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Fifty-two years ago this week, the United States Navy successfully orbited the Transit 4A navigation satellite which carried the SNAP-3A Radioisotope Thermoelectric Generator (RTG).  This historic mission marked the first time that an RTG was used as a spacecraft power source.

Transit was the first operational satellite navigation system.  More formally known as the Navy NAVSAT (Navigation Satellite) System, Transit provided accurate global position data in support of naval worldwide sea operations.  The navigation of submarines and surface ships was greatly aided by Transit-provided data as were sundry hydrographic and geodetic surveying programs.

Transit was developed for the Navy by the Applied Physics Laboratory of the Johns Hopkins University (JHU/APL).  Work began in 1958 and launch of the first prototype Transit satellite, Transit 1A, took place in September 1959.  A number of Transit satellite launches took place over the next 5 years with the system going operational in 1964.

Transit satellites provided position data that was accurate to within about 3 feet.  The system revolutionized global navigation and was ultimately used by an untold number of ships and boats.  Transit navigational operations ceased in 1996 with the advent of the Global Positioning System (GPS).  One of the great benefits of GPS is that position data are provided continuously whereas Transit provided discrete data about once an hour.

Transit 4A was unique in that an experimental Radioisotope Thermoelectric Generator (RTG) was carried onboard the spacecraft.  An RTG coverts the heat generated by the natural decay of a radioisotope fuel into electricity.  This device is especially useful for power generation applications where solar arrays are either impractical or inadequate.  An example application would be a long duration mission in deep space.

The 3-watt RTG carried onboard Transit 4A was officially known as the Systems For Nuclear Auxiliary Power (SNAP-3).  Being experimental in nature, it only provided power to instrumentation and a pair of Transit 4A’s quartet of radio transmitters.  Later versions would provide power for all spacecraft systems.

The Transit 4A satellite was cylindrical in shape, measuring 43 inches in diameter and 31 inches in height.  The spacecraft weighed 174 lbs.  The majority of its external surface was covered with solar cells that charged nickel-cadmium batteries.

On Thursday, 29 June 1961, Transit 4A was launched from Cape Canaveral’s LC-17B at 04:22 UTC.  The Thor Ablestar 315 launch vehicle successfully placed Transit 4A into a 596-nm x 515-nm earth orbit.  Two other satellites (GRAB and Injun) were also orbited on this mission.  They separated from Transit 4A, but not from each other.  GRAB 3 had a SIGINT mission.  Injun was a USN satellite investigating radiation in Earth’s magnetosphere.

Transit 4A became the longest continuous broadcasting spacecraft in 1966.  It continued to hold that distinction through 1971; the type’s 10th anniversary in space.  At that point, the satellite had traveled more than 1.7 billion miles in space and flown around the Earth more than 55,000 times.

By the end of 1996, the Transit satellites were no longer utilized for navigation purposes and were superseded by the Navstar Global Positioning System (GPS).  However, Transit satellite systems were still operating well and the spacecraft continued to transmit valuable data from orbit.  The Navy renamed the Transit Satellite System as the Navy Ionospheric Monitoring Systems (NIMS).

The RTG technology pioneered on the Transit 4A mission matured significantly over the next five decades.  During that time, RTG’s provided a safe, reliable, and maintenance-free means for generating spacecraft thermal and electrical power.  Indeed, RTG’s have proven pivotal to the success of numerous manned and unmanned space missions including those associated with the Apollo, Viking, Pioneer, Voyager, Galileo, Ulysses, and Cassini Programs.

Posted in Aerospace, History

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