Sunday, July 31, 2016

July 31, 1964 -- Ranger 7 sends back the first close-up photographs of the moon, with images 1,000 times clearer than anything ever seen from earth-bound telescopes.

Ranger lunar orbiter - Smithsonian Air and Space Museum - 2012-05-15

As the United States planned for a lunar landing, it needed high-quality maps of the lunar surface. Mapping of the Moon began with the Ranger spacecraft. This was followed up by the Surveyor spacecraft, which not only took high-quality photos but actually landed on the Moon in areas NASA thought were good for human exploration. The Lunar Orbiter spacecraft was the third mapping mission, which performed high-resolution reconnaissance just before the Apollo Moon landings.

There were three types (or "blocks") of Ranger spacecraft. Ranger 1 and 2 were part of "block one," which was designed to test the Ranger technology in Earth orbit. The basic spacecraft technology we take for granted today -- three-axis attitude stabilization (without spinning the craft), on-board thrusters, two-way communication, closed-loop tracking (in which the spacecraft measures where it is a space and moves without receiving ground signals), on-board computing, control from the ground using commands that tell the on-board computer to run complex sequences -- all had to be developed for the Ranger program.

Sadly, the Atlas-Agena rockets that launched Ranger 1 and Ranger 2 left these craft in orbits which were too low. The spacecraft could not stabilize themselves or collect solar power. (NASA would later use the designs for Ranger "block one" for the Mariner space probes of Venus.)

"Block two" Rangers were improved versions of the "block one" craft. They carried a TV camera, a radiation detector, and a seismometer. The seismometer was designed to be ejected from the craft as it crashed into the Moon. Retrorockets would slow the seismometer capsule, allowing it to land softly. The capsule would then transmit seismic data back to Earth.

Sadly, Ranger 3 missed the Moon due to a trajectory miscalculation on Earth months earlier. Ranger 4's TV camera and radiation detector failed during the flight to the Moon. Its seismometer also failed to function, but the seismometer did soft-land on the lunar surface. Ranger 5 also missed the Moon due to a trajectory error. (The Rangers used gold-plated diodes. NASA discovered that this gold cracked and flaked off in the harsh conditions of outer space. They were immediately replaced.)

The "block three" Rangers carried a high-resolution TV camera. Items as small as a 12 inches across could be discerned on the images it generated. The camera on Ranger 6 failed as it approached the Moon. But Ranger 7, 8, and 9 performed flawlessly! Ranger 7 photographed Mare Cognitum, proving that impact craters covered the Moon -- even in the supposedly smooth highlands. Ranger 8 photographed the southern part of Oceanus Procellarum and Mare Nubium, and eventually was told by NASA to crash in Mare Tranquillitatis (where Apollo 11 would land four years later). Ranger 9 was told to crash in the crater Alphonsus.

Although less scientific knowledge was gained than expected by the Ranger spacecraft, NASA engineers learned an immense amount about spacecraft and quality control which would later allow NASA to put a man on the Moon safely.

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