Taken from the NASA/JPL information summary "Our Solar System at a Glance".
Each spacecraft consists of various scientific instruments selected for a particular mission, supported by basic subsystems for electrical power, trajectory and orientation control, as well as for processing data and communicating with Earth.
Electrical power is required to operate the spacecraft instruments and systems. NASA uses both solar energy from arrays of photovoltaic cells and small nuclear generators to power its solar system missions. Rechargeable batteries are employed for backup and supplemental power.
Imagine that a spacecraft has successfully journeyed millions of miles through space to fly but one time near a planet, only to have its cameras and other sensing instruments pointed the wrong way as it speeds past the target! To help prevent such a mishap, a subsystem of small thrusters is used to control spacecraft.
The thrusters are linked with devices that maintain a constant gaze at selected stars. Just as Earth's early seafarers used the stars to navigate the oceans, spacecraft use stars to maintain their bearings in space. With the subsystem locked onto fixed points of reference, flight controllers can keep a spacecraft's scientific instruments pointed at the target body and the craft's communications antennas pointed toward Earth. The thrusters can also be used to fine-tune the flight path and speed of the spacecraft to ensure that a target body is encountered at the planned distance and on the proper trajectory.
Between 1959 and 1971, NASA spacecraft were dispatched to study the Moon and the solar environment; they also scanned the inner planets other than Earth -- Mercury, Venus and Mars. These three worlds, and our own, are known as the terrestrial planets because they share a solid-rock composition.
For the early planetary reconnaissance missions, NASA employed a highly successful series of spacecraft called the Mariners. Their flights helped shape the planning of later missions. Between 1962 and 1975, seven Mariner missions conducted the first surveys of our planetary neighbors in space.
All of the Mariners used solar panels as their primary power source. The first and the final versions of the spacecraft had two wings covered with photovoltaic cells. Other Mariners were equipped with four solar panels extending from their octagonal bodies.
Although the Mariners ranged from the Mariner 2 Venus spacecraft, weighing in at 203 kilograms (447 pounds), to the Mariner 9 Mars Orbiter, weighing in at 974 kilograms (2,147 pounds), their basic design remained quite similar throughout the program. The Mariner 5 Venus spacecraft, for example, had originally been a backup for the Mariner 4 Mars flyby. The Mariner 10 spacecraft sent to Venus and Mercury used components left over from the Mariner 9 Mars Orbiter program.
In 1972, NASA launched Pioneer 10, a Jupiter spacecraft. Interest was shifting to four of the outer planets -- Jupiter, Saturn, Uranus and Neptune -- giant balls of dense gas quite different from the terrestrial worlds we had already surveyed.
Four NASA spacecraft in all -- two Pioneers and two Voyagers -- were sent in the 1970s to tour the outer regions of our solar system. Because of the distances involved, these travelers took anywhere from 20 months to 12 years to reach their destinations. Barring faster spacecraft, they will eventually become the first human artifacts to journey to distant stars. Because the Sun's light becomes so faint in the outer solar system, these travelers do not use solar power but instead operate on electricity generated by heat from the decay of radioisotopes.
NASA also developed highly specialized spacecraft to revisit our neighbors Mars and Venus in the middle and late 1970s. Twin Viking Landers were equipped to serve as seismic and weather stations and as biology laboratories. Two advanced orbiters -- descendants of the Mariner craft -- carried the Viking Landers from Earth and then studied Martian features from above.
Two drum-shaped Pioneer spacecraft visited Venus in 1978. The Pioneer Venus Orbiter was equipped with a radar instrument that allowed it to "see" through the planet's dense cloud cover to study surface features. The Pioneer Venus Multiprobe carried four probes that were dropped through the clouds. The probes and the main body -- all of which contained scientific instruments -- radioed information about the planet's atmosphere during their descent toward the surface.
A new generation of automated spacecraft -- including Magellan, Galileo, Ulysses, Mars Observer and Cassini -- is being developed and sent out into the solar system to make detailed examinations that will increase our understanding of our neighborhood and our own planet.