The Suprathermal Ion Detector Experiment (SIDE) was a

lunar science The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

experiment, first deployed by astronauts on the lunar surface in 1969 as part of

Apollo 12 Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center in Florida. Commander Charles ...

, and later flying on

Apollo 14 Apollo 14 (January 31February 9, 1971) was the eighth crewed mission in the United States Apollo program, the third to Moon landing, land on the Moon, and the first to land in the Geology of the Moon#Highlands, lunar highlands. It was the las ...

and

Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the Apollo program and the fourth Moon landing. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a longer stay on the Moon and a greate ...

. The goal of SIDE was to study any potential lunar ionosphere and the

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

.Description of SIDE/ CCGE Experiments for ALSEP
(1966), R.M Magee

Background

The idea of landing a positive ion detector on the Moon was first proposed by the lunar science branch of NASA's Manned Space Science Program Office in 1964. If, as was assumed, the Moon did not have any kind of

bow shock In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby fl ...

and limited atmosphere then "daytime" measurements would mostly correlate with the undisturbed solar wind. However, during the lunar night it was believed that due to the presence of either shock limbs or turbulent flow of the solar wind, measurements by a positive ion detector would not simply be of undisturbed solar wind. While experiments for assessing non-ionised gases would be preferred for analysing the Moon's atmosphere, ion detectors and ion spectrometers were more mature technologies. It was feared that the

Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed s ...

could release enough gas from its engine exhaust to constitute 5% of the entire potential atmosphere of the Moon and contaminate any results from the experiment.

Instrument

SIDE consists of two positive ion detectors, a mass analyser and a total ion detector, located side by side in a parallel arrangement. The goal of the experiment was to provide mass per unit charge spectra of the positive ions present near the Moon's surface. To mitigate any possible effects of the Moon's regolith having an electric potential that might impact detection of low energy ions, the instrument would sit on top of a wire screen that would apply a variety of charges to counter any potential surface voltage. The housing for SIDE also housed the electronics for the Cold Cathode Gauge Experiment (CCGE). The SIDE command and control circuits also supported the CCGE. The CCGE was separated from SIDE by a cable.

Science

The experiment found evidence that the limb shock generated a large cloud of hot solar-wind electrons. SIDE also found atmospheric ions were accelerated by the solar wind and would be reimplanted at the terminator. Daytime observations of the lunar atmosphere was made difficult by highly variable solar-wind and extreme-ultraviolet flux. Whilst the lunar surface daytime electric potential was found to be +10 V, this became -100 V at sunset and sunrise, and -250 V at night. All deployed instruments would regularly detect exhaust gases from the Lunar Module descent and ascent engines, including when the Apollo 14 lunar module ascent stage overflew the Apollo 12 landing site at an altitude of . The SIDE instrument was key in identifying a new plasma regime in the lobes of the Earth's

magnetotail In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...

, consisting mostly of low-energy plasma. This plasma consisted most of protons and ionized atomic oxygen and nitrogen, likely derived from the

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

Lunar water

The Apollo 14 SIDE instrument provided the first signal that suggested the presence of

water vapour Water vapor, water vapour, or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor ...

on the Moon. Artificial origins from the Apollo 14 lunar module could not be ruled out. The strength and persistence of the event suggested this was not the result of contamination. The authors of the original study did later revisit and postulate that there were other possible sources and mechanisms for the entrainment and release of water vapour from Apollo 14's ascent stage. While the origins of the water detected by Apollo 14 remain in dispute, the presence of water on the Moon has been confirmed through direct observation by the

Stratospheric Observatory for Infrared Astronomy The Stratospheric Observatory For Infrared Astronomy (SOFIA) was an 80/20 joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. NASA awarded the contract for the development of the aircra ...

and the Moon Mineralogy Mapper instrument on board

Chandrayaan-1 Chandrayaan-1 (; from Sanskrit: , "Moon" and , "craft, vehicle") was the first Indian lunar probe under the Chandrayaan programme. It was launched by the Indian Space Research Organisation (ISRO) in October 2008, and operated until August 200 ...

. A 2008 study of lunar rock samples revealed evidence of water molecules trapped in volcanic glass beads.Moon Once Harbored Water, Lunar Lava Beads Show
''Scientific American'', July 9, 2008 A similar study showed how the Moon's solar wind–driven processes could entrain this on the Moon's surface.

References

{{Apollo program hardware, state=collapsed Lunar science Apollo program hardware Apollo 12 Apollo 14 Apollo 15