This attack came suddenly without a warning, but the thing was that James and others knew this attack wouldn't succeed… alone.
The base wasn't only protected by this external shield, but there was another one internal, surrounding the planets inside the base. It was said, according to the words of Anasta and other gossips from James' allies, that this shield was sturdier than the external natural one.
However, Rigo managed to crack its secret not long ago. The shield worked by harvesting the energy from the black holes surrounding the place, and it did this on pulses, with time intervals to charge and restart the shield again.