Vanguard 3

Vanguard 3 or Vanguard III was launched by a Vanguard rocket on Sep. 18, 1959, the third successful Vanguard launch out of eleven attempts.

Mission Objectives

The satellite was launched from the Eastern Test Range into a geocentric orbit. The objectives of the flight were to measure the earth's magnetic field, the solar X-ray radiation and its effects on the earth's atmosphere, and the near-earth micrometeoroid environment. Instrumentation included a proton magnetometer, X-ray ionization chambers, and various micrometeoroid detectors. The spacecraft was a 50.8-cm-diameter magnesium sphere. The magnetometer was housed in a glass fiber phenolic resin conical tube attached to the sphere. Data transmission stopped on December 11, 1959, after 84 days of operation. The data obtained provided a comprehensive survey of the earth's magnetic field over the area covered, defined the lower edge of the Van Allen radiation belt, and provided a count of micrometeoroid impacts. Vanguard 3 has an expected orbital lifetime of 300 years.

Mission Results

Proton Precessional Magnetometer

This experiment had a proton precessional magnetometer to measure the earth's magnetic field at altitudes ranging from 514 to 3714 km and at latitudes between plus or minus 33.4 deg. The measurements were made on command as the spacecraft passed seven minitrack stations in North and South America and one each in Australia and South Africa. When switched on by command, the polarization coil around the proton sample (normal hexane) was turned on for 2 s followed by a 2 s readout of the precession signal. Several readings were taken during each pass over a station. The experiment worked well during its 85-day active life, and approximately 4300 readings were recorded. The experiment is described in J. C. Cain et al., "Measurements of the geomagnetic field by the Vanguard 3 satellite," NASA TN D-1418, Goddard Space Flight Center, Greenbelt, Md., 1962. The overall accuracy of the field measurements was approximately 10 nT (gammas).

Micrometeorite Detector

This experiment contained two sealed pressure zones, extending along the interior walls of the satellite, which were designed to record the impact of micrometeorites large enough to pierce the satellite shell. These pressure zones were partial vacuums, each at a different pressure, and were protected by 0.66 mm magnesium walls that presented an exposed surface area of 0.162 m², which was 20% of the area of the shell. A puncture in the walls of either zone was detected by a differential pressure gauge mounted between them, and telemetered as a change in the length of one of the telemetry channels. Erosion of the satellite shell through bombardment by space dust, micrometeorites, and other particles was recorded by three chromium-strip erosion gauges mounted on the satellite surface, and by a photosensitive detector. Electrical resistances of the gauges changed as their surfaces were changed by erosion. The photosensitive detector, a cadmium sulfide cell protected by an opaque covering of aluminized PET film, also showed a resistance change as the covering was eroded or penetrated. Erosion measurements also were telemetered as channel lengths, which permitted estimates of the erosion rates. Four barium titanate-type microphones recorded micrometeorite impacts on the satellite's surface. The microphone output was amplified, shaped, and fed into a magnetic counter unit, which provided continuously, in three-decimal digits, the cumulative count of impacts. The unit counted up to 1000 and then reset to zero. The satellite recorded 6600 micrometeorite impacts during 66 days of operation, of which 2800 occurred during a 70-h interval from November 16 to 18, 1959. No penetrations or fractures were recorded in the sensors of the surface penetration experiment. Because the erosion sensors were not disrupted, no definite results could be drawn from that experiment.

atellite Drag Atmospheric Density

Because of its symmetrical shape, Vanguard 3 was selected by the experimenters for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity. As the spacecraft continuously orbited, it would lag its predicted positions slightly, accumulating greater and greater delay due to drag of the residual atmosphere. By measuring the rate and timing of orbital shifts, the relevant atmosphere's parameters could be back-calculated knowing the body's drag properties. It was determined that atmospheric pressures, and thus drag and orbital decay, were higher than anticipated, as Earth's upper atmosphere tapered into space gradually.

This experiment was very much planned prior to launch. Initial Naval Research Laboratory proposals for Project Vanguard included conical satellite bodies; this eliminated the need for a separate fairing and ejection mechanisms, and their associated weight and failure modes. Radio tracking would gather data and establish a position. Early in the program, optical tracking (with a Baker-Nunn camera network and human spotters) was added. A panel of scientists proposed changing the design to spheres, at least twenty inches in diameter and hopefully thirty. A sphere would have a constant optical reflection, and constant coefficient of drag, based on size alone, while a cone would vary with orientation. James Van Allen proposed a cylinder, which eventually flew. The Project finally accepted 6.4- and 20-inch satellites [C. McLaughlin Green, M. Lomask, "Vanguard, A History," Chapter 5, Battle over Vehicle Specifications. NASA SP-4202.] .

As the three Vanguard satellites are still orbiting, with their drag properties essentially unchanged, they form a baseline atmospheric dataset fifty years old and counting. Vanguard 3 has an expected lifetime of 300 yr.

X-Ray Experiment

The objective of this experiment was to measure the X-ray emission from the sun and its effects on the earth's atmosphere. The detectors were two identical ionization chambers sensitive to X-ray wavelengths produced in solar flares (2 to 8 Å, or 200 to 800 pm). The ionization chambers were located 120 deg apart in the equatorial plane of the satellite and received a maximum signal when an ion chamber tube "looked" toward the sun. The instrumentation measured the 2 to 8 Å (200 to 800 pm) X-ray flux, and recorded the peak solar flare intensity by means of a peak-reading memory device, during the daylight portion of each orbit.

References