Spacecraft that do not have to carry human
beings can be of a great variety of sizes, from a few centimetres to several
metres in diameter, and of many shapes, depending on the purposes for which
they are designed. Spacecraft that do not carry a crew have
radio-transmitting equipment, both to relay information back to Earth and to
signal the position of the spacecraft.
Manned spacecraft must fulfil more exacting requirements than unmanned
vehicles because of the needs of the human occupants. A manned space vehicle
is designed to provide air for the astronauts, food and water, navigation
and guidance equipment, seating and sleeping accommodation, and
communication equipment, so that the astronauts can send and receive
information. A distinctive feature of manned spacecraft is the heat shield
that protects the vehicle as it re-enters the atmosphere.