CHENNAI: The country's space programme is poised for a historic RPT historic milestone tomorrow when ISRO 's workhorse PSLV rocket will put two foreign satellites in orbit in the agency's 100th mission from the spaceport at Sriharikota in Andhra Pradesh, about 110 km from here.Prime Minister Manmohan Singh will witness the launch of PSLV C-21, scheduled at 9.51 AM, that will carry French advanced remote sensing satellite 720 kg SPOT-6 and its 15 kg Japanese micro satellite PROITERES. The mission is without any Indian satellite The 51-hour countdown which began at 6.51 AM on Friday 'is as per time schedule," an ISRO official said."Weather conditions are fine, though it had rained yesterday," he said.The Prime Minister, who arrived here this evening en route to Sriharikota, will stay there overnight.From the time of its humble beginnings when it launched the indigenous ' Aryabhatta ' in Russia on April 19, 1975, it has been a long and successful odyssey for ISRO.SPOT-6 is the biggest commercial lift so far since India forayed into the money-spinning commercial satellite launch services after 350kg Agile of Italy was put in orbit in 2007 by PSLV. Twelve other foreign commercial satellites were less than 300 kg.France's five earlier SPOT satellites were launched by European Araine rocket.SPOT-6, built by ASTRIUM SAS,a subsidiary of EADS, France, is an earth observation satellite, while PROITERES, developed by students and faculty of Osaka Institute of Technology, will study Kansai region of Japanese island of Honshu.