When a mage enters the Astral Realms, he creates a simulacrum of himself, which is commonly known as an Aquaster. This body interacts with the astral environment as if it were solid, with all the same rules and laws it follows in the external world. A mage can, of course, alter these laws through magic.
The Aquaster is sustained by the mage's Willpower, if the willpower drops to zero, the mage simply wakes up and cannot travel again to the Astral realms if he has not gained one willpower point.