Our Solar System at a Glance


INTRODUCTION

From our small world we have gazed upon the cosmic ocean for
untold thousands of years. Ancient astronomers observed points of
light that appeared to move among the stars. They called these
objects planets, meaning wanderers, and named them after Roman
deities -- Jupiter, king of the gods; Mars, the god of war;
Mercury, messenger of the gods; Venus, the god of love and beauty,
and Saturn, father of Jupiter and god of agriculture. The
stargazers also observed comets with sparkling tails, and meteors
or shooting stars apparently falling from the sky.

Science flourished during the European Renaissance.
Fundamental physical laws governing planetary motion were
discovered, and the orbits of the planets around the Sun were
calculated. In the 17th century, astronomers pointed a new device
called the telescope at the heavens and made startling
discoveries.

But the years since 1959 have amounted to a golden age of
solar system exploration. Advancements in rocketry after World War
II enabled our machines to break the grip of Earth's gravity and
travel to the Moon and to other planets.

The United States has sent automated spacecraft, then human-
crewed expeditions, to explore the Moon. Our automated machines
have orbited and landed on Venus and Mars; explored the Sun's
environment; observed comets, and made close-range surveys while
flying past Mercury, Jupiter, Saturn, Uranus and Neptune.

These travelers brought a quantum leap in our knowledge and
understanding of the solar system. Through the electronic sight
and other "senses" of our automated spacecraft, color and
complexion have been given to worlds that for centuries appeared
to Earth-bound eyes as fuzzy disks or indistinct points of light.
And dozens of previously unknown objects have been discovered.

Future historians will likely view these pioneering flights
through the solar system as some of the most remarkable
achievements of the 20th century.


AUTOMATED SPACECRAFT

The National Aeronautics and Space Administration's (NASA's)
automated spacecraft for solar system exploration come in many
shapes and sizes. While they are designed to fulfill separate and
specific mission objectives, the craft share much in common.

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,