Explorer 1

Spacecraft

Explorer 1 was designed and built by California Institute of Technology's JPL under the direction of Dr. William Hayward Pickering. It was the second satellite to carry a mission payload (Sputnik 2 was the first).

The total mass of the satellite was 13.97 kg (30.8 lb), of which 8.3 kg (18 lb) were instrumentation. In comparison, the mass of the first Soviet satellite Sputnik 1 was 83.6 kg (184 lb). The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

Data from the scientific instruments was transmitted to the ground by two antennas. A 60 milliwatt transmitter fed a dipole antenna consisting of two fiberglasses slot antennas in the body of the satellite operating on 108.03 MHz, and four flexible whips forming a turnstile antenna were fed by a 10 milliwatt transmitter operating on 108.00 MHz.

Because of the limited space available and the requirements for low weight, the payload instrumentation was designed and built with simplicity and high reliability in mind, using germanium and silicon transistors in its electronics. A total of 20 transistors were used in Explorer 1, plus additional ones in the Army's micrometeorite amplifier. Electrical power was provided by mercury chemical batteries that made up approximately 40% of the payload weight.

The external skin of the instrument section was sandblasted stainless steel with white stripes. Several other color schemes had been tested, resulting in backup articles, models, and photographs showing different configurations, including alternate white and green striping and blue stripes alternating with copper. The final color scheme was determined by studies of shadow–sunlight intervals based on firing time, trajectory, orbit and inclination.

Science payload

The Explorer 1 payload consisted of the Iowa Cosmic Ray Instrument without a tape data recorder which was not modified in time to make it onto the spacecraft. The real-time data received on the ground was therefore very sparse and puzzling showing normal counting rates and no counts at all. The later Explorer 3 mission, which included a tape data recorder in the payload, provided the additional data for confirmation of the earlier Explorer 1 data.

The scientific instrumentation of Explorer 1 was designed and built under the direction of Dr. James Van Allen of the University of Iowa containing:

• Anton 314 omnidirectional Geiger–Müller tube, designed by Dr. George H. Ludwig of Iowa's Cosmic Ray Laboratory, to detect cosmic rays. It could detect protons with E>30 MeV and electrons with E>3 MeV. Most of the time the instrument was saturated;

• Five temperature sensors (one internal, three external and one on the nose cone);

• Acoustic detector (crystal transducer and solid-state amplifier) to detect micrometeorite (cosmic dust) impacts. It responded to micrometeorite impacts on the spacecraft skin in such a way that each impact would be a function of mass and velocity. Its effective area was 0.075 m² and the average threshold sensitivity was 2.5×10⁻³ g cm/s;

• Wire grid detector, also to detect micrometeorite impacts. It consisted of 12 parallel connected cards mounted in a fiberglass supporting ring. Each card was wound with two layers of enameled nickel alloy wire with a diameter of 17 μm (21 μm with the enamel insulation included) in such way that a total area of 1 × 1 cm (0.39 × 0.39 in) was completely covered. If a micrometeorite of about 10 μm impacted, it would fracture the wire, destroy the electrical connection, and thus record the event.

Flight

After a jet stream-related delay on 28 January 1958, at 03:47:56 GMT on 1 February 1958 the Juno I rocket was launched, putting Explorer 1 into orbit with a perigee of 358 km (222 mi) and an apogee of 2,550 km (1,580 mi) having a period of 114.80 minutes, and an inclination of 33.24°. Goldstone Tracking Station could not report after 90 minutes as planned whether the launch had succeeded because the orbit was larger than expected. At about 06:30 GMT, after confirming that Explorer 1 was indeed in orbit, a news conference was held in the Great Hall at the National Academy of Sciences in Washington, D.C. to announce it to the world.

The original expected lifetime of the satellite before orbital decay was three years. Mercury batteries powered the high-power transmitter for 31 days and the low-power transmitter for 105 days. Explorer 1 stopped transmission of data on 23 May 1958, when its batteries died, but remained in orbit for more than 12 years. It reentered the atmosphere over the Pacific Ocean on 31 March 1970 after more than 58,400 orbits.

Results

Explorer 1 changed rotation axis after launch. The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started precessing due to energy dissipation from flexible structural elements. Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years – to address this kind of momentum-preserving energy dissipation.

Sometimes the instrumentation reported the expected cosmic ray count (approximately 30 counts per second) but other times it would show a peculiar zero counts per second. The University of Iowa (under James Van Allen) observed that all of the zero counts per second reports were from an altitude of more than 2,000 km (1,200 mi) over South America, while passes at 500 km (310 mi) would show the expected level of cosmic rays. Later, after Explorer 3, it was concluded that the original Geiger counter had been overwhelmed ("saturated") by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field. This belt of charged particles is now known as the Van Allen radiation belt. The discovery was considered to be one of the outstanding discoveries of the International Geophysical Year.

The acoustic micrometeorite detector detected 145 impacts of cosmic dust in 78,750 seconds. This calculates to an average impact rate of 8.0⁻³ impacts per second per square meter, or 29 impacts per hour per square meter, over the twelve-day period.

Legacy

Explorer 1 was the first of the long-running Explorers program. Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958. Of these, Explorer 3 and 4 were successful, while Explorer 2 and 5 failed to reach orbit. The final flight of the Juno I booster, the satellite Beacon-1, also failed. The Juno I vehicle was replaced by the Juno II launch vehicle in 1959.

A follow-up to the first mission, Explorer-1 Prime Unit 2, was successfully launched aboard a Delta II launch vehicle in late October 2011. The Prime was built using modern satellite construction techniques. The orbiting satellite was a backup, because the initial Explorer-1 Prime, launched on 4 March 2011, did not reach orbit due to a launch vehicle failure.

An identically constructed flight backup of Explorer 1 is on display in the Smithsonian Institution's National Air and Space Museum, Milestones of Flight Gallery in Washington, D.C., LC-26A was deactivated in 1963 and was designated for use as a museum in 1964, the Air Force Space and Missile Museum. Here too, a full-scale Explorer 1 is on display, but this one is a mock-up.