designed to pivot down to the ground and hold the CCIG in place a fixed distance from the SIDE, and with the proper orientation of the gauge head. This avoided the difficulties encountered with Apollo 12 and 14 units, where the extreme stiffness and springiness of the cold wrapped electrical cable, plus the low lunar gravity, made it difficult to position the gauge head. The CCIG and SIDE electronics comprised an integrated system. The power (6.5 W) and data rate (82.8 bits/second) apply to the combined instruments.

The ALSEP central station was located at 26.1341 N latitude, 3.6298 E longitude. The SIDE was deployed approximately 17 meters east-northeast of the central station. The SIDE was turned on about 19:00 UT on 31 July 1972. Electric arcing at high temperatures resulted in the SIDE being systematically cycled on-off for several days on each side of lunar noon, throughout the mission, to keep the internal temperatures below ~55 degrees C. The SIDE was commanded off on 12 March 1977. An instrument description and preliminary report is given by Hills, Meister, Vondrak, and Freeman as section 12 of the Apollo 15 Preliminary Science Report, NASA SP-289, 1972.

The results of the SIDE experiments included observations of: lunar ions accelerated by the solar wind induced field; 1-3 keV protons during lunar night, considered to be protons from the bow shock of the Earth; the energetic ion characteristics in the Earth's magnetosheath and at its boundaries, and correlation with geomagnetic activity; apparent motions of ion "clouds" related to lunar impact events, and mass spectra during the events; energetic ions during lunar night, when site is shielded from the solar wind direction; ion events near terminators suggesting a turbulent region of solar wind plasma interaction with the solid Moon; positive ion fluxes while in the geomagnetic tail, and correlation with geomagnetic storm activity; mass spectra of ions from the ambient atmosphere; the electric potential of the lunar surface in the magnetosheath or solar wind and near the terminators; solar wind during interplanetary storms; penetrating ions from solar flares (especially from the major flare event in August 1972); the effects of the LM ascent engine exhaust on magnetosheath ion fluxes, ion mass spectra due to the LM exhaust gas, and the intensity decay rate.

Alternate Names

  • Apollo15ALSEP/SIDE
  • S036
  • SIDE
  • urn:nasa:pds:context:instrument:side.a15a

Facts in Brief

Mass: 8.5 kg
Power (avg): 6.5 W
Bit rate (avg): 0.08281 kbps

Funding Agency

  • NASA-Office of Manned Space Flight (United States)


  • Space Physics: Magnetospheric Studies
  • Planetary Science: Fields and Particles

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. H. Kent HillsOther InvestigatorNASA Goddard Space Flight Center
Dr. F. Curtis MichelOther InvestigatorRice
Dr. John W. FreemanPrincipal InvestigatorRice University

Selected References

  • Hills, H. K., et al., Suprathermal Ion Detector Experiment (Lunar Ionosphere Detector), in Apollo 15 Prelim. Sci. Rept., NASA SP-289, Wash., DC, 1972.

Related SIDE Information at NSSDCA

Restored SIDE Data at NSSDCA

Apollo 12 SIDE Instrument Description
Apollo 14 SIDE Instrument Description
Apollo 15 SIDE Instrument Description

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