Far-ultraviolet spectrometer

Previous experiments on and around the Moon had shown the lunar atmosphere to be incredibly tenuous. Scientists hoped to characterise what little there was by looking at the emissions it gave off in the ultraviolet region of the spectrum as atmospheric atoms fluoresced in the presence of solar ultraviolet radiation. They were surprised that this spectrometer could find no trace of an atmosphere around the Moon except for a transient atmosphere generated by the exhaust from the LM engines.

Radar to the Moon

One of Apollo’s major contributions to planetary science was to help to push the development of radar on an orbiting vehicle as a tool to investigate the surface and subsurface of a planet or a moon. From the simple experiments conducted on Apollo, such technology has gained profound capabilities that allow’ the shape or topography of a surface to be accurately profiled. Moreover, depending on the nature of the soil it can ‘see’ buried structures to a depth of one kilometre, and since it does not require light, it allows unlit and cloud-covered terrain to be viewed.

Starting w ith Apollo 14, radar tests became a normal part of the CMP’s solo tasks when the spacecraft’s S-band (around 2,200 MHz) and VHF transmissions (around 260 MIIz) w’ere aimed at the Moon to be received by large dish antennae on Earth. These bistatic tests w’ere so called because, unlike most radar setups in w’hich the transmitting and receiving antenna is one and the same, here two antennae were used, separated by a distance similar to the target distance. As a result, these tests required no additional equipment on the CSM and w’ere simply a bonus from using what wras already available. Researchers could determine the electrical properties of the surface by seeing how’ the strength of the reflected radio wave varied with its incident angle. And the interplay between the spacecraft’s orbital motion and the resultant Doppler effect on the signal’s frequency allowed discrete lunar features to be ‘seen’ in the signal’s received spectrum.

For Apollo 17’s lunar sounder, researchers took the technology to the next level. Specialised antennae were mounted on the service module to send pulses of radio energy towards the Moon and to receive the reflection, including any modification due to its interaction with the surface. Results from the radar w’ere recorded optically on film for later analysis on Earth.

The lunar sounder w’as the prototype for radar systems that successfully imaged the cloud-covered landscapes of Venus and Titan, searched for underground geology and ice deposits on Mars, and mapped the surface topography of most of Earth.

The Moon after Apollo 393