Surveyor 7

Surveyor 7 was the final spacecraft in the United States uncrewed Surveyor Programme and was launched in 1968 on a primarily scientific and photographic mission. It became the fifth lander in the series to achieve a successful lunar soft landing, following two earlier failures within the programme. As the only Surveyor mission specifically targeted to land in the lunar highlands rather than a mare region, it provided geological and chemical data significantly different from those obtained by its predecessors. During its operational period, Surveyor 7 transmitted a total of 21,091 images to Earth.

Mission Objectives and Context

Surveyor 7’s mission objectives centred on enhancing scientific understanding of highland regions of the Moon. These aims included:

• achieving a soft landing in a terrain remote from the lunar mare;
• gathering photography of a new geological environment;
• determining relative elemental abundances;
• manipulating and sampling lunar surface material;
• acquiring touchdown dynamics data;
• and collecting thermal and radar-reflectivity measurements.

The spacecraft closely resembled earlier Surveyor models but carried a wider range of experimental equipment. Additional features included polarising filters on the television camera, auxiliary mirrors to extend visual coverage, bar magnets on two footpads and two horseshoe magnets on the sampling scoop. These magnets were intended to capture ferromagnetic particles from the regolith.
Surveyor 7 landed on 10 January 1968 on the outer rim of Tycho Crater, a location characterised by steep, rugged highland terrain. Operations commenced immediately after landing and continued until 26 January, approximately 80 hours after sunset. Although the spacecraft survived the extreme temperature drop of the first lunar night, its batteries suffered damage, resulting in intermittent contact during the second operational period in February. Communication ceased entirely on 21 February 1968.

Scientific Discoveries and Observational Achievements

One of Surveyor 7’s most notable accomplishments was the first recorded detection of the lunar horizon glow, a faint luminescence seen just after lunar sunset. This phenomenon is now attributed to sunlight scattering off electrostatically levitated dust particles, providing early evidence of active dust transport processes on the Moon.
Another important achievement occurred on 20 January 1968, when the spacecraft detected two laser beams aimed at it from Earth: one from Kitt Peak National Observatory in Arizona and the other from Table Mountain Observatory in California. These detections represented an early demonstration of Earth-to-space laser communication potential.
NASA and Bellcom engineers later considered the Surveyor 7 landing site as a candidate target for a crewed Apollo mission due to its geological interest. However, the area’s high latitude, rough terrain and the eventual cancellation of post-Apollo 17 missions prevented further crewed exploration.

Television Imaging System

The Surveyor 7 television system was an advanced version of the vidicon-based cameras used in earlier missions. It included:

• a vidicon tube;
• two lenses of 25 mm and 100 mm focal length;
• shutters and an adjustable iris;
• polarising filters designed to analyse the linear polarisation of light from the lunar surface;
• and a movable mirror driven by stepping motors to adjust azimuth and elevation.

The system could capture a full 360° azimuth view and an elevation range from 90° above the horizontal plane down to 60° below it. As with other Surveyor cameras, it used both 200-line and 600-line operational modes:

• 200-line mode transmitted through an omnidirectional antenna, scanning one frame every 6.18 seconds and requiring a 12 kHz bandwidth.
• 600-line mode transmitted through a directional antenna with higher resolution, scanning a frame every 36 seconds and using a 220 kHz bandwidth.

The images were displayed on Earth using slow-scan monitors and were recorded on magnetic tape and 70 mm film. Surveyor 7 transmitted 20,961 pictures during the first lunar day alone. During its second lunar day, it briefly operated in 200-line mode due to a malfunction in the 600-line horizontal sweep, but only 45 images were sent before power failure halted camera operations.
Camera performance remained excellent, though with slightly reduced dynamic range and sensitivity compared with Surveyor 6. Its polarisation capabilities allowed novel measurements of surface scattering properties, enhancing scientific understanding of lunar photometry.

Alpha-Scattering Surface Analyzer

Surveyor 7 carried an alpha-scattering instrument designed to determine the abundances of major chemical elements in lunar soil and rock samples. The system incorporated a curium-242 alpha source that irradiated the surface beneath it through a 10 mm aperture. Two independent detector assemblies collected energy spectra of both alpha particles scattered from the sample and protons produced through alpha–proton reactions.
Data were continuously telemetered to Earth and offered quantitative insights into all major elements except hydrogen, helium and lithium. During the mission, 46 hours of measurements were collected from three sample types: undisturbed lunar soil, a lunar rock and soil from an excavated trench.
Although the instrument failed to deploy correctly, mission controllers successfully used the soil mechanics sampler to reposition it. This demonstrated the versatility of the sampler and ensured the successful fulfilment of the experiment’s objectives.

Soil Mechanics Surface Sampler

The soil mechanics sampler, a vital tool for analysing the lunar surface, consisted of a scoop equipped with a sharpened blade, a container and a motor-driven opening mechanism. Its flat foot contained two embedded horseshoe magnets to collect magnetic particles.
The sampler was mounted on a pantograph arm capable of extending up to roughly 1.5 metres, sweeping laterally between 40° and 72° and elevating by about 130 mm. Gravity-assisted motion allowed controlled impact tests. Positioned beneath the television camera, it could place samples within the field of view and manipulate the alpha-scattering instrument as needed.
During the mission, the sampler carried out:

• 16 bearing tests;
• seven trenching tests;
• two impact tests;
• repositioning of the alpha-scattering instrument;
• shading of instruments to control temperature;
• and transport of surface material for photographic analysis.