The results of many of the space missions to the Moon and planets have already been detailed in the chapters on the Solar System. In the present section we outline some of the methods by which these results were obtained. The pattern of study of a planet by subsequent spacecraft generally parallels that applied to the Moon. Initial fly-by missions send back photographs of much higher resolution than obtainable from Earth. Apart from their obvious scientific value, such pictures are also of cultural value, being the results most easily appreciated from such a mission.
Accurate tracking of the spacecraft, usually relying on Doppler shifts of narrow-band radio beams retransmitted back to Earth, give its detailed orbit. This usually improves our knowledge of the planet itself and paves the way for orbital missions. Occultation of the spacecraft, due to the planet passing between it and the Earth, is useful in obtaining data on the atmosphere of the planet. The atmosphere acts like a radio lens, deflecting and altering the velocity of the radio beam. Thus, in the few seconds during which the spacecraft enters and exits from occupation, its path inferred from the Doppler tracking appears to waver by a few metres or so . The precise nature of the changes gives a measure of the refractive index of the atmosphere and thus its mean molecular density. Complete density profiles can be built up, at least above layers which do not severely absorb the radio waves, and from these, relationships between atmospheric pressures and temperatures can be deduced. Independent knowledge of the molecular constitution of the atmosphere allows the pressure and temperature to be separated.
Devices for measuring magnetic fields (magnetometers) and charged particles can reveal details of any planetary magnetosphere. Instruments scanning the planet at selected infrared wavelengths can deduce, for example, the water or carbon dioxide content of the atmosphere and the surface temperature, perhaps down to a frac¬tion of a degree.
The next stage of instrumentation used in the exploration of the planet is usually an orbiter, finally followed by a landing system or in the case of Jupiter or Saturn, atmospheric probes. ORBITERS Are usually designed to take adequate maps for landing craft, which often means a surface resolution of tens of metres or less. This may decide the dimension of the imaging system, which is probably similar to a television camera, having an optical system and vidicon screen. One picture may be composed of about one million picture elements (PIXELS), each of which are coded according to the degree of light or shade. This totals about 10 million bits (binary digits) per picture, which must be temporarily stored on board, for the telemetry bit rate may only be one quarter of a million bits per minute. Repeated occultations allow density profiles of the atmosphere to be constructed for a variety of diurnal and seasonal conditions. Infrared and ultraviolet spectroscopy reveal the nature of cloud cover, and enable some details of the surface composition and temperature etc. to be carefully mapped. Accurate tracking of the spacecraft permits a detailed gravitational model for the planet to be built up. This yields the overall mass distributions within the planet itself.
Landing craft have to be built specifically for each planet, or moon, taking into consideration the atmospheric density and surface conditions. Close-up television pictures may be obtained, as well as the analysis of soil samples. Seismometers monitor the internal creaking of the planet and stray meteor hits. Rocket boosters can be deliberately impacted onto the surface, as was done on the Moon, in order to provide valuable seismic information. The atmospheric composition may be measured on the descent, as well as at the landing site, by mass spectrometers. Soil samples can be analysed by the powerful technique of X-ray fluorescence.
The Moon is the only celestial body yet visited by man, and is likely to remain so for a considerable time to come. Automatic roving vehicles, such as implemented by the USSR on the Moon are a more realistic means of obtaining samples from extended regions on distant planets. The Lunokhods were controlled by television from Earth, a method which will prove impracticable over the 8 to 46-minute time delay to Mars. Such rovers will need to be sophisticated enough to make their own decisions – by computer of course. It is possible to launch craft to Mars, for example, which can land and then return soil samples to Earth. To simplify the mission and reduce weight, the final package of only a few kilograms may well return to Earth orbit, to await retrieval by the space Shuttle.
Notable planetary spacecraft of the late-1970s included the PIONEER and MARINER series of the US and the VENERA and MARS series of the USSR. The Pioneer spacecraft were designed to take approximately 30kg of scientific payload on extended missions, such as to the outer planets. Stabilization is provided by spinning the whole craft, and solar power may be supplemented by radio-isotope generators. The Mariner series carry about 70kg of scientific payload and the spacecraft are stablized by gas jets.
An outstanding achievement in 1976 was the safe landing of two Viking craft on Mars. These provided close-up photographs of the rocks on Mars and analysed the soil for traces of life.