  Since the atomic clock was invented, its performance has been improved for
one digit every decade until 90s of last century when the traditional atomic
clock almost reached its limit. With laser cooled atoms, the performance can be
further improved, and nowadays the cold atom based clocks are widely used as
primary frequency standards. Such a kind of cold atom clocks has great
applications in space. This paper presents the design and tests of a cold atom
clock (CAC) operating in space. In microgravity, the atoms are cooled, trapped,
launched and finally detected after being interrogated by microwave field with
Ramsey method. The results of laser cooling of atoms in microgravity in orbit
are presented and compared with that on ground for the first time. That the
full width at half maximum (FWHM) of obtained central Ramsey fringes varies
linearly with launching velocity of cold atoms shows the effects of
microgravity. With appropriate parameters, a closed-loop locking of the CAC is
realized in orbit and the estimated short term frequency stability of
$3.0\times 10 ^{-13}/\sqrt{\tau }$ has been reached.
