Surveyor 7 essay by Hamish Lindsay

Surveyor 7

by Hamish Lindsay.

Surveyor 7.

Launch : 0630 UT (1630 AEST) Sunday 7 January 1968
Landed : 01:05:36 UT (1105:36 AEST) Wednesday 10 January 1968
Location : Lat: 40.95°S by Long: 11.41°W in Crater Tycho’s ejecta blanket.
Final LOS : Wednesday 0024 UT (1024 AEST) 21 February 1968.

With the investigations for the Apollo missions successfully completed with Surveyor 6, the last Surveyor mission was given over to science, and the choice of the site was the rugged, rock-strewn ejecta blanket near the north rim of the comparatively young ray crater Tycho. It was believed it could be covered with debris excavated from deep beneath the surface when Tycho was formed. It was most unlikely any Apollo mission would land there, so Surveyor 7 was the first spacecraft in the program to carry all three devices – a camera, the mini lab, and the digger.

The specific objectives for this mission were to:

(1) Perform a lunar soft landing (in a highland area well removed from the maria to provide a type of terrain photography and lunar sample significantly different from those of other Surveyor missions).

(2) Obtain post landing TV pictures.

(3) Determine the relative abundances of chemical elements.

(4) Manipulate the lunar material.

(5) Obtain touchdown dynamics data.

(6) Obtain thermal and radar reflectivity data.

In view of the rugged terrain with the consequence hazards, it was necessary to reduce the target area from the 30 kilometre radius circle used before, to a 10 kilometre radius. Surveyor 7 was the only Surveyor spacecraft to land in the lunar highland region.

Prime tracking station support was provided by the Deep Space Network – Critical flight manoeuvres were commanded by DSS11 (Goldstone, California), with DSS42 (Tidbinbilla, Australia) and DSS61 (Madrid, Spain) providing precision tracking, communications, data transmission, processing and computing. Additional support was provided by the Air Force Eastern Test Range during the launch phase, DSS71 (Cape Kennedy), DSS51 (Johannesburg), DSS14 (Goldstone) and DSS12 (Goldstone). Mission Control and data reduction was all performed by the Space Flight Operations Facility (SFOF) at the Jet Propulsion Laboratory (JPL) Pasadena, California.

After landing, DSS42 and DSS61 recorded many television pictures, an abundance of alpha scattering data, as well as other engineering and scientific data.

Launch.
Watched by Eugene Shoemaker and three of his team of scientists, Surveyor 7 was launched at 06:30:00.54 UT (1630:01 AEST) on 7 January 1968 on an Atlas-Centaur SLV-3C AC-15 booster from launch complex 36A of the Eastern Test Range at Cape Kennedy. The Centaur put the spacecraft into an Earth parking orbit.

Flight.
Surveyor 7 was then transferred to a lunar trajectory by a second burn of the Centaur upper stage. It separated from the Centaur at 07:05:16 UT (1705:16 AEST). A mid-course manoeuvre was performed at 23:30:10 UT on 7 January (0930:10 AEST 8 January) 1968.

Landing.
After a journey of 66 hours 35 minutes 36 seconds, touchdown occurred at 01:05:36.3 UT (1105:36 AEST) on 10 January 1968 at Latitude 40.95°S by Longitude 11.41°W, less than 2.4 kilometres SSW from the planned target.

It landed on a 3° slope with the outboard side of the alpha-scattering sensor facing 20° west of north, on an ejecta blanket about 29 kilometres north of the rim of the Tycho Crater in the lunar highlands. The Sun was 12.5° above the eastern horizon. In the landing area were many small scaled irregular hillocks and swales with irregular low hills and depressions ranging from 100 metres to several hundred metres across, with scattered blocks and small craters and swarms of north-trending fissures. Craters visible from Surveyor 7 ranged in diameter from 13 centimetres to about 100 metres. Most of the craters in the foreground were between 13 centimetres and 4 metres in diameter. Craters larger than 30 metres across were only seen in the far field, mostly with smooth raised rims. Scattered blocks were noticed on the raised rims and within the craters.
The horizon, seen from Surveyor, was less than 200 metres distant to the east, south and west. The local surface sloped to the north, so much more distant features could be seen in that direction.

Lunar Surface Activities.
Science operations commenced shortly after landing. The TV camera rattled off 20,993 pictures on the first lunar day. The first sequence consisted of 15 images taken with 200-line mode and 20,978 images in the 600-line mode. The television camera clearly saw two 1-Watt laser beams aimed at it from the night side of the Earth, from Kitt Peak Observatory, Tucson, Arizona and Table Mountain at Wrightwood, California. Their brightness appeared comparable to the star Sirius.

Surveyor 7 was the first spacecraft to detect a faint glow on the lunar horizon after dark, thought to be light reflected from electrostatically levitated moon dust. Observations of the Sun’s faint outer corona were conducted 8 to 14 hours after sunset. Seven pictures were obtained in polarised and unpolarised light, the later ones recording the coronial image to about 50 solar radii. It covered the previously unobserved transitional region between the solar corona and the inner zodiacal light.

After completing its background measurements, which began at 16:13 UT (0213 AEST 11 January), the alpha-scattering instrument failed to deploy all the way down to the surface, so the digger was used to force the instrument down to the soil. The digger’s scoop was used to shade the sensor head from the Sun and keep its temperature below the survival limit of 75ºC. The digger was later used to set the alpha-scattering instrument on a rock and then into a trench it had dug. Approximately 66 hours of alpha-scattering data were obtained during the first lunar day on samples from three places: some undisturbed soil, a lunar rock and an area dug up by the scoop. Most of the fragments appeared to be dense coherent rock while others seemed to be less dense and porous. There were fewer craters larger than 8 metres at this site than observed at previous sites, which indicated a younger age for the Tycho rim material. An additional 34 hours of analysis was obtained on the third sample during the second lunar day.

The scoop dug a number of trenches, conducted static and dynamic bearing strength tests, picked up rocks, fractured a rock, weighed a rock plus executed manipulations of lunar material. Soil displaced by the landing footpads was noticeably darker than the undisturbed surface. The surface area around the spacecraft was nearly all covered by a powder layer. It was concluded that the rocks around Surveyor 7 were relatively hard, and would probably resist crushing if impacted by a landing spacecraft.

Operations were continued after sunset at 06:06 UT (1606 AEST) on 25 January 1968, which included pictures of the Earth, stars, and the solar corona. Operations were terminated at 14:12 UT on 26 January, (0012 AEST 27 January). Some battery damage during the lunar night resulted in intermittent transmission during the second day.

Second lunar day operations began at 19:01 UT on 12 February (0501 AEST 13 February) 1968, and included an additional 45 pictures in 200-line mode for a total of 21,038 and 34 hours of alpha-scattering data from inside the trench.

Operations were terminated on 21 February at 12:24 UT (2224 AEST), 1968, when contact with spacecraft was lost. The lunar surface sampler operated flawlessly for a total of 36 hours 21 minutes, digging trenches and moving and manipulating four rocks.

Results were generally consistent with earlier missions except that the chemical analysis of the highland crust showed it to be poorer in iron group elements than the samples from previous landings, all in lunar maria. The magnet experiments showed the presence of magnetic constituents in amounts comparable to those at the Surveyor 5 and 6 sites.

The Surveyor 7 mission objectives were fully satisfied by the spacecraft operations.

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