The icy nucleus of comet 103P/Hartley 2 measures no more than a couple of kilometers across. That tiny nugget, however, is surrounded by an vast atmosphere of gas more than 150,000 km in diameter—
Pretty unusual, it doesn’t sound like outgassing but
what kind of mass would you need to hold that atmosphere
in place, to travel along with the nucleus??
It’s not being held in place. It is diffusing out into space.