Surveyor 6

by Hamish Lindsay.

Surveyor 6.

Launched : 07:39:01 UT (1739:01 AEST) Tuesday 7 November 1967.
Landed : 0101:04 UT (1101:04 AEST) Friday 10 November 1967
Location : Lat: 0.4743°N by Long: 1.48°W in Central Bay in Sinus Medii
Final LOS : Thursday 1914 UT 14 December (0514 AEST 15 December 1967).

The specific primary objectives for this mission were to:
(1) Perform a soft landing on the Moon in the Sinus Medii region.
(2) Obtain post landing television pictures of the lunar surface.

The secondary objectives were to:
(1) Determine the relative abundance of the chemical elements in the lunar soil by operation of the alpha-scattering instrument.
(2) Obtain touchdown dynamics data.
(3) Obtain thermal and radar reflectivity data.
(4) Conduct a vernier engine erosion experiment.

Prime stations to support Surveyor 6 were DSS11 (Goldstone, California), DSS42 (Tidbinbilla, Canberra, Australia), and DSS61 (Robledo, near Madrid, Spain) to provide tracking data, telemetry, video and command functions.

Also supporting at various times were DSS71 (Cape Kennedy); DSS14 (64 metre diameter antenna, Goldstone) for mid-course correction, descent and post landing hop experiment and DSS51 (Johannesburg), the initial two-way acquisition station. Mission Control for commands, tracking and telemetry data was the Space Flight Operations Facility (SFOF) at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Launch day, on 7 November 1967, was fine with a temperature of 14ºC and an 11-knot wind from the north west. Surveyor 6 was launched at 7:39:01 UT (1739:01 AEST) from launch complex 36B of the Eastern Test range at Cape Kennedy on a flight azimuth of 83º. The Atlas-Centaur SLV-3C AC-14 booster launched the spacecraft, then the Centaur’s first burn of 5 minutes 23.9 seconds put the spacecraft into an initial Earth parking orbit with an altitude of 166.7 kilometres.

After a 12.9 minute coast the Centaur re-ignited for 1 minute 55.7 seconds to inject Surveyor 6 into a lunar transfer trajectory at 8:03:30 UT (1803:30 AEST). Following spacecraft separation at 08:04:30 UT (1804:30 AEST) the Centaur manoeuvred to increase its distance from the lander, and to miss the Moon. DSS51 (Johannesburg) was the first station to establish two-way lock and issue commands to the spacecraft at 08:13:25 UT (1813:25 AEST), and tracked for 14 hours during the first pass, and 12 hours during the second pass. The star Canopus was located by the spacecraft at 16:27:49 UT 7 November (0227:49 AEST 8 November) and Surveyor 6 settled down to cruise mode, with pitch and yaw attitudes controlled by tracking the Sun, and roll attitude by Canopus.

A 10.24 second midcourse correction of 10.12 metres per second (36.4 kph) manoeuvre was performed at 2:20:02 UT (1220:02 AEST) on 8 November 1967 to correct the target miss distance to within 10.5 kilometres of the aim point.

Beginning about 33 minutes before retro-ignition, at 00:25:20 UT (1025:20 AEST) 10 November a roll-yaw-roll manoeuvre was executed to align the retro rocket with the flight path. The terminal descent phase sequence began with a ‘mark’ signal from the spacecraft marking radar when it was 110.7 kilometre slant range from the lunar surface. After a pre-set timed delay of 5.875 seconds, the three vernier engines ignited, followed by ignition of the solid propellant main retro motor 1.1 seconds later at 00:58:12 UT (1058:12 AEST). The retro motor burned for almost 40 seconds to slow the spacecraft with a slant range of 7.54 kilometres. At a height of 4.3 metres the vernier engines shut down to let the spacecraft free-fall and hit the surface at 13 kilometres per hour. The total time of descent from vernier engine ignition to touchdown was 182.5 seconds.

After a flight of 65 hours 22 minutes 3 seconds, Surveyor 6 touched down on the lunar surface on 10 November 1967 at 01:01:04.2 UT (1101:04.2 AEST) in Sinus Medii, a flat, heavily cratered mare region, at Latitude 0.4743ºN, Longitude 1.48ºW, 10.5 kilometres from the final aim point. All three legs impacted the surface within 0.1 of a second. The spacecraft ended up with a 2º list from vertical on a 0.8º surface slope. The landing occurred just after local sunrise. The site was an almost flat, heavily cratered area about 200 metres northwest of the base of a 30 metre high ridge.

Lunar Surface Activities.
Sitting in the centre of the Moon’s visible hemisphere from Earth, Surveyor 6 conducted all the usual tests such as 27 hours of analysing the chemical elements in the lunar soil, touchdown dynamics data, thermal and reflectivity data, and tried a vernier engine erosion experiment.

During the first lunar day, in addition to DSS11 (Goldstone) operations, the camera was operated from DSS42 (Tidbinbilla) and to a lesser extent from DSS61 (Madrid), resulting in a greater number of pictures than were taken on any of the previous missions.

Between landing and 24 November 29,952 quality images were recorded. The first 200-line television picture was transmitted 50 minutes after landing. 24 quality 200-line pictures were recorded before the system was reconfigured for the 600-line mode of operation, which required the solar panel to receive maximum solar power and the antenna aligned to Earth for maximum signal strength. A 360º wide-angle survey as well as a special area survey was accomplished before DSS11 (Goldstone) handed over to DSS42 (Tidbinbilla).

At 05:39 UT (1539 AEST) television proceedings were halted and the alpha scattering operations commenced for 41 minutes of initial readings. At 06:20 UT (1620 AEST) the scatter instrument was turned off and the television picture taking resumed to take advantage of desirable light and shadow effects created by the low Sun angle of the early morning sunlight.

At 07:20 UT (1720 AEST) control of the spacecraft was passed on to DSS42 (Tidbinbilla). As far as Mission Control was concerned real time television pictures were no longer available on their video displays. As pictures were received at Tidbinbilla, the Surveyor Operations Supervisor at DSS42 described the results over the voice line to Mission Control, who recommended camera changes as necessary.

Television operations included wide and narrow angle panoramas; focus ranging; photometric sequences; star surveys; polarisation surveys; shadow progressions; a solar corona sequence; special area sequences of the lunar surface and parts of the spacecraft.
After the hop the camera recorded the surface erosion characteristics resulting from firing the vernier engines, views taken after the hop of the imprints made by the spacecraft after the initial landing, stereo frames taken before and after the hop and particle accumulation and distribution on the bar magnet.

The first spacecraft to Lift-Off from the Moon.
Scientists and mission specialists recommended that the ‘hop’ should best occur at around lunar midday because the high Sun angle would provide optimum shading for the various flight control and propulsion system components. Also, the previous missions had proved that extended exposure to the lunar environment had degraded the propulsion systems.

On 17 November at 10:32:01 UT (2032:01 AEST) the vernier engines were fired for 2.5 seconds, causing Surveyor 6 to lift about 4 metres off the lunar surface for 6.1 seconds and land about 2.4 meters north west of its original position, though rotated by 5.5º anticlockwise. A small pitch up moment was caused by a slightly delayed shutdown of Engine 1, which helped level the spacecraft for its second landing. Communication with the spacecraft was maintained throughout the manoeuvre. By 11:07 UT (2107 AEST) the spacecraft checked out operationally and was ready to send more 600-line pictures.

Project management proposed a second, longer hop, but while they were analysing the situation the oxidiser system pressure suffered a major pressure drop, causing the helium pressure regulator to open and allowed helium pressure to pass into the oxidiser system, and then overboard so the second hop was cancelled.

This lunar ‘hop’ represented the first powered take-off from the lunar surface, and furnished new information on the effects of firing rocket engines on the Moon, allowed viewing of the original landing site, and provided a baseline for stereoscopic viewing and photogrammetric mapping of the surrounding terrain, as well as extending the reach of the scoop.

The mission transmitted images until a few hours after sunset at 13:53 UT (2353 AEST) on 24 November, returning a total of 29,952 images. The alpha-scattering experiment acquired 30 hours of data on the chemical composition of the surface material before the hop. After the hop no useful data for scientific analysis was received because the sensor head was on its side.

Other information provided by Surveyor included pictures of a bar magnet installed on a footpad to determine the concentration of material in the lunar soil, views of the stars, Earth, and the solar corona. It also measured the surface temperatures up to 41 hours after sunset.

The first Surveyor 6 lunar sunset occurred at 13:53 UT (2353 AEST) on 24 November and the spacecraft was placed into hibernation for the 2-week lunar night. Before sunset the battery charge level had been brought to a maximum of 165 ampere hours, almost full capacity. The electronic components in compartments A and B had to be kept above the minimum temperature of 23°C to assure reliable operation. The spacecraft was operated on a low duty cycle as long as possible in order to prevent the battery from cooling to the low temperature of the lunar night. Interrogation of the spacecraft was continued periodically during this time for engineering assessment and to obtain thermal data. About 13 hours after sunset it became evident that some of the 76 thermal switches of the electronic compartments, designed to open when the environment cooled down, were not opening as designed. This caused heat losses from the thermal compartments to be greater than planned. As a result extra battery energy was needed to maintain the temperatures in the compartments. This forced abandonment of the plan for operations into the lunar night and final shut down for the night of the spacecraft occurred only 41 hours after sunset. The battery capacity at shut down was estimated to be 102 ampere hours.

Attempts to contact Surveyor 6 began on 13 December, 80 hours after lunar sunrise, but it wasn’t until 16:41 UT on 14 December (0241 AEST on 15 December) through DSS61 (Madrid) that an erratic signal in both signal strength and frequency was detected, and the station had difficulty locking on to it. No real useful data was returned, and the last transmission from Surveyor 6 was received at 19:14 UT on 14 December (0514 AEST on 15 December).

Mission support was terminated on 21 December 1967.

The results of the experiments showed that the surface had a basaltic composition, similar to that found at the Surveyor 5 landing site. Engineering and soil mechanics data indicated the bearing strength of the surface was more than adequate to support human landings.
The spacecraft accomplished all planned objectives for the sixth mission.

The successful accomplishment of the Surveyor 6 mission not only satisfied all obligations to Apollo, but also completed the scientific investigation of four widely separated mare regions on the Moon’s equatorial belt, spaced roughly uniformly across a longitude range between 43ºW and 23ºE.

Deputy Director at DSS42, Tidbinbilla, Mike Dinn, recalled, “Surveyor 6 was expected to survive the lunar night – but it didn’t. They thought they understood how to look after a spacecraft through the night. JPL claimed they had done all the right things when they ‘put it to bed,’ but for some reason it didn’t start up again.”



See also:

The Missions: Surveyor 1, Surveyor 2, Surveyor 3, Surveyor 4, Surveyor 5, Surveyor 6, Surveyor 7.

Surveyor Program Results Summary, The Surveyor Spacecraft and Systems.