The Mission of Ranger 8

With the threat to cut further funding for NASA’s successful and long-running Lunar Reconnaissance Orbiter mission in the news, it seems hard to believe that there was a time when successfully reaching the Moon at almost any cost was a national obsession. After a long string of failures in the wake of early Soviet successes, the American Ranger lunar program was finally beginning to deliver a half a century ago after the nation had gone five years without a single lunar mission success (see “Vintage Micro: The Pioneer 4 Lunar Probe”). With the Soviet Luna program largely silent as they struggled to develop an automated lunar lander out of the public eye, America was finally beginning to dominate the propaganda war in the race to the Moon.

Background

Officially approved at the end of 1959, Ranger was NASA’s first in-house lunar program with the Jet Propulsion Laboratory (JPL) given responsibility for the project. The first two missions, Ranger 1 and 2 launched in 1961 using the Atlas-Agena B, were engineering test flights using the Block I Ranger meant to prove the basic three-axis stabilized spacecraft design in a highly elliptical Earth orbit. Although problems with the Agena B stage stranded them in their low Earth parking orbits, enough was learned during their brief flights to verify the design which was subsequently adapted for JPL’s successful Mariner 2 mission to Venus in 1962 (see “The Prototype That Conquered the Solar System“). The objective of the three Block II Ranger flights launched in 1962 was to obtain high resolution imagery of the lunar surface before deploying a robust hard lander onto the lunar surface to make extended observations of lunar seismic activity. The flights of Ranger 3, 4 and 5 all failed due to problems with the launch vehicle or fatal malfunctions in key systems on the spacecraft (see “NASA’s First Moon Lander“).

Model of the Block II Rangers flown in 1962. The hard -lander is the sphere on the top of the spacecraft with a solid retrorocket beneath it. (NASA)

Reviews by NASA and outside groups about the string of lunar mission failures produced a laundry list of engineering changes and management reforms for the next round of Block III Ranger flights which were intended to make observations of the Moon before they impacted its surface. Among the changes was for the Block III Ranger flights to concentrate exclusively on high-resolution imaging of the Moon. All other instruments and investigations were dropped in order to better support the new Apollo program.

The main bus of the 366-kilogram Block III spacecraft consisted of an hexagonal aluminum alloy frame 1.52 meters across. Boxes attached to the side of this frame contained all of the electronic systems to control the spacecraft, provide engineering telemetry, communications and power. Mounted either side of this frame were a pair of solar panels with a total span of 4.6 meters that provided 200 watts of electrical power to keep the spacecraft’s batteries charged. A low gain antenna at the top of the 3.1-meter tall spacecraft and a 1.22-meter in diameter directional high-gain antenna on a hinged mount allowed contact with the Earth to be maintained.

Diagram showing the components of the Ranger Block III design. (NASA)

Inside the bus was a hydrazine monopropellant propulsion system meant to provide the inevitable course corrections after the Atlas-Agena B launch vehicle had sent Ranger on its way. The 220-newton engine pointed downward out of the base of the bus and had enough propellant for up to a 98.5 second burn – sufficient to provide a 60 meter per second change in velocity. A fully redundant set of a dozen nitrogen gas jets provided attitude control for the three-axis stabilize spacecraft. Four primary and two secondary Sun and Earth sensors provided celestial references for maintaining attitude. When these were not available, such as during mid-course correction maneuvers, attitude reference was provided by a trio of gyros.

The Block III Ranger’s main instruments were a pair of independent chains of slow-scan vidicon cameras developed and built by RCA (a leading electronics manufacturer of the day, bought by GE in 1986 and subsequently dismantled) which were enclosed in a 1.5-meter tall tower mounted on top of the bus. Clad in polished aluminum for thermal control, the 173-kilogram cylindrical tower tapered from 69 centimeters at its base to 41 centimeters at the top, where the low-gain antenna was mounted. A total of six cameras viewed the approaching lunar surface through a 33-centimeter square opening on the side of the tower. Their optical axes were canted at a 38° angle from the spacecraft’s long axis in order to get a clear view of the lunar surface during approach.

A simplified schematic showing the pointing direction of the Ranger cameras as well as the relative sizes and positions of their fields of view. Click on image to enlarge. (NASA)

The full-scan camera set or “F-chain” consisted of a pair vidicon cameras that provided 1,152-line images (comparable to today’s HD format) at a rate of about one image per camera each five seconds. The “A” camera had a relatively wide 25° field of view while the “B”  camera had a narrower 8° field of view. The independent “P-chain” consisted of a quartet of vidicon cameras fitted with the same lenses as the pair of F-chain cameras but performed only partial, 300-line scans of their vidicon plate. The P-chain camera images had about the same resolution as the F-chain but only covered about 7% of the area. This was done so that images could be returned at a rate of five per second in hopes of capturing at least a partial image a couple of tenths of a second before impact. At this altitude of only 300 to 600 meters, a resolution of 30 centimeters or better was possible.

To maximize the redundancy of the camera system, it included two independent sets of power supplies, camera sequencers, and batteries; one set for each chain of cameras. Each chain also possessed its own 60-watt transmitter to independently transmit analog television images back to Earth in real time via the craft’s directional high-gain antenna. If the one set of cameras were to malfunction, the other was designed to operate independently. The bus had its own 3-watt transmitter which carried only engineering telemetry.

Diagram showing the terminal maneuvers of the Block III Ranger to align its cameras to take images before it impacted the lunar surface. Click on image to enlarge. (NASA)

In the typical Block II Ranger mission timeline, the spacecraft would begin its terminal maneuver and reorient itself about an hour before impact. This maneuver would aim the cameras along Ranger’s flight path towards approaching lunar surface (which would reduce image smearing) with the high gain antenna turned to point towards Earth. The impact was planned to occur about 65 hours after launch so that the 26-meter tracking antenna at NASA’s Goldstone Station would have a clear view of the Moon during these last mission events to provide a communications link with Ranger. Some 17 minutes before impact, the F-chain of cameras would be commanded to warm up for 90 seconds and enter a standby mode. The P-chain then would take its turn and warm up. Finally, 14 minutes before impact at an altitude of about 1,900 kilometers, the F-chain’s 60-watt transmitter would start beaming images back to Earth, followed by the P-chain about 150 seconds later. Transmission would continue until the spacecraft impacted the lunar surface at a speed of about 2,600 meters per second. If everything worked perfectly, over 4,200 close-up television images of the lunar surface would be transmitted back to the Earth.

After months of delays for modification and additional testing, the first improved Block III Ranger, Ranger 6, was finally launched on January 30, 1964. While the spacecraft was successfully navigated to impact the Moon only 31 kilometers from its aim point in Mare Tranquillitatis, a heartbreaking failure in its imaging system prevented any pictures from being returned to the Earth. After more reviews and modifications to the Block III Ranger design to prevent a repeat of the camera package failure, Ranger 7 was launched on July 28, 1964. This time everything work as planned with Ranger 7 returning 4,316 images back to Earth before crashing only 13 kilometers from its aim point in Mare Nubium. The success of Ranger 7 not only saved the troubled Ranger program, but probably JPL as well (see “The Mission of Ranger 7”).

The Ranger 8 Mission

The requirements for the relative positions of the Sun, Earth and Moon as well as the need to have fairly low Sun angles in Ranger’s impact area to help maximize the visibility of surface features limited Ranger’s launch window to a week-long period around the time of the last quarter Moon. Since the lighting conditions on the slowly rotating Moon changed noticeably each day, each proposed launch date of a Ranger mission had its own impact target point chosen for it that was about 13° farther west in longitude than the previous day’s target.

With the successful Ranger 7 mission under their belts, the Ranger program’s site selection group had their work cut out for them trying to balance the various desires of its team members to come up with a list of seven sites for Ranger 8. Some wished to revisit the Ranger 7 impact site so that the crater it produced could be observed just as NASA’s Stardust spacecraft was redirected to observe the crater on Comet Tempel 1 in 2011 that had been produced 4½ years earlier during the Deep Impact mission. Since Ranger 7 came down in a fairly smooth lunar mare, there were those who wished Ranger 8 to study a rougher highland site.

The initial site list for Ranger 8 released on January 19, 1965, which included a variety of scientifically interesting targets scattered on the near side of the Moon, met with criticism from Project Apollo officials. They preferred that Ranger 8 take a closer look at more mare sites close to the equator to support manned lunar landing mission site selection. After a series of upper level meetings between various advisory boards, a compromise target list was finally approved just a week before the opening of Ranger’s launch window.

Ranger 8 Impact Site List

As it turned out, Ranger 8 managed to launch on the very first day of its eight-day launch window towards a target in Mare Tranquillitatis about 210 kilometers south of the Ranger 6 impact site. The Atlas 196D lifted off at 12:05 PM EST from Launch Complex 12 at Cape Kennedy successfully placing Ranger 8 and its Agena B upper stage into a 185-kilometer high temporary parking orbit. After coasting for about a quarter of an hour, the Agena reignited its engine at 12:26 PM sending Ranger 8 on its way to the Moon.

The launch of Ranger 8 on an Atlas-Agena B from LC-12 at Cape Kennedy on February 17, 1965. (NASA)

Initial tracking of Ranger 8 showed that it would miss the Moon by 1,828 kilometers and would require a midcourse correction to set it back on target. At 5:00 AM EST the day after launch and 159,743 kilometers from the Earth, Ranger 8 obediently turned on ground command and fired its main engine for 59 seconds to set it back on course. While the maneuver was successful and Ranger reestablished its cruise attitude as well as contact with Earth after an hour and a half, engineers back on Earth were concerned about intermittent losses in engineering telemetry during the maneuver that hinted that there might be an issue with the pointing mechanism on Ranger’s high gain antenna needed to return images back to Earth.

In order to avoid any further communication problems, the Ranger science team requested that the terminal maneuver used to align Ranger’s cameras with its velocity vector be cancelled. This would leave the craft in its cruise attitude with the cameras pointing down towards the Moon at an angle to the approach trajectory. While this would smear the last few images returned just before impact, it would allow Ranger 8 to maintain its communications lock on the Earth. A side benefit of this decision was that Ranger 8 would now take a swath of images across the lunar surface covering 2.3 million square kilometers. Some of those images would overlap with those of Ranger 7 allowing stereoscopic analysis of parts of the lunar surface. It was also decided to turn the cameras on about ten minutes early to maximize the number of images returned over a longer swath. The early start of imaging would also give some extra margin for the engineers to correct any problems that might arise before impact.

Diagram showing the approach and imaging geometry for Ranger 8. Click on image to enlarge. (JPL/NASA)

On command from controllers at JPL, the cameras on the Ranger 8 powered up and commenced returning images 23.1 minutes prior to impact in the early-morning hours of February 20, 1965. To the relief of all, the camera package operated as planned with its first images having a resolution comparable to Earth-bound telescopic views of the Moon. As the lunar probe approached the surface, the cameras scanned across the lunar surface at ever higher resolution. Ranger 8 finally impacted the Moon 49° off of the local vertical at 1:57:37 AM PST to the cheers of controllers as well as reporters and officials gathered at JPL to witness the completion of the mission. Ranger 8 had come down at 2.59°N, 24.77°E – less than 24 kilometers from its aim point and only 71 kilometers from where Apollo 11 would land 4½ years later.

One of the images returned by the Ranger 8 television system showing the craters Ritter (left) and Sabine. The craters are each about 30 kilometers in diameter and located in the southern part of Mare Tranquillitatis. (JPL/NASA)

Ranger 8 had returned a total of 7,137 pictures during its extended imaging session. The last images transmitted just before impact showed features as small as 1.5 meters despite the motion-induced smearing. While Ranger 7 had come down in a mare region filled with secondary craters and rays from the craters Copernicus and Tycho, Ranger 8 had come down in what was thought was going to be a smoother mare region. Detailed analysis of the images showed little difference in the general character of the two landing sites. While craters of all sizes and states of preservation were observed down to the resolution limit, their forms suggested that the lunar surface was strong enough to support a lander and still smooth enough for a safe landing by the Apollo Lunar Module.

Last image taken by Ranger 8 camera-A from a distance of 4.2 km, 2 seconds before impact on February 20, 1965. The area shown is at 2.7 N, 24.55 E and the image is about 1.4 km across. The right side of the image is missing because Ranger 8 crashed before completing transmission. (JPL/NASA)

With the success of Ranger 8 and the immediate needs of Apollo program planners satisfied, the last Ranger mission would be directed towards scientifically more interesting sites. With two successful lunar missions finally under their belts after years of frustrating failures, the pressure was on to prepare Ranger 9 for launch in just one month for the project’s grand finale. Afterwards, JPL’s attention would turn towards the Surveyor program with the goal of soft landing an automated spacecraft on the lunar surface.

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Related Video

Here is a 1965 film showing the images acquired by the various cameras on Ranger 8 as it approached and impacted the Moon.

Related Reading

“The Mission of Ranger 7”, Drew Ex Machina, July 29, 2014 [Post]

“The Prototype That Conquered the Solar System”, Drew Ex Machina, September 15, 2015 [Post]

“NASA’s First Moon Lander”, Drew Ex Machina, January 26, 2016 [Post]

“Vintage Micro: The Pioneer 4 Lunar Probe”, Drew Ex Machina, August 2, 2014 [Post]

General References

R. Cargill Hall, Lunar Impact: The NASA History of Project Ranger, Dover Publishing, 2010

Raymond L. Heacock, “Ranger: Its Mission and Its Results”, TRW Spacelog, Vol. 5, No. 2, pp. 2-23, Summer 1965

“Ranger 8 Lunar Reconnaissance”, Sky & Telescope, Vol. 29, No. 4, pp. 205-209, April 1965

“Ranger VIII Photographs of the Moon”, JPL, December 15, 1965