The 51-hr. countdown is under way for India’s latest Polar Satellite Launch Vehicle (PSLV-C21) mission, which will orbit two foreign satellites.

The PSLV-C21 will be launched from the Satish Dhawan space center at Sriharikota in south India at 9:51 a.m. local time on Sept. 9, an official at the Indian Space Research Organization (ISRO) says. The countdown began Sept. 7. The launch will mark ISRO’s 100th mission into space.

The PSLV will orbit two satellites—France’s 800-kg (1,800-lb.) SPOT-6 remote-sensing satellite, built by Astrium, and a 15-kg Japanese microsatellite called Proiteres (Project of OIT Electric Rocket Engine onboard small Spaceship), which is a technology demonstrator for Earth observation carrying a high-resolution camera developed by a team of students and faculty at the Osaka Institute of Technology (OIT).

Both satellites will be injected into an orbit of 655 km (407 mi.) altitude at an inclination of 98.23 deg., the official adds.

In addition to conducting compulsory tests on the launch vehicle and spacecraft, several “propellant-filling operations of liquid propellant second stage (PS2) and fourth stage (PS4) of the launch vehicle will be carried out” during the countdown, the ISRO offical says.

“Also, charging of batteries and pressurization of propellant tanks onboard the satellite will be performed. [The] readiness of various ground systems such as tracking radar systems and communication networks will also be ascertained,” he says.

The four stages of the PSLV have been assembled on the launch pad in Sriharikota. The 230-ton vehicle stands 44 meters (144 ft.) tall.

The PSLV is capable of launching 1,600-kg satellites into a 620-km, Sun-synchronous polar orbit, or 1,050-kg spacecraft into geosynchronous transfer orbit. The rocket’s stages use a mix of solid and liquid propulsion.

The first stage, powered by one of the largest solid rocket boosters in the world, carries 139 tons of propellant. A cluster of six strap-ons are attached to the first-stage motor, four of which are ignited on the ground and two in flight. For this mission, ISRO will not need the six strap-on motors.

The mission will bring ISRO’s total tally of foreign satellites launched to 29. India’s space odyssey began when the Aryabhatta satellite lifted off from the Volgograd launch station at Kapustin-Yar on April 19, 1975. Since then, ISRO has undertaken 99 missions, including 62 satellites and 37 launch vehicles. ISRO also is planning to loft its ambitious Geostationary Satellite Launch Vehicle, with an indigenously built cryogenic engine, early in 2013.

PSLV photo: ISRO