Chennai: The 28-hour countdown for the launch of ISRO’s 100th satellite along with 30 others in a single mission, from the space port of Sriharikota, about 110 km from here, began on Thursday. On its 42nd mission, ISRO trusted workhorse ‘PSLV-C40’ will carry the weather observation ‘Cartosat-2’ series satellite and 30 co-passengers (together weighing about 613 kg) at lift-off at 9.28 am on Friday. At present, the scientists are involved in propellant filling operation to carry out various stages of the flight.