Ranger 4 topped by lightweight balsa wood impact limiter that encapsulated a transmitter and a seismometer.
Credit: NASA/JPL

There’s an interesting historical side note given China’s imminent, milestone making robotic landing on the farside of the Moon.

The U.S. Ranger spacecraft series was a set of kamikaze-like missions, hurled to the Moon to take photos of the lunar surface before a high-speed crash.

NASA’s Block 2, Ranger 4 was launched on April 23, 1962, rocketed moonward via an Atlas Agena-B booster from Cape Canaveral Air Force Station, Florida. This craft carried a unique scientific experiment – a lightweight balsa wood impact limiter that encapsulated a transmitter and a seismometer designed by the Caltech Seismological Laboratory.

Credit: NASA/JPL

Rough landing

Credit: NASA/JPL

According to Julie Cooper of the Jet Propulsion Laboratory’s (JPL) Library and Archives Group, the sphere was 25.5 inches (65 centimeters) in diameter. The seismometer within the separate capsule was to be slowed by a rocket motor and separate from the spacecraft shortly before Ranger 4’s impact and survive the rough landing on the Moon.

The capsule was also vacuum-filled with a protective fluid to reduce movement during impact. After landing, the instrument was to float to an upright position. Then the fluid would be drained out so it could settle and switch on.

Credit: NASA/JPL

Seismometer signals

While Ranger 4 had a perfect launch, the craft apparently suffered a main timer failure. Ranger 4’s computer had stopped, disabling the probe’s telemetry system; preprogrammed events such as solar panel deployment did not occur, and the probe became completely unresponsive to manual commands.

JPL’s Systems Design secretary Pat McKibben holds Ranger sphere.
Credit: NASA/JPL/Julie Cooper, JPL, Library and Archives Group.

Although Ranger 4’s transponder had ceased to operate, stations within the Deep Space Network continued their radio tracking nonetheless, homing on the 50-milliwatt signal produced by the tiny battery-powered transmitter in the seismometer capsule.

The Ranger project team tracked the seismometer capsule to impact just out of sight on the lunar farside, validating the spacecraft’s communications and navigation system.

Ranger 4 impact site?
Credit: GeoHack

Rest in pieces

On April 26, 1962, Ranger 4 impacted the farside of the Moon. A guesstimate placed the crash site at 15.5°S 130.7°W.

“You could confidentially state it crashed on the western eject of the Orientale basin,” explains Mark Robinson of Arizona State University’s School of Earth and Space Exploration in Tempe, Arizona. He is also the principal investigator of NASA’s Lunar Reconnaissance Orbiter’s (LRO) LROC camera system.

To date, no LRO imagery has identified Ranger 4’s final “rest in pieces” landing spot.

“It was a blind crash on the farside,” Robinson told Inside Outer Space. “How could anybody positively identify the crash site…there is no before image. The problem is the sheer number of 10 to 20 meter diameter fresh craters. How would you ever confirm [the crash site] without a before and after image, or a precise coordinate…short of going to the crater and digging around looking for spacecraft wreckage?”

Difficult to spot

It would be very difficult to identify any specific impact crater associated with Ranger 4 says noted Moon expert, Philip Stooke. He is Professor Emeritus and Adjunct Research Professor within the Department of Geography, and Center for Planetary Science and Exploration, at the University of Western Ontario.

Possible Ranger-4 crash site somewhere in the Loffe and Fridman Friedmann crater area?
Credit: NASA/GSFC/Arizona State University

“The whole area has been imaged at high resolution by LRO so we probably have a picture of the crater,” Stooke told Inside Outer Space.

Stooke points out one thing to consider.

“What did people know about the farside at that time? Practically nothing! So, to calculate an impact location, you plot the trajectory, as well as you know it, and see where it intersects the lunar surface,” Stooke says. But if you know nothing at all about the topography, he continues, you have to use an approximation – it would have just been a sphere the mean size of the Moon. The actual point would vary a bit if there is a difference between the real topography and the assumed sphere.

“I don’t think anyone has ever gone back and recalculated the impact point for this or other missions like the NASA Lunar Orbiters using modern topography. If they did I think the orbiters could be found eventually, but I think the Ranger 4 location is probably still too uncertain to find it,” Stooke concludes.

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