The Mission of Ranger 9

Since entering orbit around Mercury four years ago, NASA’s MESSENGER spacecraft has had to use its propulsion system periodically to compensate for the effects of the Sun on its orbit. But with its propellant supply now nearly exhausted, the low point of MESSENGER’s orbit will quickly sink and cause it to crash into Mercury ending its highly successful mission around the end of April 2015. While much effort has been invested in keeping MESSENGER from impacting the Moon-like surface of Mercury, it may come as a surprise to some that the goal of NASA’s first major lunar project was to impact the Moon on purpose. The objective of this program, called Ranger, was to stream live television images of the approaching lunar surface at ever higher resolution until impact.

Unfortunately, NASA’s Ranger program was plagued by problems as the team responsible at the Jet Propulsion Laboratory (JPL) learned the hard way how to properly design, build, fly and manage a complex unmanned lunar spacecraft. The first two Ranger missions launched in 1961 were engineering test flights destined for highly elliptical Earth orbits. Unfortunately, both Rangers 1 and 2 were stranded in low Earth orbit by failures in their Agena B upper stages (see “The Prototype That Conquered the Solar System“). The next three Ranger flights launched in 1962 were meant to obtain images of the lunar surface using a simple camera 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“). The goal of the final four Rangers was to return large amounts of high-resolution imagery using a set of six television cameras. While the cameras on Ranger 6 failed to activate before impact in February 1964, the modified Ranger 7 finally succeeded that July giving NASA its first success in the program (see “50 Years Ago Today: The Launch of Ranger 7”).

Diagram showing the major components of the Ranger 6 to 9 design. Click on image to enlarge. (NASA)

The Last Ranger Flight

After Ranger 8 successfully achieved all of its objectives by returning 7,137 television images before impacting the Moon on February 20, 1965, NASA was ready to fly its final mission of the Ranger program during the next lunar launch window the very next month (see“The Mission of Ranger 8” for a complete description of the spacecraft and this mission). Since Rangers 7 and 8 had targeted the Moon’s relatively dark and smooth maria to support the selection of landing sites for the Apollo program as well as the upcoming Surveyor unmanned lunar landing missions, the Ranger science team at JPL wanted to target a scientifically more interesting highland site for this last flight. After meetings with managers at NASA headquarters, Ranger 9 was finally given the green light to target a highland site.

A recently reprocessed view of Alphonsus crater acquired in 1967 during NASA’s Lunar Orbiter 4 mission. Note the rilles and dark haloed craters which are signs of volcanic activity. Click on image to enlarge. (Lunar & Planetary Institute)

The target of greatest interest to the Ranger science team was the 120-kilometer in diameter crater called Alphonsus. Named in 1651 by the Italian Jesuit priest-astronomer Giovanni Riccioli after the 13^th century King Alfonso X of Castile (also known as “Alfonso the Wise”), Alphonsus is a relatively old crater straddling Mare Nubium and the Imbrian Highlands. At its heart is a central peak reaching about 1,500 meters above its floor which presents a range of volcanic features including rilles and dark haloed craters believed to be cinder cones. Most intriguing of all, Alphonsus appeared to display transient phenomena that have been interpreted as signs of recent selenographic activity. In 1957, the director of the Griffith Park Observatory, Dinsmore Alter, reported sighting fluorescent gas inside the crater. The following year, Soviet astronomer Nikolai Kozyrev also observed these emissions and succeeded in securing spectra of them. Obtaining high resolution images of this crater was a top priority for the Ranger science team since nearly the beginning of the program in December 1959.

Since the navigation during the previous three Ranger flights proved to be so accurate, the science team decided to target Ranger 9 when the Sun was just 10.4° above the horizon compared to 23.2° and 14.7° for Rangers 7 and 8, respectively, in order to maximize the visibility of surface topography. This dictated a launch on the afternoon of March 21, 1965 – the third day of the week-long launch window to the Moon. In order to target Alphonsus specifically, the first two days of the launch window were purposely passed over. To maintain ideal lighting conditions in the target area, however, the aim point of Ranger had to move about 12° westward in longitude for each successive day here on Earth because of the Moon’s slow rotation. This meant that Alphonsus could only be reached with the desired lighting if launched on March 21. If that launch opportunity was missed, Ranger 9 would be targeted for the crater Copernicus on March 22, Kepler on March 23 and the sinuous rille called Schroter’s Valley located on the Aristarchus plateau on March 24 and 25. The target list was sent to NASA Headquarters on March 10 and officially approved a few days later.

Diagram showing the approach and imaging geometry for Ranger 9. Click on image to enlarge. (JPL/NASA)

One potential problem for targeting Alphonsus with a March 21 launch date was that it conflicted with the launch of Gemini 3 – the first manned flight of this high priority program. While Gemini 3 was scheduled to lift off on March 22, it required the use of the same ground computers as Ranger needed starting the day before Gemini’s launch. Some on the Ranger team felt that the launch of Gemini 3 would probably be delayed as it had been several times earlier thus allowing a March 21 launch for Ranger 9 but there was no guarantee that this would be the case this time. Finally, on March 15, NASA Associate Administrator Robert Seamans interceded on the behalf of Ranger and delayed the launch of Gemini 3 by one day (see “The Mission of Gemini 3“). The way was now clear to launch Ranger 9 towards Alphonsus.

The Ranger 9 Mission

The launch vehicle for the Ranger missions was the Atlas-Agena B. The first stage of the launch vehicle that would send Ranger 9 on its way, Atlas 204D, was erected at Launch Complex 12 at Cape Kennedy (which has since returned to its original name, Cape Canaveral) on February 19, 1965 – ten days after it had arrived at the Cape and only two days after the launch of Ranger 8 from the same pad. The Agena B upper stage number 6007 was mated with Atlas 204D at LC-12 on February 24. The 368-kilogram Ranger D spacecraft, which would be renamed Ranger 9 after launch, was finally added to the stack on March 16 after three weeks of checkout following its arrival at the Cape from JPL.

The launch of Ranger 9 from LC-12 at Cape Kennedy on March 21, 1965. (NASA)

The countdown for Ranger 9 began at 7:51 AM EST on March 20, 1965. Inclement weather complicated preparations with winds gusting to 56 kilometers per hour delaying the roll back of the service tower until the last possible opportunity at the end of a scheduled hold at T-minus 60 minutes. Finally at 4:37:02 PM EST on March 21, Ranger 9 lifted off near the end of its launch window. The Atlas 204D worked almost perfectly with a short burn from the Agena B placing the upper stage and Ranger 9 into a nearly circular parking orbit at an altitude of 188 kilometers. After a short coast of just 178 seconds, Agena 6007 reignited its engine for an 86-second burn that sent Ranger 9 on its way to the Moon.

Diagram showing the launch trajectory of Ranger to the Moon. Click on Image to enlarge. (NASA)

After separating from the spent Agena, Ranger extended its solar panels and high gain antenna before automatically aligning itself with the Sun and Earth for the cruise to the Moon. Initial tracking of the receding spacecraft indicated that Ranger 9 would impact the Moon only 520 kilometers north of its intended target inside Alphonsus – the most accurate translunar injection of the Ranger series. Because of the small aiming error, the midcourse maneuver that would normally take place about 17 hours after launch during the first tracking pass at NASA’s primary deep space tracking facility at Goldstone Station in California was postponed to the second pass 22 hours later. This delay would help to improve the accuracy of the final targeting.

Diagram highlighting the steps needed for Ranger to perform a midcourse maneuver on its way to the Moon. Click on image to enlarge. (NASA)

Upon command from controllers at JPL, Ranger 9 realigned itself and fired its midcourse engine starting at 4:30:09 AM PST on March 23. The 31-second burn starting at a distance of 291,014 kilometers from the Earth changed Ranger’s velocity by 18 meters per second and placed the spacecraft on target for an impact northeast of the central peak of Alphonsus about halfway to the crater’s rim. This target would allow Ranger’s cameras to obtain images of the mare and highlands adjacent to the crater during descent and ensure that the impact would occur well away from the shadow cast on the crater floor by the central peak. With the improvements made to Ranger’s systems to reduce electronic noise and better procedures to refine the focus of its television cameras before launch, this last flight promised to return the best pictures ever of the Moon.

Diagram showing Ranger’s terminal maneuver to align its cameras to take images before it impacted the lunar surface. Click on image to enlarge. (NASA)

On the morning of March 24 at an altitude of 7,211 kilometers above the lunar surface, Ranger 9 started its terminal maneuver 65 minutes and 46 seconds before impact . The purpose of this maneuver was to align the boresight of Ranger’s six cameras roughly along its flight path to minimize image smearing caused by the accelerating motion of the spacecraft. At an altitude of 2,376 kilometers, 18 minutes and 27 seconds prior to impact, the two independent camera systems on Ranger 9 were up to full power and started returning a stream of images to controllers at JPL. A new television conversion system developed for the Surveyor program was employed for the first time that allowed some of the images to be broadcasted live on television to a nation-wide audience eager to see the Ranger program’s finale. Ranger 9 transmitted 5,814 images before it impacted the lunar surface at a speed of 2.671 kilometers per second at 6:08:20 AM PST. Ranger 9 had come down at 12.91° N, 2.38° W after a flight lasting 64 hours, 31 minutes and 18 seconds only 4.4 kilometers from its aim point – the most accurate hit of the Ranger series. Ranger 9 had imaged about 1.6 million square kilometers of the lunar surface with the last image, taken at an altitude of only 610 meters, showing features as small as 25 centimeters.

A sample of images of Alphonsus returned by Ranger 9. The spacecraft’s impact site is indicated by the small white circle. Click on image to enlarge. (JP/NASA)

The resounding success of Ranger 9 (as well as its two predecessors) stood in stark contrast to the disappointing failures that dominated the program from 1961 to the beginning of 1964. The high resolution images returned by the last three Ranger missions clearly indicated that the lunar surface was smooth enough and most likely strong enough for a safe landing by Apollo. The new images from Ranger 9 demonstrated that safe landings were possible even in the more rugged highland regions of the Moon opening many possibilities for future exploration. Tracking of the Rangers also indicated that the Moon’s geometric center was displaced from its gravitational center. This fact was required to improve the navigational accuracy of future lunar missions.

After six years of effort, a total of $267 million in funding (which would be about $2 billion in today’s money), much heartache over six failures, and much relief on three successes, NASA’s first major lunar exploration program was ended. Efforts now turned to the other two legs of NASA’s unmanned lunar mission triad: Surveyor, which would land on the Moon, and Lunar Orbiter, which would systematically map the surface from orbit.

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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

Gerard Kuiper, “Lunar Results from Rangers 7 to 9”, Sky & Telescope, Vol. 29, No. 5, pp. 293-308, May 1965

Ranger VIII and IX: Part I. Mission Description and Performance, JPL Technical Report 32-800, January 31, 1966

Ranger IX Photographs of the Moon, JPL, December 15, 1965