Sending humans into deep space isn't practical -- yet. But we can send a robot -- a spacecraft
that observes, listens, and processes information and commands. A spacecraft listens to our
commands through its antenna and radio receiver, and sends images and science data back to
Earth.
The spacecraft's brain is a computer, which controls the scientific instruments. On its journey,
the spacecraft needs fuel for its engines to burn -- a large engine is used for big changes such
as entering orbit; and small thrusters fine-tune the direction. The probe did not need fuel --- it
was released and captured by gravity as it plunged into Jupiter's atmosphere.
Planets and moons emit or reflect radiation. We use Galileo's scientific instruments to see
different kinds of radiation --- and even to look through clouds or beneath the solid surface of a
moon.
A camera sees the way we do, at wavelengths of light we've called "visible." But we are
surrounded by other wavelengths of radiated energy that we cannot see. We talk on cellular
phones -- they use radio waves -- cook in ovens that use microwaves, and change TV channels
with remote-control units that use infrared. We avoid ultraviolet by wearing dark glasses and sun
block.
There are 10 scientific instruments aboard the spacecraft. Some of the instruments are
mounted on long arms so they don't pick up unwanted signals from the spacecraft. The probe
used six instruments plus its radio to investigate Jupiter's atmosphere. In addition, we use
Galileo's radio system for experiments.
Related Links:
Next: Observing Atmospheres 
