Ranger 4

• 1. Introduction

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  At 12:49:53 UT on the 26th of April 1962, Ranger 4 struck the far side of the Moon at about 9600 km/h, after 64 hours of flight. No scientific data came back from it. The crash still carried two firsts. Ranger 4 was the first United States spacecraft to reach another celestial body. It was also the first craft from any nation to reach the surface of the far side of the Moon. So how did a spacecraft launched so cleanly end its journey as a silent, tumbling machine? What had engineers changed after the failure of Ranger 3? And how did NASA explain a mission that arrived with milestones but no results?

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• 2. The Machine Itself

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  Ranger 4 weighed 331 kg and stood 3.1 m high. It was a Block II Ranger, virtually identical to Ranger 3. At its top sat a lunar capsule, 0.65 m in diameter, wrapped in a balsawood impact-limiter. Below it were a mono-propellant mid-course motor and a retrorocket with 22.6 kN of thrust.

  A gold- and chrome-plated hexagonal base, 1.5 m across, carried the rest of the craft. A large high-gain dish antenna was attached to this base. Two wing-like solar panels, 5.2 m across, were meant to unfold early in the flight.

  In all, 8,680 solar cells made up the power system. They were meant to charge an 11.5 kg silver-zinc battery with a capacity of 1 kWh, which handled launch power and served as a backup.

  A solid-state computer and sequencer ran the spacecraft, alongside a command system controlled from Earth. Sun and Earth sensors, gyroscopes, and pitch and roll jets kept it pointed correctly. The telemetry system used two 960 MHz transmitters, one at 3 W and one at 50 mW. These shared a high-gain antenna and an omnidirectional antenna. White paint, gold and chrome plating, and a silvered plastic sheet around the retrorocket managed the heat.

  That solid-state computer and sequencer would soon become the center of the whole story.

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• 3. Five Jobs for One Spacecraft

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  Ten minutes was all the time the camera would have. Ranger 4 was designed to transmit pictures of the lunar surface to Earth stations during the last ten minutes of flight before crashing. Its vidicon television camera used a scan mechanism that produced one complete frame every ten seconds.

  A gamma-ray spectrometer on a 1.8 m boom was meant to collect data during the flight. A radar altimeter would study how the lunar surface reflects radar. It had a second duty as well. It was designed to start the separation of the capsule and ignite the retrorocket.

  Six silver-cadmium batteries powered the seismometer capsule, enough to run its transmitter for 30 days. Inside the capsule were the seismometer, an amplifier, a 50-milliwatt transmitter, voltage control, and a turnstile antenna. The whole package was designed to rough-land on the Moon at 130 to 160 km/h.

  The fifth job was less visible. Ranger 4 was also meant to continue testing the Ranger program, which aimed to develop lunar and interplanetary spacecraft. Before any of this could happen, engineers had to fix a problem inherited from the previous flight.

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• 4. Lessons From Ranger 3

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  Heat sterilization was the suspected culprit behind the malfunction of Ranger 3's computer. For Ranger 4, the team dropped that procedure entirely.

  The seismometer capsule also received a new coat of paint, a sawtooth pattern meant to give it better thermal protection.

  Rangers 1-3 had all been troubled by booster problems, and by the time of Ranger 4 those problems had been resolved. The remaining question was whether the spacecraft itself would behave once it left the Agena.

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• 5. Launch Day

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  In March, Atlas 133D and Agena 6004 arrived at Cape Canaveral and began preflight checkouts. Unlike earlier Ranger launches, no serious difficulties came up in readying the launch vehicle. The probe passed all of its systems tests with ease.

  On the 20th of April, Ranger 4 was stacked atop the booster. Liftoff followed at 3:50 pm EST on the 23rd of April.

  The Agena fired a second time to send Ranger 4 on a translunar trajectory, and neither the Atlas nor the Agena showed any anomaly. Their precision was such that Ranger 4 would not even need a midcourse correction burn to reach the Moon.

  Then the Agena let go of the spacecraft, and the tracking stations began to listen.

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• 6. Silence in Flight

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  Right after separation from the Agena, tracking stations picked up Ranger 4's radio transmitter but received no telemetry. Commands sent to the computer drew no response. The radio transponder signal fluctuated, which suggested the craft was tumbling and that the solar panels and high-gain antenna had not deployed.

  Ground controllers sent commands to unfurl the panels and antenna and to use the attitude control system by hand to stop the rolling. The probe stayed unresponsive. The Spacecraft Data Analysis Team at JPL concluded that the main timer in the computer had stopped. That one fault disabled telemetry, blocked preprogrammed events such as panel deployment, and left the probe deaf to manual commands.

  The cause was hard to pin down. The timer had failed during the coasting phase before trans-lunar injection, while Ranger 4 was passing between tracking stations in the Caribbean and South Africa.

  Without solar power, the batteries ran down on the morning of the 26th of April, and the radio transponder went quiet. The 50-milliwatt transmitter in the seismometer capsule kept sending its signal as Ranger 4 closed on the Moon.

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• 7. Where It Fell

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  The impact point was given as 229.3 degrees E, 15.5 degrees S, but those coordinates were only guesstimated. The Lunar Reconnaissance Orbiter later presumably imaged the crater, yet the exact crater has never been identified.

  NASA officials tried to put a positive spin on the mission. They called the probe "far more sophisticated" than the Soviet Luna 2 probe of 1959. Luna 2 had been little more than a pressurized sphere designed to deposit pennants on the surface at impact.

  The Atlas-Agena booster's excellent performance also raised morale. For the first time, the Atlas-Agena-Ranger combination had flown without fault, even though the spacecraft missed its primary objective.

  Somewhere on the far side of the Moon, near those guessed coordinates, lies a crater that no one has yet been able to tie to Ranger 4.

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