Navigating the Apollo Missions

The only manned spacecraft that have navigated through space and into an orbit other than Earth’s were those of the Apollo mission, when nine space­craft went into orbit around the Moon. The Apollo program also landed twelve astronauts on the Moon between 1969 and 1972. On the way to the Moon, the spacecraft’s inertial guidance system figured out its position by sensing changes in its speed and direction. The position was double-checked by taking sightings of stars using a sextant and telescope. Thirty-seven stars were used as guides to navigate the spacecraft. The sextant, telescope, inertial guidance sys­tem, and guidance com­puter provided Apollo’s primary guidance, naviga­tion, and control system (PGNCS). The astronauts called it “Pings.”

О Timing and precision are crucial to space navigation. Scientists successfully launched the probe Deep Impact to intercept the comet Tempel 1 in 2005. The probe released an impactor to create a crater, so releasing debris to gain informa­tion about the comet’s interior.

The most precise Apollo navigation system was not in the spacecraft at all. It was on Earth. The huge radio dishes of NASA’s Deep Space Network were trained on the spacecraft and relayed communications between Apollo mis­sions and Earth. Tracking a spacecraft using these dishes showed exactly where it was. This information was sent up to the spacecraft and used to correct any errors in its own guidance system. If the spacecraft lost radio contact with Earth, its own guidance system would be cor­rect and could guide it until contact with Earth was restored. Some losses of con­tact were expected. While the spacecraft

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SEE ALSO:

• Bird • Communication • Global

Positioning System • Radar

• Space Probe

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was in orbit around the Moon, it lost touch with Earth every time it disap­peared around the far side of the Moon. NASA still uses the dishes of the Deep Space Network in California, Spain, and Australia to communicate with space probes in all parts of the solar system.

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AN EXPENSIVE ERROR

When navigation goes wrong, the results can be catastrophic. In 1998 NASA sent a probe named the Mars Climate Orbiter to Mars. It was sup­posed to orbit Mars, but instead it entered the planet’s atmosphere and burned up. An inquiry found that some of the navigation data was calculated using U. S. standard units (feet/pounds/seconds), and this became mixed up with data calcu­lated in metric units (meters/ kilograms/seconds). The navigational error resulted in the loss of the multimillion-dollar spacecraft.