ISRO is ready to launch the historic PSLV-C21 on Sunday morning. The wholly commercial launch will be the space agency's 100th mission in 49 years. So far it has built 62 satellites and flown 37 launch vehicles. Prime Minister Manmohan Singh is slated to arrive at the space port of Sriharikota, some 120 km from Chennai and located in coastal Andhra Pradesh, on Saturday evening and witness the launch on 9.51 a.m. on Sunday. The PSLV will carry France’s SPOT-6 earth observation satellite as the primary payload and PROITERES, a small spacecraft built by a team of Osaka Institute of Technology in Japan, as a secondary rider. They will be put into their respective pole-to-pole orbits at a distance of 655 km from Earth. Scientists at the launch site, Satish Dhawan Space Centre at Sriharikota, began a 51-hour countdown at 6.51 a.m. on Friday, a release said. The Launch Authorisation Board met on Thursday and cleared the event. During the run-up, teams associated with the launch will complete filling liquid propellants in the second and fourth stages of the launch vehicle. The rocket and the spacecraft will be checked. Batteries will be charged and the fuel tanks on the satellites will be pressurised, the ISRO officials said.