Scientists had thought that the global magnetic fields of these big asteroids were probably created by the circulation of heat energy within the core. This process would likely have petered out relatively quickly, after a few tens of millions of years at most, researchers said.
But the new X-ray observations, along with computer simulations performed by the study team, paint a different picture: The pallasite parent body's magnetic field lasted for a long time — perhaps several hundred million years. Furthermore, this field was probably generated by the progressive solidification of the core — a phenomenon known as "compositional convection," rather than thermal convection as previously presumed, researchers said. (When the core solidified completely, the magnetic field died.)
Magnetic Fields of Asteroids Lasted Hundreds of Millions of Years
It is suggested that you can not have a magnetic field without some form of electrical current.
A magnetic field is the magnetic influence of electric currents and magnetic materials ... Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin.
If asteroids and comets (active asteroids) water vapour/jets are a form of electrochemical process then this may show how they could have an magnetic field for so long?
Especially as the Suns plasma (solar wind) is electrically charged and the asteroids are cutting through it and the suns magnetic lines/field.
Or that asteroids were created by some form of plasma discharge, a massive electrical event, and perhaps still being electrically attacked at times?