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The Transit Research And Attitude Control (TRAAC) satellite was launched by the US Navy along with the Transit 4B satellite on 15 November 1961. It was used to test the feasibility of using the Earth's gravitational attraction to orient and stabilize a vehicle in space. The concept was that a satellite with a long axis would orient itself with the gravity gradient, so that one end would always point downward towards the Earth, minimizing or eliminating the need for attitude control thrusters. Although the gravity gradient attitude control experiment did not work as planned, TRAAC played a key role in monitoring the Starfish Prime space-based nuclear test.

The original primary objectives of the mission were to; 1) back up satellite Transit 4B in improving knowledge of Earth's gravitational field; 2) demonstrate the principle of gravity gradient stabilization; 3) improve the delineation of the number density of protons in the inner Van Allen Belt, search for trapped particles heavier than proton, check the cosmic ray neutron albedo theory of the origin of the inner Van Allen Belt; and 4) test advanced engineering concepts (such as deployment of a weak "lossy" spring one coil at a time, from a subliming encapsulation, and the damping of libration by means of this spring.)

Spacecraft and Subsystems

The TRAAC satellite had a disk-shaped body, 43 in (109 cm) in diameter with an approximately 28 cm diameter cylinder protruding from the center axis, giving it a total height of 41 in (104 cm), of which approximately 2/3 is the cylinder and 1/3 is the disk. Total mass of the spacecraft was 104.76 kg. The center cylinder held the gravity attitude control mechanism, comprising an extendible boom, a long spring, and a weight. The boom is made of 2 in (5.08cm) wide by 0.002 in (0.005 cm) beryllium copper tape rolled onto a hub. When extended by a motor, it forms a 60 ft (18.3 m) cylinder with a diameter of 0.45 in (1.14 cm). Attached to the end of the boom is an ultraweak, high energy dissipating spring (for libration damping) with a 5 lb (2.27 kg) mass on the end. The spring consists of 142 turns of 0.007 in diameter beryllium copper wire, coated with a 0.0008 in thick layer of cadmium and a 0.0002 in thick layer of silver. Extended, the spring had a length of 40 to 45 feet (12.2 to 13.7 m), bringing the deployed length of the boom apparatus with the weight on the end to 100 to 105 ft (30.5 to 32 m).

Power was supplied by a ring of solar cells around the perimeter of the disk. These cells charge an array of nickel-cadmium batteries. Minimum power generation is 16 W. There is also a backup battery for the command system in case of failure of the primary system. A standard Transit-type Doppler system was onboard, consisting of a 3-MHz ultra-stable oscillator and frequency multiplier, and 54 MHz and 324 MHz transmitters, to be used as a backup for the Transit 4B satellite if needed. The communications system used a command receiver, 136 MHz transmitter, a whip antenna and an omnidirectional antenna comprising a turnstile of four single elements to transmit telemetry from a 30 channel PDM/PAM/FM/PM analog system and binary digital encoder (256 bits/sec).

Attitude knowledge was provided by a system of eight spin rate detectors - solar cell and photo diodes used to determine the spin rate and orientation of the satellite with respect to the Sun. It also had a three-axis vector magnetometer system to determine its orientation with respect to the Earth's magnetic field. The satellite had four magnetic despin rods and a set of four electromagnets for magnetic orientation. Each electromagnet was a solenoid of 3600 turns of copper wire around an iron core, directed along the symmetry axis.

In addition to the boom, the TRAAC satellite science and engineering experiment payload comprised a neutron detector, six proton and alpha particle detectors, two geiger counters, a solar cell experiment, and a solder sublimation experiment.

Mission Profile

TRAAC was launched with Transit 4B on a Thor Able-Star from the Eastern Test Range of Cape Canaveral at 22:25:39.1 UT (5:25 p.m. EST) on Nov. 15, 1961 into a 957 x 1109 km orbit with a period of 105.8 minutes and an inclination of 32.4 degrees. The Doppler transmitters were turned off in the first few days because they were interfering with spacecraft communication. Two days after launch the Z-electromagnet was turned on, causing the satellite to align within 10 degrees of the local magnetic field by 21 November. On 24 November the electromagnet was turned off and the boom was commanded to extend, but for unknown reasons it malfunctioned. Later analysis indicated that the weight and spring had deployed, and this did serve to substantially damp satellite oscillations, making the electromagnet assisted orientation much more effective.

On 9 July 1962 at 0900 UT a 1.4 megaton yield nuclear device was detonated in space at an altitude of 400 km over Johnston Atoll in the Pacific Ocean. The test, designated "Starfish Prime", caused a massive electromagnetic pulse and large increases in the energetic particle fluxes in space, particularly in the Van Allen belts. TRAAC was operational at the time and observed the effects of the blast and its aftermath. TRAAC showed a sharp drop in the large electron flux over the first 9 hours, but also identified a "Pacific hot spot", a moderately intense, very narrow trapped radiation zone emitting energetic electrons. Spacecraft components, particularly the solar cells, were damaged by the intense radiation environment, but TRAAC continued to operate and return data until its final communication on 12 August 1962, measuring the albedo neutron and energetic electron flux. Data up to that point indicated that the Starfish injected electrons would persist for much longer than originally predicted in the inner Van Allen Belt. TRAAC remains in a roughly 950 km Earth orbit with an expected lifetime of hundreds of years.

Starfish Prime

Starfish Prime was an American high-altitude nuclear test that took place on 9 July 1962. Launch took place from Johnston atoll in the Pacific Ocean (about 1330 km southwest of Honolulu) at 8:46:28 UT on 9 July 1962 (10:46:28 pm, 8 July local time) on a Thor rocket carrying a W49 thermonuclear warhead. Detonation of the warhead occurred at 09:00:09 UT on 9 July (11:00:09 pm local time, 8 July) at an altitude of 400 km. Total yield was 1.4 megatons. The explosion, occurring at a geomagnetic latitude 10.5 degrees, generated an electromagnetic pulse and large quantities of charged particles. These had the effect of damaging many operating satellites, both at the time of the blast and later, as the energetic particles remained trapped in the Earth's magnetic field, forming an artificial radiation belt that persisted for many days after the explosion. These damaged satellites include TRAAC, Transit-4B, Ariel 1, Cosmos 5, Telstar 1, Explorers 14 and 15, and possibly Injun 1, OSO-1, Alouette 1, and ANNA-1B.

Image credit: National Museum of the U.S. Navy

Alternate Names

  • 00205
  • 1961 Alpha Eta 2
  • Transit Research And Attitude Control

Facts in Brief

Launch Date: 1961-11-15
Launch Vehicle: Thor-Able-Star
Launch Site: Cape Canaveral, United States
Mass: 105 kg

Funding Agency

  • Department of Defense-Department of the Navy (United States)


  • Space Physics
  • Engineering

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. David R. Williams



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