Moscow, Aug 24 (IANS/RIA Novosti): NASA will launch two probes early Friday to study the enigmatic radiation belts around the earth that could be hazardous for spacecraft and astronauts.

An Atlas V carrier rocket with twin Radiation Belt Storm Probes(RBSP) is scheduled to lift off at 8.07 a.m. GMT from Florida's Cape Canaveral Air Force Station.

The $686-million RBSP mission will help scientists to get an insight into the physical dynamics of the Van Allen Belts - conglomerations of highly energised particles extending up to 40,000 km around the earth.

"Understanding the radiation belt environment and its variability has extremely important practical applications in the areas of spacecraft operations, spacecraft and spacecraft system design, mission planning, and astronaut safety," NASA said on its website.

The rocket has been cleared for launch after extensive additional testing of the booster engine actuator system on the Russian-made RD-180 engines and a thorough data analysis.

The belts contain electrons and protons at various energy levels, as well as heavier particles of ionised oxygen and helium, which are trapped and formed into rings by earth's magnetic field.

An inner Van Allen belt extends from an altitude of 1,600 km to 13,000 km, compared to the orbital altitude of the International Space Station (about 390 km).

The outer radiation belt extends from 19,000 to 40,000 km (12,000 to 25,000 miles) in altitude. Geosynchronous communications satellites orbit just inside the outer edge of this radiation belt.

The belts are constantly growing and shrinking in unpredictable patterns along with changes in solar activity, or "space weather."

During the two-year mission, the heavily-armoured probes will operate in tandem to help scientists better understand the complicated behaviour of the radiation belts and their reactions to solar storms.