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.