
a) spacecraft- space vehicle; to call- to name; artificial- man-made; significance- importance; area- district; satellite- moon;
purpose- aim; complicated- sophisticated;
explorer- researcher; to guide- to control; to offer- to propose;
investigation- exploration; to incorporate- to include;
to determine- to define; to gather- to collect;
b) special - conventional;
upper- lower; unmanned- manned; natural- artificial;
last- first; unequal- equal; distant- near;
civil- military; inefficien- efficien;