This is Phobos, the larger of the two Martian moons. It is about 13 miles long and 9 miles wide.
Scientists think that it and the other Martian moon called Demios are old asteroids that have been captured by Mars in the distant past.
Phobos is like a city in space. People live in tunnels that have been
dug all through the little moon. There are shops, offices, factories,
schools, hotels and even greenhouses for growing food. They are all
connected via a system of tube trains.
There are four tube tracks that run the length of the moon, called
Red Track, Yellow Track, Blue Track and Green Track. These are connected
by four circular tracks, called the A Ring, B Ring, C Ring and D Ring.
The stations on each ring are numbered from 1 to 20. At each station
there is a large open concourse area with shops, restaurants etc...
and many tunnels leading off to other areas with offices and homes.
All the living areas are just under the surface of the moon and they are
given an artificial gravity by spinning Phobos on it's long axis. This
is done with rocket motors round the edge, all computer controlled.