Vanguard 2

Vanguard 2 or Vanguard II was an earth-orbiting satellite launched 1959-02-17 by the Vanguard SLV 4 launch vehicle as part of Project Vanguard. [cite web | url = http://history.nasa.gov/SP-4202/chap12.html | title = VANGUARD - A HISTORY, CHAPTER 12, SUCCESS - AND AFTER | publisher = NASA]

Mission objectives

The satellite was designed to measure cloud-cover distribution over the daylight portion of its orbit. The spacecraft was a magnesium sphere 20 in (50.8 cm) in diameter. It contained two optical telescopes with two photocells. The sphere was internally gold-plated and externally covered with an aluminum deposit coated with silicon oxide of sufficient thickness to provide thermal control for the instrumentation. Radio communication was provided by a 1 W, 108.03 MHz telemetry transmitter and a 10 mW, 108 MHz beacon transmitter that sent a continuous signal for tracking purposes. A command receiver was used to activate a tape recorder that relayed telescope experiment data to the telemetry transmitter. Both transmitters functioned normally for 19 days. The satellite was spin stabilized at 50 rpm, but the optical instrument's data was poor because of an unsatisfactory orientation of the spin axis. The power supply for the instrumentation was provided by mercury batteries.

Mission results

atellite drag atmospheric density

Because of its symmetrical shape, Vanguard 2 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 Naval Research Lab finally accepted 6.4-inch spheres as a "test vehicle," with twenty inches for follow-on satellites. The payload weight savings, from reduced size as well as decreased instrumentation in the early satellites, was considered acceptable for the initial launches. Afterwards, the later Vanguard rockets had some test instrumentation removed, lightening them enough for the 20" bodies [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 2 has an expected orbital lifetime of 300 years.

Optical scanner

The optical scanner experiment was designed to obtain cloud-cover data between the equator and 35° to 45° N latitude. As the satellite circled Earth, two photocells, located at the focus of two optical telescopes aimed in diametrically opposite directions, measured the intensity of sunlight reflected from clouds (about 80%), from land masses (15 to 20%), and from sea areas (5%). The satellite motion and rotation caused the photocells to scan the earth in successive "lines". Separate solar batteries turned on a recorder only when the earth beneath the satellite was in sunlight and about 50 min of data per orbit were obtained. The measured reflection intensities were stored on tape. Ground stations interrogated the satellite by signaling its command receiver, which caused the entire tape to be played back in 60 s. The tape was then erased and rewound. Experiment equipment functioned normally, but data was poor because of an unsatisfactory satellite spin axis orientation. It is also a weather and observation Satellite.

ee also

* Weather satellite

References