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 fulfil separate and
specific mission objectives, the craft share much in
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
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 neighbours 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 travellers took
anywhere from 20 months to 12 years to reach their
destinations. Barring faster spacecraft, they will
eventually become the first human artefacts to journey to
distant stars. Because the Sun's light becomes so faint in
the outer solar system, these travellers 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 neighbours
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
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
neighbourhood and our own planet.