Two spacecraft will set out shortly from opposite sides of the globe to study Mars and its atmosphere. India’s Mars Orbiter Mission is scheduled to leave next Tuesday (Nov. 5) aboard a Polar Satellite Launch Vehicle (PSLV) from Sriharikota. America’s MAVEN — an acronym for ‘Mars Atmosphere and Volatile Evolution’ mission — will follow about a fortnight later.

They will, however, follow different paths to their destination. MAVEN will be making the first leg of its journey on an Atlas V, a rocket considerably more powerful than the PSLV. It will be able to put the spacecraft on a direct course for the Red Planet, a luxury the Indian probe will not enjoy.

The PSLV will leave the spacecraft in an elliptical orbit 250 km at its closest to Earth and 23,500 km at its farthest. To get to Mars, the orbiter must repeatedly fire its own liquid propellant engine. In doing so, it will become the first Indian spacecraft to cross Earth's escape velocity of 11.2 km per second, the threshold beyond which Earth’s gravity can no longer pull it back.

As the amount of propellant remaining at the end of the journey will be an important factor in determining the spacecraft’s life, scientists and engineers of the Indian Space Research Organisation (ISRO) have done their best to hold down its consumption during these manoeuvres.

The onboard engine will be fired five times, each time lengthening the spacecraft’s elliptical course around Earth. With multiple burns, the engine’s performance and the velocity imparted to the spacecraft on each occasion can be taken into account in planning the next firing.

Thus the necessary velocity can be added more accurately, reducing need for corrections later. This strategy was successfully used when India sent the Chandrayaan-1 probe to the Moon in 2008.

The sixth firing of the engine, scheduled for November 30, will push the probe beyond escape velocity and put it on a carefully chosen propellant-saving trajectory for Mars. The probe will take nearly 300 days to traverse some 400 million km.

As the spacecraft gets close to Mars, its engine must again fire, this time to reduce velocity and put it into orbit around that planet.

Charting the spacecraft’s course accurately requires elaborate modelling, according to the ISRO chairman, K. Radhakrishnan. The effect of Earth’s gravity as well as that of the Sun, Moon, Mars, the two Martian moons and the other planets are among the many factors that needed to be incorporated in the calculations.

“When the probe leaves Earth's orbit on November 30, the position of its arrival near Mars on September 21, 2014 has to be estimated with a precision of 50 km,” Dr. Radhakrishnan told this correspondent.

The results from ISRO's models had been benchmarked against computations carried out by Jet Propulsion Laboratory in the U.S., which has overseen many interplanetary missions.

Once the spacecraft successfully enters Martian orbit, its scientific instruments can be switched on. Given the high failure rate of missions to Mars, ISRO will be keeping its collective fingers crossed.

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