The interplanetary medium is a tenuous mixture of dust and ionized gas, mostly hydrogen, that pervades the Solar System and carries the expanding solar magnetic field. Dust is present as fragments from collisions in the asteroid belt and also as dust from comets that break up during their passage through the inner Solar System. Charged particles are provided by the Sun in the form of the solar wind. There are also neutral gas atoms present that are destroyed close to the Sun but are replenished from the interstellar medium.